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/test/ui/consts/const_discriminant.rs

@@ -0,0 +1,40 @@
+// run-pass
+#![feature(const_discriminant)]
+#![allow(dead_code)]
+
+use std::mem::{discriminant, Discriminant};
+
+// `discriminant(const_expr)` may get const-propagated.
+// As we want to check that const-eval is equal to ordinary exection,
+// we wrap `const_expr` with a function which is not const to prevent this.
+#[inline(never)]
+fn identity<T>(x: T) -> T { x }
+
+enum Test {
+    A(u8),
+    B,
+    C { a: u8, b: u8 },
+}
+
+const TEST_A: Discriminant<Test> = discriminant(&Test::A(5));
+const TEST_A_OTHER: Discriminant<Test> = discriminant(&Test::A(17));
+const TEST_B: Discriminant<Test> = discriminant(&Test::B);
+
+enum Void {}
+
+enum SingleVariant {
+    V,
+    Never(Void),
+}
+
+const TEST_V: Discriminant<SingleVariant> = discriminant(&SingleVariant::V);
+
+fn main() {
+    assert_eq!(TEST_A, TEST_A_OTHER);
+    assert_eq!(TEST_A, discriminant(identity(&Test::A(17))));
+    assert_eq!(TEST_B, discriminant(identity(&Test::B)));
+    assert_ne!(TEST_A, TEST_B);
+    assert_ne!(TEST_B, discriminant(identity(&Test::C { a: 42, b: 7 })));
+
+    assert_eq!(TEST_V, discriminant(identity(&SingleVariant::V)));
+}