Auto merge of #131045 - compiler-errors:remove-unnamed_fields, r=wesleywiser

Retire the `unnamed_fields` feature for now

`#![feature(unnamed_fields)]` was implemented in part in #115131 and #115367, however work on that feature has (afaict) stalled and in the mean time there have been some concerns raised (e.g.[^1][^2]) about whether `unnamed_fields` is worthwhile to have in the language, especially in its current desugaring. Because it represents a compiler implementation burden including a new kind of anonymous ADT and additional complication to field selection, and is quite prone to bugs today, I'm choosing to remove the feature.

However, since I'm not one to really write a bunch of words, I'm specifically *not* going to de-RFC this feature. This PR essentially *rolls back* the state of this feature to "RFC accepted but not yet implemented"; however if anyone wants to formally unapprove the RFC from the t-lang side, then please be my guest. I'm just not totally willing to summarize the various language-facing reasons for why this feature is or is not worthwhile, since I'm coming from the compiler side mostly.

Fixes #117942
Fixes #121161
Fixes #121263
Fixes #121299
Fixes #121722
Fixes #121799
Fixes #126969
Fixes #131041

Tracking:
* #49804

[^1]: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Unnamed.20struct.2Funion.20fields
[^2]: #49804 (comment)

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2167,10 +2167,6 @@ pub enum TyKind {
     Never,
     /// A tuple (`(A, B, C, D,...)`).
     Tup(ThinVec<P<Ty>>),
-    /// An anonymous struct type i.e. `struct { foo: Type }`.
-    AnonStruct(NodeId, ThinVec<FieldDef>),
-    /// An anonymous union type i.e. `union { bar: Type }`.
-    AnonUnion(NodeId, ThinVec<FieldDef>),
     /// A path (`module::module::...::Type`), optionally
     /// "qualified", e.g., `<Vec<T> as SomeTrait>::SomeType`.
     ///
@@ -2227,10 +2223,6 @@ impl TyKind {
             None
         }
     }
-
-    pub fn is_anon_adt(&self) -> bool {
-        matches!(self, TyKind::AnonStruct(..) | TyKind::AnonUnion(..))
-    }
 }
 
 /// Syntax used to declare a trait object.

compiler/rustc_ast/src/mut_visit.rs

@@ -519,10 +519,6 @@ pub fn walk_ty<T: MutVisitor>(vis: &mut T, ty: &mut P<Ty>) {
             visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::Impl));
         }
         TyKind::MacCall(mac) => vis.visit_mac_call(mac),
-        TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {
-            vis.visit_id(id);
-            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
-        }
     }
     visit_lazy_tts(vis, tokens);
     vis.visit_span(span);

compiler/rustc_ast/src/util/classify.rs

@@ -287,12 +287,6 @@ fn type_trailing_braced_mac_call(mut ty: &ast::Ty) -> Option<&ast::MacCall> {
             | ast::TyKind::Pat(..)
             | ast::TyKind::Dummy
             | ast::TyKind::Err(..) => break None,
-
-            // These end in brace, but cannot occur in a let-else statement.
-            // They are only parsed as fields of a data structure. For the
-            // purpose of denying trailing braces in the expression of a
-            // let-else, we can disregard these.
-            ast::TyKind::AnonStruct(..) | ast::TyKind::AnonUnion(..) => break None,
         }
     }
 }

