Rollup merge of #94457 - jhpratt:stabilize-derive_default_enum, r=davidtwco

Stabilize `derive_default_enum`

This stabilizes `#![feature(derive_default_enum)]`, as proposed in [RFC 3107](rust-lang/rfcs#3107) and tracked in #87517. In short, it permits you to `#[derive(Default)]` on `enum`s, indicating what the default should be by placing a `#[default]` attribute on the desired variant (which must be a unit variant in the interest of forward compatibility).

```````@rustbot``````` label +S-waiting-on-review +T-lang

Author: Dylan-DPC

core/src/default.rs

@@ -52,6 +52,23 @@
 /// This trait can be used with `#[derive]` if all of the type's fields implement
 /// `Default`. When `derive`d, it will use the default value for each field's type.
 ///
+/// ### `enum`s
+///
+/// When using `#[derive(Default)]` on an `enum`, you need to choose which unit variant will be
+/// default. You do this by placing the `#[default]` attribute on the variant.
+///
+/// ```
+/// #[derive(Default)]
+/// enum Kind {
+///     #[default]
+///     A,
+///     B,
+///     C,
+/// }
+/// ```
+///
+/// You cannot use the `#[default]` attribute on non-unit or non-exhaustive variants.
+///
 /// ## How can I implement `Default`?
 ///
 /// Provide an implementation for the `default()` method that returns the value of

core/src/lib.rs

@@ -167,7 +167,7 @@
 #![feature(const_precise_live_drops)]
 #![feature(const_refs_to_cell)]
 #![feature(decl_macro)]
-#![feature(derive_default_enum)]
+#![cfg_attr(bootstrap, feature(derive_default_enum))]
 #![feature(deprecated_suggestion)]
 #![feature(doc_cfg)]
 #![feature(doc_notable_trait)]