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 

3 files changed, 5 insertions, 5 deletions

diff --git a/src/libstd/path/mod.rs b/src/libstd/path/mod.rs
index 1b09fb92737..b42353e964c 100644
--- a/src/libstd/path/mod.rs
+++ b/src/libstd/path/mod.rs
@@ -59,7 +59,7 @@
 //! println!("path exists: {}", path.exists());
 //! ```
 
-#![unstable]
+#![unstable(feature = "path")]
 
 use core::marker::Sized;
 use ffi::CString;
@@ -823,14 +823,14 @@ pub struct Display<'a, P:'a> {
     filename: bool
 }
 
-#[stable]
+#[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, P: GenericPath> fmt::Debug for Display<'a, P> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Debug::fmt(&self.as_cow(), f)
     }
 }
 
-#[stable]
+#[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, P: GenericPath> fmt::Display for Display<'a, P> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.as_cow().fmt(f)
diff --git a/src/libstd/path/posix.rs b/src/libstd/path/posix.rs
index 202d81f5696..39138e14803 100644
--- a/src/libstd/path/posix.rs
+++ b/src/libstd/path/posix.rs
@@ -57,7 +57,7 @@ pub fn is_sep(c: char) -> bool {
     c == SEP
 }
 
-#[stable]
+#[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Debug for Path {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Debug::fmt(&self.display(), f)
diff --git a/src/libstd/path/windows.rs b/src/libstd/path/windows.rs
index 9f5ed177918..34faa65af75 100644
--- a/src/libstd/path/windows.rs
+++ b/src/libstd/path/windows.rs
@@ -85,7 +85,7 @@ pub struct Path {
     sepidx: Option<uint> // index of the final separator in the non-prefix portion of repr
 }
 
-#[stable]
+#[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Debug for Path {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Debug::fmt(&self.display(), f)