Update `InterpCx::project_field` to take `FieldIdx`

As suggested by Ralf in 142005.

26 files changed, 102 insertions, 80 deletions

diff --git a/compiler/rustc_abi/src/layout/ty.rs b/compiler/rustc_abi/src/layout/ty.rs
index 4f43c0e6f8e..eb49b977257 100644
--- a/compiler/rustc_abi/src/layout/ty.rs
+++ b/compiler/rustc_abi/src/layout/ty.rs
@@ -39,6 +39,13 @@ rustc_index::newtype_index! {
     pub struct FieldIdx {}
 }
 
+impl FieldIdx {
+    /// The second field.
+    ///
+    /// For use alongside [`FieldIdx::ZERO`], particularly with scalar pairs.
+    pub const ONE: FieldIdx = FieldIdx::from_u32(1);
+}
+
 rustc_index::newtype_index! {
     /// The *source-order* index of a variant in a type.
     ///
@@ -274,7 +281,7 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
 
     /// Finds the one field that is not a 1-ZST.
     /// Returns `None` if there are multiple non-1-ZST fields or only 1-ZST-fields.
-    pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(usize, Self)>
+    pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(FieldIdx, Self)>
     where
         Ty: TyAbiInterface<'a, C> + Copy,
     {
@@ -288,7 +295,7 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
                 // More than one non-1-ZST field.
                 return None;
             }
-            found = Some((field_idx, field));
+            found = Some((FieldIdx::from_usize(field_idx), field));
         }
         found
     }
diff --git a/compiler/rustc_codegen_cranelift/src/inline_asm.rs b/compiler/rustc_codegen_cranelift/src/inline_asm.rs
index afee5095549..120d6ff9e38 100644
--- a/compiler/rustc_codegen_cranelift/src/inline_asm.rs
+++ b/compiler/rustc_codegen_cranelift/src/inline_asm.rs
@@ -850,7 +850,7 @@ fn asm_clif_type<'tcx>(fx: &FunctionCx<'_, '_, 'tcx>, ty: Ty<'tcx>) -> Option<ty
         // Adapted from https://github.com/rust-lang/rust/blob/f3c66088610c1b80110297c2d9a8b5f9265b013f/compiler/rustc_hir_analysis/src/check/intrinsicck.rs#L136-L151
         ty::Adt(adt, args) if fx.tcx.is_lang_item(adt.did(), LangItem::MaybeUninit) => {
             let fields = &adt.non_enum_variant().fields;
-            let ty = fields[FieldIdx::from_u32(1)].ty(fx.tcx, args);
+            let ty = fields[FieldIdx::ONE].ty(fx.tcx, args);
             let ty::Adt(ty, args) = ty.kind() else {
                 unreachable!("expected first field of `MaybeUninit` to be an ADT")
             };
diff --git a/compiler/rustc_codegen_cranelift/src/vtable.rs b/compiler/rustc_codegen_cranelift/src/vtable.rs
index 9d9e0462a9b..05a8e3c3342 100644
--- a/compiler/rustc_codegen_cranelift/src/vtable.rs
+++ b/compiler/rustc_codegen_cranelift/src/vtable.rs
@@ -53,7 +53,7 @@ pub(crate) fn get_ptr_and_method_ref<'tcx>(
                     .layout()
                     .non_1zst_field(fx)
                     .expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type");
-                arg = arg.value_field(fx, FieldIdx::new(idx));
+                arg = arg.value_field(fx, idx);
             }
         }
 
@@ -62,8 +62,7 @@ pub(crate) fn get_ptr_and_method_ref<'tcx>(
                 let inner_layout = fx.layout_of(arg.layout().ty.builtin_deref(true).unwrap());
                 let dyn_star = CPlace::for_ptr(Pointer::new(arg.load_scalar(fx)), inner_layout);
                 let ptr = dyn_star.place_field(fx, FieldIdx::ZERO).to_ptr();
-                let vtable =
-                    dyn_star.place_field(fx, FieldIdx::new(1)).to_cvalue(fx).load_scalar(fx);
+                let vtable = dyn_star.place_field(fx, FieldIdx::ONE).to_cvalue(fx).load_scalar(fx);
                 break 'block (ptr, vtable);
             }
         }
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 1baab62ae43..7cea48ce1cc 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -1077,7 +1077,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             let (idx, _) = op.layout.non_1zst_field(bx).expect(
                                 "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
                             );
-                            op = op.extract_field(self, bx, idx);
+                            op = op.extract_field(self, bx, idx.as_usize());
                         }
 
