Auto merge of #5613 - oli-obk:master, r=matklad
authorbors <bors@rust-lang.org>
Wed, 6 Jun 2018 17:32:01 +0000 (17:32 +0000)
committerbors <bors@rust-lang.org>
Wed, 6 Jun 2018 17:32:01 +0000 (17:32 +0000)
Allow rustc bootstrap to use unstable features even though it's using a beta-cargo

This is the sane alternative to https://github.com/rust-lang/rust/pull/51322#issuecomment-394828839

cc @kennytm


Trivial merge