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](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

6 files changed, 29 insertions, 53 deletions

diff --git a/src/test/ui/deriving/deriving-default-enum.rs b/src/test/ui/deriving/deriving-default-enum.rs
index 931ff1a5847..d1a81c72c2f 100644
--- a/src/test/ui/deriving/deriving-default-enum.rs
+++ b/src/test/ui/deriving/deriving-default-enum.rs
@@ -1,7 +1,5 @@
 // run-pass
 
-#![feature(derive_default_enum)]
-
 // nb: does not impl Default
 #[derive(Debug, PartialEq)]
 struct NotDefault;
diff --git a/src/test/ui/deriving/deriving-with-helper.rs b/src/test/ui/deriving/deriving-with-helper.rs
index d8f0b27a2e5..1c30b0b6fba 100644
--- a/src/test/ui/deriving/deriving-with-helper.rs
+++ b/src/test/ui/deriving/deriving-with-helper.rs
@@ -5,7 +5,6 @@
 #![feature(lang_items)]
 #![feature(no_core)]
 #![feature(rustc_attrs)]
-#![feature(derive_default_enum)]
 
 #![no_core]
 
diff --git a/src/test/ui/feature-gates/feature-gate-derive_default_enum.rs b/src/test/ui/feature-gates/feature-gate-derive_default_enum.rs
deleted file mode 100644
index 05a5d4e1422..00000000000
--- a/src/test/ui/feature-gates/feature-gate-derive_default_enum.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-#[derive(Default)] //~ ERROR deriving `Default` on enums is experimental
-enum Foo {
-    #[default]
-    Alpha,
-}
-
-fn main() {}
diff --git a/src/test/ui/feature-gates/feature-gate-derive_default_enum.stderr b/src/test/ui/feature-gates/feature-gate-derive_default_enum.stderr
deleted file mode 100644
index 58dd4d508a7..00000000000
--- a/src/test/ui/feature-gates/feature-gate-derive_default_enum.stderr
+++ /dev/null
@@ -1,13 +0,0 @@
-error[E0658]: deriving `Default` on enums is experimental
-  --> $DIR/feature-gate-derive_default_enum.rs:1:10
-   |
-LL | #[derive(Default)]
-   |          ^^^^^^^
-   |
-   = note: see issue #86985 <https://github.com/rust-lang/rust/issues/86985> for more information
-   = help: add `#![feature(derive_default_enum)]` to the crate attributes to enable
-   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/macros/macros-nonfatal-errors.rs b/src/test/ui/macros/macros-nonfatal-errors.rs
index 98f64aa6f80..e7a01f105de 100644
--- a/src/test/ui/macros/macros-nonfatal-errors.rs
+++ b/src/test/ui/macros/macros-nonfatal-errors.rs
@@ -5,7 +5,6 @@
 
 #![feature(trace_macros, concat_idents)]
 #![feature(stmt_expr_attributes, arbitrary_enum_discriminant)]
-#![feature(derive_default_enum)]
 
 use std::arch::asm;
 
diff --git a/src/test/ui/macros/macros-nonfatal-errors.stderr b/src/test/ui/macros/macros-nonfatal-errors.stderr
index 79e8db9c1d4..b3c6d07f967 100644
--- a/src/test/ui/macros/macros-nonfatal-errors.stderr
+++ b/src/test/ui/macros/macros-nonfatal-errors.stderr
@@ -1,41 +1,41 @@
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:14:5
+  --> $DIR/macros-nonfatal-errors.rs:13:5
    |
 LL |     #[default]
    |     ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:19:36
+  --> $DIR/macros-nonfatal-errors.rs:18:36
    |
 LL | struct DefaultInnerAttrTupleStruct(#[default] ());
    |                                    ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:23:1
+  --> $DIR/macros-nonfatal-errors.rs:22:1
    |
 LL | #[default]
    | ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:27:1
+  --> $DIR/macros-nonfatal-errors.rs:26:1
    |
 LL | #[default]
    | ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:37:11
+  --> $DIR/macros-nonfatal-errors.rs:36:11
    |
 LL |     Foo = #[default] 0,
    |           ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:38:14
+  --> $DIR/macros-nonfatal-errors.rs:37:14
    |
 LL |     Bar([u8; #[default] 1]),
    |              ^^^^^^^^^^
 
 error: no default declared
-  --> $DIR/macros-nonfatal-errors.rs:43:10
+  --> $DIR/macros-nonfatal-errors.rs:42:10
    |
 LL | #[derive(Default)]
    |          ^^^^^^^
@@ -44,7 +44,7 @@ LL | #[derive(Default)]
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: multiple declared defaults
-  --> $DIR/macros-nonfatal-errors.rs:49:10
+  --> $DIR/macros-nonfatal-errors.rs:48:10
    |
 LL | #[derive(Default)]
    |          ^^^^^^^
@@ -62,7 +62,7 @@ LL |     Baz,
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: `#[default]` attribute does not accept a value
-  --> $DIR/macros-nonfatal-errors.rs:61:5
+  --> $DIR/macros-nonfatal-errors.rs:60:5
    |
 LL |     #[default = 1]
    |     ^^^^^^^^^^^^^^
@@ -70,7 +70,7 @@ LL |     #[default = 1]
    = help: try using `#[default]`
 
 error: multiple `#[default]` attributes
-  --> $DIR/macros-nonfatal-errors.rs:69:5
+  --> $DIR/macros-nonfatal-errors.rs:68:5
    |
 LL |     #[default]
    |     ---------- `#[default]` used here
@@ -81,13 +81,13 @@ LL |     Foo,
    |
    = note: only one `#[default]` attribute is needed
 help: try removing this
-  --> $DIR/macros-nonfatal-errors.rs:68:5
+  --> $DIR/macros-nonfatal-errors.rs:67:5
    |
 LL |     #[default]
    |     ^^^^^^^^^^
 
