Avoid suggesting constrain the associated type to a trait

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -934,7 +934,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
     }
 
     fn suggest_trait_and_bounds(
-        &self,
+        &mut self,
         err: &mut Diag<'_>,
         source: PathSource<'_, '_, '_>,
         res: Option<Res>,
@@ -1479,7 +1479,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
     }
 
     /// Given `where <T as Bar>::Baz: String`, suggest `where T: Bar<Baz = String>`.
-    fn restrict_assoc_type_in_where_clause(&self, span: Span, err: &mut Diag<'_>) -> bool {
+    fn restrict_assoc_type_in_where_clause(&mut self, span: Span, err: &mut Diag<'_>) -> bool {
         // Detect that we are actually in a `where` predicate.
         let (bounded_ty, bounds, where_span) = if let Some(ast::WherePredicate {
             kind:
@@ -1539,6 +1539,19 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                     &poly_trait_ref.trait_ref.path.segments[..]
                 {
                     if ident.span == span {
+                        if matches!(
+                            self.resolve_path(
+                                &[Segment::from_ident(*ident)],
+                                Some(TypeNS),
+                                None,
+                                PathSource::Type,
+                            ),
+                            PathResult::Indeterminate | PathResult::Failed { .. }
+                        ) {
+                            // if the ident doesn't resolve to a type, don't suggest constrain associated type.
+                            return false;
+                        }
+
                         let Some(new_where_bound_predicate) =
                             mk_where_bound_predicate(path, poly_trait_ref, ty)
                         else {

tests/ui/traits/associated_type_bound/assoc-type-maybe-trait-147356.rs

@@ -0,0 +1,16 @@
+use std::str::FromStr;
+fn foo<T: FromStr>() -> T
+where
+    <T as FromStr>::Err: Debug, //~ ERROR expected trait
+{
+    "".parse().unwrap()
+}
+
+fn bar<T: FromStr>() -> T
+where
+    <T as FromStr>::Err: some_unknown_name, //~ ERROR cannot find trait `some_unknown_name` in this scope
+{
+    "".parse().unwrap()
+}
+
+fn main() {}

tests/ui/traits/associated_type_bound/assoc-type-maybe-trait-147356.stderr

@@ -0,0 +1,21 @@
+error[E0404]: expected trait, found derive macro `Debug`
+  --> $DIR/assoc-type-maybe-trait-147356.rs:4:26
+   |
+LL |     <T as FromStr>::Err: Debug,
+   |                          ^^^^^ not a trait
+   |
+help: consider importing this trait instead
+   |
+LL + use std::fmt::Debug;
+   |
+
+error[E0405]: cannot find trait `some_unknown_name` in this scope
+  --> $DIR/assoc-type-maybe-trait-147356.rs:11:26
+   |
+LL |     <T as FromStr>::Err: some_unknown_name,
+   |                          ^^^^^^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0404, E0405.
+For more information about an error, try `rustc --explain E0404`.