on_unimplemented.rs

use super::{
    ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote, PredicateObligation,
};
use crate::infer::InferCtxt;
use crate::ty::subst::Subst;
use crate::ty::{self, GenericParamDefKind};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_span::symbol::sym;

impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
    fn impl_similar_to(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> Option<DefId> {
        let tcx = self.tcx;
        let param_env = obligation.param_env;
        let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
        let trait_self_ty = trait_ref.self_ty();

        let mut self_match_impls = vec![];
        let mut fuzzy_match_impls = vec![];

        self.tcx.for_each_relevant_impl(trait_ref.def_id, trait_self_ty, |def_id| {
            let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
            let impl_trait_ref = tcx.impl_trait_ref(def_id).unwrap().subst(tcx, impl_substs);

            let impl_self_ty = impl_trait_ref.self_ty();

            if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
                self_match_impls.push(def_id);

                if trait_ref
                    .substs
                    .types()
                    .skip(1)
                    .zip(impl_trait_ref.substs.types().skip(1))
                    .all(|(u, v)| self.fuzzy_match_tys(u, v))
                {
                    fuzzy_match_impls.push(def_id);
                }
            }
        });

        let impl_def_id = if self_match_impls.len() == 1 {
            self_match_impls[0]
        } else if fuzzy_match_impls.len() == 1 {
            fuzzy_match_impls[0]
        } else {
            return None;
        };

        tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented).then_some(impl_def_id)
    }

    /// Used to set on_unimplemented's `ItemContext`
    /// to be the enclosing (async) block/function/closure
    fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
        let hir = &self.tcx.hir();
        let node = hir.find(hir_id)?;
        match &node {
            hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. }) => {
                self.describe_generator(*body_id).or_else(|| {
                    Some(if let hir::FnHeader { asyncness: hir::IsAsync::Async, .. } = sig.header {
                        "an async function"
                    } else {
                        "a function"
                    })
                })
            }
            hir::Node::TraitItem(hir::TraitItem {
                kind: hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(body_id)),
                ..
            }) => self.describe_generator(*body_id).or_else(|| Some("a trait method")),
            hir::Node::ImplItem(hir::ImplItem {
                kind: hir::ImplItemKind::Method(sig, body_id),
                ..
            }) => self.describe_generator(*body_id).or_else(|| {
                Some(if let hir::FnHeader { asyncness: hir::IsAsync::Async, .. } = sig.header {
                    "an async method"
                } else {
                    "a method"
                })
            }),
            hir::Node::Expr(hir::Expr {
                kind: hir::ExprKind::Closure(_is_move, _, body_id, _, gen_movability),
                ..
            }) => self.describe_generator(*body_id).or_else(|| {
                Some(if gen_movability.is_some() { "an async closure" } else { "a closure" })
            }),
            hir::Node::Expr(hir::Expr { .. }) => {
                let parent_hid = hir.get_parent_node(hir_id);
                if parent_hid != hir_id {
                    return self.describe_enclosure(parent_hid);
                } else {
                    None
                }
            }
            _ => None,
        }
    }

    crate fn on_unimplemented_note(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> OnUnimplementedNote {
        let def_id =
            self.impl_similar_to(trait_ref, obligation).unwrap_or_else(|| trait_ref.def_id());
        let trait_ref = *trait_ref.skip_binder();

        let mut flags = vec![];
        flags.push((
            sym::item_context,
            self.describe_enclosure(obligation.cause.body_id).map(|s| s.to_owned()),
        ));

        match obligation.cause.code {
            ObligationCauseCode::BuiltinDerivedObligation(..)
            | ObligationCauseCode::ImplDerivedObligation(..) => {}
            _ => {
                // this is a "direct", user-specified, rather than derived,
                // obligation.
                flags.push((sym::direct, None));
            }
        }

        if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
            // FIXME: maybe also have some way of handling methods
            // from other traits? That would require name resolution,
            // which we might want to be some sort of hygienic.
            //
            // Currently I'm leaving it for what I need for `try`.
            if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
                let method = self.tcx.item_name(item);
                flags.push((sym::from_method, None));
                flags.push((sym::from_method, Some(method.to_string())));
            }
        }
        if let Some((t, _)) = self.get_parent_trait_ref(&obligation.cause.code) {
            flags.push((sym::parent_trait, Some(t)));
        }

        if let Some(k) = obligation.cause.span.desugaring_kind() {
            flags.push((sym::from_desugaring, None));
            flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
        }
        let generics = self.tcx.generics_of(def_id);
        let self_ty = trait_ref.self_ty();
        // This is also included through the generics list as `Self`,
        // but the parser won't allow you to use it
        flags.push((sym::_Self, Some(self_ty.to_string())));
        if let Some(def) = self_ty.ty_adt_def() {
            // We also want to be able to select self's original
            // signature with no type arguments resolved
            flags.push((sym::_Self, Some(self.tcx.type_of(def.did).to_string())));
        }

        for param in generics.params.iter() {
            let value = match param.kind {
                GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
                    trait_ref.substs[param.index as usize].to_string()
                }
                GenericParamDefKind::Lifetime => continue,
            };
            let name = param.name;
            flags.push((name, Some(value)));
        }

        if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
            flags.push((sym::crate_local, None));
        }

        // Allow targeting all integers using `{integral}`, even if the exact type was resolved
        if self_ty.is_integral() {
            flags.push((sym::_Self, Some("{integral}".to_owned())));
        }

        if let ty::Array(aty, len) = self_ty.kind {
            flags.push((sym::_Self, Some("[]".to_owned())));
            flags.push((sym::_Self, Some(format!("[{}]", aty))));
            if let Some(def) = aty.ty_adt_def() {
                // We also want to be able to select the array's type's original
                // signature with no type arguments resolved
                flags.push((
                    sym::_Self,
                    Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
                ));
                let tcx = self.tcx;
                if let Some(len) = len.try_eval_usize(tcx, ty::ParamEnv::empty()) {
                    flags.push((
                        sym::_Self,
                        Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
                    ));
                } else {
                    flags.push((
                        sym::_Self,
                        Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
                    ));
                }
            }
        }

        if let Ok(Some(command)) =
            OnUnimplementedDirective::of_item(self.tcx, trait_ref.def_id, def_id)
        {
            command.evaluate(self.tcx, trait_ref, &flags[..])
        } else {
            OnUnimplementedNote::default()
        }
    }
}