Rollup merge of #69825 - lcnr:discriminant, r=oli-obk

make `mem::discriminant` const

implements #69821, which could be used as a tracking issue for `const_discriminant`.

Should this be added to the meta tracking issue #57563?
@Lokathor

src/libcore/lib.rs

@@ -72,6 +72,7 @@
 #![feature(concat_idents)]
 #![feature(const_ascii_ctype_on_intrinsics)]
 #![feature(const_alloc_layout)]
+#![feature(const_discriminant)]
 #![feature(const_if_match)]
 #![feature(const_loop)]
 #![feature(const_checked_int_methods)]