Take empty `where` into account when suggesting predicates

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -358,11 +358,17 @@ pub fn suggest_constraining_type_params<'a>(
         //      trait Foo<T=()> {... }
         //                     - insert: `where T: Zar`
         if matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. }) {
+            // If we are here and the where clause span is of non-zero length
+            // it means we're dealing with an empty where clause like this:
+            //      fn foo<X>(x: X) where { ... }
+            // In that case we don't want to add another "where" (Fixes #120838)
+            let where_prefix = if generics.where_clause_span.is_empty() { " where" } else { "" };
+
             // Suggest a bound, but there is no existing `where` clause *and* the type param has a
             // default (`<T=Foo>`), so we suggest adding `where T: Bar`.
             suggestions.push((
                 generics.tail_span_for_predicate_suggestion(),
-                format!(" where {param_name}: {constraint}"),
+                format!("{where_prefix} {param_name}: {constraint}"),
                 SuggestChangingConstraintsMessage::RestrictTypeFurther { ty: param_name },
             ));
             continue;

tests/ui/trait-impl-bound-suggestions.fixed

@@ -17,4 +17,13 @@ trait InsufficientlyConstrainedGeneric<X=()> where X: std::marker::Copy {
     }
 }
 
+// Regression test for #120838
+#[allow(dead_code)]
+trait InsufficientlyConstrainedGenericWithEmptyWhere<X=()> where X: std::marker::Copy {
+    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+        //~^ ERROR the trait bound `X: Copy` is not satisfied
+        ConstrainedStruct { x }
+    }
+}
+
 pub fn main() { }

tests/ui/trait-impl-bound-suggestions.rs

@@ -17,4 +17,13 @@ trait InsufficientlyConstrainedGeneric<X=()> {
     }
 }
 
+// Regression test for #120838
+#[allow(dead_code)]
+trait InsufficientlyConstrainedGenericWithEmptyWhere<X=()> where {
+    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+        //~^ ERROR the trait bound `X: Copy` is not satisfied
+        ConstrainedStruct { x }
+    }
+}
+
 pub fn main() { }

tests/ui/trait-impl-bound-suggestions.stderr

@@ -14,6 +14,22 @@ help: consider further restricting type parameter `X`
 LL | trait InsufficientlyConstrainedGeneric<X=()> where X: std::marker::Copy {
    |                                              ++++++++++++++++++++++++++
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `X: Copy` is not satisfied
+  --> $DIR/trait-impl-bound-suggestions.rs:23:52
+   |
+LL |     fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+   |                                                    ^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `X`
+   |
+note: required by a bound in `ConstrainedStruct`
+  --> $DIR/trait-impl-bound-suggestions.rs:8:29
+   |
+LL | struct ConstrainedStruct<X: Copy> {
+   |                             ^^^^ required by this bound in `ConstrainedStruct`
+help: consider further restricting type parameter `X`
+   |
+LL | trait InsufficientlyConstrainedGenericWithEmptyWhere<X=()> where X: std::marker::Copy {
+   |                                                                  ++++++++++++++++++++
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.