item.rs

use super::diagnostics::{dummy_arg, ConsumeClosingDelim, Error};
use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
use super::{AttrWrapper, FollowedByType, ForceCollect, Parser, PathStyle, TrailingToken};

use rustc_ast::ast::*;
use rustc_ast::ptr::P;
use rustc_ast::token::{self, Delimiter, TokenKind};
use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
use rustc_ast::{self as ast, AttrVec, Attribute, DUMMY_NODE_ID};
use rustc_ast::{Async, Const, Defaultness, IsAuto, Mutability, Unsafe, UseTree, UseTreeKind};
use rustc_ast::{BindingMode, Block, FnDecl, FnSig, Param, SelfKind};
use rustc_ast::{EnumDef, FieldDef, Generics, TraitRef, Ty, TyKind, Variant, VariantData};
use rustc_ast::{FnHeader, ForeignItem, Path, PathSegment, Visibility, VisibilityKind};
use rustc_ast::{MacArgs, MacCall, MacDelimiter};
use rustc_ast_pretty::pprust;
use rustc_errors::{struct_span_err, Applicability, PResult, StashKey};
use rustc_span::edition::Edition;
use rustc_span::lev_distance::lev_distance;
use rustc_span::source_map::{self, Span};
use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_span::DUMMY_SP;

use std::convert::TryFrom;
use std::mem;
use tracing::debug;

impl<'a> Parser<'a> {
    /// Parses a source module as a crate. This is the main entry point for the parser.
    pub fn parse_crate_mod(&mut self) -> PResult<'a, ast::Crate> {
        let (attrs, items, spans) = self.parse_mod(&token::Eof)?;
        Ok(ast::Crate { attrs, items, spans, id: DUMMY_NODE_ID, is_placeholder: false })
    }

    /// Parses a `mod <foo> { ... }` or `mod <foo>;` item.
    fn parse_item_mod(&mut self, attrs: &mut Vec<Attribute>) -> PResult<'a, ItemInfo> {
        let unsafety = self.parse_unsafety();
        self.expect_keyword(kw::Mod)?;
        let id = self.parse_ident()?;
        let mod_kind = if self.eat(&token::Semi) {
            ModKind::Unloaded
        } else {
            self.expect(&token::OpenDelim(Delimiter::Brace))?;
            let (mut inner_attrs, items, inner_span) =
                self.parse_mod(&token::CloseDelim(Delimiter::Brace))?;
            attrs.append(&mut inner_attrs);
            ModKind::Loaded(items, Inline::Yes, inner_span)
        };
        Ok((id, ItemKind::Mod(unsafety, mod_kind)))
    }

    /// Parses the contents of a module (inner attributes followed by module items).
    pub fn parse_mod(
        &mut self,
        term: &TokenKind,
    ) -> PResult<'a, (Vec<Attribute>, Vec<P<Item>>, ModSpans)> {
        let lo = self.token.span;
        let attrs = self.parse_inner_attributes()?;

        let post_attr_lo = self.token.span;
        let mut items = vec![];
        while let Some(item) = self.parse_item(ForceCollect::No)? {
            items.push(item);
            self.maybe_consume_incorrect_semicolon(&items);
        }

        if !self.eat(term) {
            let token_str = super::token_descr(&self.token);
            if !self.maybe_consume_incorrect_semicolon(&items) {
                let msg = &format!("expected item, found {token_str}");
                let mut err = self.struct_span_err(self.token.span, msg);
                err.span_label(self.token.span, "expected item");
                return Err(err);
            }
        }

        let inject_use_span = post_attr_lo.data().with_hi(post_attr_lo.lo());
        let mod_spans = ModSpans { inner_span: lo.to(self.prev_token.span), inject_use_span };
        Ok((attrs, items, mod_spans))
    }
}

pub(super) type ItemInfo = (Ident, ItemKind);

impl<'a> Parser<'a> {
    pub fn parse_item(&mut self, force_collect: ForceCollect) -> PResult<'a, Option<P<Item>>> {
        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
        self.parse_item_(fn_parse_mode, force_collect).map(|i| i.map(P))
    }

    fn parse_item_(
        &mut self,
        fn_parse_mode: FnParseMode,
        force_collect: ForceCollect,
    ) -> PResult<'a, Option<Item>> {
        let attrs = self.parse_outer_attributes()?;
        self.parse_item_common(attrs, true, false, fn_parse_mode, force_collect)
    }

    pub(super) fn parse_item_common(
        &mut self,
        attrs: AttrWrapper,
        mac_allowed: bool,
        attrs_allowed: bool,
        fn_parse_mode: FnParseMode,
        force_collect: ForceCollect,
    ) -> PResult<'a, Option<Item>> {
        // Don't use `maybe_whole` so that we have precise control
        // over when we bump the parser
        if let token::Interpolated(nt) = &self.token.kind && let token::NtItem(item) = &**nt {
            let mut item = item.clone();
            self.bump();

            attrs.prepend_to_nt_inner(&mut item.attrs);
            return Ok(Some(item.into_inner()));
        };

        let mut unclosed_delims = vec![];
        let item =
            self.collect_tokens_trailing_token(attrs, force_collect, |this: &mut Self, attrs| {
                let item =
                    this.parse_item_common_(attrs, mac_allowed, attrs_allowed, fn_parse_mode);
                unclosed_delims.append(&mut this.unclosed_delims);
                Ok((item?, TrailingToken::None))
            })?;

        self.unclosed_delims.append(&mut unclosed_delims);
        Ok(item)
    }

    fn parse_item_common_(
        &mut self,
        mut attrs: Vec<Attribute>,
        mac_allowed: bool,
        attrs_allowed: bool,
        fn_parse_mode: FnParseMode,
    ) -> PResult<'a, Option<Item>> {
        let lo = self.token.span;
        let vis = self.parse_visibility(FollowedByType::No)?;
        let mut def = self.parse_defaultness();
        let kind =
            self.parse_item_kind(&mut attrs, mac_allowed, lo, &vis, &mut def, fn_parse_mode)?;
        if let Some((ident, kind)) = kind {
            self.error_on_unconsumed_default(def, &kind);
            let span = lo.to(self.prev_token.span);
            let id = DUMMY_NODE_ID;
            let item = Item { ident, attrs, id, kind, vis, span, tokens: None };
            return Ok(Some(item));
        }

        // At this point, we have failed to parse an item.
        self.error_on_unmatched_vis(&vis);
        self.error_on_unmatched_defaultness(def);
        if !attrs_allowed {
            self.recover_attrs_no_item(&attrs)?;
        }
        Ok(None)
    }

    /// Error in-case a non-inherited visibility was parsed but no item followed.
    fn error_on_unmatched_vis(&self, vis: &Visibility) {
        if let VisibilityKind::Inherited = vis.kind {
            return;
        }
        let vs = pprust::vis_to_string(&vis);
        let vs = vs.trim_end();
        self.struct_span_err(vis.span, &format!("visibility `{vs}` is not followed by an item"))
            .span_label(vis.span, "the visibility")
            .help(&format!("you likely meant to define an item, e.g., `{vs} fn foo() {{}}`"))
            .emit();
    }

    /// Error in-case a `default` was parsed but no item followed.
    fn error_on_unmatched_defaultness(&self, def: Defaultness) {
        if let Defaultness::Default(sp) = def {
            self.struct_span_err(sp, "`default` is not followed by an item")
                .span_label(sp, "the `default` qualifier")
                .note("only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`")
                .emit();
        }
    }

    /// Error in-case `default` was parsed in an in-appropriate context.
    fn error_on_unconsumed_default(&self, def: Defaultness, kind: &ItemKind) {
        if let Defaultness::Default(span) = def {
            let msg = format!("{} {} cannot be `default`", kind.article(), kind.descr());
            self.struct_span_err(span, &msg)
                .span_label(span, "`default` because of this")
                .note("only associated `fn`, `const`, and `type` items can be `default`")
                .emit();
        }
    }

    /// Parses one of the items allowed by the flags.
    fn parse_item_kind(
        &mut self,
        attrs: &mut Vec<Attribute>,
        macros_allowed: bool,
        lo: Span,
        vis: &Visibility,
        def: &mut Defaultness,
        fn_parse_mode: FnParseMode,
    ) -> PResult<'a, Option<ItemInfo>> {
        let def_final = def == &Defaultness::Final;
        let mut def = || mem::replace(def, Defaultness::Final);

        let info = if self.eat_keyword(kw::Use) {
            self.parse_use_item()?
        } else if self.check_fn_front_matter(def_final) {
            // FUNCTION ITEM
            let (ident, sig, generics, body) = self.parse_fn(attrs, fn_parse_mode, lo, vis)?;
            (ident, ItemKind::Fn(Box::new(Fn { defaultness: def(), sig, generics, body })))
        } else if self.eat_keyword(kw::Extern) {
            if self.eat_keyword(kw::Crate) {
                // EXTERN CRATE
                self.parse_item_extern_crate()?
            } else {
                // EXTERN BLOCK
                self.parse_item_foreign_mod(attrs, Unsafe::No)?
            }
        } else if self.is_unsafe_foreign_mod() {
            // EXTERN BLOCK
            let unsafety = self.parse_unsafety();
            self.expect_keyword(kw::Extern)?;
            self.parse_item_foreign_mod(attrs, unsafety)?
        } else if self.is_static_global() {
            // STATIC ITEM
            self.bump(); // `static`
            let m = self.parse_mutability();
            let (ident, ty, expr) = self.parse_item_global(Some(m))?;
            (ident, ItemKind::Static(ty, m, expr))
        } else if let Const::Yes(const_span) = self.parse_constness() {
            // CONST ITEM
            if self.token.is_keyword(kw::Impl) {
                // recover from `const impl`, suggest `impl const`
                self.recover_const_impl(const_span, attrs, def())?
            } else {
                self.recover_const_mut(const_span);
                let (ident, ty, expr) = self.parse_item_global(None)?;
                (ident, ItemKind::Const(def(), ty, expr))
            }
        } else if self.check_keyword(kw::Trait) || self.check_auto_or_unsafe_trait_item() {
            // TRAIT ITEM
            self.parse_item_trait(attrs, lo)?
        } else if self.check_keyword(kw::Impl)
            || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Impl])
        {
            // IMPL ITEM
            self.parse_item_impl(attrs, def())?
        } else if self.check_keyword(kw::Mod)
            || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Mod])
        {
            // MODULE ITEM
            self.parse_item_mod(attrs)?
        } else if self.eat_keyword(kw::Type) {
            // TYPE ITEM
            self.parse_type_alias(def())?
        } else if self.eat_keyword(kw::Enum) {
            // ENUM ITEM
            self.parse_item_enum()?
        } else if self.eat_keyword(kw::Struct) {
            // STRUCT ITEM
            self.parse_item_struct()?
        } else if self.is_kw_followed_by_ident(kw::Union) {
            // UNION ITEM
            self.bump(); // `union`
            self.parse_item_union()?
        } else if self.eat_keyword(kw::Macro) {
            // MACROS 2.0 ITEM
            self.parse_item_decl_macro(lo)?
        } else if let IsMacroRulesItem::Yes { has_bang } = self.is_macro_rules_item() {
            // MACRO_RULES ITEM
            self.parse_item_macro_rules(vis, has_bang)?
        } else if self.isnt_macro_invocation()
            && (self.token.is_ident_named(sym::import)
                || self.token.is_ident_named(sym::using)
                || self.token.is_ident_named(sym::include)
                || self.token.is_ident_named(sym::require))
        {
            return self.recover_import_as_use();
        } else if self.isnt_macro_invocation() && vis.kind.is_pub() {
            self.recover_missing_kw_before_item()?;
            return Ok(None);
        } else if macros_allowed && self.check_path() {
            // MACRO INVOCATION ITEM
            (Ident::empty(), ItemKind::MacCall(self.parse_item_macro(vis)?))
        } else {
            return Ok(None);
        };
        Ok(Some(info))
    }

