Update `ty::VariantDef` to use `IndexVec<FieldIdx, FieldDef>`

And while doing the updates for that, also uses `FieldIdx` in `ProjectionKind::Field` and `TypeckResults::field_indices`.

There's more places that could use it (like `rustc_const_eval` and `LayoutS`), but I tried to keep this PR from exploding to *even more* places.

Part 2/? of https://github.com/rust-lang/compiler-team/issues/606

2 files changed, 4 insertions, 3 deletions

diff --git a/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs b/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs
index 58bc083280a..85623499439 100644
--- a/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs
+++ b/compiler/rustc_mir_transform/src/elaborate_box_derefs.rs
@@ -92,13 +92,14 @@ pub struct ElaborateBoxDerefs;
 impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         if let Some(def_id) = tcx.lang_items().owned_box() {
-            let unique_did = tcx.adt_def(def_id).non_enum_variant().fields[0].did;
+            let unique_did =
+                tcx.adt_def(def_id).non_enum_variant().fields[FieldIdx::from_u32(0)].did;
 
             let Some(nonnull_def) = tcx.type_of(unique_did).subst_identity().ty_adt_def() else {
                 span_bug!(tcx.def_span(unique_did), "expected Box to contain Unique")
             };
 
-            let nonnull_did = nonnull_def.non_enum_variant().fields[0].did;
+            let nonnull_did = nonnull_def.non_enum_variant().fields[FieldIdx::from_u32(0)].did;
 
             let patch = MirPatch::new(body);
 
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 98d2df71978..15b31d38394 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -907,7 +907,7 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                 }
                 ty::Adt(adt_def, substs) => {
                     let var = parent_ty.variant_index.unwrap_or(FIRST_VARIANT);
-                    let Some(field) = adt_def.variant(var).fields.get(f.as_usize()) else {
+                    let Some(field) = adt_def.variant(var).fields.get(f) else {
                         self.validation = Err("malformed MIR");
                         return;
                     };