Auto merge of #38679 - alexcrichton:always-deny-warnings, r=nrc

Remove not(stage0) from deny(warnings)

Historically this was done to accommodate bugs in lints, but there hasn't been a
bug in a lint since this feature was added which the warnings affected. Let's
completely purge warnings from all our stages by denying warnings in all stages.
This will also assist in tracking down `stage0` code to be removed whenever
we're updating the bootstrap compiler.

Author: bors

lib.rs

@@ -52,7 +52,7 @@
        html_playground_url = "https://play.rust-lang.org/",
        test(attr(deny(warnings))))]
 #![deny(missing_docs)]
-#![cfg_attr(not(stage0), deny(warnings))]
+#![deny(warnings)]
 
 #![feature(box_syntax)]
 #![feature(staged_api)]