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 rust-lang#20661.

Closes rust-lang#16678

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

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",

src/liblibc/lib.rs

@@ -11,6 +11,7 @@
 #![crate_name = "libc"]
 #![crate_type = "rlib"]
 #![cfg_attr(not(feature = "cargo-build"), experimental)]
+#![cfg_attr(not(feature = "cargo-build"), staged_api)]
 #![no_std]
 #![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/liblog/lib.rs

@@ -157,6 +157,7 @@
 
 #![crate_name = "log"]
 #![experimental = "use the crates.io `log` 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/librand/lib.rs

@@ -25,6 +25,7 @@
 
 #![no_std]
 #![experimental]
+#![staged_api]
 
 #[macro_use]
 extern crate core;

src/librbml/lib.rs

@@ -17,6 +17,7 @@
 
 #![crate_name = "rbml"]
 #![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/libregex/lib.rs

@@ -17,6 +17,7 @@
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![experimental = "use the crates.io `regex` library instead"]
+#![staged_api]
 #![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",
        html_root_url = "http://doc.rust-lang.org/nightly/",

src/librustc/lib.rs

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

src/librustc/lint/builtin.rs

@@ -34,7 +34,7 @@ use middle::{def, pat_util, stability};
 use middle::const_eval::{eval_const_expr_partial, const_int, const_uint};
 use util::ppaux::{ty_to_string};
 use util::nodemap::{FnvHashMap, NodeSet};
-use lint::{Context, LintPass, LintArray};
+use lint::{Context, LintPass, LintArray, Lint};
 
 use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::num::SignedInt;
@@ -1643,19 +1643,27 @@ declare_lint! {
     "detects use of #[unstable] items (incl. items with no stability attribute)"
 }
 
+declare_lint!(STAGED_EXPERIMENTAL, Warn,
+              "detects use of #[experimental] items in staged builds");
+
+declare_lint!(STAGED_UNSTABLE, Warn,
+              "detects use of #[unstable] items (incl. items with no stability attribute) \
+               in staged builds");
+
 /// Checks for use of items with `#[deprecated]`, `#[experimental]` and
 /// `#[unstable]` attributes, or no stability attribute.
 #[derive(Copy)]
 pub struct Stability;
 
 impl Stability {
     fn lint(&self, cx: &Context, id: ast::DefId, span: Span) {
-        let stability = stability::lookup(cx.tcx, id);
+
+        let ref stability = stability::lookup(cx.tcx, id);
         let cross_crate = !ast_util::is_local(id);
 
         // stability attributes are promises made across crates; only
         // check DEPRECATED for crate-local usage.
-        let (lint, label) = match stability {
+        let (lint, label) = match *stability {
             // no stability attributes == Unstable
             None if cross_crate => (UNSTABLE, "unmarked"),
             Some(attr::Stability { level: attr::Unstable, .. }) if cross_crate =>
@@ -1667,24 +1675,53 @@ impl Stability {
             _ => return
         };
 
-        let msg = match stability {
-            Some(attr::Stability { text: Some(ref s), .. }) => {
-                format!("use of {} item: {}", label, *s)
+        output(cx, span, stability, lint, label);
+        if cross_crate && stability::is_staged_api(cx.tcx, id) {
+            if lint.name == UNSTABLE.name {
+                output(cx, span, stability, STAGED_UNSTABLE, label);
+            } else if lint.name == EXPERIMENTAL.name {
+                output(cx, span, stability, STAGED_EXPERIMENTAL, label);
             }
-            _ => format!("use of {} item", label)
-        };
+        }
 
-        cx.span_lint(lint, span, msg.index(&FullRange));
+        fn output(cx: &Context, span: Span, stability: &Option<attr::Stability>,
+                  lint: &'static Lint, label: &'static str) {
+            let msg = match *stability {
+                Some(attr::Stability { text: Some(ref s), .. }) => {
+                    format!("use of {} item: {}", label, *s)
+                }
+                _ => format!("use of {} item", label)
+            };
+
+            cx.span_lint(lint, span, msg.index(&FullRange));
+        }
     }
 
+
     fn is_internal(&self, cx: &Context, span: Span) -> bool {
         cx.tcx.sess.codemap().span_is_internal(span)
     }
+
 }
 
 impl LintPass for Stability {
     fn get_lints(&self) -> LintArray {
-        lint_array!(DEPRECATED, EXPERIMENTAL, UNSTABLE)
+        lint_array!(DEPRECATED, EXPERIMENTAL, UNSTABLE, STAGED_EXPERIMENTAL, STAGED_UNSTABLE)
+    }
+
+    fn check_crate(&mut self, _: &Context, c: &ast::Crate) {
+        // Just mark the #[staged_api] attribute used, though nothing else is done
+        // with it during this pass over the source.
+        for attr in c.attrs.iter() {
+            if attr.name().get() == "staged_api" {
+                match attr.node.value.node {
+                    ast::MetaWord(_) => {
+                        attr::mark_used(attr);
+                    }
+                    _ => (/*pass*/)
+                }
+            }
+        }
     }
 
     fn check_view_item(&mut self, cx: &Context, item: &ast::ViewItem) {
@@ -1746,6 +1783,7 @@ impl LintPass for Stability {
             }
             _ => return
         };
+
         self.lint(cx, id, span);
     }
 