    fn recover_import_as_use(&mut self) -> PResult<'a, Option<(Ident, ItemKind)>> {
        let span = self.token.span;
        let token_name = super::token_descr(&self.token);
        let snapshot = self.create_snapshot_for_diagnostic();
        self.bump();
        match self.parse_use_item() {
            Ok(u) => {
                self.struct_span_err(span, format!("expected item, found {token_name}"))
                    .span_suggestion_short(
                        span,
                        "items are imported using the `use` keyword",
                        "use",
                        Applicability::MachineApplicable,
                    )
                    .emit();
                Ok(Some(u))
            }
            Err(e) => {
                e.cancel();
                self.restore_snapshot(snapshot);
                Ok(None)
            }
        }
    }

    fn parse_use_item(&mut self) -> PResult<'a, (Ident, ItemKind)> {
        let tree = self.parse_use_tree()?;
        if let Err(mut e) = self.expect_semi() {
            match tree.kind {
                UseTreeKind::Glob => {
                    e.note("the wildcard token must be last on the path");
                }
                UseTreeKind::Nested(..) => {
                    e.note("glob-like brace syntax must be last on the path");
                }
                _ => (),
            }
            return Err(e);
        }
        Ok((Ident::empty(), ItemKind::Use(tree)))
    }

    /// When parsing a statement, would the start of a path be an item?
    pub(super) fn is_path_start_item(&mut self) -> bool {
        self.is_kw_followed_by_ident(kw::Union) // no: `union::b`, yes: `union U { .. }`
        || self.check_auto_or_unsafe_trait_item() // no: `auto::b`, yes: `auto trait X { .. }`
        || self.is_async_fn() // no(2015): `async::b`, yes: `async fn`
        || matches!(self.is_macro_rules_item(), IsMacroRulesItem::Yes{..}) // no: `macro_rules::b`, yes: `macro_rules! mac`
    }

    /// Are we sure this could not possibly be a macro invocation?
    fn isnt_macro_invocation(&mut self) -> bool {
        self.check_ident() && self.look_ahead(1, |t| *t != token::Not && *t != token::ModSep)
    }

    /// Recover on encountering a struct or method definition where the user
    /// forgot to add the `struct` or `fn` keyword after writing `pub`: `pub S {}`.
    fn recover_missing_kw_before_item(&mut self) -> PResult<'a, ()> {
        // Space between `pub` keyword and the identifier
        //
        //     pub   S {}
        //        ^^^ `sp` points here
        let sp = self.prev_token.span.between(self.token.span);
        let full_sp = self.prev_token.span.to(self.token.span);
        let ident_sp = self.token.span;
        if self.look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace)) {
            // possible public struct definition where `struct` was forgotten
            let ident = self.parse_ident().unwrap();
            let msg = format!("add `struct` here to parse `{ident}` as a public struct");
            let mut err = self.struct_span_err(sp, "missing `struct` for struct definition");
            err.span_suggestion_short(
                sp,
                &msg,
                " struct ",
                Applicability::MaybeIncorrect, // speculative
            );
            Err(err)
        } else if self.look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Parenthesis)) {
            let ident = self.parse_ident().unwrap();
            self.bump(); // `(`
            let kw_name = self.recover_first_param();
            self.consume_block(Delimiter::Parenthesis, ConsumeClosingDelim::Yes);
            let (kw, kw_name, ambiguous) = if self.check(&token::RArrow) {
                self.eat_to_tokens(&[&token::OpenDelim(Delimiter::Brace)]);
                self.bump(); // `{`
                ("fn", kw_name, false)
            } else if self.check(&token::OpenDelim(Delimiter::Brace)) {
                self.bump(); // `{`
                ("fn", kw_name, false)
            } else if self.check(&token::Colon) {
                let kw = "struct";
                (kw, kw, false)
            } else {
                ("fn` or `struct", "function or struct", true)
            };

            let msg = format!("missing `{kw}` for {kw_name} definition");
            let mut err = self.struct_span_err(sp, &msg);
            if !ambiguous {
                self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
                let suggestion =
                    format!("add `{kw}` here to parse `{ident}` as a public {kw_name}");
                err.span_suggestion_short(
                    sp,
                    &suggestion,
                    format!(" {kw} "),
                    Applicability::MachineApplicable,
                );
            } else if let Ok(snippet) = self.span_to_snippet(ident_sp) {
                err.span_suggestion(
                    full_sp,
                    "if you meant to call a macro, try",
                    format!("{}!", snippet),
                    // this is the `ambiguous` conditional branch
                    Applicability::MaybeIncorrect,
                );
            } else {
                err.help(
                    "if you meant to call a macro, remove the `pub` \
                              and add a trailing `!` after the identifier",
                );
            }
            Err(err)
        } else if self.look_ahead(1, |t| *t == token::Lt) {
            let ident = self.parse_ident().unwrap();
            self.eat_to_tokens(&[&token::Gt]);
            self.bump(); // `>`
            let (kw, kw_name, ambiguous) = if self.eat(&token::OpenDelim(Delimiter::Parenthesis)) {
                ("fn", self.recover_first_param(), false)
            } else if self.check(&token::OpenDelim(Delimiter::Brace)) {
                ("struct", "struct", false)
            } else {
                ("fn` or `struct", "function or struct", true)
            };
            let msg = format!("missing `{kw}` for {kw_name} definition");
            let mut err = self.struct_span_err(sp, &msg);
            if !ambiguous {
                err.span_suggestion_short(
                    sp,
                    &format!("add `{kw}` here to parse `{ident}` as a public {kw_name}"),
                    format!(" {} ", kw),
                    Applicability::MachineApplicable,
                );
            }
            Err(err)
        } else {
            Ok(())
        }
    }

    /// Parses an item macro, e.g., `item!();`.
    fn parse_item_macro(&mut self, vis: &Visibility) -> PResult<'a, MacCall> {
        let path = self.parse_path(PathStyle::Mod)?; // `foo::bar`
        self.expect(&token::Not)?; // `!`
        match self.parse_mac_args() {
            // `( .. )` or `[ .. ]` (followed by `;`), or `{ .. }`.
            Ok(args) => {
                self.eat_semi_for_macro_if_needed(&args);
                self.complain_if_pub_macro(vis, false);
                Ok(MacCall { path, args, prior_type_ascription: self.last_type_ascription })
            }

            Err(mut err) => {
                // Maybe the user misspelled `macro_rules` (issue #91227)
                if self.token.is_ident()
                    && path.segments.len() == 1
                    && lev_distance("macro_rules", &path.segments[0].ident.to_string(), 3).is_some()
                {
                    err.span_suggestion(
                        path.span,
                        "perhaps you meant to define a macro",
                        "macro_rules",
                        Applicability::MachineApplicable,
                    );
                }
                Err(err)
            }
        }
    }

    /// Recover if we parsed attributes and expected an item but there was none.
    fn recover_attrs_no_item(&mut self, attrs: &[Attribute]) -> PResult<'a, ()> {
        let ([start @ end] | [start, .., end]) = attrs else {
            return Ok(());
        };
        let msg = if end.is_doc_comment() {
            "expected item after doc comment"
        } else {
            "expected item after attributes"
        };
        let mut err = self.struct_span_err(end.span, msg);
        if end.is_doc_comment() {
            err.span_label(end.span, "this doc comment doesn't document anything");
        }
        if end.meta_kind().is_some() {
            if self.token.kind == TokenKind::Semi {
                err.span_suggestion_verbose(
                    self.token.span,
                    "consider removing this semicolon",
                    "",
                    Applicability::MaybeIncorrect,
                );
            }
        }
        if let [.., penultimate, _] = attrs {
            err.span_label(start.span.to(penultimate.span), "other attributes here");
        }
        Err(err)
    }

    fn is_async_fn(&self) -> bool {
        self.token.is_keyword(kw::Async) && self.is_keyword_ahead(1, &[kw::Fn])
    }

    fn parse_polarity(&mut self) -> ast::ImplPolarity {
        // Disambiguate `impl !Trait for Type { ... }` and `impl ! { ... }` for the never type.
        if self.check(&token::Not) && self.look_ahead(1, |t| t.can_begin_type()) {
            self.bump(); // `!`
            ast::ImplPolarity::Negative(self.prev_token.span)
        } else {
            ast::ImplPolarity::Positive
        }
    }

    /// Parses an implementation item.
    ///
    /// ```ignore (illustrative)
    /// impl<'a, T> TYPE { /* impl items */ }
    /// impl<'a, T> TRAIT for TYPE { /* impl items */ }
    /// impl<'a, T> !TRAIT for TYPE { /* impl items */ }
    /// impl<'a, T> const TRAIT for TYPE { /* impl items */ }
    /// ```
    ///
    /// We actually parse slightly more relaxed grammar for better error reporting and recovery.
    /// ```ebnf
    /// "impl" GENERICS "const"? "!"? TYPE "for"? (TYPE | "..") ("where" PREDICATES)? "{" BODY "}"
    /// "impl" GENERICS "const"? "!"? TYPE ("where" PREDICATES)? "{" BODY "}"
    /// ```
    fn parse_item_impl(
        &mut self,
        attrs: &mut Vec<Attribute>,
        defaultness: Defaultness,
    ) -> PResult<'a, ItemInfo> {
        let unsafety = self.parse_unsafety();
        self.expect_keyword(kw::Impl)?;

        // First, parse generic parameters if necessary.
        let mut generics = if self.choose_generics_over_qpath(0) {
            self.parse_generics()?
        } else {
            let mut generics = Generics::default();
            // impl A for B {}
            //    /\ this is where `generics.span` should point when there are no type params.
            generics.span = self.prev_token.span.shrink_to_hi();
            generics
        };

        let constness = self.parse_constness();
        if let Const::Yes(span) = constness {
            self.sess.gated_spans.gate(sym::const_trait_impl, span);
        }

        let polarity = self.parse_polarity();

        // Parse both types and traits as a type, then reinterpret if necessary.
        let err_path = |span| ast::Path::from_ident(Ident::new(kw::Empty, span));
        let ty_first = if self.token.is_keyword(kw::For) && self.look_ahead(1, |t| t != &token::Lt)
        {
            let span = self.prev_token.span.between(self.token.span);
            self.struct_span_err(span, "missing trait in a trait impl")
                .span_suggestion(
                    span,
                    "add a trait here",
                    " Trait ",
                    Applicability::HasPlaceholders,
                )
                .span_suggestion(
                    span.to(self.token.span),
                    "for an inherent impl, drop this `for`",
                    "",
                    Applicability::MaybeIncorrect,
                )
                .emit();
            P(Ty {
                kind: TyKind::Path(None, err_path(span)),
                span,
                id: DUMMY_NODE_ID,
                tokens: None,
            })
        } else {
            self.parse_ty_with_generics_recovery(&generics)?
        };

