Auto merge of #86860 - fee1-dead:stabilize, r=LeSeulArtichaut

Stabilize `arbitrary_enum_discriminant`

Closes #60553.

----

## Stabilization Report

_copied from https://github.com/rust-lang/rust/issues/60553#issuecomment-865922311_

### Summary

Enables a user to specify *explicit* discriminants on arbitrary enums.

Previously, this was hard to achieve:

```rust
#[repr(u8)]
enum Foo {
    A(u8) = 0,
    B(i8) = 1,
    C(bool) = 42,
}
```

Someone would need to add 41 hidden variants in between as a workaround with implicit discriminants.

In conjunction with [RFC 2195](https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md), this feature would provide more flexibility for FFI and unsafe code involving enums.

### Test cases

Most tests are in [`src/test/ui/enum-discriminant`](https://github.com/rust-lang/rust/tree/master/src/test/ui/enum-discriminant), there are two [historical](https://github.com/rust-lang/rust/blob/master/src/test/ui/parser/tag-variant-disr-non-nullary.rs) [tests](https://github.com/rust-lang/rust/blob/master/src/test/ui/parser/issue-17383.rs) that are now covered by the feature (removed by this pr due to them being obsolete).

### Edge cases

The feature is well defined and does not have many edge cases.
One [edge case](https://github.com/rust-lang/rust/issues/70509) was related to another unstable feature named `repr128` and is resolved.

### Previous PRs

The [implementation PR](https://github.com/rust-lang/rust/pull/60732) added documentation to the Unstable Book, https://github.com/rust-lang/reference/pull/1055 was opened as a continuation of https://github.com/rust-lang/reference/pull/639.

### Resolution of unresolved questions

The questions are resolved in https://github.com/rust-lang/rust/issues/60553#issuecomment-511235271.

----

(someone please add `needs-fcp`)

16 files changed, 8 insertions, 151 deletions

diff --git a/src/doc/unstable-book/src/language-features/arbitrary-enum-discriminant.md b/src/doc/unstable-book/src/language-features/arbitrary-enum-discriminant.md
deleted file mode 100644
index e0bb782270e..00000000000
--- a/src/doc/unstable-book/src/language-features/arbitrary-enum-discriminant.md
+++ /dev/null
@@ -1,37 +0,0 @@
-# `arbitrary_enum_discriminant`
-
-The tracking issue for this feature is: [#60553]
-
-[#60553]: https://github.com/rust-lang/rust/issues/60553
-
-------------------------
-
-The `arbitrary_enum_discriminant` feature permits tuple-like and
-struct-like enum variants with `#[repr(<int-type>)]` to have explicit discriminants.
-
-## Examples
-
-```rust
-#![feature(arbitrary_enum_discriminant)]
-
-#[allow(dead_code)]
-#[repr(u8)]
-enum Enum {
-    Unit = 3,
-    Tuple(u16) = 2,
-    Struct {
-        a: u8,
-        b: u16,
-    } = 1,
-}
-
-impl Enum {
-    fn tag(&self) -> u8 {
-        unsafe { *(self as *const Self as *const u8) }
-    }
-}
-
-assert_eq!(3, Enum::Unit.tag());
-assert_eq!(2, Enum::Tuple(5).tag());
-assert_eq!(1, Enum::Struct{a: 7, b: 11}.tag());
-```
diff --git a/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.rs b/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.rs
index 4da7b5ab24b..a6e5f70fdef 100644
--- a/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.rs
+++ b/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.rs
@@ -1,5 +1,4 @@
 #![crate_type="lib"]
-#![feature(arbitrary_enum_discriminant)]
 
 enum Enum {
 //~^ ERROR `#[repr(inttype)]` must be specified
diff --git a/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.stderr b/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.stderr
index 2db5372da0c..7af063c591d 100644
--- a/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.stderr
+++ b/src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.stderr
@@ -1,5 +1,5 @@
 error[E0732]: `#[repr(inttype)]` must be specified
-  --> $DIR/arbitrary_enum_discriminant-no-repr.rs:4:1
+  --> $DIR/arbitrary_enum_discriminant-no-repr.rs:3:1
    |
 LL | / enum Enum {
 LL | |
diff --git a/src/test/ui/enum-discriminant/arbitrary_enum_discriminant.rs b/src/test/ui/enum-discriminant/arbitrary_enum_discriminant.rs
index f2270602d87..360bddb7bd1 100644
--- a/src/test/ui/enum-discriminant/arbitrary_enum_discriminant.rs
+++ b/src/test/ui/enum-discriminant/arbitrary_enum_discriminant.rs
@@ -1,5 +1,5 @@
 // run-pass
-#![feature(arbitrary_enum_discriminant, const_raw_ptr_deref, test)]
+#![feature(const_raw_ptr_deref, test)]
 
