rollup merge of #23598: brson/gate

Conflicts:
	src/compiletest/compiletest.rs
	src/libcollections/lib.rs
	src/librustc_back/lib.rs
	src/libserialize/lib.rs
	src/libstd/lib.rs
	src/libtest/lib.rs
	src/test/run-make/rustdoc-default-impl/foo.rs
	src/test/run-pass/env-home-dir.rs

src/libcore/marker.rs

@@ -323,6 +323,7 @@ impl<T:?Sized> MarkerTrait for T { }
 /// `MarkerTrait`:
 ///
 /// ```
+/// # #![feature(core)]
 /// use std::marker::MarkerTrait;
 /// trait Even : MarkerTrait { }
 /// ```