Change `tag_field` to `FieldIdx` in `Variants::Multiple`

It was already available as a generic parameter anyway, and it's not like we'll ever put a tag in the 5-billionth field.

14 files changed, 33 insertions, 31 deletions

diff --git a/compiler/rustc_abi/src/layout.rs b/compiler/rustc_abi/src/layout.rs
index 42250aa173b..21fd6be39fa 100644
--- a/compiler/rustc_abi/src/layout.rs
+++ b/compiler/rustc_abi/src/layout.rs
@@ -758,7 +758,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                         niche_variants,
                         niche_start,
                     },
-                    tag_field: 0,
+                    tag_field: FieldIdx::new(0),
                     variants: IndexVec::new(),
                 },
                 fields: FieldsShape::Arbitrary {
@@ -1072,7 +1072,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             variants: Variants::Multiple {
                 tag,
                 tag_encoding: TagEncoding::Direct,
-                tag_field: 0,
+                tag_field: FieldIdx::new(0),
                 variants: IndexVec::new(),
             },
             fields: FieldsShape::Arbitrary {
diff --git a/compiler/rustc_abi/src/layout/coroutine.rs b/compiler/rustc_abi/src/layout/coroutine.rs
index 27e704d538c..2b22276d4ae 100644
--- a/compiler/rustc_abi/src/layout/coroutine.rs
+++ b/compiler/rustc_abi/src/layout/coroutine.rs
@@ -158,7 +158,7 @@ pub(super) fn layout<
     // Build a prefix layout, including "promoting" all ineligible
     // locals as part of the prefix. We compute the layout of all of
     // these fields at once to get optimal packing.
-    let tag_index = prefix_layouts.len();
+    let tag_index = prefix_layouts.next_index();
 
     // `variant_fields` already accounts for the reserved variants, so no need to add them.
     let max_discr = (variant_fields.len() - 1) as u128;
@@ -187,7 +187,7 @@ pub(super) fn layout<
 
             // "a" (`0..b_start`) and "b" (`b_start..`) correspond to
             // "outer" and "promoted" fields respectively.
-            let b_start = FieldIdx::new(tag_index + 1);
+            let b_start = tag_index.plus(1);
             let offsets_b = IndexVec::from_raw(offsets.raw.split_off(b_start.index()));
             let offsets_a = offsets;
 
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index b806d0aba31..46b7a0c1e77 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -1573,7 +1573,7 @@ pub enum Variants<FieldIdx: Idx, VariantIdx: Idx> {
     Multiple {
         tag: Scalar,
         tag_encoding: TagEncoding<VariantIdx>,
-        tag_field: usize,
+        tag_field: FieldIdx,
         variants: IndexVec<VariantIdx, LayoutData<FieldIdx, VariantIdx>>,
     },
 }
diff --git a/compiler/rustc_codegen_cranelift/src/discriminant.rs b/compiler/rustc_codegen_cranelift/src/discriminant.rs
index 4d0d5dc60eb..a08b0e0cbfc 100644
--- a/compiler/rustc_codegen_cranelift/src/discriminant.rs
+++ b/compiler/rustc_codegen_cranelift/src/discriminant.rs
@@ -28,7 +28,7 @@ pub(crate) fn codegen_set_discriminant<'tcx>(
             tag_encoding: TagEncoding::Direct,
             variants: _,
         } => {
-            let ptr = place.place_field(fx, FieldIdx::new(tag_field));
+            let ptr = place.place_field(fx, tag_field);
             let to = layout.ty.discriminant_for_variant(fx.tcx, variant_index).unwrap().val;
             let to = match ptr.layout().ty.kind() {
                 ty::Uint(UintTy::U128) | ty::Int(IntTy::I128) => {
@@ -53,7 +53,7 @@ pub(crate) fn codegen_set_discriminant<'tcx>(
             variants: _,
         } => {
             if variant_index != untagged_variant {
-                let niche = place.place_field(fx, FieldIdx::new(tag_field));
+                let niche = place.place_field(fx, tag_field);
                 let niche_type = fx.clif_type(niche.layout().ty).unwrap();
                 let niche_value = variant_index.as_u32() - niche_variants.start().as_u32();
                 let niche_value = (niche_value as u128).wrapping_add(niche_start);
@@ -118,7 +118,7 @@ pub(crate) fn codegen_get_discriminant<'tcx>(
     let cast_to = fx.clif_type(dest_layout.ty).unwrap();
 
