extended information for E0554 feature attributes only work on nightlies

It's more pleasing to use the inner-attribute syntax (`#!` rather than
`#`) in the error message, as that is how `feature` attributes in
particular will be declared (as they apply to the entire crate).

src/libsyntax/diagnostic_list.rs

@@ -162,6 +162,21 @@ pub fn main() {}
 https://doc.rust-lang.org/reference.html#conditional-compilation
 "##,
 
+E0554: r##"
+Feature attributes are only allowed on the nightly release channel. Stable or
+beta compilers will not comply.
+
+Example of erroneous code (on a stable compiler):
+
+```ignore (depends on release channel)
+#![feature(non_ascii_idents)] // error: #![feature] may not be used on the
+                              //        stable release channel
+```
+
+If you need the feature, make sure to use a nightly release of the compiler
+(but be warned that the feature may be removed or altered in the future).
+"##,
+
 E0558: r##"
 The `export_name` attribute was malformed.
 
@@ -301,7 +316,6 @@ fn main() {
     E0550, // multiple deprecated attributes
     E0551, // incorrect meta item
     E0552, // unrecognized representation hint
-    E0554, // #[feature] may not be used on the [] release channel
     E0555, // malformed feature attribute, expected #![feature(...)]
     E0556, // malformed feature, expected just one word
     E0557, // feature has been removed

src/libsyntax/feature_gate.rs

@@ -1602,7 +1602,7 @@ fn maybe_stage_features(span_handler: &Handler, krate: &ast::Crate,
             if attr.check_name("feature") {
                 let release_channel = option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)");
                 span_err!(span_handler, attr.span, E0554,
-                          "#[feature] may not be used on the {} release channel",
+                          "#![feature] may not be used on the {} release channel",
                           release_channel);
             }
         }