rollup merge of #21830: japaric/for-cleanup

Conflicts:
	src/librustc/metadata/filesearch.rs
	src/librustc_back/target/mod.rs
	src/libstd/os.rs
	src/libstd/sys/windows/os.rs
	src/libsyntax/ext/tt/macro_parser.rs
	src/libsyntax/print/pprust.rs
	src/test/compile-fail/issue-2149.rs

src/libstd/lib.rs

@@ -120,14 +120,13 @@
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
-#![cfg_attr(not(stage0), feature(macro_reexport))]
+#![feature(macro_reexport)]
 #![cfg_attr(test, feature(test))]
 
 // Don't link to std. We are std.
 #![no_std]
 
 #![deny(missing_docs)]
-#![cfg_attr(not(stage0), allow(unused_mut))] // NOTE: remove after stage0 snap
 
 #[cfg(test)]
 #[macro_use]