     // Read the tag/niche-encoded discriminant from memory.
-    let tag = value.value_field(fx, FieldIdx::new(tag_field));
+    let tag = value.value_field(fx, tag_field);
     let tag = tag.load_scalar(fx);
 
     // Decode the discriminant (specifically if it's niche-encoded).
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/cpp_like.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/cpp_like.rs
index e9574108696..a5c80895741 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/cpp_like.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/cpp_like.rs
@@ -1,7 +1,7 @@
 use std::borrow::Cow;
 
 use libc::c_uint;
-use rustc_abi::{Align, Endian, Size, TagEncoding, VariantIdx, Variants};
+use rustc_abi::{Align, Endian, FieldIdx, Size, TagEncoding, VariantIdx, Variants};
 use rustc_codegen_ssa::debuginfo::type_names::compute_debuginfo_type_name;
 use rustc_codegen_ssa::debuginfo::{tag_base_type, wants_c_like_enum_debuginfo};
 use rustc_codegen_ssa::traits::{ConstCodegenMethods, MiscCodegenMethods};
@@ -401,7 +401,7 @@ fn build_union_fields_for_enum<'ll, 'tcx>(
     enum_type_and_layout: TyAndLayout<'tcx>,
     enum_type_di_node: &'ll DIType,
     variant_indices: impl Iterator<Item = VariantIdx> + Clone,
-    tag_field: usize,
+    tag_field: FieldIdx,
     untagged_variant_index: Option<VariantIdx>,
 ) -> SmallVec<&'ll DIType> {
     let tag_base_type = tag_base_type(cx.tcx, enum_type_and_layout);
@@ -805,7 +805,7 @@ fn build_union_fields_for_direct_tag_enum_or_coroutine<'ll, 'tcx>(
     variant_field_infos: &[VariantFieldInfo<'ll>],
     discr_type_di_node: &'ll DIType,
     tag_base_type: Ty<'tcx>,
-    tag_field: usize,
+    tag_field: FieldIdx,
     untagged_variant_index: Option<VariantIdx>,
     di_flags: DIFlags,
 ) -> SmallVec<&'ll DIType> {
@@ -858,7 +858,7 @@ fn build_union_fields_for_direct_tag_enum_or_coroutine<'ll, 'tcx>(
     }));
 
     assert_eq!(
-        cx.size_and_align_of(enum_type_and_layout.field(cx, tag_field).ty),
+        cx.size_and_align_of(enum_type_and_layout.field(cx, tag_field.as_usize()).ty),
         cx.size_and_align_of(self::tag_base_type(cx.tcx, enum_type_and_layout))
     );
 
@@ -875,7 +875,7 @@ fn build_union_fields_for_direct_tag_enum_or_coroutine<'ll, 'tcx>(
             Endian::Big => (8, 0),
         };
 
-        let tag_field_offset = enum_type_and_layout.fields.offset(tag_field).bytes();
+        let tag_field_offset = enum_type_and_layout.fields.offset(tag_field.as_usize()).bytes();
         let lo_offset = Size::from_bytes(tag_field_offset + lo_offset);
         let hi_offset = Size::from_bytes(tag_field_offset + hi_offset);
 
