Auto merge of #118118 - spastorino:do-not-erase-late-bound-regions-on-iat, r=compiler-errors

Do not erase late bound regions when selecting inherent associated types

In the fix for #97156 we would want the following code:

```rust
#![feature(inherent_associated_types)]
#![allow(incomplete_features)]

struct Foo<T>(T);

impl Foo<fn(&'static ())> {
    type Assoc = u32;
}

trait Other {}
impl Other for u32 {}

// FIXME(inherent_associated_types): Avoid emitting two diagnostics (they only differ in span).
// FIXME(inherent_associated_types): Enhancement: Spruce up the diagnostic by saying something like
// "implementation is not general enough" as is done for traits via
// `try_report_trait_placeholder_mismatch`.

fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}
//~^ ERROR mismatched types
//~| ERROR mismatched types

fn main() {}
```

to fail with ...

```
error[E0220]: associated type `Assoc` not found for `Foo<for<'a> fn(&'a ())>` in the current scope
  --> tests/ui/associated-inherent-types/issue-109789.rs:18:36
   |
4  | struct Foo<T>(T);
   | ------------- associated item `Assoc` not found for this struct
...
18 | fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}
   |                                    ^^^^^ associated item not found in `Foo<for<'a> fn(&'a ())>`
   |
   = note: the associated type was found for
           - `Foo<fn(&'static ())>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0220`.
```

This PR fixes the ICE we are currently getting "was a subtype of Foo<Binder(fn(&ReStatic ()), [])> during selection but now it is not"

Also fixes #112631

r? `@lcnr`

3 files changed, 24 insertions, 1 deletions

diff --git a/tests/ui/associated-inherent-types/assoc-inherent-late-bound.rs b/tests/ui/associated-inherent-types/assoc-inherent-late-bound.rs
new file mode 100644
index 00000000000..b6993f66fe7
--- /dev/null
+++ b/tests/ui/associated-inherent-types/assoc-inherent-late-bound.rs
@@ -0,0 +1,14 @@
+// check-pass
+
+#![feature(inherent_associated_types)]
+#![allow(incomplete_features)]
+
+struct Foo<T>(T);
+
+impl<'a> Foo<fn(&'a ())> {
+    type Assoc = &'a ();
+}
+
+fn bar(_: for<'a> fn(Foo<fn(Foo<fn(&'static ())>::Assoc)>::Assoc)) {}
+
+fn main() {}
diff --git a/tests/ui/associated-inherent-types/issue-111404-1.rs b/tests/ui/associated-inherent-types/issue-111404-1.rs
index f4ad5d7ff6c..dd62e59f07d 100644
--- a/tests/ui/associated-inherent-types/issue-111404-1.rs
+++ b/tests/ui/associated-inherent-types/issue-111404-1.rs
@@ -9,5 +9,6 @@ impl<'a> Foo<fn(&'a ())> {
 
 fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
 //~^ ERROR higher-ranked subtype error
+//~| ERROR higher-ranked subtype error
 
 fn main() {}
diff --git a/tests/ui/associated-inherent-types/issue-111404-1.stderr b/tests/ui/associated-inherent-types/issue-111404-1.stderr
index 2c78e3a1fb7..cf4d4a5f19b 100644
--- a/tests/ui/associated-inherent-types/issue-111404-1.stderr
+++ b/tests/ui/associated-inherent-types/issue-111404-1.stderr
@@ -4,5 +4,13 @@ error: higher-ranked subtype error
 LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 1 previous error
+error: higher-ranked subtype error
+  --> $DIR/issue-111404-1.rs:10:1
+   |
+LL | fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 2 previous errors