                         // Now that we have `*dyn Trait` or `&dyn Trait`, split it up into its
@@ -1109,7 +1109,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             let (idx, _) = op.layout.non_1zst_field(bx).expect(
                                 "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
                             );
-                            op = op.extract_field(self, bx, idx);
+                            op = op.extract_field(self, bx, idx.as_usize());
                         }
 
                         // Make sure that we've actually unwrapped the rcvr down
diff --git a/compiler/rustc_const_eval/src/const_eval/mod.rs b/compiler/rustc_const_eval/src/const_eval/mod.rs
index c0438fb3ff8..6fd0b9d26e3 100644
--- a/compiler/rustc_const_eval/src/const_eval/mod.rs
+++ b/compiler/rustc_const_eval/src/const_eval/mod.rs
@@ -1,6 +1,6 @@
 // Not in interpret to make sure we do not use private implementation details
 
-use rustc_abi::VariantIdx;
+use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_middle::query::Key;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -60,7 +60,7 @@ pub(crate) fn try_destructure_mir_constant_for_user_output<'tcx>(
 
     let fields_iter = (0..field_count)
         .map(|i| {
-            let field_op = ecx.project_field(&down, i).discard_err()?;
+            let field_op = ecx.project_field(&down, FieldIdx::from_usize(i)).discard_err()?;
             let val = op_to_const(&ecx, &field_op, /* for diagnostics */ true);
             Some((val, field_op.layout.ty))
         })
diff --git a/compiler/rustc_const_eval/src/const_eval/valtrees.rs b/compiler/rustc_const_eval/src/const_eval/valtrees.rs
index 34239ae1d15..58d230af683 100644
--- a/compiler/rustc_const_eval/src/const_eval/valtrees.rs
+++ b/compiler/rustc_const_eval/src/const_eval/valtrees.rs
@@ -1,4 +1,4 @@
-use rustc_abi::{BackendRepr, VariantIdx};
+use rustc_abi::{BackendRepr, FieldIdx, VariantIdx};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_middle::mir::interpret::{EvalToValTreeResult, GlobalId, ReportedErrorInfo};
 use rustc_middle::ty::layout::{LayoutCx, LayoutOf, TyAndLayout};
@@ -40,7 +40,7 @@ fn branches<'tcx>(
     }
 
     for i in 0..field_count {
-        let field = ecx.project_field(&place, i).unwrap();
+        let field = ecx.project_field(&place, FieldIdx::from_usize(i)).unwrap();
         let valtree = const_to_valtree_inner(ecx, &field, num_nodes)?;
         branches.push(valtree);
     }
@@ -437,7 +437,7 @@ fn valtree_into_mplace<'tcx>(
                     ty::Str | ty::Slice(_) | ty::Array(..) => {
                         ecx.project_index(place, i as u64).unwrap()
                     }
-                    _ => ecx.project_field(&place_adjusted, i).unwrap(),
+                    _ => ecx.project_field(&place_adjusted, FieldIdx::from_usize(i)).unwrap(),
                 };
 
                 debug!(?place_inner);
diff --git a/compiler/rustc_const_eval/src/interpret/call.rs b/compiler/rustc_const_eval/src/interpret/call.rs
index 789baea0734..37677f9e048 100644
--- a/compiler/rustc_const_eval/src/interpret/call.rs
+++ b/compiler/rustc_const_eval/src/interpret/call.rs
@@ -62,7 +62,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     pub(super) fn fn_arg_field(
         &self,
         arg: &FnArg<'tcx, M::Provenance>,
-        field: usize,
+        field: FieldIdx,
     ) -> InterpResult<'tcx, FnArg<'tcx, M::Provenance>> {
         interp_ok(match arg {
             FnArg::Copy(op) => FnArg::Copy(self.project_field(op, field)?),
@@ -600,10 +600,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                         Cow::from(
                             args.iter()
                                 .map(|a| interp_ok(a.clone()))
-                                .chain(
-                                    (0..untuple_arg.layout().fields.count())
-                                        .map(|i| self.fn_arg_field(untuple_arg, i)),
-                                )
+                                .chain((0..untuple_arg.layout().fields.count()).map(|i| {
+                                    self.fn_arg_field(untuple_arg, FieldIdx::from_usize(i))
+                                }))
                                 .collect::<InterpResult<'_, Vec<_>>>()?,
                         )
                     } else {
diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index 643a5805019..9e15f4572d7 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -1,6 +1,6 @@
 use std::assert_matches::assert_matches;
 