 extern crate test;
 
diff --git a/src/test/ui/enum-discriminant/discriminant_value.rs b/src/test/ui/enum-discriminant/discriminant_value.rs
index eb60aaf4b2d..7ed1d9660a6 100644
--- a/src/test/ui/enum-discriminant/discriminant_value.rs
+++ b/src/test/ui/enum-discriminant/discriminant_value.rs
@@ -1,6 +1,6 @@
 // run-pass
 #![allow(stable_features)]
-#![feature(arbitrary_enum_discriminant, core, core_intrinsics)]
+#![feature(core, core_intrinsics)]
 
 extern crate core;
 use core::intrinsics::discriminant_value;
diff --git a/src/test/ui/enum-discriminant/feature-gate-arbitrary_enum_discriminant.rs b/src/test/ui/enum-discriminant/feature-gate-arbitrary_enum_discriminant.rs
deleted file mode 100644
index 3e90af4d36a..00000000000
--- a/src/test/ui/enum-discriminant/feature-gate-arbitrary_enum_discriminant.rs
+++ /dev/null
@@ -1,10 +0,0 @@
-#![crate_type="lib"]
-
-enum Enum {
-  Unit = 1,
-  //~^ ERROR custom discriminant values are not allowed in enums with tuple or struct variants
-  Tuple() = 2,
-  //~^ ERROR discriminants on non-unit variants are experimental
-  Struct{} = 3,
-  //~^ ERROR discriminants on non-unit variants are experimental
-}
diff --git a/src/test/ui/enum-discriminant/feature-gate-arbitrary_enum_discriminant.stderr b/src/test/ui/enum-discriminant/feature-gate-arbitrary_enum_discriminant.stderr
deleted file mode 100644
index b5f61e6e991..00000000000
--- a/src/test/ui/enum-discriminant/feature-gate-arbitrary_enum_discriminant.stderr
+++ /dev/null
@@ -1,36 +0,0 @@
-error[E0658]: discriminants on non-unit variants are experimental
-  --> $DIR/feature-gate-arbitrary_enum_discriminant.rs:6:13
-   |
-LL |   Tuple() = 2,
-   |             ^
-   |
-   = note: see issue #60553 <https://github.com/rust-lang/rust/issues/60553> for more information
-   = help: add `#![feature(arbitrary_enum_discriminant)]` to the crate attributes to enable
-
-error[E0658]: discriminants on non-unit variants are experimental
-  --> $DIR/feature-gate-arbitrary_enum_discriminant.rs:8:14
-   |
-LL |   Struct{} = 3,
-   |              ^
-   |
-   = note: see issue #60553 <https://github.com/rust-lang/rust/issues/60553> for more information
-   = help: add `#![feature(arbitrary_enum_discriminant)]` to the crate attributes to enable
-
-error[E0658]: custom discriminant values are not allowed in enums with tuple or struct variants
-  --> $DIR/feature-gate-arbitrary_enum_discriminant.rs:4:10
-   |
-LL |   Unit = 1,
-   |          ^ disallowed custom discriminant
-LL |
-LL |   Tuple() = 2,
-   |   ----------- tuple variant defined here
-LL |
-LL |   Struct{} = 3,
-   |   ------------ struct variant defined here
-   |
-   = note: see issue #60553 <https://github.com/rust-lang/rust/issues/60553> for more information
-   = help: add `#![feature(arbitrary_enum_discriminant)]` to the crate attributes to enable
-
-error: aborting due to 3 previous errors
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs
index f927dd18903..ad9fcc25b41 100644
--- a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs
+++ b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice-2.rs
@@ -1,4 +1,4 @@
-#![feature(arbitrary_enum_discriminant, core_intrinsics)]
+#![feature(core_intrinsics)]
 
 extern crate core;
 use core::intrinsics::discriminant_value;
diff --git a/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs b/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs
index e62582fb516..42a062239d3 100644
--- a/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs
+++ b/src/test/ui/enum-discriminant/issue-70453-polymorphic-ctfe.rs
@@ -1,4 +1,4 @@
-#![feature(arbitrary_enum_discriminant, core_intrinsics)]
+#![feature(core_intrinsics)]
 
 extern crate core;
 use core::intrinsics::discriminant_value;
diff --git a/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs b/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs
index ae389e11466..3adac7b7262 100644
--- a/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs
+++ b/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs
@@ -1,5 +1,5 @@
 // run-pass
-#![feature(repr128, arbitrary_enum_discriminant)]
+#![feature(repr128)]
 //~^ WARN the feature `repr128` is incomplete
 
