Do not emit invalid suggestion in E0191 when spans overlap

Fix #115019.

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -597,7 +597,21 @@ pub(crate) fn complain_about_missing_associated_types(
                 }
             }
         }
-        if !suggestions.is_empty() {
+        suggestions.sort_by_key(|&(span, _)| span);
+        // There are cases where one bound points to a span within another bound's span, like when
+        // you have code like the following (#115019), so we skip providing a suggestion in those
+        // cases to avoid having a malformed suggestion.
+        //
+        // pub struct Flatten<I> {
+        //     inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>::core,
+        //             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+        //             |                  ^^^^^^^^^^^^^^^^^^^^^
+        //             |                  |
+        //             |                  associated types `Item`, `IntoIter` must be specified
+        //             associated types `Item`, `IntoIter` must be specified
+        // }
+        let overlaps = suggestions.windows(2).any(|pair| pair[0].0.overlaps(pair[1].0));
+        if !suggestions.is_empty() && !overlaps {
             err.multipart_suggestion(
                 format!("specify the associated type{}", pluralize!(types_count)),
                 suggestions,

tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr

@@ -6,11 +6,6 @@ LL |     inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>
    |             |                  |
    |             |                  associated types `Item`, `IntoIter` must be specified
    |             associated types `Item`, `IntoIter` must be specified
-   |
-help: specify the associated types
-   |
-LL |     inner: <IntoIterator<Item: IntoIterator<Item: >, Item = Type, IntoIter = Type>IntoIterator<Item: , Item = Type, IntoIter = Type>>::IntoIterator as Item>::Core,
-   |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/overlaping-bound-suggestion.rs:7:13