@@ -1878,3 +1916,22 @@ impl LintPass for HardwiredLints {
         )
     }
 }
+
+/// Forbids using the `#[feature(...)]` attribute
+#[deriving(Copy)]
+pub struct UnstableFeatures;
+
+declare_lint!(UNSTABLE_FEATURES, Allow,
+              "enabling unstable features");
+
+impl LintPass for UnstableFeatures {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(UNSTABLE_FEATURES)
+    }
+    fn check_attribute(&mut self, ctx: &Context, attr: &ast::Attribute) {
+        use syntax::attr;
+        if attr::contains_name(&[attr.node.value.clone()], "feature") {
+            ctx.span_lint(UNSTABLE_FEATURES, attr.span, "unstable feature");
+        }
+    }
+}

src/librustc/lint/context.rs

@@ -28,8 +28,9 @@ use self::TargetLint::*;
 use middle::privacy::ExportedItems;
 use middle::ty::{self, Ty};
 use session::{early_error, Session};
+use session::config::UnstableFeatures;
 use lint::{Level, LevelSource, Lint, LintId, LintArray, LintPass, LintPassObject};
-use lint::{Default, CommandLine, Node, Allow, Warn, Deny, Forbid};
+use lint::{Default, CommandLine, Node, Allow, Warn, Deny, Forbid, ReleaseChannel};
 use lint::builtin;
 use util::nodemap::FnvHashMap;
 
@@ -210,6 +211,7 @@ impl LintStore {
                      UnusedAllocation,
                      Stability,
                      MissingCopyImplementations,
+                     UnstableFeatures,
         );
 
         add_builtin_with_new!(sess,
@@ -298,6 +300,29 @@ impl LintStore {
             }
         }
     }
+
+    fn maybe_stage_features(&mut self, sess: &Session) {
+        let lvl = match sess.opts.unstable_features {
+            UnstableFeatures::Default => return,
+            UnstableFeatures::Disallow => Warn,
+            UnstableFeatures::Cheat => Allow
+        };
+        match self.by_name.get("unstable_features") {
+            Some(&Id(lint_id)) => self.set_level(lint_id, (lvl, ReleaseChannel)),
+            Some(&Renamed(_, lint_id)) => self.set_level(lint_id, (lvl, ReleaseChannel)),
+            None => unreachable!()
+        }
+        match self.by_name.get("staged_unstable") {
+            Some(&Id(lint_id)) => self.set_level(lint_id, (lvl, ReleaseChannel)),
+            Some(&Renamed(_, lint_id)) => self.set_level(lint_id, (lvl, ReleaseChannel)),
+            None => unreachable!()
+        }
+        match self.by_name.get("staged_experimental") {
+            Some(&Id(lint_id)) => self.set_level(lint_id, (lvl, ReleaseChannel)),
+            Some(&Renamed(_, lint_id)) => self.set_level(lint_id, (lvl, ReleaseChannel)),
+            None => unreachable!()
+        }
+    }
 }
 
 /// Context for lint checking.
@@ -380,6 +405,7 @@ pub fn raw_emit_lint(sess: &Session, lint: &'static Lint,
     if level == Allow { return }
 
     let name = lint.name_lower();
+    let mut def = None;
     let mut note = None;
     let msg = match source {
         Default => {
@@ -394,7 +420,13 @@ pub fn raw_emit_lint(sess: &Session, lint: &'static Lint,
                     }, name.replace("_", "-"))
         },
         Node(src) => {
-            note = Some(src);
+            def = Some(src);
+            msg.to_string()
+        }
+        ReleaseChannel => {
+            let release_channel = option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)");
+            note = Some(format!("this feature may not be used in the {} release channel",
+                                release_channel));
             msg.to_string()
         }
     };
@@ -410,7 +442,11 @@ pub fn raw_emit_lint(sess: &Session, lint: &'static Lint,
         _ => sess.bug("impossible level in raw_emit_lint"),
     }
 
-    for span in note.into_iter() {
+    for note in note.into_iter() {
+        sess.note(note.index(&FullRange));
+    }
+
+    for span in def.into_iter() {
         sess.span_note(span, "lint level defined here");
     }
 }
@@ -767,6 +803,10 @@ impl LintPass for GatherNodeLevels {
 /// Consumes the `lint_store` field of the `Session`.
 pub fn check_crate(tcx: &ty::ctxt,
                    exported_items: &ExportedItems) {
+
+    // If this is a feature-staged build of rustc then flip several lints to 'forbid'
+    tcx.sess.lint_store.borrow_mut().maybe_stage_features(&tcx.sess);
+
     let krate = tcx.map.krate();
     let mut cx = Context::new(tcx, krate, exported_items);