compiler/rustc_ast/src/visit.rs

@@ -535,9 +535,6 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) -> V::Result {
         TyKind::Err(_guar) => {}
         TyKind::MacCall(mac) => try_visit!(visitor.visit_mac_call(mac)),
         TyKind::Never | TyKind::CVarArgs => {}
-        TyKind::AnonStruct(_id, ref fields) | TyKind::AnonUnion(_id, ref fields) => {
-            walk_list!(visitor, visit_field_def, fields);
-        }
     }
     V::Result::output()
 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -1259,46 +1259,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let kind = match &t.kind {
             TyKind::Infer => hir::TyKind::Infer,
             TyKind::Err(guar) => hir::TyKind::Err(*guar),
-            // Lower the anonymous structs or unions in a nested lowering context.
-            //
-            // ```
-            // struct Foo {
-            //     _: union {
-            // //     ^__________________  <-- within the nested lowering context,
-            //         /* fields */ //   |     we lower all fields defined into an
-            //     } //                  |     owner node of struct or union item
-            // //  ^_____________________|
-            // }
-            // ```
-            TyKind::AnonStruct(node_id, fields) | TyKind::AnonUnion(node_id, fields) => {
-                // Here its `def_id` is created in `build_reduced_graph`.
-                let def_id = self.local_def_id(*node_id);
-                debug!(?def_id);
-                let owner_id = hir::OwnerId { def_id };
-                self.with_hir_id_owner(*node_id, |this| {
-                    let fields = this.arena.alloc_from_iter(
-                        fields.iter().enumerate().map(|f| this.lower_field_def(f)),
-                    );
-                    let span = t.span;
-                    let variant_data =
-                        hir::VariantData::Struct { fields, recovered: ast::Recovered::No };
-                    // FIXME: capture the generics from the outer adt.
-                    let generics = hir::Generics::empty();
-                    let kind = match t.kind {
-                        TyKind::AnonStruct(..) => hir::ItemKind::Struct(variant_data, generics),
-                        TyKind::AnonUnion(..) => hir::ItemKind::Union(variant_data, generics),
-                        _ => unreachable!(),
-                    };
-                    hir::OwnerNode::Item(this.arena.alloc(hir::Item {
-                        ident: Ident::new(kw::Empty, span),
-                        owner_id,
-                        kind,
-                        span: this.lower_span(span),
-                        vis_span: this.lower_span(span.shrink_to_lo()),
-                    }))
-                });
-                hir::TyKind::AnonAdt(hir::ItemId { owner_id })
-            }
             TyKind::Slice(ty) => hir::TyKind::Slice(self.lower_ty(ty, itctx)),
             TyKind::Ptr(mt) => hir::TyKind::Ptr(self.lower_mt(mt, itctx)),
             TyKind::Ref(region, mt) => {

compiler/rustc_ast_passes/messages.ftl

@@ -1,7 +1,3 @@
-ast_passes_anon_struct_or_union_not_allowed =
-    anonymous {$struct_or_union}s are not allowed outside of unnamed struct or union fields
-    .label = anonymous {$struct_or_union} declared here
-
 ast_passes_assoc_const_without_body =
     associated constant in `impl` without body
     .suggestion = provide a definition for the constant
@@ -160,14 +156,6 @@ ast_passes_inherent_cannot_be = inherent impls cannot be {$annotation}
     .type = inherent impl for this type
     .only_trait = only trait implementations may be annotated with {$annotation}
 
-ast_passes_invalid_unnamed_field =
-    unnamed fields are not allowed outside of structs or unions
-    .label = unnamed field declared here
-
-ast_passes_invalid_unnamed_field_ty =
-    unnamed fields can only have struct or union types
-    .label = not a struct or union
-
 ast_passes_item_invalid_safety = items outside of `unsafe extern {"{ }"}` cannot be declared with `safe` safety qualifier
     .suggestion = remove safe from this item
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -244,9 +244,6 @@ impl<'a> AstValidator<'a> {
                     }
                 }
             }
-            TyKind::AnonStruct(_, ref fields) | TyKind::AnonUnion(_, ref fields) => {
-                walk_list!(self, visit_struct_field_def, fields)
-            }
             _ => visit::walk_ty(self, t),
         }
     }
@@ -255,7 +252,6 @@ impl<'a> AstValidator<'a> {
         if let Some(ident) = field.ident
             && ident.name == kw::Underscore
         {
-            self.check_unnamed_field_ty(&field.ty, ident.span);
             self.visit_vis(&field.vis);
             self.visit_ident(ident);
             self.visit_ty_common(&field.ty);
@@ -294,39 +290,6 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_unnamed_field_ty(&self, ty: &Ty, span: Span) {
-        if matches!(
-            &ty.kind,
-            // We already checked for `kw::Underscore` before calling this function,
-            // so skip the check
-            TyKind::AnonStruct(..) | TyKind::AnonUnion(..)
-            // If the anonymous field contains a Path as type, we can't determine
-            // if the path is a valid struct or union, so skip the check
-            | TyKind::Path(..)
-        ) {
-            return;
-        }
-        self.dcx().emit_err(errors::InvalidUnnamedFieldTy { span, ty_span: ty.span });
-    }
-
-    fn deny_anon_struct_or_union(&self, ty: &Ty) {
-        let struct_or_union = match &ty.kind {
-            TyKind::AnonStruct(..) => "struct",
-            TyKind::AnonUnion(..) => "union",
-            _ => return,
-        };
-        self.dcx().emit_err(errors::AnonStructOrUnionNotAllowed { struct_or_union, span: ty.span });
-    }
-
-    fn deny_unnamed_field(&self, field: &FieldDef) {
-        if let Some(ident) = field.ident
-            && ident.name == kw::Underscore
-        {
-            self.dcx()
-                .emit_err(errors::InvalidUnnamedField { span: field.span, ident_span: ident.span });
-        }
-    }
-
     fn check_trait_fn_not_const(&self, constness: Const, parent: &TraitOrTraitImpl) {
         let Const::Yes(span) = constness else {
             return;
@@ -890,15 +853,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
     fn visit_ty(&mut self, ty: &'a Ty) {
         self.visit_ty_common(ty);
-        self.deny_anon_struct_or_union(ty);
         self.walk_ty(ty)
     }
 
