Turn the duplicate feature lint into an error

Author: varkor

src/librustc/diagnostics.rs

@@ -1918,6 +1918,19 @@ fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
 ```
 "##,
 
+E0636: r##"
+A `#![feature]` attribute was declared multiple times.
+
+Erroneous code example:
+
+```compile_fail,E0636
+#![allow(stable_features)]
+#![feature(rust1)]
+#![feature(rust1)] // error: the feature `rust1` has already been declared
+```
+
+"##,
+
 E0644: r##"
 A closure or generator was constructed that references its own type.
 

src/librustc/lint/builtin.rs

@@ -111,12 +111,6 @@ declare_lint! {
     "unknown features found in crate-level #[feature] directives"
 }
 
-declare_lint! {
-    pub DUPLICATE_FEATURES,
-    Deny,
-    "duplicate features found in crate-level #[feature] directives"
-}
-
 declare_lint! {
     pub STABLE_FEATURES,
     Warn,
@@ -375,7 +369,6 @@ impl LintPass for HardwiredLints {
             WARNINGS,
             UNUSED_FEATURES,
             UNKNOWN_FEATURES,
-            DUPLICATE_FEATURES,
             STABLE_FEATURES,
             UNKNOWN_CRATE_TYPES,
             TRIVIAL_CASTS,

src/librustc/middle/stability.rs

@@ -18,7 +18,7 @@ use hir::def::Def;
 use hir::def_id::{CrateNum, CRATE_DEF_INDEX, DefId, LOCAL_CRATE};
 use ty::{self, TyCtxt};
 use middle::privacy::AccessLevels;
-use session::DiagnosticMessageId;
+use session::{DiagnosticMessageId, Session};
 use syntax::symbol::Symbol;
 use syntax_pos::{Span, MultiSpan};
 use syntax::ast;
@@ -816,33 +816,24 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
 
     let declared_lang_features = &tcx.features().declared_lang_features;
     let mut lang_features = FxHashSet();
-    for &(ref feature, span, since) in declared_lang_features {
+    for &(feature, span, since) in declared_lang_features {
         if let Some(since) = since {
             // Warn if the user has enabled an already-stable lang feature.
-            tcx.lint_node(lint::builtin::STABLE_FEATURES,
-                        ast::CRATE_NODE_ID,
-                        span,
-                        &format_stable_since_msg(*feature, since));
+            unnecessary_stable_feature_lint(tcx, span, feature, since);
         }
         if lang_features.contains(&feature) {
             // Warn if the user enables a lang feature multiple times.
-            tcx.lint_node(lint::builtin::DUPLICATE_FEATURES,
-                          ast::CRATE_NODE_ID,
-                          span,
-                          &format!("duplicate `{}` feature attribute", feature));
+            duplicate_feature_err(tcx.sess, span, feature);
         }
         lang_features.insert(feature);
     }
 
     let declared_lib_features = &tcx.features().declared_lib_features;
     let mut remaining_lib_features = FxHashMap();
     for (feature, span) in declared_lib_features {
-        // Warn if the user enables a lib feature multiple times.
         if remaining_lib_features.contains_key(&feature) {
-            tcx.lint_node(lint::builtin::DUPLICATE_FEATURES,
-                          ast::CRATE_NODE_ID,
-                          *span,
-                          &format!("duplicate `{}` feature attribute", feature));
+            // Warn if the user enables a lib feature multiple times.
+            duplicate_feature_err(tcx.sess, *span, *feature);
         }
         remaining_lib_features.insert(feature, span.clone());
     }
@@ -851,16 +842,12 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     remaining_lib_features.remove(&Symbol::intern("libc"));
 
     for (feature, stable) in tcx.lib_features().iter() {
-        // Warn if the user has enabled an already-stable lib feature.
         if let Some(since) = stable {
             if let Some(span) = remaining_lib_features.get(&feature) {
-                tcx.lint_node(lint::builtin::STABLE_FEATURES,
-                              ast::CRATE_NODE_ID,
-                              *span,
-                              &format_stable_since_msg(feature, since));
+                // Warn if the user has enabled an already-stable lib feature.
+                unnecessary_stable_feature_lint(tcx, *span, feature, since);
             }
         }
-
         remaining_lib_features.remove(&feature);
     }
 
@@ -875,8 +862,20 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     // don't lint about unused features. We should reenable this one day!
 }
 
-fn format_stable_since_msg(feature: Symbol, since: Symbol) -> String {
-    // "this feature has been stable since {}. Attribute no longer needed"
-    format!("the feature `{}` has been stable since {} and no longer requires \
-             an attribute to enable", feature, since)
+fn unnecessary_stable_feature_lint<'a, 'tcx>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    span: Span,
+    feature: Symbol,
+    since: Symbol
+) {
+    tcx.lint_node(lint::builtin::STABLE_FEATURES,
+        ast::CRATE_NODE_ID,
+        span,
+        &format!("the feature `{}` has been stable since {} and no longer requires \
+                  an attribute to enable", feature, since));
+}
+
+fn duplicate_feature_err(sess: &Session, span: Span, feature: Symbol) {
+    struct_span_err!(sess, span, E0636, "the feature `{}` has already been declared", feature)
+        .emit();
 }

src/librustc_lint/lib.rs

@@ -189,7 +189,6 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_EXTERN_CRATES,
                     UNUSED_FEATURES,
                     UNKNOWN_FEATURES,
-                    DUPLICATE_FEATURES,
                     UNUSED_LABELS,
                     UNUSED_PARENS);
 

src/test/ui/feature-gate/duplicate-features.rs

@@ -9,12 +9,11 @@
 // except according to those terms.
 
 #![allow(stable_features)]
-#![deny(duplicate_features)]
 
 #![feature(rust1)]
-#![feature(rust1)] //~ ERROR duplicate `rust1` feature attribute
+#![feature(rust1)] //~ ERROR the feature `rust1` has already been declared
 
 #![feature(if_let)]
-#![feature(if_let)] //~ ERROR duplicate `if_let` feature attribute
+#![feature(if_let)] //~ ERROR the feature `if_let` has already been declared
 
 fn main() {}

src/test/ui/feature-gate/duplicate-features.stderr

@@ -1,20 +1,15 @@
-error: duplicate `if_let` feature attribute
-  --> $DIR/duplicate-features.rs:18:12
+error[E0636]: the feature `if_let` has already been declared
+  --> $DIR/duplicate-features.rs:17:12
    |
-LL | #![feature(if_let)] //~ ERROR duplicate `if_let` feature attribute
+LL | #![feature(if_let)] //~ ERROR the feature `if_let` has already been declared
    |            ^^^^^^
-   |
-note: lint level defined here
-  --> $DIR/duplicate-features.rs:12:9
-   |
-LL | #![deny(duplicate_features)]
-   |         ^^^^^^^^^^^^^^^^^^
 
-error: duplicate `rust1` feature attribute
-  --> $DIR/duplicate-features.rs:15:12
+error[E0636]: the feature `rust1` has already been declared
+  --> $DIR/duplicate-features.rs:14:12
    |
-LL | #![feature(rust1)] //~ ERROR duplicate `rust1` feature attribute
+LL | #![feature(rust1)] //~ ERROR the feature `rust1` has already been declared
    |            ^^^^^
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0636`.