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'.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/librustdoc/test.rs b/src/librustdoc/test.rs
index bbe35eb0e9c..8e0f4b2d443 100644
--- a/src/librustdoc/test.rs
+++ b/src/librustdoc/test.rs
@@ -22,6 +22,7 @@ use std::collections::{HashSet, HashMap};
 use testing;
 use rustc::session::{self, config};
 use rustc::session::search_paths::{SearchPaths, PathKind};
+use rustc_driver::get_unstable_features_setting;
 use rustc_driver::driver;
 use syntax::ast;
 use syntax::codemap::{CodeMap, dummy_spanned};
@@ -52,6 +53,7 @@ pub fn run(input: &str,
         search_paths: libs.clone(),
         crate_types: vec!(config::CrateTypeDylib),
         externs: externs.clone(),
+        unstable_features: get_unstable_features_setting(),
         ..config::basic_options().clone()
     };
 
@@ -128,6 +130,7 @@ fn runtest(test: &str, cratename: &str, libs: SearchPaths,
             .. config::basic_codegen_options()
         },
         test: as_test_harness,
+        unstable_features: get_unstable_features_setting(),
         ..config::basic_options().clone()
     };