 error: multiple `#[default]` attributes
-  --> $DIR/macros-nonfatal-errors.rs:79:5
+  --> $DIR/macros-nonfatal-errors.rs:78:5
    |
 LL |     #[default]
    |     ---------- `#[default]` used here
@@ -99,7 +99,7 @@ LL |     Foo,
    |
    = note: only one `#[default]` attribute is needed
 help: try removing these
-  --> $DIR/macros-nonfatal-errors.rs:76:5
+  --> $DIR/macros-nonfatal-errors.rs:75:5
    |
 LL |     #[default]
    |     ^^^^^^^^^^
@@ -109,7 +109,7 @@ LL |     #[default]
    |     ^^^^^^^^^^
 
 error: the `#[default]` attribute may only be used on unit enum variants
-  --> $DIR/macros-nonfatal-errors.rs:86:5
+  --> $DIR/macros-nonfatal-errors.rs:85:5
    |
 LL |     Foo {},
    |     ^^^
@@ -117,7 +117,7 @@ LL |     Foo {},
    = help: consider a manual implementation of `Default`
 
 error: default variant must be exhaustive
-  --> $DIR/macros-nonfatal-errors.rs:94:5
+  --> $DIR/macros-nonfatal-errors.rs:93:5
    |
 LL |     #[non_exhaustive]
    |     ----------------- declared `#[non_exhaustive]` here
@@ -127,37 +127,37 @@ LL |     Foo,
    = help: consider a manual implementation of `Default`
 
 error: asm template must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:99:10
+  --> $DIR/macros-nonfatal-errors.rs:98:10
    |
 LL |     asm!(invalid);
    |          ^^^^^^^
 
 error: concat_idents! requires ident args
-  --> $DIR/macros-nonfatal-errors.rs:102:5
+  --> $DIR/macros-nonfatal-errors.rs:101:5
    |
 LL |     concat_idents!("not", "idents");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:104:17
+  --> $DIR/macros-nonfatal-errors.rs:103:17
    |
 LL |     option_env!(invalid);
    |                 ^^^^^^^
 
 error: expected string literal
-  --> $DIR/macros-nonfatal-errors.rs:105:10
+  --> $DIR/macros-nonfatal-errors.rs:104:10
    |
 LL |     env!(invalid);
    |          ^^^^^^^
 
 error: expected string literal
-  --> $DIR/macros-nonfatal-errors.rs:106:10
+  --> $DIR/macros-nonfatal-errors.rs:105:10
    |
 LL |     env!(foo, abr, baz);
    |          ^^^
 
 error: environment variable `RUST_HOPEFULLY_THIS_DOESNT_EXIST` not defined
-  --> $DIR/macros-nonfatal-errors.rs:107:5
+  --> $DIR/macros-nonfatal-errors.rs:106:5
    |
 LL |     env!("RUST_HOPEFULLY_THIS_DOESNT_EXIST");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -165,7 +165,7 @@ LL |     env!("RUST_HOPEFULLY_THIS_DOESNT_EXIST");
    = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: format argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:109:13
+  --> $DIR/macros-nonfatal-errors.rs:108:13
    |
 LL |     format!(invalid);
    |             ^^^^^^^
@@ -176,19 +176,19 @@ LL |     format!("{}", invalid);
    |             +++++
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:111:14
+  --> $DIR/macros-nonfatal-errors.rs:110:14
    |
 LL |     include!(invalid);
    |              ^^^^^^^
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:113:18
+  --> $DIR/macros-nonfatal-errors.rs:112:18
    |
 LL |     include_str!(invalid);
    |                  ^^^^^^^
 
 error: couldn't read $DIR/i'd be quite surprised if a file with this name existed: $FILE_NOT_FOUND_MSG (os error 2)
-  --> $DIR/macros-nonfatal-errors.rs:114:5
+  --> $DIR/macros-nonfatal-errors.rs:113:5
    |
 LL |     include_str!("i'd be quite surprised if a file with this name existed");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -196,13 +196,13 @@ LL |     include_str!("i'd be quite surprised if a file with this name existed")
    = note: this error originates in the macro `include_str` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: argument must be a string literal
-  --> $DIR/macros-nonfatal-errors.rs:115:20
+  --> $DIR/macros-nonfatal-errors.rs:114:20
    |
 LL |     include_bytes!(invalid);
    |                    ^^^^^^^
 
 error: couldn't read $DIR/i'd be quite surprised if a file with this name existed: $FILE_NOT_FOUND_MSG (os error 2)
-  --> $DIR/macros-nonfatal-errors.rs:116:5
+  --> $DIR/macros-nonfatal-errors.rs:115:5
    |
 LL |     include_bytes!("i'd be quite surprised if a file with this name existed");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -210,13 +210,13 @@ LL |     include_bytes!("i'd be quite surprised if a file with this name existed
    = note: this error originates in the macro `include_bytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: trace_macros! accepts only `true` or `false`
-  --> $DIR/macros-nonfatal-errors.rs:118:5
+  --> $DIR/macros-nonfatal-errors.rs:117:5
    |
 LL |     trace_macros!(invalid);
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 error: cannot find macro `llvm_asm` in this scope
-  --> $DIR/macros-nonfatal-errors.rs:100:5
+  --> $DIR/macros-nonfatal-errors.rs:99:5
    |
 LL |     llvm_asm!(invalid);
    |     ^^^^^^^^