Auto merge of rust-lang#116240 - dtolnay:constdiscriminant, r=thomcc

Const stabilize mem::discriminant

Tracking issue: rust-lang#69821.

This PR is a rebase of rust-lang#103893 to resolve conflicts in library/core/src/lib.rs (against rust-lang#102470 and rust-lang#110393).

Author: bors

core/src/intrinsics.rs

@@ -2279,7 +2279,7 @@ extern "rust-intrinsic" {
     /// any safety invariants.
     ///
     /// The stabilized version of this intrinsic is [`core::mem::discriminant`].
-    #[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
+    #[rustc_const_stable(feature = "const_discriminant", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_safe_intrinsic]
     #[rustc_nounwind]
     pub fn discriminant_value<T>(v: &T) -> <T as DiscriminantKind>::Discriminant;

core/src/lib.rs

@@ -125,7 +125,6 @@
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_char_from_u32_unchecked)]
-#![feature(const_discriminant)]
 #![feature(const_eval_select)]
 #![feature(const_exact_div)]
 #![feature(const_float_bits_conv)]

core/src/mem/mod.rs

@@ -1206,7 +1206,7 @@ impl<T> fmt::Debug for Discriminant<T> {
 /// // assert_eq!(0, unsafe { std::mem::transmute::<_, u8>(std::mem::discriminant(&unit_like)) });
 /// ```
 #[stable(feature = "discriminant_value", since = "1.21.0")]
-#[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
+#[rustc_const_stable(feature = "const_discriminant", since = "CURRENT_RUSTC_VERSION")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_discriminant")]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 pub const fn discriminant<T>(v: &T) -> Discriminant<T> {