Auto merge of #21248 - brson:feature-staging, r=alexcrichton

This implements the remaining bits of 'feature staging', as described in [RFC 507](https://github.com/rust-lang/rfcs/blob/master/text/0507-release-channels.md).

This is not quite done, but the substance of the work is complete so submitting for early review.

Key changes:
* `unstable`, `stable` and `deprecated` attributes all require 'feature' and 'since', and support an optional 'reason'.
* The `unstable` lint is removed.
* A new 'stability checking' pass warns when a used unstable library feature has not been activated with the `feature` attribute. At 1.0 beta this will become an error.
* A new 'unused feature checking' pass emits a lint ('unused_feature', renamed from 'unknown_feature') for any features that were activated but not used.
* A new tidy script `featureck.py` performs some global sanity checking, particularly that 'since' numbers agree, and also prints out a summary of features.

Differences from RFC:
* As implemented `unstable` requires a `since` attribute. I do not know if this is useful. I included it in the original sed script and just left it.
* RFC didn't specify the name of the optional 'reason' attribute.
* This continues to use 'unstable', 'stable' and 'deprecated' names (the 'nice' names) instead of 'staged_unstable', but only activates them with the crate-level 'staged_api' attribute.

I intend to update the RFC based on the outcome of this PR.

Issues:
* The unused feature check doesn't account for language features - i.e. you can activate a language feature, not use it, and not get the error.

Open questions:
* All unstable and deprecated features are named 'unnamed_feature', which i picked just because it is uniquely greppable. This is the 'catch-all' feature. What should it be?
* All stable features are named 'grandfathered'. What should this be?

TODO:
* Add check that all `deprecated` attributes are paired with a `stable` attribute in order to preserve the knowledge about when a feature became stable.
* Update rustdoc in various ways.
* Remove obsolete stability discussion from reference.
* Add features for 'path', 'io', 'os', 'hash' and 'rand'.

cc #20445 @alexcrichton @aturon 

2 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/rt/mod.rs b/src/libstd/rt/mod.rs
index e3e4e132b81..7e19f1cac2c 100644
--- a/src/libstd/rt/mod.rs
+++ b/src/libstd/rt/mod.rs
@@ -16,7 +16,7 @@
 //! and should be considered as private implementation details for the
 //! time being.
 
-#![unstable]
+#![unstable(feature = "std_misc")]
 
 // FIXME: this should not be here.
 #![allow(missing_docs)]
diff --git a/src/libstd/rt/unwind.rs b/src/libstd/rt/unwind.rs
index b313a5312bc..18298f1c7f4 100644
--- a/src/libstd/rt/unwind.rs
+++ b/src/libstd/rt/unwind.rs
@@ -582,7 +582,7 @@ fn begin_unwind_inner(msg: Box<Any + Send>, file_line: &(&'static str, uint)) ->
 /// Only a limited number of callbacks can be registered, and this function
 /// returns whether the callback was successfully registered or not. It is not
 /// currently possible to unregister a callback once it has been registered.
-#[unstable]
+#[unstable(feature = "std_misc")]
 pub unsafe fn register(f: Callback) -> bool {
     match CALLBACK_CNT.fetch_add(1, Ordering::SeqCst) {
         // The invocation code has knowledge of this window where the count has