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

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 {

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -701,7 +701,8 @@ impl<'tcx> CPlace<'tcx> {
                     };
                 }
                 ty::Adt(adt_def, substs) if layout.ty.is_simd() => {
-                    let f0_ty = adt_def.non_enum_variant().fields[0].ty(fx.tcx, substs);
+                    let f0 = &adt_def.non_enum_variant().fields[FieldIdx::from_u32(0)];
+                    let f0_ty = f0.ty(fx.tcx, substs);
 
                     match f0_ty.kind() {
                         ty::Array(_, _) => {

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/mod.rs

@@ -274,7 +274,8 @@ fn build_enum_variant_struct_type_di_node<'ll, 'tcx>(
                 .map(|field_index| {
                     let field_name = if variant_def.ctor_kind() != Some(CtorKind::Fn) {
                         // Fields have names
-                        Cow::from(variant_def.fields[field_index].name.as_str())
+                        let field = &variant_def.fields[FieldIdx::from_usize(field_index)];
+                        Cow::from(field.name.as_str())
                     } else {
                         // Tuple-like
                         super::tuple_field_name(field_index)

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -8,7 +8,7 @@ use crate::interpret::{
 use crate::interpret::{MPlaceTy, Value};
 use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
 use rustc_span::source_map::DUMMY_SP;
-use rustc_target::abi::{Align, VariantIdx, FIRST_VARIANT};
+use rustc_target::abi::{Align, FieldIdx, VariantIdx, FIRST_VARIANT};
 
 #[instrument(skip(ecx), level = "debug")]
 fn branches<'tcx>(
@@ -412,6 +412,7 @@ fn valtree_into_mplace<'tcx>(
 
                         let inner_ty = match ty.kind() {
                             ty::Adt(def, substs) => {
+                                let i = FieldIdx::from_usize(i);
                                 def.variant(FIRST_VARIANT).fields[i].ty(tcx, substs)
                             }
                             ty::Tuple(inner_tys) => inner_tys[i],

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -16,7 +16,9 @@ use rustc_middle::mir::interpret::InterpError;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_span::symbol::{sym, Symbol};
-use rustc_target::abi::{Abi, Scalar as ScalarAbi, Size, VariantIdx, Variants, WrappingRange};
+use rustc_target::abi::{
+    Abi, FieldIdx, Scalar as ScalarAbi, Size, VariantIdx, Variants, WrappingRange,
+};
 
 use std::hash::Hash;
 
@@ -269,14 +271,16 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 match layout.variants {
                     Variants::Single { index } => {
                         // Inside a variant
-                        PathElem::Field(def.variant(index).fields[field].name)
+                        PathElem::Field(def.variant(index).fields[FieldIdx::from_usize(field)].name)
                     }
                     Variants::Multiple { .. } => bug!("we handled variants above"),
                 }
             }
 
             // other ADTs
-            ty::Adt(def, _) => PathElem::Field(def.non_enum_variant().fields[field].name),
+            ty::Adt(def, _) => {
+                PathElem::Field(def.non_enum_variant().fields[FieldIdx::from_usize(field)].name)
+            }
 
             // arrays/slices
             ty::Array(..) | ty::Slice(..) => PathElem::ArrayElem(field),

compiler/rustc_const_eval/src/transform/validate.rs

@@ -360,7 +360,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, '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 {
                             fail_out_of_bounds(self, location);
                             return;
                         };

