Preliminary feature staging

This partially implements the feature staging described in the
[release channel RFC][rc]. It does not yet fully conform to the RFC as
written, but does accomplish its goals sufficiently for the 1.0 alpha
release.

It has three primary user-visible effects:

* On the nightly channel, use of unstable APIs generates a warning.
* On the beta channel, use of unstable APIs generates a warning.
* On the beta channel, use of feature gates generates a warning.

Code that does not trigger these warnings is considered 'stable',
modulo pre-1.0 bugs.

Disabling the warnings for unstable APIs continues to be done in the
existing (i.e. old) style, via `#[allow(...)]`, not that specified in
the RFC. I deem this marginally acceptable since any code that must do
this is not using the stable dialect of Rust.

Use of feature gates is itself gated with the new 'unstable_features'
lint, on nightly set to 'allow', and on beta 'warn'.

The attribute scheme used here corresponds to an older version of the
RFC, with the `#[staged_api]` crate attribute toggling the staging
behavior of the stability attributes, but the user impact is only
in-tree so I'm not concerned about having to make design changes later
(and I may ultimately prefer the scheme here after all, with the
`#[staged_api]` crate attribute).

Since the Rust codebase itself makes use of unstable features the
compiler and build system to a midly elaborate dance to allow it to
bootstrap while disobeying these lints (which would otherwise be
errors because Rust builds with `-D warnings`).

This patch includes one significant hack that causes a
regression. Because the `format_args!` macro emits calls to unstable
APIs it would trigger the lint.  I added a hack to the lint to make it
not trigger, but this in turn causes arguments to `println!` not to be
checked for feature gates. I don't presently understand macro
expansion well enough to fix. This is bug #20661.

Closes #16678

[rc]: https://github.com/rust-lang/rfcs/blob/master/text/0507-release-channels.md

Author: brson

configure

@@ -599,6 +599,18 @@ then
 fi
 putvar CFG_RELEASE_CHANNEL
 
+# A magic value that allows the compiler to use unstable features
+# during the bootstrap even when doing so would normally be an error
+# because of feature staging or because the build turns on
+# warnings-as-errors and unstable features default to warnings.  The
+# build has to match this key in an env var. Meant to be a mild
+# deterrent from users just turning on unstable features on the stable
+# channel.
+# Basing CFG_BOOTSTRAP_KEY on CFG_BOOTSTRAP_KEY lets it get picked up
+# during a Makefile reconfig.
+CFG_BOOTSTRAP_KEY="${CFG_BOOTSTRAP_KEY-`date +%N`}"
+putvar CFG_BOOTSTRAP_KEY
+
 step_msg "looking for build programs"
 
 probe_need CFG_PERL        perl

mk/main.mk

@@ -25,11 +25,13 @@ ifeq ($(CFG_RELEASE_CHANNEL),stable)
 CFG_RELEASE=$(CFG_RELEASE_NUM)
 # This is the string used in dist artifact file names, e.g. "0.12.0", "nightly"
 CFG_PACKAGE_VERS=$(CFG_RELEASE_NUM)
+CFG_DISABLE_UNSTABLE_FEATURES=1
 endif
 ifeq ($(CFG_RELEASE_CHANNEL),beta)
 # The beta channel is temporarily called 'alpha'
 CFG_RELEASE=$(CFG_RELEASE_NUM)-alpha$(CFG_BETA_CYCLE)
 CFG_PACKAGE_VERS=$(CFG_RELEASE_NUM)-alpha$(CFG_BETA_CYCLE)
+CFG_DISABLE_UNSTABLE_FEATURES=1
 endif
 ifeq ($(CFG_RELEASE_CHANNEL),nightly)
 CFG_RELEASE=$(CFG_RELEASE_NUM)-nightly
@@ -319,11 +321,20 @@ export CFG_VERSION_WIN
 export CFG_RELEASE
 export CFG_PACKAGE_NAME
 export CFG_BUILD
+export CFG_RELEASE_CHANNEL
 export CFG_LLVM_ROOT
 export CFG_PREFIX
 export CFG_LIBDIR
 export CFG_LIBDIR_RELATIVE
 export CFG_DISABLE_INJECT_STD_VERSION
+ifdef CFG_DISABLE_UNSTABLE_FEATURES
+CFG_INFO := $(info cfg: disabling unstable features (CFG_DISABLE_UNSTABLE_FEATURES))
+# Turn on feature-staging
+export CFG_DISABLE_UNSTABLE_FEATURES
+endif
+# Subvert unstable feature lints to do the self-build
+export CFG_BOOTSTRAP_KEY
+export RUSTC_BOOTSTRAP_KEY:=$(CFG_BOOTSTRAP_KEY)
 
 ######################################################################
 # Per-stage targets and runner

src/liballoc/lib.rs

@@ -58,6 +58,7 @@
 
 #![crate_name = "alloc"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",

src/libarena/lib.rs

@@ -21,6 +21,7 @@
 
 #![crate_name = "arena"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",

src/libcollections/lib.rs

@@ -15,6 +15,7 @@
 
 #![crate_name = "collections"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",

src/libcore/lib.rs

@@ -49,6 +49,7 @@
 
 #![crate_name = "core"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",

src/libflate/lib.rs

@@ -16,6 +16,7 @@
 
 #![crate_name = "flate"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",

src/libfmt_macros/lib.rs

@@ -16,6 +16,7 @@
 
 #![crate_name = "fmt_macros"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",

src/libgetopts/lib.rs

@@ -79,6 +79,7 @@
 
 #![crate_name = "getopts"]
 #![experimental = "use the crates.io `getopts` library instead"]
+#![staged_api]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",

src/libgraphviz/lib.rs

@@ -266,6 +266,7 @@
 
 #![crate_name = "graphviz"]
 #![experimental]
+#![staged_api]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",