adjust E0303 error code docs

diff --git a/src/librustc_error_codes/error_codes/E0303.md b/src/librustc_error_codes/error_codes/E0303.md
index 20a6c078f4fa279bee7779da3e323f1068a0d32c..700a66438e060269f0200aea39313a72a977c5cc 100644 (file)
--- a/src/librustc_error_codes/error_codes/E0303.md
+++ b/src/librustc_error_codes/error_codes/E0303.md
@@ -1,10 +1,18 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+Sub-bindings, e.g. `ref x @ Some(ref y)` are now allowed under
+`#![feature(bindings_after_at)]` and checked to make sure that
+memory safety is upheld.
+
+--------------
+
 In certain cases it is possible for sub-bindings to violate memory safety.
 Updates to the borrow checker in a future version of Rust may remove this
 restriction, but for now patterns must be rewritten without sub-bindings.
 
 Before:
 
-```compile_fail,E0303
+```compile_fail
 match Some("hi".to_string()) {
     ref op_string_ref @ Some(s) => {},
     None => {},