Test that RPITITs have RPIT scope and not impl-wide scope

tests/ui/impl-trait/in-trait/sibling-function-constraint.rs

@@ -0,0 +1,21 @@
+// Checks that a sibling function (i.e. `foo`) cannot constrain
+// an RPITIT from another function (`bar`).
+
+trait Trait {
+    fn foo();
+
+    fn bar() -> impl Sized;
+}
+
+impl Trait for () {
+    fn foo() {
+        let _: String = Self::bar();
+        //~^ ERROR mismatched types
+    }
+
+    fn bar() -> impl Sized {
+        loop {}
+    }
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/sibling-function-constraint.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/sibling-function-constraint.rs:12:25
+   |
+LL |         let _: String = Self::bar();
+   |                ------   ^^^^^^^^^^^ expected `String`, found opaque type
+   |                |
+   |                expected due to this
+...
+LL |     fn bar() -> impl Sized {
+   |                 ---------- the found opaque type
+   |
+   = note:   expected struct `String`
+           found opaque type `impl Sized`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.