Make a better error message for using #[feature] on stable rust

It now says '#[feature] may not be used on the stable release channel'.

I had to convert this error from a lint to a normal compiler error.

I left the lint previously-used for this in place since removing it is
a breaking change. It will just go unused until the end of time.

Fixes #24125

src/librustc/lint/context.rs

@@ -28,9 +28,8 @@ 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, ReleaseChannel};
+use lint::{Default, CommandLine, Node, Allow, Warn, Deny, Forbid};
 use lint::builtin;
 use util::nodemap::FnvHashMap;
 
@@ -208,23 +207,6 @@ impl LintStore {
             }
         }
     }
-
-    fn maybe_stage_features(&mut self, sess: &Session) {
-        let lvl = match sess.opts.unstable_features {
-            UnstableFeatures::Default => return,
-            UnstableFeatures::Disallow => Forbid,
-            UnstableFeatures::Cheat => Allow
-        };
-        match self.by_name.get("unstable_features") {
-            Some(&Id(lint_id)) => if self.get_level_source(lint_id).0 != Forbid {
-                self.set_level(lint_id, (lvl, ReleaseChannel))
-            },
-            Some(&Renamed(_, lint_id)) => if self.get_level_source(lint_id).0 != Forbid {
-                self.set_level(lint_id, (lvl, ReleaseChannel))
-            },
-            None => unreachable!()
-        }
-    }
 }
 
 /// Context for lint checking.
@@ -308,7 +290,6 @@ pub fn raw_emit_lint(sess: &Session, lint: &'static Lint,
 
     let name = lint.name_lower();
     let mut def = None;
-    let mut note = None;
     let msg = match source {
         Default => {
             format!("{}, #[{}({})] on by default", msg,
@@ -325,12 +306,6 @@ pub fn raw_emit_lint(sess: &Session, lint: &'static Lint,
             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()
-        }
     };
 
     // For purposes of printing, we can treat forbid as deny.
@@ -344,10 +319,6 @@ pub fn raw_emit_lint(sess: &Session, lint: &'static Lint,
         _ => sess.bug("impossible level in raw_emit_lint"),
     }
 
-    if let Some(note) = note {
-        sess.note(&note[..]);
-    }
-
     if let Some(span) = def {
         sess.span_note(span, "lint level defined here");
     }
@@ -689,9 +660,6 @@ impl LintPass for GatherNodeLevels {
 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);
 

src/librustc/lint/mod.rs

@@ -247,9 +247,6 @@ pub enum LintSource {
 
     /// Lint level was set by a command-line flag.
     CommandLine,
-
-    /// Lint level was set by the release channel.
-    ReleaseChannel
 }
 
 pub type LevelSource = (Level, LintSource);

src/librustc/session/config.rs

@@ -32,6 +32,7 @@ use syntax::attr::AttrMetaMethods;
 use syntax::diagnostic::{ColorConfig, Auto, Always, Never, SpanHandler};
 use syntax::parse;
 use syntax::parse::token::InternedString;
+use syntax::feature_gate::UnstableFeatures;
 
 use getopts;
 use std::collections::HashMap;
@@ -119,21 +120,6 @@ pub struct Options {
     pub unstable_features: UnstableFeatures
 }
 
-#[derive(Clone, Copy)]
-pub enum UnstableFeatures {
-    /// Hard errors for unstable features are active, as on
-    /// beta/stable channels.
-    Disallow,
-    /// Use the default lint levels
-    Default,
-    /// Errors are bypassed for bootstrapping. This is required any time
-    /// during the build that feature-related lints are set to warn or above
-    /// because the build turns on warnings-as-errors and uses lots of unstable
-    /// features. As a result, this this is always required for building Rust
-    /// itself.
-    Cheat
-}
-
 #[derive(Clone, PartialEq, Eq)]
 pub enum PrintRequest {
     FileNames,
@@ -1074,7 +1060,7 @@ pub fn get_unstable_features_setting() -> UnstableFeatures {
     match (disable_unstable_features, bootstrap_secret_key, bootstrap_provided_key) {
         (_, Some(ref s), Some(ref p)) if s == p => UnstableFeatures::Cheat,
         (true, _, _) => UnstableFeatures::Disallow,
-        (false, _, _) => UnstableFeatures::Default
+        (false, _, _) => UnstableFeatures::Allow
     }
 }
 