        // If `for` is missing we try to recover.
        let has_for = self.eat_keyword(kw::For);
        let missing_for_span = self.prev_token.span.between(self.token.span);

        let ty_second = if self.token == token::DotDot {
            // We need to report this error after `cfg` expansion for compatibility reasons
            self.bump(); // `..`, do not add it to expected tokens
            Some(self.mk_ty(self.prev_token.span, TyKind::Err))
        } else if has_for || self.token.can_begin_type() {
            Some(self.parse_ty()?)
        } else {
            None
        };

        generics.where_clause = self.parse_where_clause()?;

        let impl_items = self.parse_item_list(attrs, |p| p.parse_impl_item(ForceCollect::No))?;

        let item_kind = match ty_second {
            Some(ty_second) => {
                // impl Trait for Type
                if !has_for {
                    self.struct_span_err(missing_for_span, "missing `for` in a trait impl")
                        .span_suggestion_short(
                            missing_for_span,
                            "add `for` here",
                            " for ",
                            Applicability::MachineApplicable,
                        )
                        .emit();
                }

                let ty_first = ty_first.into_inner();
                let path = match ty_first.kind {
                    // This notably includes paths passed through `ty` macro fragments (#46438).
                    TyKind::Path(None, path) => path,
                    _ => {
                        self.struct_span_err(ty_first.span, "expected a trait, found type").emit();
                        err_path(ty_first.span)
                    }
                };
                let trait_ref = TraitRef { path, ref_id: ty_first.id };

                ItemKind::Impl(Box::new(Impl {
                    unsafety,
                    polarity,
                    defaultness,
                    constness,
                    generics,
                    of_trait: Some(trait_ref),
                    self_ty: ty_second,
                    items: impl_items,
                }))
            }
            None => {
                // impl Type
                ItemKind::Impl(Box::new(Impl {
                    unsafety,
                    polarity,
                    defaultness,
                    constness,
                    generics,
                    of_trait: None,
                    self_ty: ty_first,
                    items: impl_items,
                }))
            }
        };

        Ok((Ident::empty(), item_kind))
    }

    fn parse_item_list<T>(
        &mut self,
        attrs: &mut Vec<Attribute>,
        mut parse_item: impl FnMut(&mut Parser<'a>) -> PResult<'a, Option<Option<T>>>,
    ) -> PResult<'a, Vec<T>> {
        let open_brace_span = self.token.span;
        self.expect(&token::OpenDelim(Delimiter::Brace))?;
        attrs.append(&mut self.parse_inner_attributes()?);

        let mut items = Vec::new();
        while !self.eat(&token::CloseDelim(Delimiter::Brace)) {
            if self.recover_doc_comment_before_brace() {
                continue;
            }
            match parse_item(self) {
                Ok(None) => {
                    // We have to bail or we'll potentially never make progress.
                    let non_item_span = self.token.span;
                    self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
                    self.struct_span_err(non_item_span, "non-item in item list")
                        .span_label(open_brace_span, "item list starts here")
                        .span_label(non_item_span, "non-item starts here")
                        .span_label(self.prev_token.span, "item list ends here")
                        .emit();
                    break;
                }
                Ok(Some(item)) => items.extend(item),
                Err(mut err) => {
                    self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
                    err.span_label(open_brace_span, "while parsing this item list starting here")
                        .span_label(self.prev_token.span, "the item list ends here")
                        .emit();
                    break;
                }
            }
        }
        Ok(items)
    }

    /// Recover on a doc comment before `}`.
    fn recover_doc_comment_before_brace(&mut self) -> bool {
        if let token::DocComment(..) = self.token.kind {
            if self.look_ahead(1, |tok| tok == &token::CloseDelim(Delimiter::Brace)) {
                struct_span_err!(
                    self.diagnostic(),
                    self.token.span,
                    E0584,
                    "found a documentation comment that doesn't document anything",
                )
                .span_label(self.token.span, "this doc comment doesn't document anything")
                .help(
                    "doc comments must come before what they document, maybe a \
                    comment was intended with `//`?",
                )
                .emit();
                self.bump();
                return true;
            }
        }
        false
    }

    /// Parses defaultness (i.e., `default` or nothing).
    fn parse_defaultness(&mut self) -> Defaultness {
        // We are interested in `default` followed by another identifier.
        // However, we must avoid keywords that occur as binary operators.
        // Currently, the only applicable keyword is `as` (`default as Ty`).
        if self.check_keyword(kw::Default)
            && self.look_ahead(1, |t| t.is_non_raw_ident_where(|i| i.name != kw::As))
        {
            self.bump(); // `default`
            Defaultness::Default(self.prev_token.uninterpolated_span())
        } else {
            Defaultness::Final
        }
    }

    /// Is this an `(unsafe auto? | auto) trait` item?
    fn check_auto_or_unsafe_trait_item(&mut self) -> bool {
        // auto trait
        self.check_keyword(kw::Auto) && self.is_keyword_ahead(1, &[kw::Trait])
            // unsafe auto trait
            || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Trait, kw::Auto])
    }

    /// Parses `unsafe? auto? trait Foo { ... }` or `trait Foo = Bar;`.
    fn parse_item_trait(&mut self, attrs: &mut Vec<Attribute>, lo: Span) -> PResult<'a, ItemInfo> {
        let unsafety = self.parse_unsafety();
        // Parse optional `auto` prefix.
        let is_auto = if self.eat_keyword(kw::Auto) { IsAuto::Yes } else { IsAuto::No };

        self.expect_keyword(kw::Trait)?;
        let ident = self.parse_ident()?;
        let mut generics = self.parse_generics()?;

        // Parse optional colon and supertrait bounds.
        let had_colon = self.eat(&token::Colon);
        let span_at_colon = self.prev_token.span;
        let bounds = if had_colon {
            self.parse_generic_bounds(Some(self.prev_token.span))?
        } else {
            Vec::new()
        };

        let span_before_eq = self.prev_token.span;
        if self.eat(&token::Eq) {
            // It's a trait alias.
            if had_colon {
                let span = span_at_colon.to(span_before_eq);
                self.struct_span_err(span, "bounds are not allowed on trait aliases").emit();
            }

            let bounds = self.parse_generic_bounds(None)?;
            generics.where_clause = self.parse_where_clause()?;
            self.expect_semi()?;

            let whole_span = lo.to(self.prev_token.span);
            if is_auto == IsAuto::Yes {
                let msg = "trait aliases cannot be `auto`";
                self.struct_span_err(whole_span, msg).span_label(whole_span, msg).emit();
            }
            if let Unsafe::Yes(_) = unsafety {
                let msg = "trait aliases cannot be `unsafe`";
                self.struct_span_err(whole_span, msg).span_label(whole_span, msg).emit();
            }

            self.sess.gated_spans.gate(sym::trait_alias, whole_span);

            Ok((ident, ItemKind::TraitAlias(generics, bounds)))
        } else {
            // It's a normal trait.
            generics.where_clause = self.parse_where_clause()?;
            let items = self.parse_item_list(attrs, |p| p.parse_trait_item(ForceCollect::No))?;
            Ok((
                ident,
                ItemKind::Trait(Box::new(Trait { is_auto, unsafety, generics, bounds, items })),
            ))
        }
    }

    pub fn parse_impl_item(
        &mut self,
        force_collect: ForceCollect,
    ) -> PResult<'a, Option<Option<P<AssocItem>>>> {
        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
        self.parse_assoc_item(fn_parse_mode, force_collect)
    }

    pub fn parse_trait_item(
        &mut self,
        force_collect: ForceCollect,
    ) -> PResult<'a, Option<Option<P<AssocItem>>>> {
        let fn_parse_mode =
            FnParseMode { req_name: |edition| edition >= Edition::Edition2018, req_body: false };
        self.parse_assoc_item(fn_parse_mode, force_collect)
    }

    /// Parses associated items.
    fn parse_assoc_item(
        &mut self,
        fn_parse_mode: FnParseMode,
        force_collect: ForceCollect,
    ) -> PResult<'a, Option<Option<P<AssocItem>>>> {
        Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
            |Item { attrs, id, span, vis, ident, kind, tokens }| {
                let kind = match AssocItemKind::try_from(kind) {
                    Ok(kind) => kind,
                    Err(kind) => match kind {
                        ItemKind::Static(a, _, b) => {
                            self.struct_span_err(span, "associated `static` items are not allowed")
                                .emit();
                            AssocItemKind::Const(Defaultness::Final, a, b)
                        }
                        _ => return self.error_bad_item_kind(span, &kind, "`trait`s or `impl`s"),
                    },
                };
                Some(P(Item { attrs, id, span, vis, ident, kind, tokens }))
            },
        ))
    }

    /// Parses a `type` alias with the following grammar:
    /// ```ebnf
    /// TypeAlias = "type" Ident Generics {":" GenericBounds}? {"=" Ty}? ";" ;
    /// ```
    /// The `"type"` has already been eaten.
    fn parse_type_alias(&mut self, defaultness: Defaultness) -> PResult<'a, ItemInfo> {
        let ident = self.parse_ident()?;
        let mut generics = self.parse_generics()?;

        // Parse optional colon and param bounds.
        let bounds =
            if self.eat(&token::Colon) { self.parse_generic_bounds(None)? } else { Vec::new() };
        let before_where_clause = self.parse_where_clause()?;

        let ty = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };

        let after_where_clause = self.parse_where_clause()?;

        let where_clauses = (
            TyAliasWhereClause(before_where_clause.has_where_token, before_where_clause.span),
            TyAliasWhereClause(after_where_clause.has_where_token, after_where_clause.span),
        );
        let where_predicates_split = before_where_clause.predicates.len();
        let mut predicates = before_where_clause.predicates;
        predicates.extend(after_where_clause.predicates.into_iter());
        let where_clause = WhereClause {
            has_where_token: before_where_clause.has_where_token
                || after_where_clause.has_where_token,
            predicates,
            span: DUMMY_SP,
        };
        generics.where_clause = where_clause;

        self.expect_semi()?;

        Ok((
            ident,
            ItemKind::TyAlias(Box::new(TyAlias {
                defaultness,
                generics,
                where_clauses,
                where_predicates_split,
                bounds,
                ty,
            })),
        ))
    }

    /// Parses a `UseTree`.
    ///
    /// ```text
    /// USE_TREE = [`::`] `*` |
    ///            [`::`] `{` USE_TREE_LIST `}` |
    ///            PATH `::` `*` |
    ///            PATH `::` `{` USE_TREE_LIST `}` |
    ///            PATH [`as` IDENT]
    /// ```
    fn parse_use_tree(&mut self) -> PResult<'a, UseTree> {
        let lo = self.token.span;

        let mut prefix = ast::Path { segments: Vec::new(), span: lo.shrink_to_lo(), tokens: None };
        let kind = if self.check(&token::OpenDelim(Delimiter::Brace))
            || self.check(&token::BinOp(token::Star))
            || self.is_import_coupler()
        {
            // `use *;` or `use ::*;` or `use {...};` or `use ::{...};`
            let mod_sep_ctxt = self.token.span.ctxt();
            if self.eat(&token::ModSep) {
                prefix
                    .segments
                    .push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
            }

            self.parse_use_tree_glob_or_nested()?
        } else {
            // `use path::*;` or `use path::{...};` or `use path;` or `use path as bar;`
            prefix = self.parse_path(PathStyle::Mod)?;

            if self.eat(&token::ModSep) {
                self.parse_use_tree_glob_or_nested()?
            } else {
                UseTreeKind::Simple(self.parse_rename()?, DUMMY_NODE_ID, DUMMY_NODE_ID)
            }
        };

