Fix incorrect trait bound restriction suggestion

[estebank]

Suggest

error[E0308]: mismatched types
  --> $DIR/restrict-assoc-type-of-generic-bound.rs:9:12
   |
LL | pub fn foo<A: MyTrait, B>(a: A) -> B {
   |                        -           - expected `B` because of return type
   |                        |
   |                        expected this type parameter
LL |     return a.bar();
   |            ^^^^^^^ expected type parameter `B`, found associated type
   |
   = note: expected type parameter `B`
             found associated type `<A as MyTrait>::T`
help: consider further restricting this bound
   |
LL | pub fn foo<A: MyTrait<T = B>, B>(a: A) -> B {
   |                      +++++++

instead of

error[E0308]: mismatched types
  --> $DIR/restrict-assoc-type-of-generic-bound.rs:9:12
   |
LL | pub fn foo<A: MyTrait, B>(a: A) -> B {
   |                        -           - expected `B` because of return type
   |                        |
   |                        expected this type parameter
LL |     return a.bar();
   |            ^^^^^^^ expected type parameter `B`, found associated type
   |
   = note: expected type parameter `B`
             found associated type `<A as MyTrait>::T`
help: consider further restricting this bound
   |
LL | pub fn foo<A: MyTrait + <T = B>, B>(a: A) -> B {
   |                      +++++++++

Fix #117501.

[rustbot]

r? @TaKO8Ki

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[TaKO8Ki]

After resolving CI errors, r=me

[polina4096]

I think the message is still misleading, as it suggests restricting a trait bound instead of an associated type. The correct message is shown when a concrete type is expected.

Incorrect message:

help: consider further restricting this bound
  |
7 | fn foo<A: MyTrait<T = B>, B>(a: A) -> B {
  |                  +++++++

Correct message:

help: consider constraining the associated type `<A as MyTrait>::T` to `B`
  |
7 | fn foo<A: MyTrait<T = B>, B>(a: A) -> B {
  |                  +++++++

[estebank]

@polina4096 the wording is technically correct: we're asking to further constrain A to MyTrait<T = B> from MyTrait. It is still part of the trait bound. We should pass in more information to suggest_constraining_type_param so that the message can be more accurate in all cases.

@bors r=TaKO8Ki

[bors]

📌 Commit 9e7345b has been approved by TaKO8Ki

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.