Add error code for unstable feature errors

Author: GuillaumeGomez

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)*) => ({