-    fn visit_field_def(&mut self, field: &'a FieldDef) {
-        self.deny_unnamed_field(field);
-        visit::walk_field_def(self, field)
-    }
-
     fn visit_item(&mut self, item: &'a Item) {
         if item.attrs.iter().any(|attr| attr.is_proc_macro_attr()) {
             self.has_proc_macro_decls = true;

compiler/rustc_ast_passes/src/errors.rs

@@ -814,33 +814,6 @@ pub(crate) struct NegativeBoundWithParentheticalNotation {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_invalid_unnamed_field_ty)]
-pub(crate) struct InvalidUnnamedFieldTy {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub ty_span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(ast_passes_invalid_unnamed_field)]
-pub(crate) struct InvalidUnnamedField {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub ident_span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(ast_passes_anon_struct_or_union_not_allowed)]
-pub(crate) struct AnonStructOrUnionNotAllowed {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-    pub struct_or_union: &'static str,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_match_arm_with_no_body)]
 pub(crate) struct MatchArmWithNoBody {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -541,7 +541,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
-    gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(postfix_match, "postfix match is experimental");
     gate_all!(mut_ref, "mutable by-reference bindings are experimental");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1174,14 +1174,6 @@ impl<'a> State<'a> {
                 }
                 self.pclose();
             }
-            ast::TyKind::AnonStruct(_, fields) => {
-                self.head("struct");
-                self.print_record_struct_body(fields, ty.span);
-            }
-            ast::TyKind::AnonUnion(_, fields) => {
-                self.head("union");
-                self.print_record_struct_body(fields, ty.span);
-            }
             ast::TyKind::Paren(typ) => {
                 self.popen();
                 self.print_type(typ);

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -113,7 +113,7 @@ fn cs_clone_simple(
                 // Already produced an assertion for this type.
                 // Anonymous structs or unions must be eliminated as they cannot be
                 // type parameters.
-            } else if !field.ty.kind.is_anon_adt() {
+            } else {
                 // let _: AssertParamIsClone<FieldTy>;
                 super::assert_ty_bounds(cx, &mut stmts, field.ty.clone(), field.span, &[
                     sym::clone,

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -124,8 +124,6 @@ fn assert_ty_bounds(
     span: Span,
     assert_path: &[Symbol],
 ) {
-    // Deny anonymous structs or unions to avoid weird errors.
-    assert!(!ty.kind.is_anon_adt(), "Anonymous structs or unions cannot be type parameters");
     // Generate statement `let _: assert_path<ty>;`.
     let span = cx.with_def_site_ctxt(span);
     let assert_path = cx.path_all(span, true, cx.std_path(assert_path), vec![GenericArg::Type(ty)]);

compiler/rustc_feature/src/removed.rs

@@ -220,6 +220,8 @@ declare_features! (
     (removed, test_removed_feature, "1.0.0", None, None),
     /// Allows using items which are missing stability attributes
     (removed, unmarked_api, "1.0.0", None, None),
+    /// Allows unnamed fields of struct and union type
+    (removed, unnamed_fields, "CURRENT_RUSTC_VERSION", Some(49804), Some("feature needs redesign")),
     (removed, unsafe_no_drop_flag, "1.0.0", None, None),
     /// Allows `union` fields that don't implement `Copy` as long as they don't have any drop glue.
     (removed, untagged_unions, "1.13.0", Some(55149),

compiler/rustc_feature/src/unstable.rs

@@ -623,8 +623,6 @@ declare_features! (
     /// Allows creation of instances of a struct by moving fields that have
     /// not changed from prior instances of the same struct (RFC #2528)
     (unstable, type_changing_struct_update, "1.58.0", Some(86555)),
-    /// Allows unnamed fields of struct and union type
-    (incomplete, unnamed_fields, "1.74.0", Some(49804)),
     /// Allows const generic parameters to be defined with types that
     /// are not `Sized`, e.g. `fn foo<const N: [u8]>() {`.
     (incomplete, unsized_const_params, "1.82.0", Some(95174)),

compiler/rustc_hir_analysis/messages.ftl

@@ -248,8 +248,6 @@ hir_analysis_invalid_union_field =
 hir_analysis_invalid_union_field_sugg =
     wrap the field type in `ManuallyDrop<...>`
 
