Rollup merge of #115662 - ShE3py:E0220-note, r=compiler-errors

Improve "associated type not found" diagnostics

```rs
use core::ops::Deref;

fn foo<T>() where T: Deref<Output = u32> {}
```

Before:
```
error[E0220]: associated type `Output` not found for `Deref`
 --> E0220.rs:5:28
  |
5 | fn foo<T>() where T: Deref<Output = u32> {}
  |                            ^^^^^^ associated type `Output` not found
```

After:
```
error[E0220]: associated type `Output` not found for `Deref`
 --> E0220.rs:5:28
  |
5 | fn foo<T>() where T: Deref<Output = u32> {}
  |                            ^^^^^^ help: `Deref` has the following associated type: `Target`
```

---

`@rustbot` label +A-diagnostics +D-papercut

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -110,16 +110,22 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
  {
  // The fallback span is needed because `assoc_name` might be an `Fn()`'s `Output` without a
  // valid span, so we point at the whole path segment instead.
- let span = if assoc_name.span != DUMMY_SP { assoc_name.span } else { span };
+ let is_dummy = assoc_name.span == DUMMY_SP;
+
  let mut err = struct_span_err!(
  self.tcx().sess,
- span,
+ if is_dummy { span } else { assoc_name.span },
  E0220,
  "associated type `{}` not found for `{}`",
  assoc_name,
  ty_param_name
  );
 
+ if is_dummy {
+ err.span_label(span, format!("associated type `{assoc_name}` not found"));
+ return err.emit();
+ }
+
  let all_candidate_names: Vec<_> = all_candidates()
  .flat_map(|r| self.tcx().associated_items(r.def_id()).in_definition_order())
  .filter_map(|item| {
@@ -131,10 +137,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
  })
  .collect();
 
- if let (Some(suggested_name), true) = (
- find_best_match_for_name(&all_candidate_names, assoc_name.name, None),
- assoc_name.span != DUMMY_SP,
- ) {
+ if let Some(suggested_name) =
+ find_best_match_for_name(&all_candidate_names, assoc_name.name, None)
+ {
  err.span_suggestion(
  assoc_name.span,
  "there is an associated type with a similar name",
@@ -172,10 +177,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
  })
  .collect();
 
- if let (Some(suggested_name), true) = (
- find_best_match_for_name(&wider_candidate_names, assoc_name.name, None),
- assoc_name.span != DUMMY_SP,
- ) {
+ if let Some(suggested_name) =
+ find_best_match_for_name(&wider_candidate_names, assoc_name.name, None)
+ {
  if let [best_trait] = visible_traits
  .iter()
  .filter(|trait_def_id| {
@@ -197,7 +201,28 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
  }
  }
 
- err.span_label(span, format!("associated type `{assoc_name}` not found"));
+ // If we still couldn't find any associated type, and only one associated type exists,
+ // suggests using it.
+
+ if all_candidate_names.len() == 1 {
+ // this should still compile, except on `#![feature(associated_type_defaults)]`
+ // where it could suggests `type A = Self::A`, thus recursing infinitely
+ let applicability = if self.tcx().features().associated_type_defaults {
+ Applicability::Unspecified
+ } else {
+ Applicability::MaybeIncorrect
+ };
+
+ err.span_suggestion(
+ assoc_name.span,
+ format!("`{ty_param_name}` has the following associated type"),
+ all_candidate_names.first().unwrap().to_string(),
+ applicability,
+ );
+ } else {
+ err.span_label(assoc_name.span, format!("associated type `{assoc_name}` not found"));
+ }
+
  err.emit()
  }
 

tests/ui/associated-types/issue-23595-2.stderr

@@ -2,7 +2,7 @@ error[E0220]: associated type `anything_here_kills_it` not found for `Self`
  --> $DIR/issue-23595-2.rs:6:22
  |
 LL | type B = C<Self::anything_here_kills_it>;
- | ^^^^^^^^^^^^^^^^^^^^^^ associated type `anything_here_kills_it` not found
+ | ^^^^^^^^^^^^^^^^^^^^^^ help: `Self` has the following associated type: `B`
 
 error: aborting due to previous error
 

tests/ui/error-codes/E0220.stderr

@@ -2,7 +2,7 @@ error[E0220]: associated type `F` not found for `Trait`
  --> $DIR/E0220.rs:5:22
  |
 LL | type Foo = dyn Trait<F=i32>;
- | ^ associated type `F` not found
+ | ^ help: `Trait` has the following associated type: `Bar`
 
 error[E0191]: the value of the associated type `Bar` (from trait `Trait`) must be specified
  --> $DIR/E0220.rs:5:16

tests/ui/lifetimes/issue-95023.stderr

@@ -36,7 +36,7 @@ error[E0220]: associated type `B` not found for `Self`
  --> $DIR/issue-95023.rs:6:44
  |
 LL | fn foo<const N: usize>(&self) -> Self::B<{N}>;
- | ^ associated type `B` not found
+ | ^ help: `Self` has the following associated type: `Output`
 
 error: aborting due to 5 previous errors