Rollup merge of #22809 - nikomatsakis:issue-22655-infinite-recursion, r=pnkfelix

Change MarkerTrait to be invariant. This is a (small) loss of expressiveness, but is necessary for now to work around #22806. Fixes #22655.

r? @pnkfelix

src/libcore/marker.rs

@@ -275,7 +275,13 @@ macro_rules! impls{
 /// any methods, but instead is used to gate access to data.
 ///
 /// FIXME. Better documentation needed here!
-pub trait MarkerTrait : PhantomFn<Self> { }
+pub trait MarkerTrait : PhantomFn<Self,Self> { }
+//                                    ~~~~~ <-- FIXME(#22806)?
+//
+// Marker trait has been made invariant so as to avoid inf recursion,
+// but we should ideally solve the underlying problem. That's a bit
+// complicated.
+
 impl<T:?Sized> MarkerTrait for T { }
 
 /// `PhantomFn` is a marker trait for use with traits that contain

src/test/run-pass/traits-issue-22655.rs

@@ -0,0 +1,29 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for issue #22655: This test should not lead to
+// infinite recursion.
+
+unsafe impl<T: Send + ?Sized> Send for Unique<T> { }
+
+pub struct Unique<T:?Sized> {
+    pointer: *const T,
+}
+
+pub struct Node<V> {
+    vals: V,
+    edges: Unique<Node<V>>,
+}
+
+fn is_send<T: Send>() {}
+
+fn main() {
+    is_send::<Node<&'static ()>>();
+}