Add a simple extension trait derive

compiler/rustc_macros/src/lib.rs

@@ -14,6 +14,7 @@ use proc_macro::TokenStream;
 
 mod current_version;
 mod diagnostics;
+mod extension;
 mod hash_stable;
 mod lift;
 mod query;
@@ -40,6 +41,11 @@ pub fn symbols(input: TokenStream) -> TokenStream {
     symbols::symbols(input.into()).into()
 }
 
+#[proc_macro_attribute]
+pub fn extension(_attr: TokenStream, input: TokenStream) -> TokenStream {
+    extension::extension(input)
+}
+
 decl_derive!([HashStable, attributes(stable_hasher)] => hash_stable::hash_stable_derive);
 decl_derive!(
     [HashStable_Generic, attributes(stable_hasher)] =>