compiler/rustc_hir_analysis/src/check/check.rs

@@ -27,6 +27,7 @@ use rustc_middle::ty::{
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_span::symbol::sym;
 use rustc_span::{self, Span};
+use rustc_target::abi::FieldIdx;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::on_unimplemented::OnUnimplementedDirective;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
@@ -474,7 +475,7 @@ fn is_enum_of_nonnullable_ptr<'tcx>(
     let [var_one, var_two] = &adt_def.variants().raw[..] else {
         return false;
     };
-    let (([], [field]) | ([field], [])) = (&var_one.fields[..], &var_two.fields[..]) else {
+    let (([], [field]) | ([field], [])) = (&var_one.fields.raw[..], &var_two.fields.raw[..]) else {
         return false;
     };
     matches!(field.ty(tcx, substs).kind(), ty::FnPtr(..) | ty::Ref(..))
@@ -893,7 +894,7 @@ pub fn check_simd(tcx: TyCtxt<'_>, sp: Span, def_id: LocalDefId) {
             struct_span_err!(tcx.sess, sp, E0075, "SIMD vector cannot be empty").emit();
             return;
         }
-        let e = fields[0].ty(tcx, substs);
+        let e = fields[FieldIdx::from_u32(0)].ty(tcx, substs);
         if !fields.iter().all(|f| f.ty(tcx, substs) == e) {
             struct_span_err!(tcx.sess, sp, E0076, "SIMD vector should be homogeneous")
                 .span_label(sp, "SIMD elements must have the same type")

compiler/rustc_hir_analysis/src/check/intrinsicck.rs

@@ -5,6 +5,7 @@ use rustc_middle::ty::{self, Article, FloatTy, IntTy, Ty, TyCtxt, TypeVisitableE
 use rustc_session::lint;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{Symbol, DUMMY_SP};
+use rustc_target::abi::FieldIdx;
 use rustc_target::asm::{InlineAsmReg, InlineAsmRegClass, InlineAsmRegOrRegClass, InlineAsmType};
 
 pub struct InlineAsmCtxt<'a, 'tcx> {
@@ -82,7 +83,7 @@ impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
             }
             ty::Adt(adt, substs) if adt.repr().simd() => {
                 let fields = &adt.non_enum_variant().fields;
-                let elem_ty = fields[0].ty(self.tcx, substs);
+                let elem_ty = fields[FieldIdx::from_u32(0)].ty(self.tcx, substs);
                 match elem_ty.kind() {
                     ty::Never | ty::Error(_) => return None,
                     ty::Int(IntTy::I8) | ty::Uint(UintTy::U8) => {

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1030,7 +1030,7 @@ fn check_type_defn<'tcx>(tcx: TyCtxt<'tcx>, item: &hir::Item<'tcx>, all_sized: b
             // intermediate types must be sized.
             let needs_drop_copy = || {
                 packed && {
-                    let ty = tcx.type_of(variant.fields.last().unwrap().did).subst_identity();
+                    let ty = tcx.type_of(variant.fields.raw.last().unwrap().did).subst_identity();
                     let ty = tcx.erase_regions(ty);
                     if ty.needs_infer() {
                         tcx.sess
@@ -1046,7 +1046,7 @@ fn check_type_defn<'tcx>(tcx: TyCtxt<'tcx>, item: &hir::Item<'tcx>, all_sized: b
             let all_sized = all_sized || variant.fields.is_empty() || needs_drop_copy();
             let unsized_len = if all_sized { 0 } else { 1 };
             for (idx, field) in
-                variant.fields[..variant.fields.len() - unsized_len].iter().enumerate()
+                variant.fields.raw[..variant.fields.len() - unsized_len].iter().enumerate()
             {
                 let last = idx == variant.fields.len() - 1;
                 let field_id = field.did.expect_local();

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -486,8 +486,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
             // U` can be coerced to `*mut V` if `U: Unsize<V>`.
             let fields = &def_a.non_enum_variant().fields;
             let diff_fields = fields
-                .iter()
-                .enumerate()
+                .iter_enumerated()
                 .filter_map(|(i, f)| {
                     let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 

compiler/rustc_hir_typeck/src/cast.rs

@@ -103,13 +103,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         Ok(match *t.kind() {
             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
             ty::Dynamic(ref tty, _, ty::Dyn) => Some(PointerKind::VTable(tty.principal_def_id())),
-            ty::Adt(def, substs) if def.is_struct() => match def.non_enum_variant().fields.last() {
-                None => Some(PointerKind::Thin),
-                Some(f) => {
-                    let field_ty = self.field_ty(span, f, substs);
-                    self.pointer_kind(field_ty, span)?
+            ty::Adt(def, substs) if def.is_struct() => {
+                match def.non_enum_variant().fields.raw.last() {
+                    None => Some(PointerKind::Thin),
+                    Some(f) => {
+                        let field_ty = self.field_ty(span, f, substs);
+                        self.pointer_kind(field_ty, span)?
+                    }
                 }
-            },
+            }
             ty::Tuple(fields) => match fields.last() {
                 None => Some(PointerKind::Thin),
                 Some(&f) => self.pointer_kind(f, span)?,

