Auto merge of #109762 - scottmcm:variantdef-indexvec, r=WaffleLapkin

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 rust-lang/compiler-team#606

Author: bors

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -350,7 +350,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     if !including_tuple_field.0 && variant.ctor_kind() == Some(CtorKind::Fn) {
                         return None;
                     }
-                    Some(variant.fields[field.index()].name.to_string())
+                    Some(variant.fields[field].name.to_string())
                 }
                 ty::Tuple(_) => Some(field.index().to_string()),
                 ty::Ref(_, ty, _) | ty::RawPtr(ty::TypeAndMut { ty, .. }) => {

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -854,7 +854,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             },
         };
 
-        if let Some(field) = variant.fields.get(field.index()) {
+        if let Some(field) = variant.fields.get(field) {
             Ok(self.cx.normalize(field.ty(tcx, substs), location))
         } else {
             Err(FieldAccessError::OutOfRange { field_count: variant.fields.len() })
@@ -1725,7 +1725,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             AggregateKind::Adt(adt_did, variant_index, substs, _, active_field_index) => {
                 let def = tcx.adt_def(adt_did);
                 let variant = &def.variant(variant_index);
-                let adj_field_index = active_field_index.unwrap_or(field_index);
+                let adj_field_index =
+                    FieldIdx::from_usize(active_field_index.unwrap_or(field_index));
                 if let Some(field) = variant.fields.get(adj_field_index) {
                     Ok(self.normalize(field.ty(tcx, substs), location))
                 } else {