Auto merge of rust-lang#86735 - jhpratt:rfc-3107, r=petrochenkov

bors · bors · commit 310eb551f42e · 2021-07-28T05:59:30.000Z
Implement RFC 3107: `#[derive(Default)]` on enums with a `#[default]` attribute

This PR implements RFC 3107, which permits `#[derive(Default)]` on enums where a unit variant has a `#[default]` attribute. See comments for current status.
diff --git a/core/src/default.rs b/core/src/default.rs
@@ -161,7 +161,8 @@ pub fn default<T: Default>() -> T {
 }
 
 /// Derive macro generating an impl of the trait `Default`.
-#[rustc_builtin_macro]
+#[cfg_attr(not(bootstrap), rustc_builtin_macro(Default, attributes(default)))]
+#[cfg_attr(bootstrap, rustc_builtin_macro)]
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow_internal_unstable(core_intrinsics)]
 pub macro Default($item:item) {

Original file line number	Diff line number	Diff line change
`@@ -161,7 +161,8 @@ pub fn default<T: Default>() -> T {`
`161`	`161`	`}`
`162`	`162`
`163`	`163`	/// Derive macro generating an impl of the trait `Default`.
`164`		`-#[rustc_builtin_macro]`
	`164`	`+#[cfg_attr(not(bootstrap), rustc_builtin_macro(Default, attributes(default)))]`
	`165`	`+#[cfg_attr(bootstrap, rustc_builtin_macro)]`
`165`	`166`	`#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]`
`166`	`167`	`#[allow_internal_unstable(core_intrinsics)]`
`167`	`168`	`pub macro Default($item:item) {`