Don't compute name of associated item if it's an RPITIT

tests/ui/impl-trait/in-trait/dont-probe-missing-item-name.rs

@@ -0,0 +1,12 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/139873>.
+
+// Test that we don't try to get the (nonexistent) name of the RPITIT in `Trait::foo`
+// when emitting an error for a missing associated item `Trait::Output`.
+
+trait Trait {
+    fn foo() -> impl Sized;
+    fn bar() -> Self::Output;
+    //~^ ERROR associated type `Output` not found for `Self`
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/dont-probe-missing-item-name.stderr

@@ -0,0 +1,9 @@
+error[E0220]: associated type `Output` not found for `Self`
+  --> $DIR/dont-probe-missing-item-name.rs:8:23
+   |
+LL |     fn bar() -> Self::Output;
+   |                       ^^^^^^ associated type `Output` not found
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0220`.