rollup merge of #20663: brson/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 do 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

Next steps are to disable the existing out-of-tree behavior for stability attributes, and convert the remaining system to be feature-based per the RFC. During the first beta cycle we will set these lints to 'forbid'.

src/libsyntax/codemap.rs

@@ -590,7 +590,12 @@ impl CodeMap {
                 Some(ref info) => {
                     // save the parent expn_id for next loop iteration
                     expnid = info.call_site.expn_id;
-                    if info.callee.span.is_none() {
+                    if info.callee.name == "format_args" {
+                        // This is a hack because the format_args builtin calls unstable APIs.
+                        // I spent like 6 hours trying to solve this more generally but am stupid.
+                        is_internal = true;
+                        false
+                    } else if info.callee.span.is_none() {
                         // it's a compiler built-in, we *really* don't want to mess with it
                         // so we skip it, unless it was called by a regular macro, in which case
                         // we will handle the caller macro next turn

src/libsyntax/lib.rs

@@ -16,6 +16,7 @@
 
 #![crate_name = "syntax"]
 #![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/libsyntax/std_inject.rs

@@ -48,7 +48,7 @@ fn no_prelude(attrs: &[ast::Attribute]) -> bool {
 }
 
 struct StandardLibraryInjector<'a> {
-    alt_std_name: Option<String>,
+    alt_std_name: Option<String>
 }
 
 impl<'a> fold::Folder for StandardLibraryInjector<'a> {
@@ -84,14 +84,13 @@ impl<'a> fold::Folder for StandardLibraryInjector<'a> {
 
 fn inject_crates_ref(krate: ast::Crate, alt_std_name: Option<String>) -> ast::Crate {
     let mut fold = StandardLibraryInjector {
-        alt_std_name: alt_std_name,
+        alt_std_name: alt_std_name
     };
     fold.fold_crate(krate)
 }
 
 struct PreludeInjector<'a>;
 
-
 impl<'a> fold::Folder for PreludeInjector<'a> {
     fn fold_crate(&mut self, mut krate: ast::Crate) -> ast::Crate {
         // Add #![no_std] here, so we don't re-inject when compiling pretty-printed source.
@@ -104,20 +103,10 @@ impl<'a> fold::Folder for PreludeInjector<'a> {
         attr::mark_used(&no_std_attr);
         krate.attrs.push(no_std_attr);
 
+        // only add `use std::prelude::*;` if there wasn't a
+        // `#![no_implicit_prelude]` at the crate level.
+        // fold_mod() will insert glob path.
         if !no_prelude(krate.attrs.index(&FullRange)) {
-            // only add `use std::prelude::*;` if there wasn't a
-            // `#![no_implicit_prelude]` at the crate level.
-            // fold_mod() will insert glob path.
-            let globs_attr = attr::mk_attr_inner(attr::mk_attr_id(),
-                                                 attr::mk_list_item(
-                InternedString::new("feature"),
-                vec!(
-                    attr::mk_word_item(InternedString::new("globs")),
-                )));
-            // std_inject runs after feature checking so manually mark this attr
-            attr::mark_used(&globs_attr);
-            krate.attrs.push(globs_attr);
-
             krate.module = self.fold_mod(krate.module);
         }
         krate