        Ok(UseTree { prefix, kind, span: lo.to(self.prev_token.span) })
    }

    /// Parses `*` or `{...}`.
    fn parse_use_tree_glob_or_nested(&mut self) -> PResult<'a, UseTreeKind> {
        Ok(if self.eat(&token::BinOp(token::Star)) {
            UseTreeKind::Glob
        } else {
            UseTreeKind::Nested(self.parse_use_tree_list()?)
        })
    }

    /// Parses a `UseTreeKind::Nested(list)`.
    ///
    /// ```text
    /// USE_TREE_LIST = Ø | (USE_TREE `,`)* USE_TREE [`,`]
    /// ```
    fn parse_use_tree_list(&mut self) -> PResult<'a, Vec<(UseTree, ast::NodeId)>> {
        self.parse_delim_comma_seq(Delimiter::Brace, |p| Ok((p.parse_use_tree()?, DUMMY_NODE_ID)))
            .map(|(r, _)| r)
    }

    fn parse_rename(&mut self) -> PResult<'a, Option<Ident>> {
        if self.eat_keyword(kw::As) { self.parse_ident_or_underscore().map(Some) } else { Ok(None) }
    }

    fn parse_ident_or_underscore(&mut self) -> PResult<'a, Ident> {
        match self.token.ident() {
            Some((ident @ Ident { name: kw::Underscore, .. }, false)) => {
                self.bump();
                Ok(ident)
            }
            _ => self.parse_ident(),
        }
    }

    /// Parses `extern crate` links.
    ///
    /// # Examples
    ///
    /// ```ignore (illustrative)
    /// extern crate foo;
    /// extern crate bar as foo;
    /// ```
    fn parse_item_extern_crate(&mut self) -> PResult<'a, ItemInfo> {
        // Accept `extern crate name-like-this` for better diagnostics
        let orig_name = self.parse_crate_name_with_dashes()?;
        let (item_name, orig_name) = if let Some(rename) = self.parse_rename()? {
            (rename, Some(orig_name.name))
        } else {
            (orig_name, None)
        };
        self.expect_semi()?;
        Ok((item_name, ItemKind::ExternCrate(orig_name)))
    }

    fn parse_crate_name_with_dashes(&mut self) -> PResult<'a, Ident> {
        let error_msg = "crate name using dashes are not valid in `extern crate` statements";
        let suggestion_msg = "if the original crate name uses dashes you need to use underscores \
                              in the code";
        let mut ident = if self.token.is_keyword(kw::SelfLower) {
            self.parse_path_segment_ident()
        } else {
            self.parse_ident()
        }?;
        let mut idents = vec![];
        let mut replacement = vec![];
        let mut fixed_crate_name = false;
        // Accept `extern crate name-like-this` for better diagnostics.
        let dash = token::BinOp(token::BinOpToken::Minus);
        if self.token == dash {
            // Do not include `-` as part of the expected tokens list.
            while self.eat(&dash) {
                fixed_crate_name = true;
                replacement.push((self.prev_token.span, "_".to_string()));
                idents.push(self.parse_ident()?);
            }
        }
        if fixed_crate_name {
            let fixed_name_sp = ident.span.to(idents.last().unwrap().span);
            let mut fixed_name = ident.name.to_string();
            for part in idents {
                fixed_name.push_str(&format!("_{}", part.name));
            }
            ident = Ident::from_str_and_span(&fixed_name, fixed_name_sp);

            self.struct_span_err(fixed_name_sp, error_msg)
                .span_label(fixed_name_sp, "dash-separated idents are not valid")
                .multipart_suggestion(suggestion_msg, replacement, Applicability::MachineApplicable)
                .emit();
        }
        Ok(ident)
    }

    /// Parses `extern` for foreign ABIs modules.
    ///
    /// `extern` is expected to have been consumed before calling this method.
    ///
    /// # Examples
    ///
    /// ```ignore (only-for-syntax-highlight)
    /// extern "C" {}
    /// extern {}
    /// ```
    fn parse_item_foreign_mod(
        &mut self,
        attrs: &mut Vec<Attribute>,
        mut unsafety: Unsafe,
    ) -> PResult<'a, ItemInfo> {
        let abi = self.parse_abi(); // ABI?
        if unsafety == Unsafe::No
            && self.token.is_keyword(kw::Unsafe)
            && self.look_ahead(1, |t| t.kind == token::OpenDelim(Delimiter::Brace))
        {
            let mut err = self.expect(&token::OpenDelim(Delimiter::Brace)).unwrap_err();
            err.emit();
            unsafety = Unsafe::Yes(self.token.span);
            self.eat_keyword(kw::Unsafe);
        }
        let module = ast::ForeignMod {
            unsafety,
            abi,
            items: self.parse_item_list(attrs, |p| p.parse_foreign_item(ForceCollect::No))?,
        };
        Ok((Ident::empty(), ItemKind::ForeignMod(module)))
    }

    /// Parses a foreign item (one in an `extern { ... }` block).
    pub fn parse_foreign_item(
        &mut self,
        force_collect: ForceCollect,
    ) -> PResult<'a, Option<Option<P<ForeignItem>>>> {
        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: false };
        Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
            |Item { attrs, id, span, vis, ident, kind, tokens }| {
                let kind = match ForeignItemKind::try_from(kind) {
                    Ok(kind) => kind,
                    Err(kind) => match kind {
                        ItemKind::Const(_, a, b) => {
                            self.error_on_foreign_const(span, ident);
                            ForeignItemKind::Static(a, Mutability::Not, b)
                        }
                        _ => return self.error_bad_item_kind(span, &kind, "`extern` blocks"),
                    },
                };
                Some(P(Item { attrs, id, span, vis, ident, kind, tokens }))
            },
        ))
    }

    fn error_bad_item_kind<T>(&self, span: Span, kind: &ItemKind, ctx: &str) -> Option<T> {
        let span = self.sess.source_map().guess_head_span(span);
        let descr = kind.descr();
        self.struct_span_err(span, &format!("{descr} is not supported in {ctx}"))
            .help(&format!("consider moving the {descr} out to a nearby module scope"))
            .emit();
        None
    }

    fn error_on_foreign_const(&self, span: Span, ident: Ident) {
        self.struct_span_err(ident.span, "extern items cannot be `const`")
            .span_suggestion(
                span.with_hi(ident.span.lo()),
                "try using a static value",
                "static ",
                Applicability::MachineApplicable,
            )
            .note("for more information, visit https://doc.rust-lang.org/std/keyword.extern.html")
            .emit();
    }

    fn is_unsafe_foreign_mod(&self) -> bool {
        self.token.is_keyword(kw::Unsafe)
            && self.is_keyword_ahead(1, &[kw::Extern])
            && self.look_ahead(
                2 + self.look_ahead(2, |t| t.can_begin_literal_maybe_minus() as usize),
                |t| t.kind == token::OpenDelim(Delimiter::Brace),
            )
    }

    fn is_static_global(&mut self) -> bool {
        if self.check_keyword(kw::Static) {
            // Check if this could be a closure.
            !self.look_ahead(1, |token| {
                if token.is_keyword(kw::Move) {
                    return true;
                }
                matches!(token.kind, token::BinOp(token::Or) | token::OrOr)
            })
        } else {
            false
        }
    }

    /// Recover on `const mut` with `const` already eaten.
    fn recover_const_mut(&mut self, const_span: Span) {
        if self.eat_keyword(kw::Mut) {
            let span = self.prev_token.span;
            self.struct_span_err(span, "const globals cannot be mutable")
                .span_label(span, "cannot be mutable")
                .span_suggestion(
                    const_span,
                    "you might want to declare a static instead",
                    "static",
                    Applicability::MaybeIncorrect,
                )
                .emit();
        }
    }

    /// Recover on `const impl` with `const` already eaten.
    fn recover_const_impl(
        &mut self,
        const_span: Span,
        attrs: &mut Vec<Attribute>,
        defaultness: Defaultness,
    ) -> PResult<'a, ItemInfo> {
        let impl_span = self.token.span;
        let mut err = self.expected_ident_found();

        // Only try to recover if this is implementing a trait for a type
        let mut impl_info = match self.parse_item_impl(attrs, defaultness) {
            Ok(impl_info) => impl_info,
            Err(recovery_error) => {
                // Recovery failed, raise the "expected identifier" error
                recovery_error.cancel();
                return Err(err);
            }
        };

        match impl_info.1 {
            ItemKind::Impl(box Impl { of_trait: Some(ref trai), ref mut constness, .. }) => {
                *constness = Const::Yes(const_span);

                let before_trait = trai.path.span.shrink_to_lo();
                let const_up_to_impl = const_span.with_hi(impl_span.lo());
                err.multipart_suggestion(
                    "you might have meant to write a const trait impl",
                    vec![(const_up_to_impl, "".to_owned()), (before_trait, "const ".to_owned())],
                    Applicability::MaybeIncorrect,
                )
                .emit();
            }
            ItemKind::Impl { .. } => return Err(err),
            _ => unreachable!(),
        }

        Ok(impl_info)
    }

    /// Parse `["const" | ("static" "mut"?)] $ident ":" $ty (= $expr)?` with
    /// `["const" | ("static" "mut"?)]` already parsed and stored in `m`.
    ///
    /// When `m` is `"const"`, `$ident` may also be `"_"`.
    fn parse_item_global(
        &mut self,
        m: Option<Mutability>,
    ) -> PResult<'a, (Ident, P<Ty>, Option<P<ast::Expr>>)> {
        let id = if m.is_none() { self.parse_ident_or_underscore() } else { self.parse_ident() }?;

        // Parse the type of a `const` or `static mut?` item.
        // That is, the `":" $ty` fragment.
        let ty = match (self.eat(&token::Colon), self.check(&token::Eq) | self.check(&token::Semi))
        {
            // If there wasn't a `:` or the colon was followed by a `=` or `;` recover a missing type.
            (true, false) => self.parse_ty()?,
            (colon, _) => self.recover_missing_const_type(colon, m),
        };

        let expr = if self.eat(&token::Eq) { Some(self.parse_expr()?) } else { None };
        self.expect_semi()?;
        Ok((id, ty, expr))
    }

    /// We were supposed to parse `":" $ty` but the `:` or the type was missing.
    /// This means that the type is missing.
    fn recover_missing_const_type(&mut self, colon_present: bool, m: Option<Mutability>) -> P<Ty> {
        // Construct the error and stash it away with the hope
        // that typeck will later enrich the error with a type.
        let kind = match m {
            Some(Mutability::Mut) => "static mut",
            Some(Mutability::Not) => "static",
            None => "const",
        };

        let colon = match colon_present {
            true => "",
            false => ":",
        };

        let span = self.prev_token.span.shrink_to_hi();
        let mut err = self.struct_span_err(span, &format!("missing type for `{kind}` item"));
        err.span_suggestion(
            span,
            "provide a type for the item",
            format!("{colon} <type>"),
            Applicability::HasPlaceholders,
        );
        err.stash(span, StashKey::ItemNoType);

