Account for trailing comma in removal suggestion

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -5100,11 +5100,14 @@ fn point_at_assoc_type_restriction(
         return;
     };
     let name = tcx.item_name(proj.projection_ty.def_id);
-    for pred in generics.predicates {
+    let mut predicates = generics.predicates.iter().peekable();
+    let mut prev: Option<&hir::WhereBoundPredicate<'_>> = None;
+    while let Some(pred) = predicates.next() {
         let hir::WherePredicate::BoundPredicate(pred) = pred else {
             continue;
         };
-        for bound in pred.bounds {
+        let mut bounds = pred.bounds.iter().peekable();
+        while let Some(bound) = bounds.next() {
             let Some(trait_ref) = bound.trait_ref() else {
                 continue;
             };
@@ -5118,8 +5121,27 @@ fn point_at_assoc_type_restriction(
                 && let hir::QPath::Resolved(None, inner_path) = inner_path
                 && let Res::SelfTyAlias { .. } = inner_path.res
             {
+                // The following block is to determine the right span to delete for this bound
+                // that will leave valid code after the suggestion is applied.
+                let span = if let Some(hir::WherePredicate::BoundPredicate(next)) =
+                    predicates.peek()
+                    && pred.origin == next.origin
+                {
+                    // There's another bound, include the comma for the current one.
+                    pred.span.until(next.span)
+                } else if let Some(prev) = prev
+                    && pred.origin == prev.origin
+                {
+                    // Last bound, try to remove the previous comma.
+                    prev.span.shrink_to_hi().to(pred.span)
+                } else if pred.origin == hir::PredicateOrigin::WhereClause {
+                    pred.span.with_hi(generics.where_clause_span.hi())
+                } else {
+                    pred.span
+                };
+
                 err.span_suggestion_verbose(
-                    pred.span, // FIXME: include the trailing comma.
+                    span,
                     "associated type for the current `impl` cannot be restricted in `where` \
                      clauses, remove this bound",
                     "",
@@ -5168,6 +5190,7 @@ fn point_at_assoc_type_restriction(
                 );
             }
         }
+        prev = Some(pred);
     }
 }
 

tests/ui/associated-types/impl-wf-cycle-1.stderr

@@ -24,7 +24,6 @@ LL |     Self::A: Baz,
 help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
    |
 LL -     Self::A: Baz,
-LL +     ,
    |
 
 error: aborting due to 1 previous error

tests/ui/associated-types/impl-wf-cycle-2.stderr

@@ -21,7 +21,7 @@ LL |     Self::A: Copy,
 help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
    |
 LL -     Self::A: Copy,
-LL +     ,
+LL +     
    |
 
 error: aborting due to 1 previous error

tests/ui/associated-types/impl-wf-cycle-5.fixed

@@ -0,0 +1,31 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+    T: Grault,
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}

tests/ui/associated-types/impl-wf-cycle-5.rs

@@ -0,0 +1,32 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+    T: Grault,
+    Self::A: Baz,
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}

tests/ui/associated-types/impl-wf-cycle-5.stderr

@@ -0,0 +1,31 @@
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-5.rs:20:1
+   |
+LL | / impl<T> Grault for (T,)
+LL | |
+LL | | where
+LL | |     T: Grault,
+LL | |     Self::A: Baz,
+   | |_________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
+   |
+note: required for `(T,)` to implement `Grault`
+  --> $DIR/impl-wf-cycle-5.rs:20:9
+   |
+LL | impl<T> Grault for (T,)
+   |         ^^^^^^     ^^^^
+...
+LL |     Self::A: Baz,
+   |              --- unsatisfied trait bound introduced here
+help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
+   |
+LL -     T: Grault,
+LL -     Self::A: Baz,
+LL +     T: Grault,
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/associated-types/impl-wf-cycle-6.fixed

@@ -0,0 +1,31 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T: Grault> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}

tests/ui/associated-types/impl-wf-cycle-6.rs

@@ -0,0 +1,31 @@
+// run-rustfix
+
+trait Baz {}
+impl Baz for () {}
+impl<T> Baz for (T,) {}
+
+trait Fiz {}
+impl Fiz for bool {}
+
+trait Grault {
+    type A;
+    type B;
+}
+
+impl Grault for () {
+    type A = ();
+    type B = bool;
+}
+
+impl<T: Grault> Grault for (T,)
+//~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
+where
+    Self::A: Baz,
+{
+    type A = ();
+    type B = bool;
+}
+
+fn main() {
+    let _: <((),) as Grault>::A = ();
+}

tests/ui/associated-types/impl-wf-cycle-6.stderr

@@ -0,0 +1,29 @@
+error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
+  --> $DIR/impl-wf-cycle-6.rs:20:1
+   |
+LL | / impl<T: Grault> Grault for (T,)
+LL | |
+LL | | where
+LL | |     Self::A: Baz,
+   | |_________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
+   |
+note: required for `(T,)` to implement `Grault`
+  --> $DIR/impl-wf-cycle-6.rs:20:17
+   |
+LL | impl<T: Grault> Grault for (T,)
+   |                 ^^^^^^     ^^^^
+...
+LL |     Self::A: Baz,
+   |              --- unsatisfied trait bound introduced here
+help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
+   |
+LL -     Self::A: Baz,
+LL +     
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.