 #[derive(PartialEq, Debug)]
diff --git a/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr b/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr
index 5bf6ea56ebc..04fb13f37a0 100644
--- a/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr
+++ b/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr
@@ -1,7 +1,7 @@
 warning: the feature `repr128` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/issue-70509-partial_eq.rs:2:12
    |
-LL | #![feature(repr128, arbitrary_enum_discriminant)]
+LL | #![feature(repr128)]
    |            ^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
diff --git a/src/test/ui/intrinsics/panic-uninitialized-zeroed.rs b/src/test/ui/intrinsics/panic-uninitialized-zeroed.rs
index 72c0d7913e5..88ef8182f02 100644
--- a/src/test/ui/intrinsics/panic-uninitialized-zeroed.rs
+++ b/src/test/ui/intrinsics/panic-uninitialized-zeroed.rs
@@ -5,7 +5,7 @@
 
 // This test checks panic emitted from `mem::{uninitialized,zeroed}`.
 
-#![feature(never_type, arbitrary_enum_discriminant)]
+#![feature(never_type)]
 #![allow(deprecated, invalid_value)]
 
 use std::{
diff --git a/src/test/ui/parser/issue-17383.rs b/src/test/ui/parser/issue-17383.rs
deleted file mode 100644
index 7bf0e64f2c0..00000000000
--- a/src/test/ui/parser/issue-17383.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-enum X {
-    A = 3,
-    //~^ ERROR custom discriminant values are not allowed in enums with tuple or struct variants
-    B(usize)
-}
-
-fn main() {}
diff --git a/src/test/ui/parser/issue-17383.stderr b/src/test/ui/parser/issue-17383.stderr
deleted file mode 100644
index 265d6e14866..00000000000
--- a/src/test/ui/parser/issue-17383.stderr
+++ /dev/null
@@ -1,15 +0,0 @@
-error[E0658]: custom discriminant values are not allowed in enums with tuple or struct variants
-  --> $DIR/issue-17383.rs:2:9
-   |
-LL |     A = 3,
-   |         ^ disallowed custom discriminant
-LL |
-LL |     B(usize)
-   |     -------- tuple variant defined here
-   |
-   = note: see issue #60553 <https://github.com/rust-lang/rust/issues/60553> for more information
-   = help: add `#![feature(arbitrary_enum_discriminant)]` to the crate attributes to enable
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/parser/tag-variant-disr-non-nullary.rs b/src/test/ui/parser/tag-variant-disr-non-nullary.rs
deleted file mode 100644
index a9cfdd549c7..00000000000
--- a/src/test/ui/parser/tag-variant-disr-non-nullary.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-enum Color {
-    Red = 0xff0000,
-    //~^ ERROR custom discriminant values are not allowed in enums with tuple or struct variants
-    Green = 0x00ff00,
-    Blue = 0x0000ff,
-    Black = 0x000000,
-    White = 0xffffff,
-    Other(usize),
-    Other2(usize, usize),
-}
-
-fn main() {}
diff --git a/src/test/ui/parser/tag-variant-disr-non-nullary.stderr b/src/test/ui/parser/tag-variant-disr-non-nullary.stderr
deleted file mode 100644
index 79f044a0675..00000000000
--- a/src/test/ui/parser/tag-variant-disr-non-nullary.stderr
+++ /dev/null
@@ -1,25 +0,0 @@
-error[E0658]: custom discriminant values are not allowed in enums with tuple or struct variants
-  --> $DIR/tag-variant-disr-non-nullary.rs:2:11
-   |
-LL |     Red = 0xff0000,
-   |           ^^^^^^^^ disallowed custom discriminant
-LL |
-LL |     Green = 0x00ff00,
-   |             ^^^^^^^^ disallowed custom discriminant
-LL |     Blue = 0x0000ff,
-   |            ^^^^^^^^ disallowed custom discriminant
-LL |     Black = 0x000000,
-   |             ^^^^^^^^ disallowed custom discriminant
-LL |     White = 0xffffff,
-   |             ^^^^^^^^ disallowed custom discriminant
-LL |     Other(usize),
-   |     ------------ tuple variant defined here
-LL |     Other2(usize, usize),
-   |     -------------------- tuple variant defined here
-   |
-   = note: see issue #60553 <https://github.com/rust-lang/rust/issues/60553> for more information
-   = help: add `#![feature(arbitrary_enum_discriminant)]` to the crate attributes to enable
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0658`.