src/librustc_driver/driver.rs

@@ -513,7 +513,8 @@ pub fn phase_2_configure_and_expand(sess: &Session,
         let features =
             syntax::feature_gate::check_crate(sess.codemap(),
                                               &sess.parse_sess.span_diagnostic,
-                                              &krate, &attributes);
+                                              &krate, &attributes,
+                                              sess.opts.unstable_features);
         *sess.features.borrow_mut() = features;
         sess.abort_if_errors();
     });
@@ -543,7 +544,8 @@ pub fn phase_2_configure_and_expand(sess: &Session,
         let features =
             syntax::feature_gate::check_crate(sess.codemap(),
                                               &sess.parse_sess.span_diagnostic,
-                                              &krate, &attributes);
+                                              &krate, &attributes,
+                                              sess.opts.unstable_features);
         *sess.features.borrow_mut() = features;
         sess.abort_if_errors();
     });

src/librustc_driver/test.rs

@@ -36,6 +36,7 @@ use syntax::codemap;
 use syntax::codemap::{Span, CodeMap, DUMMY_SP};
 use syntax::diagnostic::{Level, RenderSpan, Bug, Fatal, Error, Warning, Note, Help};
 use syntax::parse::token;
+use syntax::feature_gate::UnstableFeatures;
 
 struct Env<'a, 'tcx: 'a> {
     infcx: &'a infer::InferCtxt<'a, 'tcx>,
@@ -103,6 +104,7 @@ fn test_env<F>(source_string: &str,
     let mut options =
         config::basic_options();
     options.debugging_opts.verbose = true;
+    options.unstable_features = UnstableFeatures::Allow;
     let codemap =
         CodeMap::new();
     let diagnostic_handler =

src/librustc_lint/builtin.rs

@@ -2215,7 +2215,7 @@ pub struct UnstableFeatures;
 declare_lint! {
     UNSTABLE_FEATURES,
     Allow,
-    "enabling unstable features"
+    "enabling unstable features (deprecated. do not use)"
 }
 
 impl LintPass for UnstableFeatures {

src/librustdoc/core.rs

@@ -12,7 +12,6 @@ pub use self::MaybeTyped::*;
 use rustc_lint;
 use rustc_driver::driver;
 use rustc::session::{self, config};
-use rustc::session::config::UnstableFeatures;
 use rustc::middle::{privacy, ty};
 use rustc::ast_map;
 use rustc::lint;
@@ -20,6 +19,7 @@ use rustc_trans::back::link;
 use rustc_resolve as resolve;
 
 use syntax::{ast, codemap, diagnostic};
+use syntax::feature_gate::UnstableFeatures;
 
 use std::cell::{RefCell, Cell};
 use std::collections::{HashMap, HashSet};
@@ -106,7 +106,7 @@ pub fn run_core(search_paths: SearchPaths, cfgs: Vec<String>, externs: Externs,
         target_triple: triple.unwrap_or(config::host_triple().to_string()),
         cfg: config::parse_cfgspecs(cfgs),
         // Ensure that rustdoc works even if rustc is feature-staged
-        unstable_features: UnstableFeatures::Default,
+        unstable_features: UnstableFeatures::Allow,
         ..config::basic_options().clone()
     };
 

src/libsyntax/feature_gate.rs

@@ -799,9 +799,46 @@ pub fn check_crate_macros(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast:
 }
 
 pub fn check_crate(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::Crate,
-                   plugin_attributes: &[(String, AttributeType)]) -> Features
+                   plugin_attributes: &[(String, AttributeType)],
+                   unstable: UnstableFeatures) -> Features
 {
+    maybe_stage_features(span_handler, krate, unstable);
+
     check_crate_inner(cm, span_handler, krate, plugin_attributes,
                       |ctx, krate| visit::walk_crate(&mut PostExpansionVisitor { context: ctx },
                                                      krate))
 }
+
+#[derive(Clone, Copy)]
+pub enum UnstableFeatures {
+    /// Hard errors for unstable features are active, as on
+    /// beta/stable channels.
+    Disallow,
+    /// Allow features to me activated, as on nightly.
+    Allow,
+    /// Errors are bypassed for bootstrapping. This is required any time
+    /// during the build that feature-related lints are set to warn or above
+    /// because the build turns on warnings-as-errors and uses lots of unstable
+    /// features. As a result, this this is always required for building Rust
+    /// itself.
+    Cheat
+}
+
+fn maybe_stage_features(span_handler: &SpanHandler, krate: &ast::Crate,
+                        unstable: UnstableFeatures) {
+    let allow_features = match unstable {
+        UnstableFeatures::Allow => true,
+        UnstableFeatures::Disallow => false,
+        UnstableFeatures::Cheat => true
+    };
+    if !allow_features {
+        for attr in &krate.attrs {
+            if attr.check_name("feature") {
+                let release_channel = option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)");
+                let ref msg = format!("#[feature] may not be used on the {} release channel",
+                                      release_channel);
+                span_handler.span_err(attr.span, msg);
+            }
+        }
+    }
+}