Arbitrary self types v2: no deshadow pre feature.

The arbitrary self types v2 work introduces a check for shadowed
methods, whereby a method in some "outer" smart pointer type may called
in preference to a method in the inner referent. This is bad if the
outer pointer adds a method later, as it may change behavior, so we
ensure we error in this circumstance.

It was intended that this new shadowing detection system only comes into
play for users who enable the `arbitrary_self_types` feature (or of
course everyone later if it's stabilized). It was believed that the
new deshadowing code couldn't be reached without building the custom
smart pointers that `arbitrary_self_types` enables, and therefore there
was no risk of this code impacting existing users.

However, it turns out that cunning use of `Pin::get_ref` can cause
this type of shadowing error to be emitted now. This commit adds a test
for this case.

As we want this test to pass without arbitrary_self_types, but fail with
it, I've split it into two files (one with run-pass and one without). If
there's a better way I can amend it.

Author: adetaylor

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1337,6 +1337,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         mutbl: hir::Mutability,
         track_unstable_candidates: bool,
     ) -> Result<(), MethodError<'tcx>> {
+        // The errors emitted by this function are part of
+        // the arbitrary self types work, and should not impact
+        // other users.
+        if !self.tcx.features().arbitrary_self_types()
+            && !self.tcx.features().arbitrary_self_types_pointers()
+        {
+            return Ok(());
+        }
+
         // We don't want to remember any of the diagnostic hints from this
         // shadow search, but we do need to provide Some/None for the
         // unstable_candidates in order to reflect the behavior of the

tests/ui/self/arbitrary_self_types_pin_getref_default.rs

@@ -0,0 +1,22 @@
+// Confirms that Pin::get_ref can shadow methods in pointees, so long
+// as we don't enable arbitrary self types.
+// This will turn into an error if arbitrary_self_types feature is enabled.
+
+//@ run-pass
+
+#![allow(dead_code)]
+
+use std::pin::Pin;
+use std::pin::pin;
+
+struct A;
+
+impl A {
+    fn get_ref(self: &Pin<&A>) {}  // note &Pin
+}
+
+fn main() {
+    let pinned_a: Pin<&mut A> = pin!(A);
+    let pinned_a: Pin<&A> = pinned_a.as_ref();
+    let _ = pinned_a.get_ref(); // ambiguous. No error yet.
+}

tests/ui/self/arbitrary_self_types_pin_getref_feature.rs

@@ -0,0 +1,21 @@
+// Confirms that Pin::get_ref can no longer shadow methods in pointees
+// once arbitrary_self_types is enabled.
+
+#![feature(arbitrary_self_types)]
+#![allow(dead_code)]
+
+use std::pin::Pin;
+use std::pin::pin;
+
+struct A;
+
+impl A {
+    fn get_ref(self: &Pin<&A>) {}  // note &Pin
+}
+
+fn main() {
+    let pinned_a: Pin<&mut A> = pin!(A);
+    let pinned_a: Pin<&A> = pinned_a.as_ref();
+    let _ = pinned_a.get_ref();
+    //~^ ERROR: multiple applicable items
+}

tests/ui/self/arbitrary_self_types_pin_getref_feature.stderr

@@ -0,0 +1,16 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_pin_getref_feature.rs:19:22
+   |
+LL |     let _ = pinned_a.get_ref();
+   |                      ^^^^^^^ multiple `get_ref` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_pin_getref_feature.rs:13:5
+   |
+LL |     fn get_ref(self: &Pin<&A>) {}  // note &Pin
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: candidate #2 is defined in an impl for the type `Pin<&'a T>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0034`.