Auto merge of #114208 - GKFX:offset_of_enum, r=wesleywiser

Support enum variants in offset_of!

This MR implements support for navigating through enum variants in `offset_of!`, placing the enum variant name in the second argument to `offset_of!`. The RFC placed it in the first argument, but I think it interacts better with nested field access in the second, as you can then write things like

```rust
offset_of!(Type, field.Variant.field)
```

Alternatively, a syntactic distinction could be made between variants and fields (e.g. `field::Variant.field`) but I'm not convinced this would be helpful.

[RFC 3308 # Enum Support](https://rust-lang.github.io/rfcs/3308-offset_of.html#enum-support-offset_ofsomeenumstructvariant-field_on_variant)
Tracking Issue #106655.

2 files changed, 12 insertions, 13 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 8c34d05042b..b6993d939c5 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -275,7 +275,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     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()
+                        layout.offset_of_subfield(self, fields.iter()).bytes()
                     }
                 };
                 self.write_scalar(Scalar::from_target_usize(val, self), &dest)?;
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index b70a342aa15..4d71f763667 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -1056,16 +1056,23 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     }
                 }
             }
-            Rvalue::NullaryOp(NullOp::OffsetOf(fields), container) => {
+            Rvalue::NullaryOp(NullOp::OffsetOf(indices), container) => {
                 let fail_out_of_bounds = |this: &mut Self, location, field, ty| {
                     this.fail(location, format!("Out of bounds field {field:?} for {ty:?}"));
                 };
 
                 let mut current_ty = *container;
 
-                for field in fields.iter() {
+                for (variant, field) in indices.iter() {
                     match current_ty.kind() {
                         ty::Tuple(fields) => {
+                            if variant != FIRST_VARIANT {
+                                self.fail(
+                                    location,
+                                    format!("tried to get variant {variant:?} of tuple"),
+                                );
+                                return;
+                            }
                             let Some(&f_ty) = fields.get(field.as_usize()) else {
                                 fail_out_of_bounds(self, location, field, current_ty);
                                 return;
@@ -1074,15 +1081,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             current_ty = self.tcx.normalize_erasing_regions(self.param_env, f_ty);
                         }
                         ty::Adt(adt_def, args) => {
-                            if adt_def.is_enum() {
-                                self.fail(
-                                    location,
-                                    format!("Cannot get field offset from enum {current_ty:?}"),
-                                );
-                                return;
-                            }
-
-                            let Some(field) = adt_def.non_enum_variant().fields.get(field) else {
+                            let Some(field) = adt_def.variant(variant).fields.get(field) else {
                                 fail_out_of_bounds(self, location, field, current_ty);
                                 return;
                             };
@@ -1093,7 +1092,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         _ => {
                             self.fail(
                                 location,
-                                format!("Cannot get field offset from non-adt type {current_ty:?}"),
+                                format!("Cannot get offset ({variant:?}, {field:?}) from type {current_ty:?}"),
                             );
                             return;
                         }