compiler/rustc_hir_typeck/src/demand.rs

@@ -19,6 +19,7 @@ use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut, TypeVisitableExt};
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{BytePos, Span};
+use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::method_chain::CollectAllMismatches;
 use rustc_trait_selection::traits::ObligationCause;
@@ -850,7 +851,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     variant.fields.len() == 1
                 })
                 .filter_map(|variant| {
-                    let sole_field = &variant.fields[0];
+                    let sole_field = &variant.fields[FieldIdx::from_u32(0)];
 
                     let field_is_local = sole_field.did.is_local();
                     let field_is_accessible =

compiler/rustc_hir_typeck/src/expr.rs

@@ -50,6 +50,7 @@ use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::{Span, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_target::abi::FieldIdx;
 use rustc_target::spec::abi::Abi::RustIntrinsic;
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::{self, ObligationCauseCode};
@@ -1561,8 +1562,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let mut remaining_fields = variant
             .fields
-            .iter()
-            .enumerate()
+            .iter_enumerated()
             .map(|(i, field)| (field.ident(tcx).normalize_to_macros_2_0(), (i, field)))
             .collect::<FxHashMap<_, _>>();
 
@@ -1815,7 +1815,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         adt_ty: Ty<'tcx>,
         span: Span,
-        remaining_fields: FxHashMap<Ident, (usize, &ty::FieldDef)>,
+        remaining_fields: FxHashMap<Ident, (FieldIdx, &ty::FieldDef)>,
         variant: &'tcx ty::VariantDef,
         ast_fields: &'tcx [hir::ExprField<'tcx>],
         substs: SubstsRef<'tcx>,
@@ -2209,11 +2209,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let (ident, def_scope) =
                         self.tcx.adjust_ident_and_get_scope(field, base_def.did(), body_hir_id);
                     let fields = &base_def.non_enum_variant().fields;
-                    if let Some(index) = fields
-                        .iter()
-                        .position(|f| f.ident(self.tcx).normalize_to_macros_2_0() == ident)
+                    if let Some((index, field)) = fields
+                        .iter_enumerated()
+                        .find(|(_, f)| f.ident(self.tcx).normalize_to_macros_2_0() == ident)
                     {
-                        let field = &fields[index];
                         let field_ty = self.field_ty(expr.span, field, substs);
                         // Save the index of all fields regardless of their visibility in case
                         // of error recovery.
@@ -2230,15 +2229,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 }
                 ty::Tuple(tys) => {
-                    let fstr = field.as_str();
-                    if let Ok(index) = fstr.parse::<usize>() {
-                        if fstr == index.to_string() {
+                    if let Ok(index) = field.as_str().parse::<usize>() {
+                        if field.name == sym::integer(index) {
                             if let Some(&field_ty) = tys.get(index) {
                                 let adjustments = self.adjust_steps(&autoderef);
                                 self.apply_adjustments(base, adjustments);
                                 self.register_predicates(autoderef.into_obligations());
 
-                                self.write_field_index(expr.hir_id, index);
+                                self.write_field_index(expr.hir_id, FieldIdx::from_usize(index));
                                 return field_ty;
                             }
                         }

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -539,7 +539,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
         match with_place.place.ty().kind() {
             ty::Adt(adt, substs) if adt.is_struct() => {
                 // Consume those fields of the with expression that are needed.
-                for (f_index, with_field) in adt.non_enum_variant().fields.iter().enumerate() {
+                for (f_index, with_field) in adt.non_enum_variant().fields.iter_enumerated() {
                     let is_mentioned = fields
                         .iter()
                         .any(|f| self.mc.typeck_results.opt_field_index(f.hir_id) == Some(f_index));
@@ -548,7 +548,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                             &*with_expr,
                             with_place.clone(),
                             with_field.ty(self.tcx(), substs),
-                            ProjectionKind::Field(f_index as u32, FIRST_VARIANT),
+                            ProjectionKind::Field(f_index, FIRST_VARIANT),
                         );
                         self.delegate_consume(&field_place, field_place.hir_id);
                     }