        // The user intended that the type be inferred,
        // so treat this as if the user wrote e.g. `const A: _ = expr;`.
        P(Ty { kind: TyKind::Infer, span, id: ast::DUMMY_NODE_ID, tokens: None })
    }

    /// Parses an enum declaration.
    fn parse_item_enum(&mut self) -> PResult<'a, ItemInfo> {
        if self.token.is_keyword(kw::Struct) {
            let mut err = self.struct_span_err(
                self.prev_token.span.to(self.token.span),
                "`enum` and `struct` are mutually exclusive",
            );
            err.span_suggestion(
                self.prev_token.span.to(self.token.span),
                "replace `enum struct` with",
                "enum",
                Applicability::MachineApplicable,
            );
            if self.look_ahead(1, |t| t.is_ident()) {
                self.bump();
                err.emit();
            } else {
                return Err(err);
            }
        }

        let id = self.parse_ident()?;
        let mut generics = self.parse_generics()?;
        generics.where_clause = self.parse_where_clause()?;

        let (variants, _) = self
            .parse_delim_comma_seq(Delimiter::Brace, |p| p.parse_enum_variant())
            .map_err(|e| {
                self.recover_stmt();
                e
            })?;

        let enum_definition = EnumDef { variants: variants.into_iter().flatten().collect() };
        Ok((id, ItemKind::Enum(enum_definition, generics)))
    }

    fn parse_enum_variant(&mut self) -> PResult<'a, Option<Variant>> {
        let variant_attrs = self.parse_outer_attributes()?;
        self.collect_tokens_trailing_token(
            variant_attrs,
            ForceCollect::No,
            |this, variant_attrs| {
                let vlo = this.token.span;

                let vis = this.parse_visibility(FollowedByType::No)?;
                if !this.recover_nested_adt_item(kw::Enum)? {
                    return Ok((None, TrailingToken::None));
                }
                let ident = this.parse_field_ident("enum", vlo)?;

                let struct_def = if this.check(&token::OpenDelim(Delimiter::Brace)) {
                    // Parse a struct variant.
                    let (fields, recovered) = this.parse_record_struct_body("struct", false)?;
                    VariantData::Struct(fields, recovered)
                } else if this.check(&token::OpenDelim(Delimiter::Parenthesis)) {
                    VariantData::Tuple(this.parse_tuple_struct_body()?, DUMMY_NODE_ID)
                } else {
                    VariantData::Unit(DUMMY_NODE_ID)
                };

                let disr_expr =
                    if this.eat(&token::Eq) { Some(this.parse_anon_const_expr()?) } else { None };

                let vr = ast::Variant {
                    ident,
                    vis,
                    id: DUMMY_NODE_ID,
                    attrs: variant_attrs.into(),
                    data: struct_def,
                    disr_expr,
                    span: vlo.to(this.prev_token.span),
                    is_placeholder: false,
                };

                Ok((Some(vr), TrailingToken::MaybeComma))
            },
        )
    }

    /// Parses `struct Foo { ... }`.
    fn parse_item_struct(&mut self) -> PResult<'a, ItemInfo> {
        let class_name = self.parse_ident()?;

        let mut generics = self.parse_generics()?;

        // There is a special case worth noting here, as reported in issue #17904.
        // If we are parsing a tuple struct it is the case that the where clause
        // should follow the field list. Like so:
        //
        // struct Foo<T>(T) where T: Copy;
        //
        // If we are parsing a normal record-style struct it is the case
        // that the where clause comes before the body, and after the generics.
        // So if we look ahead and see a brace or a where-clause we begin
        // parsing a record style struct.
        //
        // Otherwise if we look ahead and see a paren we parse a tuple-style
        // struct.

        let vdata = if self.token.is_keyword(kw::Where) {
            generics.where_clause = self.parse_where_clause()?;
            if self.eat(&token::Semi) {
                // If we see a: `struct Foo<T> where T: Copy;` style decl.
                VariantData::Unit(DUMMY_NODE_ID)
            } else {
                // If we see: `struct Foo<T> where T: Copy { ... }`
                let (fields, recovered) =
                    self.parse_record_struct_body("struct", generics.where_clause.has_where_token)?;
                VariantData::Struct(fields, recovered)
            }
        // No `where` so: `struct Foo<T>;`
        } else if self.eat(&token::Semi) {
            VariantData::Unit(DUMMY_NODE_ID)
        // Record-style struct definition
        } else if self.token == token::OpenDelim(Delimiter::Brace) {
            let (fields, recovered) =
                self.parse_record_struct_body("struct", generics.where_clause.has_where_token)?;
            VariantData::Struct(fields, recovered)
        // Tuple-style struct definition with optional where-clause.
        } else if self.token == token::OpenDelim(Delimiter::Parenthesis) {
            let body = VariantData::Tuple(self.parse_tuple_struct_body()?, DUMMY_NODE_ID);
            generics.where_clause = self.parse_where_clause()?;
            self.expect_semi()?;
            body
        } else {
            let token_str = super::token_descr(&self.token);
            let msg = &format!(
                "expected `where`, `{{`, `(`, or `;` after struct name, found {token_str}"
            );
            let mut err = self.struct_span_err(self.token.span, msg);
            err.span_label(self.token.span, "expected `where`, `{`, `(`, or `;` after struct name");
            return Err(err);
        };

        Ok((class_name, ItemKind::Struct(vdata, generics)))
    }

    /// Parses `union Foo { ... }`.
    fn parse_item_union(&mut self) -> PResult<'a, ItemInfo> {
        let class_name = self.parse_ident()?;

        let mut generics = self.parse_generics()?;

        let vdata = if self.token.is_keyword(kw::Where) {
            generics.where_clause = self.parse_where_clause()?;
            let (fields, recovered) =
                self.parse_record_struct_body("union", generics.where_clause.has_where_token)?;
            VariantData::Struct(fields, recovered)
        } else if self.token == token::OpenDelim(Delimiter::Brace) {
            let (fields, recovered) =
                self.parse_record_struct_body("union", generics.where_clause.has_where_token)?;
            VariantData::Struct(fields, recovered)
        } else {
            let token_str = super::token_descr(&self.token);
            let msg = &format!("expected `where` or `{{` after union name, found {token_str}");
            let mut err = self.struct_span_err(self.token.span, msg);
            err.span_label(self.token.span, "expected `where` or `{` after union name");
            return Err(err);
        };

        Ok((class_name, ItemKind::Union(vdata, generics)))
    }

    fn parse_record_struct_body(
        &mut self,
        adt_ty: &str,
        parsed_where: bool,
    ) -> PResult<'a, (Vec<FieldDef>, /* recovered */ bool)> {
        let mut fields = Vec::new();
        let mut recovered = false;
        if self.eat(&token::OpenDelim(Delimiter::Brace)) {
            while self.token != token::CloseDelim(Delimiter::Brace) {
                let field = self.parse_field_def(adt_ty).map_err(|e| {
                    self.consume_block(Delimiter::Brace, ConsumeClosingDelim::No);
                    recovered = true;
                    e
                });
                match field {
                    Ok(field) => fields.push(field),
                    Err(mut err) => {
                        err.emit();
                        break;
                    }
                }
            }
            self.eat(&token::CloseDelim(Delimiter::Brace));
        } else {
            let token_str = super::token_descr(&self.token);
            let msg = &format!(
                "expected {}`{{` after struct name, found {}",
                if parsed_where { "" } else { "`where`, or " },
                token_str
            );
            let mut err = self.struct_span_err(self.token.span, msg);
            err.span_label(
                self.token.span,
                format!(
                    "expected {}`{{` after struct name",
                    if parsed_where { "" } else { "`where`, or " }
                ),
            );
            return Err(err);
        }

        Ok((fields, recovered))
    }

    fn parse_tuple_struct_body(&mut self) -> PResult<'a, Vec<FieldDef>> {
        // This is the case where we find `struct Foo<T>(T) where T: Copy;`
        // Unit like structs are handled in parse_item_struct function
        self.parse_paren_comma_seq(|p| {
            let attrs = p.parse_outer_attributes()?;
            p.collect_tokens_trailing_token(attrs, ForceCollect::No, |p, attrs| {
                let lo = p.token.span;
                let vis = p.parse_visibility(FollowedByType::Yes)?;
                let ty = p.parse_ty()?;

                Ok((
                    FieldDef {
                        span: lo.to(ty.span),
                        vis,
                        ident: None,
                        id: DUMMY_NODE_ID,
                        ty,
                        attrs: attrs.into(),
                        is_placeholder: false,
                    },
                    TrailingToken::MaybeComma,
                ))
            })
        })
        .map(|(r, _)| r)
    }

    /// Parses an element of a struct declaration.
    fn parse_field_def(&mut self, adt_ty: &str) -> PResult<'a, FieldDef> {
        let attrs = self.parse_outer_attributes()?;
        self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
            let lo = this.token.span;
            let vis = this.parse_visibility(FollowedByType::No)?;
            Ok((this.parse_single_struct_field(adt_ty, lo, vis, attrs)?, TrailingToken::None))
        })
    }

    /// Parses a structure field declaration.
    fn parse_single_struct_field(
        &mut self,
        adt_ty: &str,
        lo: Span,
        vis: Visibility,
        attrs: Vec<Attribute>,
    ) -> PResult<'a, FieldDef> {
        let mut seen_comma: bool = false;
        let a_var = self.parse_name_and_ty(adt_ty, lo, vis, attrs)?;
        if self.token == token::Comma {
            seen_comma = true;
        }
        match self.token.kind {
            token::Comma => {
                self.bump();
            }
            token::CloseDelim(Delimiter::Brace) => {}
            token::DocComment(..) => {
                let previous_span = self.prev_token.span;
                let mut err = self.span_err(self.token.span, Error::UselessDocComment);
                self.bump(); // consume the doc comment
                let comma_after_doc_seen = self.eat(&token::Comma);
                // `seen_comma` is always false, because we are inside doc block
                // condition is here to make code more readable
                if !seen_comma && comma_after_doc_seen {
                    seen_comma = true;
                }
                if comma_after_doc_seen || self.token == token::CloseDelim(Delimiter::Brace) {
                    err.emit();
                } else {
                    if !seen_comma {
                        let sp = self.sess.source_map().next_point(previous_span);
                        err.span_suggestion(
                            sp,
                            "missing comma here",
                            ",",
                            Applicability::MachineApplicable,
                        );
                    }
                    return Err(err);
                }
            }
            _ => {
                let sp = self.prev_token.span.shrink_to_hi();
                let mut err = self.struct_span_err(
                    sp,
                    &format!("expected `,`, or `}}`, found {}", super::token_descr(&self.token)),
                );

