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

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Stabilize `derive_default_enum`

This stabilizes `#![feature(derive_default_enum)]`, as proposed in [RFC 3107](https://github.com/rust-lang/rfcs/pull/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
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Dylan DPC 2022-04-15 20:50:43 +02:00 • committed by GitHub
commit 27e2d811e6
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16 changed files with 55 additions and 73 deletions
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13
compiler/rustc_builtin_macros/src/deriving/default.rs
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@ -46,18 +46,7 @@ pub fn expand_deriving_default(
StaticStruct(_, fields) => {
default_struct_substructure(cx, trait_span, substr, fields)
}
StaticEnum(enum_def, _) => {
if !cx.sess.features_untracked().derive_default_enum {
rustc_session::parse::feature_err(
cx.parse_sess(),
sym::derive_default_enum,
span,
"deriving `Default` on enums is experimental",
)
.emit();
}
default_enum_substructure(cx, trait_span, enum_def)
}
StaticEnum(enum_def, _) => default_enum_substructure(cx, trait_span, enum_def),
_ => cx.span_bug(trait_span, "method in `derive(Default)`"),
}
})),