Auto merge of #5359 - matklad:rustc-cache, r=alexcrichton
authorbors <bors@rust-lang.org>
Mon, 16 Apr 2018 17:23:53 +0000 (17:23 +0000)
committerbors <bors@rust-lang.org>
Mon, 16 Apr 2018 17:23:53 +0000 (17:23 +0000)
Rustc cache

This implements rustc caching, to speed-up no-op builds.

The cache is per-project, and stored in `target` directory. To implement this, I had to move `rustc` from `Config` down to `BuildConfig`.

closes https://github.com/rust-lang/cargo/issues/5315


Trivial merge