                // Try to recover extra trailing angle brackets
                let mut recovered = false;
                if let TyKind::Path(_, Path { segments, .. }) = &a_var.ty.kind {
                    if let Some(last_segment) = segments.last() {
                        recovered = self.check_trailing_angle_brackets(
                            last_segment,
                            &[&token::Comma, &token::CloseDelim(Delimiter::Brace)],
                        );
                        if recovered {
                            // Handle a case like `Vec<u8>>,` where we can continue parsing fields
                            // after the comma
                            self.eat(&token::Comma);
                            // `check_trailing_angle_brackets` already emitted a nicer error
                            // NOTE(eddyb) this was `.cancel()`, but `err`
                            // gets returned, so we can't fully defuse it.
                            err.delay_as_bug();
                        }
                    }
                }

                if self.token.is_ident()
                    || (self.token.kind == TokenKind::Pound
                        && (self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Bracket))))
                {
                    // This is likely another field, TokenKind::Pound is used for `#[..]` attribute for next field,
                    // emit the diagnostic and keep going
                    err.span_suggestion(
                        sp,
                        "try adding a comma",
                        ",",
                        Applicability::MachineApplicable,
                    );
                    err.emit();
                    recovered = true;
                }

                if recovered {
                    // Make sure an error was emitted (either by recovering an angle bracket,
                    // or by finding an identifier as the next token), since we're
                    // going to continue parsing
                    assert!(self.sess.span_diagnostic.has_errors().is_some());
                } else {
                    return Err(err);
                }
            }
        }
        Ok(a_var)
    }

    fn expect_field_ty_separator(&mut self) -> PResult<'a, ()> {
        if let Err(mut err) = self.expect(&token::Colon) {
            let sm = self.sess.source_map();
            let eq_typo = self.token.kind == token::Eq && self.look_ahead(1, |t| t.is_path_start());
            let semi_typo = self.token.kind == token::Semi
                && self.look_ahead(1, |t| {
                    t.is_path_start()
                    // We check that we are in a situation like `foo; bar` to avoid bad suggestions
                    // when there's no type and `;` was used instead of a comma.
                    && match (sm.lookup_line(self.token.span.hi()), sm.lookup_line(t.span.lo())) {
                        (Ok(l), Ok(r)) => l.line == r.line,
                        _ => true,
                    }
                });
            if eq_typo || semi_typo {
                self.bump();
                // Gracefully handle small typos.
                err.span_suggestion_short(
                    self.prev_token.span,
                    "field names and their types are separated with `:`",
                    ":",
                    Applicability::MachineApplicable,
                );
                err.emit();
            } else {
                return Err(err);
            }
        }
        Ok(())
    }

    /// Parses a structure field.
    fn parse_name_and_ty(
        &mut self,
        adt_ty: &str,
        lo: Span,
        vis: Visibility,
        attrs: Vec<Attribute>,
    ) -> PResult<'a, FieldDef> {
        let name = self.parse_field_ident(adt_ty, lo)?;
        self.expect_field_ty_separator()?;
        let ty = self.parse_ty()?;
        if self.token.kind == token::Colon && self.look_ahead(1, |tok| tok.kind != token::Colon) {
            self.struct_span_err(self.token.span, "found single colon in a struct field type path")
                .span_suggestion_verbose(
                    self.token.span,
                    "write a path separator here",
                    "::",
                    Applicability::MaybeIncorrect,
                )
                .emit();
        }
        if self.token.kind == token::Eq {
            self.bump();
            let const_expr = self.parse_anon_const_expr()?;
            let sp = ty.span.shrink_to_hi().to(const_expr.value.span);
            self.struct_span_err(sp, "default values on `struct` fields aren't supported")
                .span_suggestion(
                    sp,
                    "remove this unsupported default value",
                    "",
                    Applicability::MachineApplicable,
                )
                .emit();
        }
        Ok(FieldDef {
            span: lo.to(self.prev_token.span),
            ident: Some(name),
            vis,
            id: DUMMY_NODE_ID,
            ty,
            attrs: attrs.into(),
            is_placeholder: false,
        })
    }

    /// Parses a field identifier. Specialized version of `parse_ident_common`
    /// for better diagnostics and suggestions.
    fn parse_field_ident(&mut self, adt_ty: &str, lo: Span) -> PResult<'a, Ident> {
        let (ident, is_raw) = self.ident_or_err()?;
        if !is_raw && ident.is_reserved() {
            let err = if self.check_fn_front_matter(false) {
                let inherited_vis = Visibility {
                    span: rustc_span::DUMMY_SP,
                    kind: VisibilityKind::Inherited,
                    tokens: None,
                };
                // We use `parse_fn` to get a span for the function
                let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
                if let Err(mut db) =
                    self.parse_fn(&mut Vec::new(), fn_parse_mode, lo, &inherited_vis)
                {
                    db.delay_as_bug();
                }
                let mut err = self.struct_span_err(
                    lo.to(self.prev_token.span),
                    &format!("functions are not allowed in {adt_ty} definitions"),
                );
                err.help("unlike in C++, Java, and C#, functions are declared in `impl` blocks");
                err.help("see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information");
                err
            } else {
                self.expected_ident_found()
            };
            return Err(err);
        }
        self.bump();
        Ok(ident)
    }

    /// Parses a declarative macro 2.0 definition.
    /// The `macro` keyword has already been parsed.
    /// ```ebnf
    /// MacBody = "{" TOKEN_STREAM "}" ;
    /// MacParams = "(" TOKEN_STREAM ")" ;
    /// DeclMac = "macro" Ident MacParams? MacBody ;
    /// ```
    fn parse_item_decl_macro(&mut self, lo: Span) -> PResult<'a, ItemInfo> {
        let ident = self.parse_ident()?;
        let body = if self.check(&token::OpenDelim(Delimiter::Brace)) {
            self.parse_mac_args()? // `MacBody`
        } else if self.check(&token::OpenDelim(Delimiter::Parenthesis)) {
            let params = self.parse_token_tree(); // `MacParams`
            let pspan = params.span();
            if !self.check(&token::OpenDelim(Delimiter::Brace)) {
                return self.unexpected();
            }
            let body = self.parse_token_tree(); // `MacBody`
            // Convert `MacParams MacBody` into `{ MacParams => MacBody }`.
            let bspan = body.span();
            let arrow = TokenTree::token_alone(token::FatArrow, pspan.between(bspan)); // `=>`
            let tokens = TokenStream::new(vec![params, arrow, body]);
            let dspan = DelimSpan::from_pair(pspan.shrink_to_lo(), bspan.shrink_to_hi());
            P(MacArgs::Delimited(dspan, MacDelimiter::Brace, tokens))
        } else {
            return self.unexpected();
        };

        self.sess.gated_spans.gate(sym::decl_macro, lo.to(self.prev_token.span));
        Ok((ident, ItemKind::MacroDef(ast::MacroDef { body, macro_rules: false })))
    }

    /// Is this a possibly malformed start of a `macro_rules! foo` item definition?
    fn is_macro_rules_item(&mut self) -> IsMacroRulesItem {
        if self.check_keyword(kw::MacroRules) {
            let macro_rules_span = self.token.span;

            if self.look_ahead(1, |t| *t == token::Not) && self.look_ahead(2, |t| t.is_ident()) {
                return IsMacroRulesItem::Yes { has_bang: true };
            } else if self.look_ahead(1, |t| (t.is_ident())) {
                // macro_rules foo
                self.struct_span_err(macro_rules_span, "expected `!` after `macro_rules`")
                    .span_suggestion(
                        macro_rules_span,
                        "add a `!`",
                        "macro_rules!",
                        Applicability::MachineApplicable,
                    )
                    .emit();

                return IsMacroRulesItem::Yes { has_bang: false };
            }
        }

        IsMacroRulesItem::No
    }

    /// Parses a `macro_rules! foo { ... }` declarative macro.
    fn parse_item_macro_rules(
        &mut self,
        vis: &Visibility,
        has_bang: bool,
    ) -> PResult<'a, ItemInfo> {
        self.expect_keyword(kw::MacroRules)?; // `macro_rules`

        if has_bang {
            self.expect(&token::Not)?; // `!`
        }
        let ident = self.parse_ident()?;

        if self.eat(&token::Not) {
            // Handle macro_rules! foo!
            let span = self.prev_token.span;
            self.struct_span_err(span, "macro names aren't followed by a `!`")
                .span_suggestion(span, "remove the `!`", "", Applicability::MachineApplicable)
                .emit();
        }

        let body = self.parse_mac_args()?;
        self.eat_semi_for_macro_if_needed(&body);
        self.complain_if_pub_macro(vis, true);

        Ok((ident, ItemKind::MacroDef(ast::MacroDef { body, macro_rules: true })))
    }

    /// Item macro invocations or `macro_rules!` definitions need inherited visibility.
    /// If that's not the case, emit an error.
    fn complain_if_pub_macro(&self, vis: &Visibility, macro_rules: bool) {
        if let VisibilityKind::Inherited = vis.kind {
            return;
        }

        let vstr = pprust::vis_to_string(vis);
        let vstr = vstr.trim_end();
        if macro_rules {
            let msg = format!("can't qualify macro_rules invocation with `{vstr}`");
            self.struct_span_err(vis.span, &msg)
                .span_suggestion(
                    vis.span,
                    "try exporting the macro",
                    "#[macro_export]",
                    Applicability::MaybeIncorrect, // speculative
                )
                .emit();
        } else {
            self.struct_span_err(vis.span, "can't qualify macro invocation with `pub`")
                .span_suggestion(
                    vis.span,
                    "remove the visibility",
                    "",
                    Applicability::MachineApplicable,
                )
                .help(&format!("try adjusting the macro to put `{vstr}` inside the invocation"))
                .emit();
        }
    }

    fn eat_semi_for_macro_if_needed(&mut self, args: &MacArgs) {
        if args.need_semicolon() && !self.eat(&token::Semi) {
            self.report_invalid_macro_expansion_item(args);
        }
    }

    fn report_invalid_macro_expansion_item(&self, args: &MacArgs) {
        let span = args.span().expect("undelimited macro call");
        let mut err = self.struct_span_err(
            span,
            "macros that expand to items must be delimited with braces or followed by a semicolon",
        );
        // FIXME: This will make us not emit the help even for declarative
        // macros within the same crate (that we can fix), which is sad.
        if !span.from_expansion() {
            if self.unclosed_delims.is_empty() {
                let DelimSpan { open, close } = match args {
                    MacArgs::Empty | MacArgs::Eq(..) => unreachable!(),
                    MacArgs::Delimited(dspan, ..) => *dspan,
                };
                err.multipart_suggestion(
                    "change the delimiters to curly braces",
                    vec![(open, "{".to_string()), (close, '}'.to_string())],
                    Applicability::MaybeIncorrect,
                );
            } else {
                err.span_suggestion(
                    span,
                    "change the delimiters to curly braces",
                    " { /* items */ }",
                    Applicability::HasPlaceholders,
                );
            }
            err.span_suggestion(
                span.shrink_to_hi(),
                "add a semicolon",
                ';',
                Applicability::MaybeIncorrect,
            );
        }
        err.emit();
    }

    /// Checks if current token is one of tokens which cannot be nested like `kw::Enum`. In case
    /// it is, we try to parse the item and report error about nested types.
    fn recover_nested_adt_item(&mut self, keyword: Symbol) -> PResult<'a, bool> {
        if (self.token.is_keyword(kw::Enum)
            || self.token.is_keyword(kw::Struct)
            || self.token.is_keyword(kw::Union))
            && self.look_ahead(1, |t| t.is_ident())
        {
            let kw_token = self.token.clone();
            let kw_str = pprust::token_to_string(&kw_token);
            let item = self.parse_item(ForceCollect::No)?;

            self.struct_span_err(
                kw_token.span,
                &format!("`{kw_str}` definition cannot be nested inside `{keyword}`"),
            )
            .span_suggestion(
                item.unwrap().span,
                &format!("consider creating a new `{kw_str}` definition instead of nesting"),
                "",
                Applicability::MaybeIncorrect,
            )
            .emit();
            // We successfully parsed the item but we must inform the caller about nested problem.
            return Ok(false);
        }
        Ok(true)
    }
}

