Create new error code E0734 for stability attributes used outside of standard library

Author: GuillaumeGomez

src/librustc/error_codes.rs

@@ -2247,4 +2247,5 @@ static X: u32 = 42;
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
     E0727, // `async` generators are not yet supported
     E0728, // `await` must be in an `async` function or block
+    E0734, // stability attributes may not be used outside of the standard library
 }

src/librustc/middle/stability.rs

@@ -199,8 +199,12 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
                 let name = attr.name_or_empty();
                 if [sym::unstable, sym::stable, sym::rustc_deprecated].contains(&name) {
                     attr::mark_used(attr);
-                    self.tcx.sess.span_err(attr.span, "stability attributes may not be used \
-                                                        outside of the standard library");
+                    struct_span_err!(
+                        self.tcx.sess,
+                        attr.span,
+                        E0734,
+                        "stability attributes may not be used outside of the standard library",
+                    ).emit();
                 }
             }