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -33,6 +33,7 @@ use rustc_span::def_id::LocalDefId;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
+use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::{self, NormalizeExt, ObligationCauseCode, ObligationCtxt};
 
@@ -147,7 +148,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    pub fn write_field_index(&self, hir_id: hir::HirId, index: usize) {
+    pub fn write_field_index(&self, hir_id: hir::HirId, index: FieldIdx) {
         self.typeck_results.borrow_mut().field_indices_mut().insert(hir_id, index);
     }
 

compiler/rustc_hir_typeck/src/intrinsicck.rs

@@ -4,7 +4,7 @@ use rustc_hir as hir;
 use rustc_index::vec::Idx;
 use rustc_middle::ty::layout::{LayoutError, SizeSkeleton};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
-use rustc_target::abi::{Pointer, VariantIdx};
+use rustc_target::abi::{FieldIdx, Pointer, VariantIdx};
 
 use super::FnCtxt;
 
@@ -28,7 +28,7 @@ fn unpack_option_like<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
         }
 
         if def.variant(data_idx).fields.len() == 1 {
-            return def.variant(data_idx).fields[0].ty(tcx, substs);
+            return def.variant(data_idx).fields[FieldIdx::from_u32(0)].ty(tcx, substs);
         }
     }
 

compiler/rustc_hir_typeck/src/mem_categorization.rs

@@ -61,7 +61,7 @@ use rustc_hir::pat_util::EnumerateAndAdjustIterator;
 use rustc_hir::PatKind;
 use rustc_infer::infer::InferCtxt;
 use rustc_span::Span;
-use rustc_target::abi::{VariantIdx, FIRST_VARIANT};
+use rustc_target::abi::{FieldIdx, VariantIdx, FIRST_VARIANT};
 use rustc_trait_selection::infer::InferCtxtExt;
 
 pub(crate) trait HirNode {
@@ -330,7 +330,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
                     expr,
                     base,
                     expr_ty,
-                    ProjectionKind::Field(field_idx as u32, FIRST_VARIANT),
+                    ProjectionKind::Field(field_idx, FIRST_VARIANT),
                 ))
             }
 
@@ -674,7 +674,8 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
 
                 for (i, subpat) in subpats.iter().enumerate_and_adjust(total_fields, dots_pos) {
                     let subpat_ty = self.pat_ty_adjusted(subpat)?;
-                    let projection_kind = ProjectionKind::Field(i as u32, FIRST_VARIANT);
+                    let projection_kind =
+                        ProjectionKind::Field(FieldIdx::from_usize(i), FIRST_VARIANT);
                     let sub_place =
                         self.cat_projection(pat, place_with_id.clone(), subpat_ty, projection_kind);
                     self.cat_pattern_(sub_place, subpat, op)?;
@@ -689,7 +690,8 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
 
                 for (i, subpat) in subpats.iter().enumerate_and_adjust(total_fields, dots_pos) {
                     let subpat_ty = self.pat_ty_adjusted(subpat)?;
-                    let projection_kind = ProjectionKind::Field(i as u32, variant_index);
+                    let projection_kind =
+                        ProjectionKind::Field(FieldIdx::from_usize(i), variant_index);
                     let sub_place =
                         self.cat_projection(pat, place_with_id.clone(), subpat_ty, projection_kind);
                     self.cat_pattern_(sub_place, subpat, op)?;
@@ -714,7 +716,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
                         pat,
                         place_with_id.clone(),
                         field_ty,
-                        ProjectionKind::Field(field_index as u32, variant_index),
+                        ProjectionKind::Field(field_index, variant_index),
                     );
                     self.cat_pattern_(field_place, fp.pat, op)?;
                 }

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -1815,7 +1815,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     .variants()
                     .iter()
                     .flat_map(|variant| {
-                        let [field] = &variant.fields[..] else { return None; };
+                        let [field] = &variant.fields.raw[..] else { return None; };
                         let field_ty = field.ty(tcx, substs);
 
                         // Skip `_`, since that'll just lead to ambiguity.