/// The parsing configuration used to parse a parameter list (see `parse_fn_params`).
///
/// The function decides if, per-parameter `p`, `p` must have a pattern or just a type.
///
/// This function pointer accepts an edition, because in edition 2015, trait declarations
/// were allowed to omit parameter names. In 2018, they became required.
type ReqName = fn(Edition) -> bool;

/// Parsing configuration for functions.
///
/// The syntax of function items is slightly different within trait definitions,
/// impl blocks, and modules. It is still parsed using the same code, just with
/// different flags set, so that even when the input is wrong and produces a parse
/// error, it still gets into the AST and the rest of the parser and
/// type checker can run.
#[derive(Clone, Copy)]
pub(crate) struct FnParseMode {
    /// A function pointer that decides if, per-parameter `p`, `p` must have a
    /// pattern or just a type. This field affects parsing of the parameters list.
    ///
    /// ```text
    /// fn foo(alef: A) -> X { X::new() }
    ///        -----^^ affects parsing this part of the function signature
    ///        |
    ///        if req_name returns false, then this name is optional
    ///
    /// fn bar(A) -> X;
    ///        ^
    ///        |
    ///        if req_name returns true, this is an error
    /// ```
    ///
    /// Calling this function pointer should only return false if:
    ///
    ///   * The item is being parsed inside of a trait definition.
    ///     Within an impl block or a module, it should always evaluate
    ///     to true.
    ///   * The span is from Edition 2015. In particular, you can get a
    ///     2015 span inside a 2021 crate using macros.
    pub req_name: ReqName,
    /// If this flag is set to `true`, then plain, semicolon-terminated function
    /// prototypes are not allowed here.
    ///
    /// ```text
    /// fn foo(alef: A) -> X { X::new() }
    ///                      ^^^^^^^^^^^^
    ///                      |
    ///                      this is always allowed
    ///
    /// fn bar(alef: A, bet: B) -> X;
    ///                             ^
    ///                             |
    ///                             if req_body is set to true, this is an error
    /// ```
    ///
    /// This field should only be set to false if the item is inside of a trait
    /// definition or extern block. Within an impl block or a module, it should
    /// always be set to true.
    pub req_body: bool,
}

/// Parsing of functions and methods.
impl<'a> Parser<'a> {
    /// Parse a function starting from the front matter (`const ...`) to the body `{ ... }` or `;`.
    fn parse_fn(
        &mut self,
        attrs: &mut Vec<Attribute>,
        fn_parse_mode: FnParseMode,
        sig_lo: Span,
        vis: &Visibility,
    ) -> PResult<'a, (Ident, FnSig, Generics, Option<P<Block>>)> {
        let header = self.parse_fn_front_matter(vis)?; // `const ... fn`
        let ident = self.parse_ident()?; // `foo`
        let mut generics = self.parse_generics()?; // `<'a, T, ...>`
        let decl =
            self.parse_fn_decl(fn_parse_mode.req_name, AllowPlus::Yes, RecoverReturnSign::Yes)?; // `(p: u8, ...)`
        generics.where_clause = self.parse_where_clause()?; // `where T: Ord`

        let mut sig_hi = self.prev_token.span;
        let body = self.parse_fn_body(attrs, &ident, &mut sig_hi, fn_parse_mode.req_body)?; // `;` or `{ ... }`.
        let fn_sig_span = sig_lo.to(sig_hi);
        Ok((ident, FnSig { header, decl, span: fn_sig_span }, generics, body))
    }

    /// Parse the "body" of a function.
    /// This can either be `;` when there's no body,
    /// or e.g. a block when the function is a provided one.
    fn parse_fn_body(
        &mut self,
        attrs: &mut Vec<Attribute>,
        ident: &Ident,
        sig_hi: &mut Span,
        req_body: bool,
    ) -> PResult<'a, Option<P<Block>>> {
        let has_semi = if req_body {
            self.token.kind == TokenKind::Semi
        } else {
            // Only include `;` in list of expected tokens if body is not required
            self.check(&TokenKind::Semi)
        };
        let (inner_attrs, body) = if has_semi {
            // Include the trailing semicolon in the span of the signature
            self.expect_semi()?;
            *sig_hi = self.prev_token.span;
            (Vec::new(), None)
        } else if self.check(&token::OpenDelim(Delimiter::Brace)) || self.token.is_whole_block() {
            self.parse_inner_attrs_and_block().map(|(attrs, body)| (attrs, Some(body)))?
        } else if self.token.kind == token::Eq {
            // Recover `fn foo() = $expr;`.
            self.bump(); // `=`
            let eq_sp = self.prev_token.span;
            let _ = self.parse_expr()?;
            self.expect_semi()?; // `;`
            let span = eq_sp.to(self.prev_token.span);
            self.struct_span_err(span, "function body cannot be `= expression;`")
                .multipart_suggestion(
                    "surround the expression with `{` and `}` instead of `=` and `;`",
                    vec![(eq_sp, "{".to_string()), (self.prev_token.span, " }".to_string())],
                    Applicability::MachineApplicable,
                )
                .emit();
            (Vec::new(), Some(self.mk_block_err(span)))
        } else {
            let expected = if req_body {
                &[token::OpenDelim(Delimiter::Brace)][..]
            } else {
                &[token::Semi, token::OpenDelim(Delimiter::Brace)]
            };
            if let Err(mut err) = self.expected_one_of_not_found(&[], &expected) {
                if self.token.kind == token::CloseDelim(Delimiter::Brace) {
                    // The enclosing `mod`, `trait` or `impl` is being closed, so keep the `fn` in
                    // the AST for typechecking.
                    err.span_label(ident.span, "while parsing this `fn`");
                    err.emit();
                } else {
                    return Err(err);
                }
            }
            (Vec::new(), None)
        };
        attrs.extend(inner_attrs);
        Ok(body)
    }

    /// Is the current token the start of an `FnHeader` / not a valid parse?
    ///
    /// `check_pub` adds additional `pub` to the checks in case users place it
    /// wrongly, can be used to ensure `pub` never comes after `default`.
    pub(super) fn check_fn_front_matter(&mut self, check_pub: bool) -> bool {
        // We use an over-approximation here.
        // `const const`, `fn const` won't parse, but we're not stepping over other syntax either.
        // `pub` is added in case users got confused with the ordering like `async pub fn`,
        // only if it wasn't preceded by `default` as `default pub` is invalid.
        let quals: &[Symbol] = if check_pub {
            &[kw::Pub, kw::Const, kw::Async, kw::Unsafe, kw::Extern]
        } else {
            &[kw::Const, kw::Async, kw::Unsafe, kw::Extern]
        };
        self.check_keyword(kw::Fn) // Definitely an `fn`.
            // `$qual fn` or `$qual $qual`:
            || quals.iter().any(|&kw| self.check_keyword(kw))
                && self.look_ahead(1, |t| {
                    // `$qual fn`, e.g. `const fn` or `async fn`.
                    t.is_keyword(kw::Fn)
                    // Two qualifiers `$qual $qual` is enough, e.g. `async unsafe`.
                    || t.is_non_raw_ident_where(|i| quals.contains(&i.name)
                        // Rule out 2015 `const async: T = val`.
                        && i.is_reserved()
                        // Rule out unsafe extern block.
                        && !self.is_unsafe_foreign_mod())
                })
            // `extern ABI fn`
            || self.check_keyword(kw::Extern)
                && self.look_ahead(1, |t| t.can_begin_literal_maybe_minus())
                && self.look_ahead(2, |t| t.is_keyword(kw::Fn))
    }

    /// Parses all the "front matter" (or "qualifiers") for a `fn` declaration,
    /// up to and including the `fn` keyword. The formal grammar is:
    ///
    /// ```text
    /// Extern = "extern" StringLit? ;
    /// FnQual = "const"? "async"? "unsafe"? Extern? ;
    /// FnFrontMatter = FnQual "fn" ;
    /// ```
    ///
    /// `vis` represents the visibility that was already parsed, if any. Use
    /// `Visibility::Inherited` when no visibility is known.
    pub(super) fn parse_fn_front_matter(&mut self, orig_vis: &Visibility) -> PResult<'a, FnHeader> {
        let sp_start = self.token.span;
        let constness = self.parse_constness();

        let async_start_sp = self.token.span;
        let asyncness = self.parse_asyncness();

        let unsafe_start_sp = self.token.span;
        let unsafety = self.parse_unsafety();

        let ext_start_sp = self.token.span;
        let ext = self.parse_extern();

        if let Async::Yes { span, .. } = asyncness {
            self.ban_async_in_2015(span);
        }

        if !self.eat_keyword(kw::Fn) {
            // It is possible for `expect_one_of` to recover given the contents of
            // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't
            // account for this.
            match self.expect_one_of(&[], &[]) {
                Ok(true) => {}
                Ok(false) => unreachable!(),
                Err(mut err) => {
                    // Qualifier keywords ordering check
                    enum WrongKw {
                        Duplicated(Span),
                        Misplaced(Span),
                    }

                    // This will allow the machine fix to directly place the keyword in the correct place or to indicate
                    // that the keyword is already present and the second instance should be removed.
                    let wrong_kw = if self.check_keyword(kw::Const) {
                        match constness {
                            Const::Yes(sp) => Some(WrongKw::Duplicated(sp)),
                            Const::No => Some(WrongKw::Misplaced(async_start_sp)),
                        }
                    } else if self.check_keyword(kw::Async) {
                        match asyncness {
                            Async::Yes { span, .. } => Some(WrongKw::Duplicated(span)),
                            Async::No => Some(WrongKw::Misplaced(unsafe_start_sp)),
                        }
                    } else if self.check_keyword(kw::Unsafe) {
                        match unsafety {
                            Unsafe::Yes(sp) => Some(WrongKw::Duplicated(sp)),
                            Unsafe::No => Some(WrongKw::Misplaced(ext_start_sp)),
                        }
                    } else {
                        None
                    };

                    // The keyword is already present, suggest removal of the second instance
                    if let Some(WrongKw::Duplicated(original_sp)) = wrong_kw {
                        let original_kw = self
                            .span_to_snippet(original_sp)
                            .expect("Span extracted directly from keyword should always work");

                        err.span_suggestion(
                            self.token.uninterpolated_span(),
                            &format!("`{original_kw}` already used earlier, remove this one"),
                            "",
                            Applicability::MachineApplicable,
                        )
                        .span_note(original_sp, &format!("`{original_kw}` first seen here"));
                    }
                    // The keyword has not been seen yet, suggest correct placement in the function front matter
                    else if let Some(WrongKw::Misplaced(correct_pos_sp)) = wrong_kw {
                        let correct_pos_sp = correct_pos_sp.to(self.prev_token.span);
                        if let Ok(current_qual) = self.span_to_snippet(correct_pos_sp) {
                            let misplaced_qual_sp = self.token.uninterpolated_span();
                            let misplaced_qual = self.span_to_snippet(misplaced_qual_sp).unwrap();

