coverage: Forbid multiple #[coverage(..)] attributes

It might make sense to allow this in the future, if we add values that aren't
mutually exclusive, but for now having multiple coverage attributes on one item
is useless.

Author: Zalathar

compiler/rustc_feature/src/builtin_attrs.rs

@@ -479,7 +479,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     gated!(
         coverage, Normal, template!(Word, List: "on|off"),
-        WarnFollowing, EncodeCrossCrate::No,
+        ErrorPreceding, EncodeCrossCrate::No,
         coverage_attribute, experimental!(coverage)
     ),
 

tests/ui/coverage-attr/bad-syntax.rs

@@ -3,15 +3,15 @@
 // Tests the error messages produced (or not produced) by various unusual
 // uses of the `#[coverage(..)]` attribute.
 
-// FIXME(#126658): Multiple coverage attributes with the same value are useless,
-// and should probably produce a diagnostic.
 #[coverage(off)]
+//~^ ERROR multiple `coverage` attributes
+//~| HELP remove this attribute
 #[coverage(off)]
 fn multiple_consistent() {}
 
-// FIXME(#126658): When there are multiple inconsistent coverage attributes,
-// it's unclear which one will prevail.
 #[coverage(off)]
+//~^ ERROR multiple `coverage` attributes
+//~| HELP remove this attribute
 #[coverage(on)]
 fn multiple_inconsistent() {}
 

tests/ui/coverage-attr/bad-syntax.stderr

@@ -20,6 +20,30 @@ LL | #[coverage(,off)]
    |            expected identifier
    |            help: remove this comma
 
+error: multiple `coverage` attributes
+  --> $DIR/bad-syntax.rs:6:1
+   |
+LL | #[coverage(off)]
+   | ^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+note: attribute also specified here
+  --> $DIR/bad-syntax.rs:9:1
+   |
+LL | #[coverage(off)]
+   | ^^^^^^^^^^^^^^^^
+
+error: multiple `coverage` attributes
+  --> $DIR/bad-syntax.rs:12:1
+   |
+LL | #[coverage(off)]
+   | ^^^^^^^^^^^^^^^^ help: remove this attribute
+   |
+note: attribute also specified here
+  --> $DIR/bad-syntax.rs:15:1
+   |
+LL | #[coverage(on)]
+   | ^^^^^^^^^^^^^^^
+
 error: expected `coverage(off)` or `coverage(on)`
   --> $DIR/bad-syntax.rs:18:1
    |
@@ -74,5 +98,5 @@ error: expected `coverage(off)` or `coverage(on)`
 LL | #[coverage(,off)]
    | ^^^^^^^^^^^^^^^^^
 
-error: aborting due to 11 previous errors
+error: aborting due to 13 previous errors