Add error code for unstable feature errors

src/libsyntax/diagnostic_list.rs

@@ -317,6 +317,31 @@ fn main() {
 ```
 "##,
 
+E0658: r##"
+An unstable feature was used.
+
+Erroneous code example:
+
+```compile_fail,E658
+let x = ::std::u128::MAX; // error: use of unstable library feature 'i128'
+```
+
+If you're using a stable or a beta version of rustc, you won't be able to use
+any unstable features. In order to do so, please switch to a nightly version of
+rustc (by using rustup).
+
+If you're using a nightly version of rustc, just add the corresponding feature
+to be able to use it:
+
+```
+#![feature(i128)]
+
+fn main() {
+    let x = ::std::u128::MAX; // ok!
+}
+```
+"##,
+
 }
 
 register_diagnostics! {

src/libsyntax/diagnostics/macros.rs

@@ -105,6 +105,14 @@ macro_rules! struct_span_err {
     })
 }
 
+#[macro_export]
+macro_rules! stringify_error_code {
+    ($code:ident) => ({
+        __diagnostic_used!($code);
+        $crate::errors::DiagnosticId::Error(stringify!($code).to_owned())
+    })
+}
+
 #[macro_export]
 macro_rules! type_error_struct {
     ($session:expr, $span:expr, $typ:expr, $code:ident, $($message:tt)*) => ({

src/libsyntax/feature_gate.rs

@@ -1179,7 +1179,9 @@ fn leveled_feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue
     };
 
     let mut err = match level {
-        GateStrength::Hard => diag.struct_span_err(span, &explanation),
+        GateStrength::Hard => {
+            diag.struct_span_err_with_code(span, &explanation, stringify_error_code!(E0658))
+        }
         GateStrength::Soft => diag.struct_span_warn(span, &explanation),
     };
 

src/test/compile-fail/E0658.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let _ = ::std::u128::MAX; //~ ERROR E0658
+}

src/test/ui/feature-gate-abi-msp430-interrupt.stderr

@@ -1,4 +1,4 @@
-error: msp430-interrupt ABI is experimental and subject to change (see issue #38487)
+error[E0658]: msp430-interrupt ABI is experimental and subject to change (see issue #38487)
   --> $DIR/feature-gate-abi-msp430-interrupt.rs:14:1
    |
 14 | extern "msp430-interrupt" fn foo() {}