-use rustc_abi::Integer;
+use rustc_abi::{FieldIdx, Integer};
 use rustc_apfloat::ieee::{Double, Half, Quad, Single};
 use rustc_apfloat::{Float, FloatConvert};
 use rustc_middle::mir::CastKind;
@@ -484,6 +484,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let mut found_cast_field = false;
                 for i in 0..src.layout.fields.count() {
                     let cast_ty_field = cast_ty.field(self, i);
+                    let i = FieldIdx::from_usize(i);
                     let src_field = self.project_field(src, i)?;
                     let dst_field = self.project_field(dest, i)?;
                     if src_field.layout.is_1zst() && cast_ty_field.is_1zst() {
diff --git a/compiler/rustc_const_eval/src/interpret/discriminant.rs b/compiler/rustc_const_eval/src/interpret/discriminant.rs
index 020cd65d75d..6c4b000e16b 100644
--- a/compiler/rustc_const_eval/src/interpret/discriminant.rs
+++ b/compiler/rustc_const_eval/src/interpret/discriminant.rs
@@ -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.as_usize())?;
+                let tag_dest = self.project_field(dest, tag_field)?;
                 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.as_usize())?)?;
+        let tag_val = self.read_immediate(&self.project_field(op, tag_field)?)?;
         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);
diff --git a/compiler/rustc_const_eval/src/interpret/projection.rs b/compiler/rustc_const_eval/src/interpret/projection.rs
index 8ecb3e13d5c..ad47a19a14d 100644
--- a/compiler/rustc_const_eval/src/interpret/projection.rs
+++ b/compiler/rustc_const_eval/src/interpret/projection.rs
@@ -10,7 +10,7 @@
 use std::marker::PhantomData;
 use std::ops::Range;
 
-use rustc_abi::{self as abi, Size, VariantIdx};
+use rustc_abi::{self as abi, FieldIdx, Size, VariantIdx};
 use rustc_middle::ty::Ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::{bug, mir, span_bug, ty};
@@ -144,22 +144,22 @@ where
     /// always possible without allocating, so it can take `&self`. Also return the field's layout.
     /// This supports both struct and array fields, but not slices!
     ///
-    /// This also works for arrays, but then the `usize` index type is restricting.
-    /// For indexing into arrays, use `mplace_index`.
+    /// This also works for arrays, but then the `FieldIdx` index type is restricting.
+    /// For indexing into arrays, use [`Self::project_index`].
     pub fn project_field<P: Projectable<'tcx, M::Provenance>>(
         &self,
         base: &P,
-        field: usize,
+        field: FieldIdx,
     ) -> InterpResult<'tcx, P> {
         // Slices nominally have length 0, so they will panic somewhere in `fields.offset`.
         debug_assert!(
             !matches!(base.layout().ty.kind(), ty::Slice(..)),
             "`field` projection called on a slice -- call `index` projection instead"
         );
-        let offset = base.layout().fields.offset(field);
+        let offset = base.layout().fields.offset(field.as_usize());
         // Computing the layout does normalization, so we get a normalized type out of this
         // even if the field type is non-normalized (possible e.g. via associated types).
-        let field_layout = base.layout().field(self, field);
+        let field_layout = base.layout().field(self, field.as_usize());
 
