Account for single where bound being removed

Author: estebank

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

@@ -5115,8 +5115,26 @@ fn point_at_assoc_type_restriction(
             {
                 // 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()
+                let span = if pred.origin == hir::PredicateOrigin::WhereClause
+                    && generics
+                        .predicates
+                        .iter()
+                        .filter(|p| {
+                            if let hir::WherePredicate::BoundPredicate(p) = p
+                                && hir::PredicateOrigin::WhereClause == p.origin
+                            {
+                                true
+                            } else {
+                                false
+                            }
+                        })
+                        .count()
+                        == 1
+                {
+                    // There's only one `where` bound, that needs to be removed. Remove the whole
+                    // `where` clause.
+                    generics.where_clause_span
+                } else if let Some(hir::WherePredicate::BoundPredicate(next)) = predicates.peek()
                     && pred.origin == next.origin
                 {
                     // There's another bound, include the comma for the current one.

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

@@ -20,8 +20,8 @@ LL |     Self::A: Copy,
    |              ---- unsatisfied trait bound introduced here
 help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
    |
+LL - where
 LL -     Self::A: Copy,
-LL +     
    |
 
 error: aborting due to 1 previous error

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

@@ -19,8 +19,7 @@ impl Grault for () {
 
 impl<T: Grault> Grault for (T,)
 //~^ ERROR overflow evaluating the requirement `<(T,) as Grault>::A == _`
-where
-    
+
 {
     type A = ();
     type B = bool;

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

@@ -20,8 +20,8 @@ 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 - where
 LL -     Self::A: Baz,
-LL +     
    |
 
 error: aborting due to 1 previous error