                            err.span_suggestion(
                                    correct_pos_sp.to(misplaced_qual_sp),
                                    &format!("`{misplaced_qual}` must come before `{current_qual}`"),
                                    format!("{misplaced_qual} {current_qual}"),
                                    Applicability::MachineApplicable,
                                ).note("keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`");
                        }
                    }
                    // Recover incorrect visibility order such as `async pub`
                    else if self.check_keyword(kw::Pub) {
                        let sp = sp_start.to(self.prev_token.span);
                        if let Ok(snippet) = self.span_to_snippet(sp) {
                            let current_vis = match self.parse_visibility(FollowedByType::No) {
                                Ok(v) => v,
                                Err(d) => {
                                    d.cancel();
                                    return Err(err);
                                }
                            };
                            let vs = pprust::vis_to_string(&current_vis);
                            let vs = vs.trim_end();

                            // There was no explicit visibility
                            if matches!(orig_vis.kind, VisibilityKind::Inherited) {
                                err.span_suggestion(
                                    sp_start.to(self.prev_token.span),
                                    &format!("visibility `{vs}` must come before `{snippet}`"),
                                    format!("{vs} {snippet}"),
                                    Applicability::MachineApplicable,
                                );
                            }
                            // There was an explicit visibility
                            else {
                                err.span_suggestion(
                                    current_vis.span,
                                    "there is already a visibility modifier, remove one",
                                    "",
                                    Applicability::MachineApplicable,
                                )
                                .span_note(orig_vis.span, "explicit visibility first seen here");
                            }
                        }
                    }
                    return Err(err);
                }
            }
        }

        Ok(FnHeader { constness, unsafety, asyncness, ext })
    }

    /// We are parsing `async fn`. If we are on Rust 2015, emit an error.
    fn ban_async_in_2015(&self, span: Span) {
        if span.rust_2015() {
            let diag = self.diagnostic();
            struct_span_err!(diag, span, E0670, "`async fn` is not permitted in Rust 2015")
                .span_label(span, "to use `async fn`, switch to Rust 2018 or later")
                .help_use_latest_edition()
                .emit();
        }
    }

    /// Parses the parameter list and result type of a function declaration.
    pub(super) fn parse_fn_decl(
        &mut self,
        req_name: ReqName,
        ret_allow_plus: AllowPlus,
        recover_return_sign: RecoverReturnSign,
    ) -> PResult<'a, P<FnDecl>> {
        Ok(P(FnDecl {
            inputs: self.parse_fn_params(req_name)?,
            output: self.parse_ret_ty(ret_allow_plus, RecoverQPath::Yes, recover_return_sign)?,
        }))
    }

    /// Parses the parameter list of a function, including the `(` and `)` delimiters.
    fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, Vec<Param>> {
        let mut first_param = true;
        // Parse the arguments, starting out with `self` being allowed...
        let (mut params, _) = self.parse_paren_comma_seq(|p| {
            let param = p.parse_param_general(req_name, first_param).or_else(|mut e| {
                e.emit();
                let lo = p.prev_token.span;
                // Skip every token until next possible arg or end.
                p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(Delimiter::Parenthesis)]);
                // Create a placeholder argument for proper arg count (issue #34264).
                Ok(dummy_arg(Ident::new(kw::Empty, lo.to(p.prev_token.span))))
            });
            // ...now that we've parsed the first argument, `self` is no longer allowed.
            first_param = false;
            param
        })?;
        // Replace duplicated recovered params with `_` pattern to avoid unnecessary errors.
        self.deduplicate_recovered_params_names(&mut params);
        Ok(params)
    }

    /// Parses a single function parameter.
    ///
    /// - `self` is syntactically allowed when `first_param` holds.
    fn parse_param_general(&mut self, req_name: ReqName, first_param: bool) -> PResult<'a, Param> {
        let lo = self.token.span;
        let attrs = self.parse_outer_attributes()?;
        self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
            // Possibly parse `self`. Recover if we parsed it and it wasn't allowed here.
            if let Some(mut param) = this.parse_self_param()? {
                param.attrs = attrs.into();
                let res = if first_param { Ok(param) } else { this.recover_bad_self_param(param) };
                return Ok((res?, TrailingToken::None));
            }

            let is_name_required = match this.token.kind {
                token::DotDotDot => false,
                _ => req_name(this.token.span.edition()),
            };
            let (pat, ty) = if is_name_required || this.is_named_param() {
                debug!("parse_param_general parse_pat (is_name_required:{})", is_name_required);

                let (pat, colon) = this.parse_fn_param_pat_colon()?;
                if !colon {
                    let mut err = this.unexpected::<()>().unwrap_err();
                    return if let Some(ident) =
                        this.parameter_without_type(&mut err, pat, is_name_required, first_param)
                    {
                        err.emit();
                        Ok((dummy_arg(ident), TrailingToken::None))
                    } else {
                        Err(err)
                    };
                }

                this.eat_incorrect_doc_comment_for_param_type();
                (pat, this.parse_ty_for_param()?)
            } else {
                debug!("parse_param_general ident_to_pat");
                let parser_snapshot_before_ty = this.clone();
                this.eat_incorrect_doc_comment_for_param_type();
                let mut ty = this.parse_ty_for_param();
                if ty.is_ok()
                    && this.token != token::Comma
                    && this.token != token::CloseDelim(Delimiter::Parenthesis)
                {
                    // This wasn't actually a type, but a pattern looking like a type,
                    // so we are going to rollback and re-parse for recovery.
                    ty = this.unexpected();
                }
                match ty {
                    Ok(ty) => {
                        let ident = Ident::new(kw::Empty, this.prev_token.span);
                        let bm = BindingMode::ByValue(Mutability::Not);
                        let pat = this.mk_pat_ident(ty.span, bm, ident);
                        (pat, ty)
                    }
                    // If this is a C-variadic argument and we hit an error, return the error.
                    Err(err) if this.token == token::DotDotDot => return Err(err),
                    // Recover from attempting to parse the argument as a type without pattern.
                    Err(err) => {
                        err.cancel();
                        *this = parser_snapshot_before_ty;
                        this.recover_arg_parse()?
                    }
                }
            };

            let span = lo.until(this.token.span);

            Ok((
                Param {
                    attrs: attrs.into(),
                    id: ast::DUMMY_NODE_ID,
                    is_placeholder: false,
                    pat,
                    span,
                    ty,
                },
                TrailingToken::None,
            ))
        })
    }

    /// Returns the parsed optional self parameter and whether a self shortcut was used.
    fn parse_self_param(&mut self) -> PResult<'a, Option<Param>> {
        // Extract an identifier *after* having confirmed that the token is one.
        let expect_self_ident = |this: &mut Self| match this.token.ident() {
            Some((ident, false)) => {
                this.bump();
                ident
            }
            _ => unreachable!(),
        };
        // Is `self` `n` tokens ahead?
        let is_isolated_self = |this: &Self, n| {
            this.is_keyword_ahead(n, &[kw::SelfLower])
                && this.look_ahead(n + 1, |t| t != &token::ModSep)
        };
        // Is `mut self` `n` tokens ahead?
        let is_isolated_mut_self =
            |this: &Self, n| this.is_keyword_ahead(n, &[kw::Mut]) && is_isolated_self(this, n + 1);
        // Parse `self` or `self: TYPE`. We already know the current token is `self`.
        let parse_self_possibly_typed = |this: &mut Self, m| {
            let eself_ident = expect_self_ident(this);
            let eself_hi = this.prev_token.span;
            let eself = if this.eat(&token::Colon) {
                SelfKind::Explicit(this.parse_ty()?, m)
            } else {
                SelfKind::Value(m)
            };
            Ok((eself, eself_ident, eself_hi))
        };
        // Recover for the grammar `*self`, `*const self`, and `*mut self`.
        let recover_self_ptr = |this: &mut Self| {
            let msg = "cannot pass `self` by raw pointer";
            let span = this.token.span;
            this.struct_span_err(span, msg).span_label(span, msg).emit();

            Ok((SelfKind::Value(Mutability::Not), expect_self_ident(this), this.prev_token.span))
        };

        // Parse optional `self` parameter of a method.
        // Only a limited set of initial token sequences is considered `self` parameters; anything
        // else is parsed as a normal function parameter list, so some lookahead is required.
        let eself_lo = self.token.span;
        let (eself, eself_ident, eself_hi) = match self.token.uninterpolate().kind {
            token::BinOp(token::And) => {
                let eself = if is_isolated_self(self, 1) {
                    // `&self`
                    self.bump();
                    SelfKind::Region(None, Mutability::Not)
                } else if is_isolated_mut_self(self, 1) {
                    // `&mut self`
                    self.bump();
                    self.bump();
                    SelfKind::Region(None, Mutability::Mut)
                } else if self.look_ahead(1, |t| t.is_lifetime()) && is_isolated_self(self, 2) {
                    // `&'lt self`
                    self.bump();
                    let lt = self.expect_lifetime();
                    SelfKind::Region(Some(lt), Mutability::Not)
                } else if self.look_ahead(1, |t| t.is_lifetime()) && is_isolated_mut_self(self, 2) {
                    // `&'lt mut self`
                    self.bump();
                    let lt = self.expect_lifetime();
                    self.bump();
                    SelfKind::Region(Some(lt), Mutability::Mut)
                } else {
                    // `&not_self`
                    return Ok(None);
                };
                (eself, expect_self_ident(self), self.prev_token.span)
            }
            // `*self`
            token::BinOp(token::Star) if is_isolated_self(self, 1) => {
                self.bump();
                recover_self_ptr(self)?
            }
            // `*mut self` and `*const self`
            token::BinOp(token::Star)
                if self.look_ahead(1, |t| t.is_mutability()) && is_isolated_self(self, 2) =>
            {
                self.bump();
                self.bump();
                recover_self_ptr(self)?
            }
            // `self` and `self: TYPE`
            token::Ident(..) if is_isolated_self(self, 0) => {
                parse_self_possibly_typed(self, Mutability::Not)?
            }
            // `mut self` and `mut self: TYPE`
            token::Ident(..) if is_isolated_mut_self(self, 0) => {
                self.bump();
                parse_self_possibly_typed(self, Mutability::Mut)?
            }
            _ => return Ok(None),
        };

        let eself = source_map::respan(eself_lo.to(eself_hi), eself);
        Ok(Some(Param::from_self(AttrVec::default(), eself, eself_ident)))
    }

    fn is_named_param(&self) -> bool {
        let offset = match self.token.kind {
            token::Interpolated(ref nt) => match **nt {
                token::NtPat(..) => return self.look_ahead(1, |t| t == &token::Colon),
                _ => 0,
            },
            token::BinOp(token::And) | token::AndAnd => 1,
            _ if self.token.is_keyword(kw::Mut) => 1,
            _ => 0,
        };

        self.look_ahead(offset, |t| t.is_ident())
            && self.look_ahead(offset + 1, |t| t == &token::Colon)
    }

    fn recover_first_param(&mut self) -> &'static str {
        match self
            .parse_outer_attributes()
            .and_then(|_| self.parse_self_param())
            .map_err(|e| e.cancel())
        {
            Ok(Some(_)) => "method",
            _ => "function",
        }
    }
}

enum IsMacroRulesItem {
    Yes { has_bang: bool },
    No,
}