Provide more context on recursive impl evaluation overflow

When an associated type `Self::Assoc` is part of a `where` clause,
we end up unable to evaluate the requirement and emit a E0275.

We now point at the associated type if specified in the `impl`. If
so, we also suggest using that type instead of `Self::Assoc`.
Otherwise, we explain that these are not allowed.

```
error[E0275]: overflow evaluating the requirement `<(T,) as Grault>::A == _`
  --> $DIR/impl-wf-cycle-1.rs:15:1
   |
LL | / impl<T: Grault> Grault for (T,)
LL | |
LL | | where
LL | |     Self::A: Baz,
LL | |     Self::B: Fiz,
   | |_________________^
LL |   {
LL |       type A = ();
   |       ------ associated type `<(T,) as Grault>::A` is specified here
   |
note: required for `(T,)` to implement `Grault`
  --> $DIR/impl-wf-cycle-1.rs:15:17
   |
LL | impl<T: Grault> Grault for (T,)
   |                 ^^^^^^     ^^^^
...
LL |     Self::A: Baz,
   |              --- unsatisfied trait bound introduced here
   = note: 1 redundant requirement hidden
   = note: required for `(T,)` to implement `Grault`
help: associated type for the current `impl` cannot be restricted in `where` clauses, remove this bound
   |
LL -     Self::A: Baz,
LL +     ,
   |
```
```
error[E0275]: overflow evaluating the requirement `<T as B>::Type == <T as B>::Type`
  --> $DIR/impl-wf-cycle-3.rs:7:1
   |
LL | / impl<T> B for T
LL | | where
LL | |     T: A<Self::Type>,
   | |_____________________^
LL |   {
LL |       type Type = bool;
   |       --------- associated type `<T as B>::Type` is specified here
   |
note: required for `T` to implement `B`
  --> $DIR/impl-wf-cycle-3.rs:7:9
   |
LL | impl<T> B for T
   |         ^     ^
LL | where
LL |     T: A<Self::Type>,
   |        ------------- unsatisfied trait bound introduced here
help: replace the associated type with the type specified in this `impl`
   |
LL |     T: A<bool>,
   |          ~~~~
```
```
error[E0275]: overflow evaluating the requirement `<T as Filter>::ToMatch == <T as Filter>::ToMatch`
  --> $DIR/impl-wf-cycle-4.rs:5:1
   |
LL | / impl<T> Filter for T
LL | | where
LL | |     T: Fn(Self::ToMatch),
   | |_________________________^
   |
note: required for `T` to implement `Filter`
  --> $DIR/impl-wf-cycle-4.rs:5:9
   |
LL | impl<T> Filter for T
   |         ^^^^^^     ^
LL | where
LL |     T: Fn(Self::ToMatch),
   |        ----------------- unsatisfied trait bound introduced here
note: associated types for the current `impl` cannot be restricted in `where` clauses
  --> $DIR/impl-wf-cycle-4.rs:7:11
   |
LL |     T: Fn(Self::ToMatch),
   |           ^^^^^^^^^^^^^
```

Fix rust-lang#116925

Author: estebank

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

@@ -7,6 +7,9 @@ LL | | where
 LL | |     Self::A: Baz,
 LL | |     Self::B: Fiz,
    | |_________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
    |
 note: required for `(T,)` to implement `Grault`
   --> $DIR/impl-wf-cycle-1.rs:15:17
@@ -18,6 +21,11 @@ LL |     Self::A: Baz,
    |              --- unsatisfied trait bound introduced here
    = note: 1 redundant requirement hidden
    = note: required for `(T,)` to implement `Grault`
+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-1.stderr

@@ -6,6 +6,9 @@ LL | |
 LL | | where
 LL | |     Self::A: Copy,
    | |__________________^
+LL |   {
+LL |       type A = ();
+   |       ------ associated type `<(T,) as Grault>::A` is specified here
    |
 note: required for `(T,)` to implement `Grault`
   --> $DIR/impl-wf-cycle-2.rs:7:17
@@ -15,6 +18,11 @@ LL | impl<T: Grault> Grault for (T,)
 ...
 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 -     Self::A: Copy,
+LL +     ,
+   |
 
 error: aborting due to 1 previous error
 

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

@@ -0,0 +1,13 @@
+trait A<T> {}
+
+trait B {
+    type Type;
+}
+
+impl<T> B for T //~ ERROR overflow evaluating the requirement
+where
+    T: A<Self::Type>,
+{
+    type Type = bool;
+}
+fn main() {}

tests/ui/associated-types/impl-wf-cycle-3.rs

@@ -0,0 +1,27 @@
+error[E0275]: overflow evaluating the requirement `<T as B>::Type == <T as B>::Type`
+  --> $DIR/impl-wf-cycle-3.rs:7:1
+   |
+LL | / impl<T> B for T
+LL | | where
+LL | |     T: A<Self::Type>,
+   | |_____________________^
+LL |   {
+LL |       type Type = bool;
+   |       --------- associated type `<T as B>::Type` is specified here
+   |
+note: required for `T` to implement `B`
+  --> $DIR/impl-wf-cycle-3.rs:7:9
+   |
+LL | impl<T> B for T
+   |         ^     ^
+LL | where
+LL |     T: A<Self::Type>,
+   |        ------------- unsatisfied trait bound introduced here
+help: replace the associated type with the type specified in this `impl`
+   |
+LL |     T: A<bool>,
+   |          ~~~~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/associated-types/impl-wf-cycle-3.stderr

@@ -0,0 +1,15 @@
+trait Filter {
+    type ToMatch;
+}
+
+impl<T> Filter for T //~ ERROR overflow evaluating the requirement
+where
+    T: Fn(Self::ToMatch),
+{
+}
+
+struct JustFilter<F: Filter> {
+    filter: F,
+}
+
+fn main() {}

tests/ui/associated-types/impl-wf-cycle-4.rs

@@ -0,0 +1,25 @@
+error[E0275]: overflow evaluating the requirement `<T as Filter>::ToMatch == <T as Filter>::ToMatch`
+  --> $DIR/impl-wf-cycle-4.rs:5:1
+   |
+LL | / impl<T> Filter for T
+LL | | where
+LL | |     T: Fn(Self::ToMatch),
+   | |_________________________^
+   |
+note: required for `T` to implement `Filter`
+  --> $DIR/impl-wf-cycle-4.rs:5:9
+   |
+LL | impl<T> Filter for T
+   |         ^^^^^^     ^
+LL | where
+LL |     T: Fn(Self::ToMatch),
+   |        ----------------- unsatisfied trait bound introduced here
+note: associated types for the current `impl` cannot be restricted in `where` clauses
+  --> $DIR/impl-wf-cycle-4.rs:7:11
+   |
+LL |     T: Fn(Self::ToMatch),
+   |           ^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.