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/libcoretest/lib.rs

@@ -24,6 +24,8 @@
 #![feature(io)]
 #![feature(collections)]
 #![feature(debug_builders)]
+#![feature(unique)]
+#![feature(step_by)]
 #![allow(deprecated)] // rand
 
 extern crate core;