Auto merge of #106934 - DrMeepster:offset_of, r=WaffleLapkin

Add offset_of! macro (RFC 3308)

Implements https://github.com/rust-lang/rfcs/pull/3308 (tracking issue #106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

1 files changed, 6 insertions, 3 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 9a366364e76..4d3a1193508 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -280,20 +280,23 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 self.write_immediate(*val, &dest)?;
             }
 
-            NullaryOp(null_op, ty) => {
+            NullaryOp(ref null_op, ty) => {
                 let ty = self.subst_from_current_frame_and_normalize_erasing_regions(ty)?;
                 let layout = self.layout_of(ty)?;
-                if layout.is_unsized() {
+                if let mir::NullOp::SizeOf | mir::NullOp::AlignOf = null_op && layout.is_unsized() {
                     // FIXME: This should be a span_bug (#80742)
                     self.tcx.sess.delay_span_bug(
                         self.frame().current_span(),
-                        &format!("Nullary MIR operator called for unsized type {}", ty),
+                        &format!("{null_op:?} MIR operator called for unsized type {ty}"),
                     );
                     throw_inval!(SizeOfUnsizedType(ty));
                 }
                 let val = match null_op {
                     mir::NullOp::SizeOf => layout.size.bytes(),
                     mir::NullOp::AlignOf => layout.align.abi.bytes(),
+                    mir::NullOp::OffsetOf(fields) => {
+                        layout.offset_of_subfield(self, fields.iter().map(|f| f.index())).bytes()
+                    }
                 };
                 self.write_scalar(Scalar::from_target_usize(val, self), &dest)?;
             }