Auto merge of #103390 - compiler-errors:metadata-mod-regions, r=eholk

Check fat pointer metadata compatibility modulo regions

Regions don't really mean anything anyways during hir typeck.

If this `erase_regions` makes anyone nervous, it's probably equally valid to just equate the types using a type relation, but regardless we should _not_ be using strict type equality while region variables are present.

Fixes #103384

src/test/ui/cast/cast-pointee-projection.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+trait Tag<'a> {
+    type Type: ?Sized;
+}
+
+trait IntoRaw: for<'a> Tag<'a> {
+    fn into_raw(this: *const <Self as Tag<'_>>::Type) -> *mut <Self as Tag<'_>>::Type;
+}
+
+impl<T: for<'a> Tag<'a>> IntoRaw for T {
+    fn into_raw(this: *const <Self as Tag<'_>>::Type) -> *mut <Self as Tag<'_>>::Type {
+        this as *mut T::Type
+    }
+}
+
+fn main() {}