-hir_analysis_invalid_unnamed_field_ty = unnamed fields can only have struct or union types
-
 hir_analysis_late_bound_const_in_apit = `impl Trait` can only mention const parameters from an fn or impl
     .label = const parameter declared here
 
@@ -538,19 +536,6 @@ hir_analysis_unconstrained_generic_parameter = the {$param_def_kind} `{$param_na
 hir_analysis_unconstrained_opaque_type = unconstrained opaque type
     .note = `{$name}` must be used in combination with a concrete type within the same {$what}
 
-hir_analysis_unnamed_fields_repr_field_defined = unnamed field defined here
-
-hir_analysis_unnamed_fields_repr_field_missing_repr_c =
-    named type of unnamed field must have `#[repr(C)]` representation
-    .label = unnamed field defined here
-    .field_ty_label = `{$field_ty}` defined here
-    .suggestion = add `#[repr(C)]` to this {$field_adt_kind}
-
-hir_analysis_unnamed_fields_repr_missing_repr_c =
-    {$adt_kind} with unnamed fields must have `#[repr(C)]` representation
-    .label = {$adt_kind} `{$adt_name}` defined here
-    .suggestion = add `#[repr(C)]` to this {$adt_kind}
-
 hir_analysis_unrecognized_atomic_operation =
     unrecognized atomic operation function: `{$op}`
     .label = unrecognized atomic operation

compiler/rustc_hir_analysis/src/check/check.rs

@@ -76,7 +76,6 @@ fn check_struct(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 
     check_transparent(tcx, def);
     check_packed(tcx, span, def);
-    check_unnamed_fields(tcx, def);
 }
 
 fn check_union(tcx: TyCtxt<'_>, def_id: LocalDefId) {
@@ -86,61 +85,6 @@ fn check_union(tcx: TyCtxt<'_>, def_id: LocalDefId) {
     check_transparent(tcx, def);
     check_union_fields(tcx, span, def_id);
     check_packed(tcx, span, def);
-    check_unnamed_fields(tcx, def);
-}
-
-/// Check the representation of adts with unnamed fields.
-fn check_unnamed_fields(tcx: TyCtxt<'_>, def: ty::AdtDef<'_>) {
-    if def.is_enum() {
-        return;
-    }
-    let variant = def.non_enum_variant();
-    if !variant.has_unnamed_fields() {
-        return;
-    }
-    if !def.is_anonymous() {
-        let adt_kind = def.descr();
-        let span = tcx.def_span(def.did());
-        let unnamed_fields = variant
-            .fields
-            .iter()
-            .filter(|f| f.is_unnamed())
-            .map(|f| {
-                let span = tcx.def_span(f.did);
-                errors::UnnamedFieldsReprFieldDefined { span }
-            })
-            .collect::<Vec<_>>();
-        debug_assert_ne!(unnamed_fields.len(), 0, "expect unnamed fields in this adt");
-        let adt_name = tcx.item_name(def.did());
-        if !def.repr().c() {
-            tcx.dcx().emit_err(errors::UnnamedFieldsRepr::MissingReprC {
-                span,
-                adt_kind,
-                adt_name,
-                unnamed_fields,
-                sugg_span: span.shrink_to_lo(),
-            });
-        }
-    }
-    for field in variant.fields.iter().filter(|f| f.is_unnamed()) {
-        let field_ty = tcx.type_of(field.did).instantiate_identity();
-        if let Some(adt) = field_ty.ty_adt_def()
-            && !adt.is_enum()
-        {
-            if !adt.is_anonymous() && !adt.repr().c() {
-                let field_ty_span = tcx.def_span(adt.did());
-                tcx.dcx().emit_err(errors::UnnamedFieldsRepr::FieldMissingReprC {
-                    span: tcx.def_span(field.did),
-                    field_ty_span,
-                    field_ty,
-                    field_adt_kind: adt.descr(),
-                    sugg_span: field_ty_span.shrink_to_lo(),
-                });
-            }
-        } else {
-            tcx.dcx().emit_err(errors::InvalidUnnamedFieldTy { span: tcx.def_span(field.did) });
-        }
-    }
 }
 
 /// Check that the fields of the `union` do not need dropping.