@@ -905,8 +905,8 @@ fn build_union_fields_for_direct_tag_enum_or_coroutine<'ll, 'tcx>(
             cx,
             enum_type_di_node,
             TAG_FIELD_NAME,
-            enum_type_and_layout.field(cx, tag_field),
-            enum_type_and_layout.fields.offset(tag_field),
+            enum_type_and_layout.field(cx, tag_field.as_usize()),
+            enum_type_and_layout.fields.offset(tag_field.as_usize()),
             di_flags,
             tag_base_type_di_node,
             None,
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs
index 20a841f2287..62d38d463ab 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs
@@ -373,7 +373,7 @@ fn build_discr_member_di_node<'ll, 'tcx>(
                 file,
                 UNKNOWN_LINE_NUMBER,
                 layout,
-                enum_or_coroutine_type_and_layout.fields.offset(tag_field),
+                enum_or_coroutine_type_and_layout.fields.offset(tag_field.as_usize()),
                 DIFlags::FlagArtificial,
                 ty,
             ))
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
index eade9e52de9..b7f2277bfda 100644
--- a/compiler/rustc_codegen_ssa/src/mir/operand.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -462,10 +462,10 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
         let tag_op = match self.val {
             OperandValue::ZeroSized => bug!(),
             OperandValue::Immediate(_) | OperandValue::Pair(_, _) => {
-                self.extract_field(fx, bx, tag_field)
+                self.extract_field(fx, bx, tag_field.as_usize())
             }
             OperandValue::Ref(place) => {
-                let tag = place.with_type(self.layout).project_field(bx, tag_field);
+                let tag = place.with_type(self.layout).project_field(bx, tag_field.as_usize());
                 bx.load_operand(tag)
             }
         };
diff --git a/compiler/rustc_codegen_ssa/src/mir/place.rs b/compiler/rustc_codegen_ssa/src/mir/place.rs
index 31db7fa9a18..937063c24a6 100644
--- a/compiler/rustc_codegen_ssa/src/mir/place.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/place.rs
@@ -250,7 +250,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
             Variants::Single { index } => assert_eq!(index, variant_index),
 
             Variants::Multiple { tag_encoding: TagEncoding::Direct, tag_field, .. } => {
-                let ptr = self.project_field(bx, tag_field);
+                let ptr = self.project_field(bx, tag_field.as_usize());
                 let to =
                     self.layout.ty.discriminant_for_variant(bx.tcx(), variant_index).unwrap().val;
                 bx.store_to_place(
@@ -265,7 +265,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
                 ..
             } => {
                 if variant_index != untagged_variant {
-                    let niche = self.project_field(bx, tag_field);
+                    let niche = self.project_field(bx, tag_field.as_usize());
                     let niche_llty = bx.cx().immediate_backend_type(niche.layout);
                     let BackendRepr::Scalar(scalar) = niche.layout.backend_repr else {
                         bug!("expected a scalar placeref for the niche");
diff --git a/compiler/rustc_const_eval/src/interpret/discriminant.rs b/compiler/rustc_const_eval/src/interpret/discriminant.rs
index 2f0b1cb6d1e..020cd65d75d 100644
--- a/compiler/rustc_const_eval/src/interpret/discriminant.rs
+++ b/compiler/rustc_const_eval/src/interpret/discriminant.rs
@@ -1,6 +1,6 @@
 //! Functions for reading and writing discriminants of multi-variant layouts (enums and coroutines).
 
-use rustc_abi::{self as abi, TagEncoding, VariantIdx, Variants};
+use rustc_abi::{self as abi, FieldIdx, TagEncoding, VariantIdx, Variants};
 use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt, TyAndLayout};
 use rustc_middle::ty::{self, CoroutineArgsExt, ScalarInt, Ty};
 use rustc_middle::{mir, span_bug};
@@ -26,7 +26,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 // No need to validate that the discriminant here because the
                 // `TyAndLayout::for_variant()` call earlier already checks the
                 // variant is valid.
-                let tag_dest = self.project_field(dest, tag_field)?;
+                let tag_dest = self.project_field(dest, tag_field.as_usize())?;
                 self.write_scalar(tag, &tag_dest)
             }
             None => {
@@ -96,7 +96,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         let tag_layout = self.layout_of(tag_scalar_layout.primitive().to_int_ty(*self.tcx))?;
 
