Support RTN on associated methods from supertraits

compiler/rustc_hir_analysis/messages.ftl

@@ -192,7 +192,7 @@ hir_analysis_return_type_notation_equality_bound =
     return type notation is not allowed to use type equality
 
 hir_analysis_return_type_notation_missing_method =
-    cannot find associated function `{$assoc_name}` in trait `{$trait_name}`
+    cannot find associated function `{$assoc_name}` for `{$ty_name}`
 
 hir_analysis_placeholder_not_allowed_item_signatures = the placeholder `_` is not allowed within types on item signatures for {$kind}
     .label = not allowed in type signatures

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1118,11 +1118,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             ) {
                 trait_ref
             } else {
-                return Err(tcx.sess.emit_err(crate::errors::ReturnTypeNotationMissingMethod {
-                    span: binding.span,
-                    trait_name: tcx.item_name(trait_ref.def_id()),
-                    assoc_name: binding.item_name.name,
-                }));
+                self.one_bound_for_assoc_method(
+                    traits::supertraits(tcx, trait_ref),
+                    trait_ref.print_only_trait_path(),
+                    binding.item_name,
+                    path_span,
+                )?
             }
         } else if self.trait_defines_associated_item_named(
             trait_ref.def_id(),
@@ -2057,6 +2058,38 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         Ok(bound)
     }
 
+    #[instrument(level = "debug", skip(self, all_candidates, ty_name), ret)]
+    fn one_bound_for_assoc_method(
+        &self,
+        all_candidates: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
+        ty_name: impl Display,
+        assoc_name: Ident,
+        span: Span,
+    ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed> {
+        let mut matching_candidates = all_candidates.filter(|r| {
+            self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Fn, assoc_name)
+        });
+
+        let candidate = match matching_candidates.next() {
+            Some(candidate) => candidate,
+            None => {
+                return Err(self.tcx().sess.emit_err(
+                    crate::errors::ReturnTypeNotationMissingMethod {
+                        span,
+                        ty_name: ty_name.to_string(),
+                        assoc_name: assoc_name.name,
+                    },
+                ));
+            }
+        };
+
+        if let Some(_conflicting_candidate) = matching_candidates.next() {
+            todo!()
+        }
+
+        Ok(candidate)
+    }
+
     // Create a type from a path to an associated type or to an enum variant.
     // For a path `A::B::C::D`, `qself_ty` and `qself_def` are the type and def for `A::B::C`
     // and item_segment is the path segment for `D`. We return a type and a def for

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -1652,17 +1652,16 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             if binding.gen_args.parenthesized == hir::GenericArgsParentheses::ReturnTypeNotation {
                 let bound_vars = if let Some(type_def_id) = type_def_id
                     && self.tcx.def_kind(type_def_id) == DefKind::Trait
-                    // FIXME(return_type_notation): We could bound supertrait methods.
-                    && let Some(assoc_fn) = self
-                        .tcx
-                        .associated_items(type_def_id)
-                        .find_by_name_and_kind(self.tcx, binding.ident, ty::AssocKind::Fn, type_def_id)
+                    && let Some((mut bound_vars, assoc_fn)) =
+                        BoundVarContext::supertrait_hrtb_vars(
+                            self.tcx,
+                            type_def_id,
+                            binding.ident,
+                            ty::AssocKind::Fn,
+                        )
                 {
-                    self.tcx
-                        .generics_of(assoc_fn.def_id)
-                        .params
-                        .iter()
-                        .map(|param| match param.kind {
+                    bound_vars.extend(self.tcx.generics_of(assoc_fn.def_id).params.iter().map(
+                        |param| match param.kind {
                             ty::GenericParamDefKind::Lifetime => ty::BoundVariableKind::Region(
                                 ty::BoundRegionKind::BrNamed(param.def_id, param.name),
                             ),
@@ -1670,9 +1669,11 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                                 ty::BoundTyKind::Param(param.def_id, param.name),
                             ),
                             ty::GenericParamDefKind::Const { .. } => ty::BoundVariableKind::Const,
-                        })
-                        .chain(self.tcx.fn_sig(assoc_fn.def_id).subst_identity().bound_vars())
-                        .collect()
+                        },
+                    ));
+                    bound_vars
+                        .extend(self.tcx.fn_sig(assoc_fn.def_id).subst_identity().bound_vars());
+                    bound_vars
                 } else {
                     self.tcx.sess.delay_span_bug(
                         binding.ident.span,
@@ -1689,8 +1690,13 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                     });
                 });
             } else if let Some(type_def_id) = type_def_id {
-                let bound_vars =
-                    BoundVarContext::supertrait_hrtb_vars(self.tcx, type_def_id, binding.ident);
+                let bound_vars = BoundVarContext::supertrait_hrtb_vars(
+                    self.tcx,
+                    type_def_id,
+                    binding.ident,
+                    ty::AssocKind::Type,
+                )
+                .map(|(bound_vars, _)| bound_vars);
                 self.with(scope, |this| {
                     let scope = Scope::Supertrait {
                         bound_vars: bound_vars.unwrap_or_default(),
@@ -1720,11 +1726,15 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
         tcx: TyCtxt<'tcx>,
         def_id: DefId,
         assoc_name: Ident,
-    ) -> Option<Vec<ty::BoundVariableKind>> {
+        assoc_kind: ty::AssocKind,
+    ) -> Option<(Vec<ty::BoundVariableKind>, &'tcx ty::AssocItem)> {
         let trait_defines_associated_type_named = |trait_def_id: DefId| {
-            tcx.associated_items(trait_def_id)
-                .find_by_name_and_kind(tcx, assoc_name, ty::AssocKind::Type, trait_def_id)
-                .is_some()
+            tcx.associated_items(trait_def_id).find_by_name_and_kind(
+                tcx,
+                assoc_name,
+                assoc_kind,
+                trait_def_id,
+            )
         };
 