         // Offset may need adjustment for unsized fields.
         let (meta, offset) = if field_layout.is_unsized() {
@@ -244,7 +244,7 @@ where
             }
             _ => span_bug!(
                 self.cur_span(),
-                "`mplace_index` called on non-array type {:?}",
+                "`project_index` called on non-array type {:?}",
                 base.layout().ty
             ),
         };
@@ -260,7 +260,7 @@ where
     ) -> InterpResult<'tcx, (P, u64)> {
         assert!(base.layout().ty.ty_adt_def().unwrap().repr().simd());
         // SIMD types must be newtypes around arrays, so all we have to do is project to their only field.
-        let array = self.project_field(base, 0)?;
+        let array = self.project_field(base, FieldIdx::ZERO)?;
         let len = array.len(self)?;
         interp_ok((array, len))
     }
@@ -384,7 +384,7 @@ where
             UnwrapUnsafeBinder(target) => base.transmute(self.layout_of(target)?, self)?,
             // We don't want anything happening here, this is here as a dummy.
             Subtype(_) => base.transmute(base.layout(), self)?,
-            Field(field, _) => self.project_field(base, field.index())?,
+            Field(field, _) => self.project_field(base, field)?,
             Downcast(_, variant) => self.project_downcast(base, variant)?,
             Deref => self.deref_pointer(&base.to_op(self)?)?.into(),
             Index(local) => {
diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 975325b0c1e..833fcc38817 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -333,7 +333,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         }
         for (field_index, operand) in operands.iter_enumerated() {
             let field_index = active_field_index.unwrap_or(field_index);
-            let field_dest = self.project_field(&variant_dest, field_index.as_usize())?;
+            let field_dest = self.project_field(&variant_dest, field_index)?;
             let op = self.eval_operand(operand, Some(field_dest.layout))?;
             self.copy_op(&op, &field_dest)?;
         }
diff --git a/compiler/rustc_const_eval/src/interpret/traits.rs b/compiler/rustc_const_eval/src/interpret/traits.rs
index a5029eea5a7..7249ef23bf6 100644
--- a/compiler/rustc_const_eval/src/interpret/traits.rs
+++ b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -1,4 +1,4 @@
-use rustc_abi::{Align, Size};
+use rustc_abi::{Align, FieldIdx, Size};
 use rustc_middle::mir::interpret::{InterpResult, Pointer};
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::{self, ExistentialPredicateStableCmpExt, Ty, TyCtxt, VtblEntry};
@@ -137,8 +137,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             matches!(val.layout().ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
             "`unpack_dyn_star` only makes sense on `dyn*` types"
         );
-        let data = self.project_field(val, 0)?;
-        let vtable = self.project_field(val, 1)?;
+        let data = self.project_field(val, FieldIdx::ZERO)?;
+        let vtable = self.project_field(val, FieldIdx::ONE)?;
         let vtable = self.read_pointer(&vtable.to_op(self)?)?;
         let ty = self.get_ptr_vtable_ty(vtable, Some(expected_trait))?;
         // `data` is already the right thing but has the wrong type. So we transmute it.
diff --git a/compiler/rustc_const_eval/src/interpret/visitor.rs b/compiler/rustc_const_eval/src/interpret/visitor.rs
index 3647c109a6e..5aea91233bd 100644
--- a/compiler/rustc_const_eval/src/interpret/visitor.rs
+++ b/compiler/rustc_const_eval/src/interpret/visitor.rs
@@ -112,8 +112,10 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
                 // So we transmute it to a raw pointer.
                 let raw_ptr_ty = Ty::new_mut_ptr(*self.ecx().tcx, self.ecx().tcx.types.unit);
                 let raw_ptr_ty = self.ecx().layout_of(raw_ptr_ty)?;
-                let vtable_field =
-                    self.ecx().project_field(v, 1)?.transmute(raw_ptr_ty, self.ecx())?;
+                let vtable_field = self
+                    .ecx()
+                    .project_field(v, FieldIdx::ONE)?
+                    .transmute(raw_ptr_ty, self.ecx())?;
                 self.visit_field(v, 1, &vtable_field)?;
 
                 // Then unpack the first field, and continue.
@@ -140,14 +142,16 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
 
                 // `Box` has two fields: the pointer we care about, and the allocator.
                 assert_eq!(v.layout().fields.count(), 2, "`Box` must have exactly 2 fields");
-                let (unique_ptr, alloc) =
-                    (self.ecx().project_field(v, 0)?, self.ecx().project_field(v, 1)?);
+                let (unique_ptr, alloc) = (
+                    self.ecx().project_field(v, FieldIdx::ZERO)?,
+                    self.ecx().project_field(v, FieldIdx::ONE)?,
+                );
                 // Unfortunately there is some type junk in the way here: `unique_ptr` is a `Unique`...
                 // (which means another 2 fields, the second of which is a `PhantomData`)
                 assert_eq!(unique_ptr.layout().fields.count(), 2);
                 let (nonnull_ptr, phantom) = (
-                    self.ecx().project_field(&unique_ptr, 0)?,
-                    self.ecx().project_field(&unique_ptr, 1)?,
+                    self.ecx().project_field(&unique_ptr, FieldIdx::ZERO)?,
+                    self.ecx().project_field(&unique_ptr, FieldIdx::ONE)?,
                 );
                 assert!(
                     phantom.layout().ty.ty_adt_def().is_some_and(|adt| adt.is_phantom_data()),
@@ -156,7 +160,7 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
                 );
                 // ... that contains a `NonNull`... (gladly, only a single field here)
                 assert_eq!(nonnull_ptr.layout().fields.count(), 1);
-                let raw_ptr = self.ecx().project_field(&nonnull_ptr, 0)?; // the actual raw ptr
+                let raw_ptr = self.ecx().project_field(&nonnull_ptr, FieldIdx::ZERO)?; // the actual raw ptr
                 // ... whose only field finally is a raw ptr we can dereference.
                 self.visit_box(ty, &raw_ptr)?;
 