         // Read tag and sanity-check `tag_layout`.
-        let tag_val = self.read_immediate(&self.project_field(op, tag_field)?)?;
+        let tag_val = self.read_immediate(&self.project_field(op, tag_field.as_usize())?)?;
         assert_eq!(tag_layout.size, tag_val.layout.size);
         assert_eq!(tag_layout.backend_repr.is_signed(), tag_val.layout.backend_repr.is_signed());
         trace!("tag value: {}", tag_val);
@@ -231,7 +231,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         &self,
         layout: TyAndLayout<'tcx>,
         variant_index: VariantIdx,
-    ) -> InterpResult<'tcx, Option<(ScalarInt, usize)>> {
+    ) -> InterpResult<'tcx, Option<(ScalarInt, FieldIdx)>> {
         // Layout computation excludes uninhabited variants from consideration.
         // Therefore, there's no way to represent those variants in the given layout.
         // Essentially, uninhabited variants do not have a tag that corresponds to their
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index 8f39afa642a..7d76d925ef2 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -294,7 +294,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
         // First, check if we are projecting to a variant.
         match layout.variants {
             Variants::Multiple { tag_field, .. } => {
-                if tag_field == field {
+                if tag_field.as_usize() == field {
                     return match layout.ty.kind() {
                         ty::Adt(def, ..) if def.is_enum() => PathElem::EnumTag,
                         ty::Coroutine(..) => PathElem::CoroutineTag,
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 7ebfebea44e..c2ae6b06192 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -934,7 +934,7 @@ where
                             .unwrap(),
                     ),
                     Variants::Multiple { tag, tag_field, .. } => {
-                        if i == tag_field {
+                        if FieldIdx::from_usize(i) == tag_field {
                             return TyMaybeWithLayout::TyAndLayout(tag_layout(tag));
                         }
                         TyMaybeWithLayout::Ty(args.as_coroutine().prefix_tys()[i])
@@ -1060,8 +1060,10 @@ where
                         tag_field,
                         variants,
                         ..
-                    } if variants.len() == 2 && this.fields.offset(*tag_field) == offset => {
-                        let tagged_variant = if untagged_variant.as_u32() == 0 {
+                    } if variants.len() == 2
+                        && this.fields.offset(tag_field.as_usize()) == offset =>
+                    {
+                        let tagged_variant = if *untagged_variant == VariantIdx::ZERO {
                             VariantIdx::from_u32(1)
                         } else {
                             VariantIdx::from_u32(0)
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/abi.rs b/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
index 06dfaf079a3..64901ee0502 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
@@ -180,7 +180,7 @@ impl<'tcx> Stable<'tcx> for rustc_abi::Variants<rustc_abi::FieldIdx, rustc_abi::
                 VariantsShape::Multiple {
                     tag: tag.stable(tables),
                     tag_encoding: tag_encoding.stable(tables),
-                    tag_field: *tag_field,
+                    tag_field: tag_field.stable(tables),
                     variants: variants.iter().as_slice().stable(tables),
                 }
             }
diff --git a/compiler/rustc_transmute/src/layout/tree.rs b/compiler/rustc_transmute/src/layout/tree.rs
index 7cf712ce9e9..ff665695b5a 100644
--- a/compiler/rustc_transmute/src/layout/tree.rs
+++ b/compiler/rustc_transmute/src/layout/tree.rs
@@ -451,7 +451,7 @@ pub(crate) mod rustc {
 
                     // For enums (but not coroutines), the tag field is
                     // currently always the first field of the layout.
-                    assert_eq!(*tag_field, 0);
+                    assert_eq!(*tag_field, FieldIdx::ZERO);
 
                     let variants = def.discriminants(cx.tcx()).try_fold(
                         Self::uninhabited(),
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index ad57555bd24..9774263e4c9 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -932,7 +932,7 @@ fn variant_info_for_coroutine<'tcx>(
             // However, if the discriminant is placed past the end of the variant, then we need
             // to factor in the size of the discriminant manually. This really should be refactored
             // better, but this "works" for now.
-            if layout.fields.offset(tag_field) >= variant_size {
+            if layout.fields.offset(tag_field.as_usize()) >= variant_size {
                 variant_size += match tag_encoding {
                     TagEncoding::Direct => tag.size(cx),
                     _ => Size::ZERO,