Provide more context on resolve error caused from incorrect RTN

estebank · estebank · commit adf5464965ec · 2025-02-28T23:01:08.000Z
When encountering a resolve E0575 error for an associated method (when a type was expected), see if it could have been an intended return type notation bound.

```
error[E0575]: expected associated type, found associated function `Trait::method`
  --&gt; $DIR/bad-inputs-and-output.rs:31:36
   |
LL | fn foo_qualified&lt;T: Trait&gt;() where &lt;T as Trait&gt;::method(i32): Send {}
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
   |
help: you might have meant to use the return type notation syntax
   |
LL - fn foo_qualified&lt;T: Trait&gt;() where &lt;T as Trait&gt;::method(i32): Send {}
LL + fn foo_qualified&lt;T: Trait&gt;() where T::method(..): Send {}
   |
```
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
@@ -3125,6 +3125,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 PathSource::Trait(AliasPossibility::No),
                 Finalize::new(trait_ref.ref_id, trait_ref.path.span),
                 RecordPartialRes::Yes,
+                &None,
             );
             self.diag_metadata.currently_processing_impl_trait = None;
             if let Some(def_id) = res.expect_full_res().opt_def_id() {
@@ -4051,6 +4052,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             source,
             Finalize::new(id, path.span),
             RecordPartialRes::Yes,
+            qself,
         );
     }
 
@@ -4062,14 +4064,21 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         source: PathSource<'ast>,
         finalize: Finalize,
         record_partial_res: RecordPartialRes,
+        parent_qself: &Option<P<QSelf>>,
     ) -> PartialRes {
         let ns = source.namespace();
 
         let Finalize { node_id, path_span, .. } = finalize;
         let report_errors = |this: &mut Self, res: Option<Res>| {
             if this.should_report_errs() {
-                let (err, candidates) =
-                    this.smart_resolve_report_errors(path, None, path_span, source, res);
+                let (err, candidates) = this.smart_resolve_report_errors(
+                    path,
+                    None,
+                    path_span,
+                    source,
+                    res,
+                    parent_qself,
+                );
 
                 let def_id = this.parent_scope.module.nearest_parent_mod();
                 let instead = res.is_some();
@@ -4138,6 +4147,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     path_span,
                     PathSource::Type,
                     None,
+                    parent_qself,
                 );
 
                 // There are two different error messages user might receive at
@@ -4386,17 +4396,17 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     /// Handles paths that may refer to associated items.
     fn resolve_qpath(
         &mut self,
-        qself: &Option<P<QSelf>>,
+        qself_: &Option<P<QSelf>>,
         path: &[Segment],
         ns: Namespace,
         finalize: Finalize,
     ) -> Result<Option<PartialRes>, Spanned<ResolutionError<'ra>>> {
         debug!(
             "resolve_qpath(qself={:?}, path={:?}, ns={:?}, finalize={:?})",
-            qself, path, ns, finalize,
+            qself_, path, ns, finalize,
         );
 
-        if let Some(qself) = qself {
+        if let Some(qself) = &qself_ {
             if qself.position == 0 {
                 // This is a case like `<T>::B`, where there is no
                 // trait to resolve. In that case, we leave the `B`
@@ -4415,6 +4425,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 PathSource::Trait(AliasPossibility::No),
                 Finalize::new(finalize.node_id, qself.path_span),
                 RecordPartialRes::No,
+                qself_,
             );
 
             if trait_res.expect_full_res() == Res::Err {
@@ -4433,12 +4444,14 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             // `qself.position`). And then we recursively resolve that,
             // but with `qself` set to `None`.
             let ns = if qself.position + 1 == path.len() { ns } else { TypeNS };
+            tracing::info!(?qself, ?path, ?ns, "method");
             let partial_res = self.smart_resolve_path_fragment(
                 &None,
                 &path[..=qself.position],
                 PathSource::TraitItem(ns),
                 Finalize::with_root_span(finalize.node_id, finalize.path_span, qself.path_span),
                 RecordPartialRes::No,
+                qself_,
             );
 
