Implement `discriminant_value` intrinsic

Implements an intrinsic for extracting the value of the discriminant
enum variant values. For non-enum types, this returns zero, otherwise it
returns the value we use for discriminant comparisons. This means that
enum types that do not have a discriminant will also work in this
arrangement.

This is (at least part of) the work on Issue #24263

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 0e91eafce18..80f506ebc06 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -569,4 +569,10 @@ extern "rust-intrinsic" {
     pub fn overflowing_sub<T>(a: T, b: T) -> T;
     /// Returns (a * b) mod 2^N, where N is the width of N in bits.
     pub fn overflowing_mul<T>(a: T, b: T) -> T;
+
+    /// Returns the value of the discriminant for the variant in 'v',
+    /// cast to a `u64`; if `T` has no discriminant, returns 0.
+    // SNAP 5520801
+    #[cfg(not(stage0))]
+    pub fn discriminant_value<T>(v: &T) -> u64;
 }