Auto merge of #43776 - zackmdavis:feature_gate_fn_must_use, r=alexcrichton
feature-gate #[must_use] for functions as `fn_must_use`
@eddyb I [was](https://github.com/rust-lang/rust/pull/43728#issuecomment-320854120) [dithering](https://github.com/rust-lang/rust/pull/43728#issuecomment-320856407) on this, but [your comment](https://github.com/rust-lang/rust/issues/43302#issuecomment-321174989) makes it sound like we do want a feature gate for this? Please advise.
r? @eddyb