         use smallvec::{smallvec, SmallVec};
@@ -1742,8 +1752,8 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                 _ => break None,
             }
 
-            if trait_defines_associated_type_named(def_id) {
-                break Some(bound_vars.into_iter().collect());
+            if let Some(assoc_item) = trait_defines_associated_type_named(def_id) {
+                break Some((bound_vars.into_iter().collect(), assoc_item));
             }
             let predicates = tcx.super_predicates_that_define_assoc_type((def_id, assoc_name));
             let obligations = predicates.predicates.iter().filter_map(|&(pred, _)| {

compiler/rustc_hir_analysis/src/errors.rs

@@ -512,7 +512,7 @@ pub(crate) struct ReturnTypeNotationEqualityBound {
 pub(crate) struct ReturnTypeNotationMissingMethod {
     #[primary_span]
     pub span: Span,
-    pub trait_name: Symbol,
+    pub ty_name: String,
     pub assoc_name: Symbol,
 }
 

compiler/rustc_middle/src/ty/context.rs

@@ -1570,7 +1570,12 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn trait_may_define_assoc_type(self, trait_def_id: DefId, assoc_name: Ident) -> bool {
         self.super_traits_of(trait_def_id).any(|trait_did| {
             self.associated_items(trait_did)
-                .find_by_name_and_kind(self, assoc_name, ty::AssocKind::Type, trait_did)
+                .find_by_name_and_kinds(
+                    self,
+                    assoc_name,
+                    &[ty::AssocKind::Type, ty::AssocKind::Const, ty::AssocKind::Fn],
+                    trait_did,
+                )
                 .is_some()
         })
     }

tests/ui/associated-type-bounds/return-type-notation/missing.rs

@@ -8,6 +8,6 @@ trait Trait {
 }
 
 fn bar<T: Trait<methid(): Send>>() {}
-//~^ ERROR cannot find associated function `methid` in trait `Trait`
+//~^ ERROR cannot find associated function `methid` for `Trait`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/missing.stderr

@@ -7,7 +7,7 @@ LL | #![feature(return_type_notation, async_fn_in_trait)]
    = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: cannot find associated function `methid` in trait `Trait`
+error: cannot find associated function `methid` for `Trait`
   --> $DIR/missing.rs:10:17
    |
 LL | fn bar<T: Trait<methid(): Send>>() {}

tests/ui/async-await/return-type-notation/super-method-bound.rs

@@ -0,0 +1,25 @@
+// edition:2021
+// check-pass
+
+#![feature(async_fn_in_trait, return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait Super<'a> {
+    async fn test();
+}
+impl Super<'_> for () {
+    async fn test() {}
+}
+
+trait Foo: for<'a> Super<'a> {}
+impl Foo for () {}
+
+fn test<T>()
+where
+    T: Foo<test(): Send>,
+{
+}
+
+fn main() {
+    test::<()>();
+}

tests/ui/async-await/return-type-notation/super-method-bound.stderr

@@ -0,0 +1,11 @@
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/super-method-bound.rs:4:31
+   |
+LL | #![feature(async_fn_in_trait, return_type_notation)]
+   |                               ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+