Replace OffsetOf by an actual sum.

Author: cjgillot

core/src/intrinsics/mod.rs

@@ -2791,6 +2791,23 @@ pub const fn size_of<T>() -> usize;
 #[rustc_intrinsic]
 pub const fn align_of<T>() -> usize;
 
+/// The offset of a field inside a type.
+///
+/// Note that, unlike most intrinsics, this is safe to call;
+/// it does not require an `unsafe` block.
+/// Therefore, implementations must not require the user to uphold
+/// any safety invariants.
+///
+/// The stabilized version of this intrinsic is [`core::mem::offset_of`].
+#[rustc_nounwind]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_const_unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_const_stable_indirect]
+#[rustc_intrinsic_const_stable_indirect]
+#[rustc_intrinsic]
+#[lang = "offset_of"]
+pub const fn offset_of<T: PointeeSized>(variant: u32, field: u32) -> usize;
+
 /// Returns the number of variants of the type `T` cast to a `usize`;
 /// if `T` has no variants, returns `0`. Uninhabited variants will be counted.
 ///

core/src/mem/mod.rs

@@ -1424,10 +1424,10 @@ impl<T> SizedTypeProperties for T {}
 /// [`offset_of_enum`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-enum.html
 /// [`offset_of_slice`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-slice.html
 #[stable(feature = "offset_of", since = "1.77.0")]
-#[allow_internal_unstable(builtin_syntax)]
+#[allow_internal_unstable(builtin_syntax, core_intrinsics)]
 pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
     // The `{}` is for better error messages
-    {builtin # offset_of($Container, $($fields)+)}
+    const {builtin # offset_of($Container, $($fields)+)}
 }
 
 /// Create a fresh instance of the inhabited ZST type `T`.