Arbitrary self types v2: niche deshadowing test

Arbitrary self types v2 attempts to detect cases where methods in an
"outer" type (e.g. a smart pointer) might "shadow" methods in the
referent.

There are a couple of cases where the current code makes no attempt to
detect such shadowing. Both of these cases only apply if other unstable
features are enabled.

Add a test, mostly for illustrative purposes, so we can see the
shadowing cases that can occur.

tests/ui/self/arbitrary_self_types_niche_deshadowing.rs

@@ -0,0 +1,63 @@
+//@ run-pass
+
+#![allow(dead_code)]
+#![allow(incomplete_features)]
+
+#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
+#![feature(pin_ergonomics)]
+
+use std::pin::Pin;
+use std::pin::pin;
+use std::marker::PhantomData;
+
+struct A;
+
+impl A {
+    fn m(self: *const SmartPtr<Self>) -> usize { 2 }
+    fn n(self: *const SmartPtr<Self>) -> usize { 2 }
+
+    fn o(self: Pin<&SmartPtr2<Self>>) -> usize { 2 }
+    fn p(self: Pin<&SmartPtr2<Self>>) -> usize { 2 }
+}
+
+struct SmartPtr<T>(T);
+
+impl<T> core::ops::Receiver for SmartPtr<T> {
+    type Target = *mut T;
+}
+
+impl<T> SmartPtr<T> {
+    // In general we try to detect cases where a method in a smart pointer
+    // "shadows" a method in the referent (in this test, A).
+    // This method "shadows" the 'n' method in the inner type A
+    // We do not attempt to produce an error in these shadowing cases
+    // since the type signature of this method and the corresponding
+    // method in A are pretty unlikely to occur in practice,
+    // and because it shows up conflicts between *const::cast and *mut::cast.
+    fn n(self: *mut Self) -> usize { 1 }
+}
+
+struct SmartPtr2<'a, T>(T, PhantomData<&'a T>);
+
+impl<'a, T> core::ops::Receiver for SmartPtr2<'a, T> {
+    type Target = Pin<&'a mut T>;
+}
+
+impl<T> SmartPtr2<'_, T> {
+    // Similarly, this method shadows the method in A
+    // Can only happen with the unstable feature pin_ergonomics
+    fn p(self: Pin<&mut Self>) -> usize { 1 }
+}
+
+fn main() {
+    let mut sm = SmartPtr(A);
+    let smp: *mut SmartPtr<A> = &mut sm as *mut SmartPtr<A>;
+    assert_eq!(smp.m(), 2);
+    assert_eq!(smp.n(), 1);
+
+    let smp: Pin<&mut SmartPtr2<A>> = pin!(SmartPtr2(A, PhantomData));
+    assert_eq!(smp.o(), 2);
+    let smp: Pin<&mut SmartPtr2<A>> = pin!(SmartPtr2(A, PhantomData));
+    assert_eq!(smp.p(), 1);
+}