             // The remaining segments (the `C` in our example) will
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -35,7 +35,7 @@ use super::NoConstantGenericsReason;
 use crate::diagnostics::{ImportSuggestion, LabelSuggestion, TypoSuggestion};
 use crate::late::{
     AliasPossibility, LateResolutionVisitor, LifetimeBinderKind, LifetimeRes, LifetimeRibKind,
-    LifetimeUseSet, RibKind,
+    LifetimeUseSet, QSelf, RibKind,
 };
 use crate::ty::fast_reject::SimplifiedType;
 use crate::{
@@ -421,6 +421,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         span: Span,
         source: PathSource<'_>,
         res: Option<Res>,
+        qself: &Option<P<QSelf>>,
     ) -> (Diag<'tcx>, Vec<ImportSuggestion>) {
         debug!(?res, ?source);
         let base_error = self.make_base_error(path, span, source, res);
@@ -453,6 +454,15 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
 
         self.suggest_self_or_self_ref(&mut err, path, span);
         self.detect_assoc_type_constraint_meant_as_path(&mut err, &base_error);
+        self.detect_rtn_with_fully_qualified_path(
+            &mut err,
+            path,
+            following_seg,
+            span,
+            source,
+            res,
+            qself,
+        );
         if self.suggest_self_ty(&mut err, source, path, span)
             || self.suggest_self_value(&mut err, source, path, span)
         {
@@ -500,6 +510,32 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
 
         (err, candidates)
     }
+    fn detect_rtn_with_fully_qualified_path(
+        &self,
+        err: &mut Diag<'_>,
+        path: &[Segment],
+        following_seg: Option<&Segment>,
+        span: Span,
+        source: PathSource<'_>,
+        res: Option<Res>,
+        qself: &Option<P<QSelf>>,
+    ) {
+        if let Some(Res::Def(DefKind::AssocFn, _)) = res
+            && let PathSource::TraitItem(TypeNS) = source
+            && let None = following_seg
+            && let Some(qself) = qself
+            && let TyKind::Path(None, ty_path) = &qself.ty.kind
+            && ty_path.segments.len() == 1
+            && self.diag_metadata.current_where_predicate.is_some()
+        {
+            err.span_suggestion_verbose(
+                span,
+                "you might have meant to use the return type notation syntax",
+                format!("{}::{}(..)", ty_path.segments[0].ident, path[path.len() - 1].ident),
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
 
     fn detect_assoc_type_constraint_meant_as_path(
         &self,
diff --git a/tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.fixed b/tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.fixed
@@ -0,0 +1,40 @@
+//@ edition: 2021
+//@ run-rustfix
+#![feature(return_type_notation)]
+#![allow(dead_code)]
+
+trait Trait {
+    async fn method() {}
+}
+
+fn foo<T: Trait<method(..): Send>>() {}
+//~^ ERROR argument types not allowed with return type notation
+
+fn bar<T: Trait<method(..): Send>>() {}
+//~^ ERROR return type not allowed with return type notation
+
+fn baz<T: Trait<method(..): Send>>() {}
+//~^ ERROR return type notation arguments must be elided with `..`
+
+fn foo_path<T: Trait>() where T::method(..): Send {}
+//~^ ERROR argument types not allowed with return type notation
+
+fn bar_path<T: Trait>() where T::method(..): Send {}
+//~^ ERROR return type not allowed with return type notation
+
+fn bay_path<T: Trait>() where T::method(..): Send {}
+//~^ ERROR return type not allowed with return type notation
+
+fn baz_path<T: Trait>() where T::method(..): Send {}
+//~^ ERROR return type notation arguments must be elided with `..`
+
+fn foo_qualified<T: Trait>() where T::method(..): Send {}
+//~^ ERROR expected associated type
+
+fn bar_qualified<T: Trait>() where T::method(..): Send {}
+//~^ ERROR expected associated type
+
+fn baz_qualified<TParam: Trait>() where TParam::method(..): Send {}
+//~^ ERROR expected associated type
+
+fn main() {}
diff --git a/tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.rs b/tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.rs
@@ -1,6 +1,7 @@
 //@ edition: 2021
-
+//@ run-rustfix
 #![feature(return_type_notation)]
+#![allow(dead_code)]
 
 trait Trait {
     async fn method() {}
@@ -33,7 +34,7 @@ fn foo_qualified<T: Trait>() where <T as Trait>::method(i32): Send {}
 fn bar_qualified<T: Trait>() where <T as Trait>::method() -> (): Send {}
 //~^ ERROR expected associated type
 
