Tweak wording of "missing stability attribute" error

Author: camelid

compiler/rustc_passes/src/stability.rs

@@ -587,7 +587,7 @@ impl<'tcx> MissingStabilityAnnotations<'tcx> {
         if is_error {
             let def_id = self.tcx.hir().local_def_id(hir_id);
             let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
-            self.tcx.sess.span_err(span, &format!("{} has missing stability attribute", descr));
+            self.tcx.sess.span_err(span, &format!("{} is missing a stability attribute", descr));
         }
     }
 

src/test/ui/missing/missing-stability.rs

@@ -6,7 +6,7 @@
 #![stable(feature = "stable_test_feature", since = "1.0.0")]
 
 pub fn unmarked() {
-    //~^ ERROR function has missing stability attribute
+    //~^ ERROR function is missing a stability attribute
     ()
 }
 
@@ -20,5 +20,5 @@ pub mod foo {
 pub mod bar {
     // #[stable] is not inherited
     pub fn unmarked() {}
-    //~^ ERROR function has missing stability attribute
+    //~^ ERROR function is missing a stability attribute
 }

src/test/ui/missing/missing-stability.stderr

@@ -1,4 +1,4 @@
-error: function has missing stability attribute
+error: function is missing a stability attribute
   --> $DIR/missing-stability.rs:8:1
    |
 LL | / pub fn unmarked() {
@@ -7,7 +7,7 @@ LL | |     ()
 LL | | }
    | |_^
 
-error: function has missing stability attribute
+error: function is missing a stability attribute
   --> $DIR/missing-stability.rs:22:5
    |
 LL |     pub fn unmarked() {}

src/test/ui/stability-attribute/missing-stability-attr-at-top-level.rs

@@ -1,4 +1,4 @@
 #![feature(staged_api)]
-//~^ ERROR module has missing stability attribute
+//~^ ERROR module is missing a stability attribute
 
 fn main() {}

src/test/ui/stability-attribute/missing-stability-attr-at-top-level.stderr

@@ -1,4 +1,4 @@
-error: module has missing stability attribute
+error: module is missing a stability attribute
   --> $DIR/missing-stability-attr-at-top-level.rs:1:1
    |
 LL | / #![feature(staged_api)]

src/test/ui/stability-attribute/stability-attribute-sanity-3.rs

@@ -5,7 +5,7 @@
 #![stable(feature = "stable_test_feature", since = "1.0.0")]
 
 #[macro_export]
-macro_rules! mac { //~ ERROR macro has missing stability attribute
+macro_rules! mac { //~ ERROR macro is missing a stability attribute
     () => ()
 }
 

src/test/ui/stability-attribute/stability-attribute-sanity-3.stderr

@@ -1,4 +1,4 @@
-error: macro has missing stability attribute
+error: macro is missing a stability attribute
   --> $DIR/stability-attribute-sanity-3.rs:8:1
    |
 LL | / macro_rules! mac {