Make proc_macro Level #[non_exhaustive]

src/libproc_macro/diagnostic.rs

@@ -16,6 +16,7 @@ use syntax_pos::MultiSpan;
 /// An enum representing a diagnostic level.
 #[unstable(feature = "proc_macro_diagnostic", issue = "38356")]
 #[derive(Copy, Clone, Debug)]
+#[non_exhaustive]
 pub enum Level {
     /// An error.
     Error,
@@ -25,8 +26,6 @@ pub enum Level {
     Note,
     /// A help message.
     Help,
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 /// A structure representing a diagnostic message and associated children

src/libproc_macro/lib.rs

@@ -36,6 +36,7 @@
 #![feature(staged_api)]
 #![feature(lang_items)]
 #![feature(optin_builtin_traits)]
+#![feature(non_exhaustive)]
 
 #![recursion_limit="256"]
 

src/libproc_macro/rustc.rs

@@ -278,7 +278,6 @@ impl Level {
             Level::Warning => errors::Level::Warning,
             Level::Note => errors::Level::Note,
             Level::Help => errors::Level::Help,
-            Level::__Nonexhaustive => unreachable!("Level::__Nonexhaustive"),
         }
     }
 }