Account for RPITIT in E0310 explicit lifetime constraint suggestion

When given

```rust
trait Original {
    fn f() -> impl Fn();
}

trait Erased {
    fn f(&self) -> Box<dyn Fn()>;
}

impl<T: Original> Erased for T {
    fn f(&self) -> Box<dyn Fn()> {
        Box::new(<T as Original>::f())
    }
}
```

emit a suggestion to further constrain the RPITIT, instead of what we did previously, suggest restricting the `Trait::{opaque}` type in a `where` clause:

```
error[E0310]: the associated type `<T as Original>::{opaque#0}` may not live long enough
  --> $DIR/missing-static-bound-from-impl.rs:11:9
   |
LL |         Box::new(<T as Original>::f())
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |         |
   |         the associated type `<T as Original>::{opaque#0}` must be valid for the static lifetime...
   |         ...so that the type `impl Fn()` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound
   |
LL |     fn f() -> impl Fn() + 'static;
   |                         +++++++++
```

Fix #119773.

Author: estebank

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2433,6 +2433,17 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 let suggestion =
                     if has_lifetimes { format!(" + {lt_name}") } else { format!(": {lt_name}") };
                 suggs.push((sp, suggestion))
+            } else if let GenericKind::Alias(ref p) = bound_kind
+                && let ty::Projection = p.kind(self.tcx)
+                && let DefKind::AssocTy = self.tcx.def_kind(p.def_id)
+                && let Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) =
+                    self.tcx.opt_rpitit_info(p.def_id)
+            {
+                // The lifetime found in the `impl` is longer than the one on the RPITIT.
+                suggs.push((
+                    self.tcx.def_span(opaque_def_id).shrink_to_hi(),
+                    format!(" + {lt_name}"),
+                ));
             } else if let Some(generics) = self.tcx.hir().get_generics(suggestion_scope) {
                 let pred = format!("{bound_kind}: {lt_name}");
                 let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred);

tests/ui/impl-trait/in-trait/async-and-ret-ref.stderr

@@ -7,7 +7,10 @@ LL |     async fn foo() -> &'static impl T;
    |     the associated type `<Self as MyTrait>::{opaque#0}` must be valid for the static lifetime...
    |     ...so that the reference type `&'static impl T` does not outlive the data it points at
    |
-   = help: consider adding an explicit lifetime bound `<Self as MyTrait>::{opaque#0}: 'static`...
+help: consider adding an explicit lifetime bound
+   |
+LL |     async fn foo() -> &'static impl T + 'static;
+   |                                       +++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/in-trait/missing-static-bound-from-impl.rs

@@ -0,0 +1,16 @@
+trait Original {
+    fn f() -> impl Fn();
+}
+
+trait Erased {
+    fn f(&self) -> Box<dyn Fn()>;
+}
+
+impl<T: Original> Erased for T {
+    fn f(&self) -> Box<dyn Fn()> {
+        Box::new(<T as Original>::f())
+        //~^ ERROR the associated type `<T as Original>::{opaque#0}` may not live long enough
+    }
+}
+
+fn main () {}

tests/ui/impl-trait/in-trait/missing-static-bound-from-impl.stderr

@@ -0,0 +1,17 @@
+error[E0310]: the associated type `<T as Original>::{opaque#0}` may not live long enough
+  --> $DIR/missing-static-bound-from-impl.rs:11:9
+   |
+LL |         Box::new(<T as Original>::f())
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         the associated type `<T as Original>::{opaque#0}` must be valid for the static lifetime...
+   |         ...so that the type `impl Fn()` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL |     fn f() -> impl Fn() + 'static;
+   |                         +++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0310`.