Rollup merge of #95970 - WaffleLapkin:nicer_trait_suggestions, r=compiler-errors

Fix suggestions in case of `T:` bounds

This PR fixes a corner case in `suggest_constraining_type_params` that was causing incorrect suggestions.

For the following functions:
```rust
fn a<T:>(t: T) { [t, t]; }
fn b<T>(t: T) where T: { [t, t]; }
```

We previously suggested the following:
```text
...
help: consider restricting type parameter `T`
  |
1 | fn a<T: Copy:>(t: T) { [t, t]; }
  |       ++++++
...
help: consider further restricting this bound
  |
2 | fn b<T>(t: T) where T: + Copy { [t, t]; }
  |                        ++++++
```

Note that neither `T: Copy:` not `where T: + Copy` is a correct bound.

With this commit the suggestions are correct:
```text
...
help: consider restricting type parameter `T`
  |
1 | fn a<T: Copy>(t: T) { [t, t]; }
  |         ++++
...
help: consider further restricting this bound
  |
2 | fn b<T>(t: T) where T: Copy { [t, t]; }
  |                        ++++
```

r? `@compiler-errors`

I've tried fixing #95898 here too, but got too confused with how `suggest_traits_to_import` works and what it does 😅

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -336,10 +336,14 @@ pub fn suggest_constraining_type_params<'a>(
         }
 
         let constraint = constraints.iter().map(|&(c, _)| c).collect::<Vec<_>>().join(" + ");
-        let mut suggest_restrict = |span| {
+        let mut suggest_restrict = |span, bound_list_non_empty| {
             suggestions.push((
                 span,
-                format!(" + {}", constraint),
+                if bound_list_non_empty {
+                    format!(" + {}", constraint)
+                } else {
+                    format!(" {}", constraint)
+                },
                 SuggestChangingConstraintsMessage::RestrictBoundFurther,
             ))
         };
@@ -360,7 +364,10 @@ pub fn suggest_constraining_type_params<'a>(
             //             |
             //             replace with: `impl Foo + Bar`
 
-            suggest_restrict(param.span.shrink_to_hi());
+            // `impl Trait` must have at least one trait in the list
+            let bound_list_non_empty = true;
+
+            suggest_restrict(param.span.shrink_to_hi(), bound_list_non_empty);
             continue;
         }
 
@@ -383,15 +390,25 @@ pub fn suggest_constraining_type_params<'a>(
                 //          --
                 //          |
                 //          replace with: `T: Bar +`
-                suggest_restrict(span);
+
+                // `bounds_span_for_suggestions` returns `None` if the list is empty
+                let bound_list_non_empty = true;
+
+                suggest_restrict(span, bound_list_non_empty);
             } else {
+                let (colon, span) = match param.colon_span_for_suggestions(tcx.sess.source_map()) {
+                    // If there is already a colon after generic, do not suggest adding it again
+                    Some(sp) => ("", sp.shrink_to_hi()),
+                    None => (":", param.span.shrink_to_hi()),
+                };
+
                 // If user hasn't provided any bounds, suggest adding a new one:
                 //
                 //   fn foo<T>(t: T) { ... }
                 //          - help: consider restricting this type parameter with `T: Foo`
                 suggestions.push((
-                    param.span.shrink_to_hi(),
-                    format!(": {}", constraint),
+                    span,
+                    format!("{colon} {constraint}"),
                     SuggestChangingConstraintsMessage::RestrictType { ty: param_name },
                 ));
             }
@@ -459,17 +476,21 @@ pub fn suggest_constraining_type_params<'a>(
                 ));
             } else {
                 let mut param_spans = Vec::new();
+                let mut non_empty = false;
 
                 for predicate in generics.where_clause.predicates {
                     if let WherePredicate::BoundPredicate(WhereBoundPredicate {
                         span,
                         bounded_ty,
+                        bounds,
                         ..
                     }) = predicate
                     {
                         if let TyKind::Path(QPath::Resolved(_, path)) = &bounded_ty.kind {
                             if let Some(segment) = path.segments.first() {
                                 if segment.ident.to_string() == param_name {
+                                    non_empty = !bounds.is_empty();
+
                                     param_spans.push(span);
                                 }
                             }
@@ -478,7 +499,7 @@ pub fn suggest_constraining_type_params<'a>(
                 }
 
                 match param_spans[..] {
-                    [&param_span] => suggest_restrict(param_span.shrink_to_hi()),
+                    [&param_span] => suggest_restrict(param_span.shrink_to_hi(), non_empty),
                     _ => {
                         suggestions.push((
                             generics.where_clause.tail_span_for_suggestion(),