-fn baz_qualified<T: Trait>() where <T as Trait>::method(): Send {}
+fn baz_qualified<TParam: Trait>() where <TParam as Trait>::method(): Send {}
 //~^ ERROR expected associated type
 
 fn main() {}
diff --git a/tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.stderr b/tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.stderr
@@ -1,5 +1,5 @@
 error: return type not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:24:45
+  --> $DIR/bad-inputs-and-output.rs:25:45
    |
 LL | fn bay_path<T: Trait>() where T::method(..) -> (): Send {}
    |                                             ^^^^^
@@ -11,25 +11,43 @@ LL + fn bay_path<T: Trait>() where T::method(..): Send {}
    |
 
 error[E0575]: expected associated type, found associated function `Trait::method`
-  --> $DIR/bad-inputs-and-output.rs:30:36
+  --> $DIR/bad-inputs-and-output.rs:31:36
    |
 LL | fn foo_qualified<T: Trait>() where <T as Trait>::method(i32): Send {}
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+   |
+help: you might have meant to use the return type notation syntax
+   |
+LL - fn foo_qualified<T: Trait>() where <T as Trait>::method(i32): Send {}
+LL + fn foo_qualified<T: Trait>() where T::method(..): Send {}
+   |
 
 error[E0575]: expected associated type, found associated function `Trait::method`
-  --> $DIR/bad-inputs-and-output.rs:33:36
+  --> $DIR/bad-inputs-and-output.rs:34:36
    |
 LL | fn bar_qualified<T: Trait>() where <T as Trait>::method() -> (): Send {}
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+   |
+help: you might have meant to use the return type notation syntax
+   |
+LL - fn bar_qualified<T: Trait>() where <T as Trait>::method() -> (): Send {}
+LL + fn bar_qualified<T: Trait>() where T::method(..): Send {}
+   |
 
 error[E0575]: expected associated type, found associated function `Trait::method`
-  --> $DIR/bad-inputs-and-output.rs:36:36
+  --> $DIR/bad-inputs-and-output.rs:37:41
+   |
+LL | fn baz_qualified<TParam: Trait>() where <TParam as Trait>::method(): Send {}
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+   |
+help: you might have meant to use the return type notation syntax
+   |
+LL - fn baz_qualified<TParam: Trait>() where <TParam as Trait>::method(): Send {}
+LL + fn baz_qualified<TParam: Trait>() where TParam::method(..): Send {}
    |
-LL | fn baz_qualified<T: Trait>() where <T as Trait>::method(): Send {}
-   |                                    ^^^^^^^^^^^^^^^^^^^^^^ not a associated type
 
 error: argument types not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:9:23
+  --> $DIR/bad-inputs-and-output.rs:10:23
    |
 LL | fn foo<T: Trait<method(i32): Send>>() {}
    |                       ^^^^^
@@ -41,7 +59,7 @@ LL + fn foo<T: Trait<method(..): Send>>() {}
    |
 
 error: return type not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:12:25
+  --> $DIR/bad-inputs-and-output.rs:13:25
    |
 LL | fn bar<T: Trait<method() -> (): Send>>() {}
    |                         ^^^^^^
@@ -53,7 +71,7 @@ LL + fn bar<T: Trait<method(..): Send>>() {}
    |
 
 error: return type notation arguments must be elided with `..`
-  --> $DIR/bad-inputs-and-output.rs:15:23
+  --> $DIR/bad-inputs-and-output.rs:16:23
    |
 LL | fn baz<T: Trait<method(): Send>>() {}
    |                       ^^
@@ -64,7 +82,7 @@ LL | fn baz<T: Trait<method(..): Send>>() {}
    |                        ++
 
 error: argument types not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:18:40
+  --> $DIR/bad-inputs-and-output.rs:19:40
    |
 LL | fn foo_path<T: Trait>() where T::method(i32): Send {}
    |                                        ^^^^^
@@ -76,7 +94,7 @@ LL + fn foo_path<T: Trait>() where T::method(..): Send {}
    |
 
 error: return type not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:21:42
+  --> $DIR/bad-inputs-and-output.rs:22:42
    |
 LL | fn bar_path<T: Trait>() where T::method() -> (): Send {}
    |                                          ^^^^^^
@@ -88,7 +106,7 @@ LL + fn bar_path<T: Trait>() where T::method(..): Send {}
    |
 
 error: return type notation arguments must be elided with `..`
-  --> $DIR/bad-inputs-and-output.rs:27:40
+  --> $DIR/bad-inputs-and-output.rs:28:40
    |
 LL | fn baz_path<T: Trait>() where T::method(): Send {}
    |                                        ^^
