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

4 files changed, 39 insertions, 14 deletions

diff --git a/compiler/rustc_hir_analysis/src/astconv/errors.rs b/compiler/rustc_hir_analysis/src/astconv/errors.rs
index 6082d446979..ed4dde419c4 100644
--- a/compiler/rustc_hir_analysis/src/astconv/errors.rs
+++ b/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()
     }
 
diff --git a/tests/ui/associated-types/issue-23595-2.stderr b/tests/ui/associated-types/issue-23595-2.stderr
index dded673f6ee..73effa9f955 100644
--- a/tests/ui/associated-types/issue-23595-2.stderr
+++ b/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
 
diff --git a/tests/ui/error-codes/E0220.stderr b/tests/ui/error-codes/E0220.stderr
index 11763ce788d..e03eadacae4 100644
--- a/tests/ui/error-codes/E0220.stderr
+++ b/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
diff --git a/tests/ui/lifetimes/issue-95023.stderr b/tests/ui/lifetimes/issue-95023.stderr
index 5b93eff8614..6361d8ad30b 100644
--- a/tests/ui/lifetimes/issue-95023.stderr
+++ b/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