@@ -188,9 +192,8 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
             }
             FieldsShape::Arbitrary { memory_index, .. } => {
                 for idx in Self::aggregate_field_iter(memory_index) {
-                    let idx = idx.as_usize();
                     let field = self.ecx().project_field(v, idx)?;
-                    self.visit_field(v, idx, &field)?;
+                    self.visit_field(v, idx.as_usize(), &field)?;
                 }
             }
             FieldsShape::Array { .. } => {
diff --git a/compiler/rustc_const_eval/src/util/caller_location.rs b/compiler/rustc_const_eval/src/util/caller_location.rs
index 9c867cc615e..65d02d3bafa 100644
--- a/compiler/rustc_const_eval/src/util/caller_location.rs
+++ b/compiler/rustc_const_eval/src/util/caller_location.rs
@@ -1,3 +1,4 @@
+use rustc_abi::FieldIdx;
 use rustc_hir::LangItem;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::{self, TyCtxt};
@@ -41,11 +42,14 @@ fn alloc_caller_location<'tcx>(
     let location = ecx.allocate(loc_layout, MemoryKind::CallerLocation).unwrap();
 
     // Initialize fields.
-    ecx.write_immediate(file_wide_ptr, &ecx.project_field(&location, 0).unwrap())
+    ecx.write_immediate(
+        file_wide_ptr,
+        &ecx.project_field(&location, FieldIdx::from_u32(0)).unwrap(),
+    )
+    .expect("writing to memory we just allocated cannot fail");
+    ecx.write_scalar(line, &ecx.project_field(&location, FieldIdx::from_u32(1)).unwrap())
         .expect("writing to memory we just allocated cannot fail");
-    ecx.write_scalar(line, &ecx.project_field(&location, 1).unwrap())
-        .expect("writing to memory we just allocated cannot fail");
-    ecx.write_scalar(col, &ecx.project_field(&location, 2).unwrap())
+    ecx.write_scalar(col, &ecx.project_field(&location, FieldIdx::from_u32(2)).unwrap())
         .expect("writing to memory we just allocated cannot fail");
 
     location
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index 846eacce9e1..5d42de74f6c 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -1100,7 +1100,7 @@ fn check_simd(tcx: TyCtxt<'_>, sp: Span, def_id: LocalDefId) {
             return;
         };
 
-        if let Some(second_field) = fields.get(FieldIdx::from_u32(1)) {
+        if let Some(second_field) = fields.get(FieldIdx::ONE) {
             struct_span_code_err!(tcx.dcx(), sp, E0075, "SIMD vector cannot have multiple fields")
                 .with_span_label(tcx.def_span(second_field.did), "excess field")
                 .emit();
diff --git a/compiler/rustc_hir_typeck/src/inline_asm.rs b/compiler/rustc_hir_typeck/src/inline_asm.rs
index 6399f0a78ae..b59c1752c25 100644
--- a/compiler/rustc_hir_typeck/src/inline_asm.rs
+++ b/compiler/rustc_hir_typeck/src/inline_asm.rs
@@ -171,7 +171,7 @@ impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
             _ if ty.references_error() => return None,
             ty::Adt(adt, args) if self.tcx().is_lang_item(adt.did(), LangItem::MaybeUninit) => {
                 let fields = &adt.non_enum_variant().fields;
-                let ty = fields[FieldIdx::from_u32(1)].ty(self.tcx(), args);
+                let ty = fields[FieldIdx::ONE].ty(self.tcx(), args);
                 // FIXME: Are we just trying to map to the `T` in `MaybeUninit<T>`?
                 // If so, just get it from the args.
                 let ty::Adt(ty, args) = ty.kind() else {
diff --git a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
index 99b95e7312b..0cf8142a560 100644
--- a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
+++ b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
@@ -616,7 +616,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             place,
             operand,
             &mut |elem, op| match elem {
-                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).discard_err(),
+                TrackElem::Field(idx) => self.ecx.project_field(op, idx).discard_err(),
                 TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).discard_err(),
                 TrackElem::Discriminant => {
                     let variant = self.ecx.read_discriminant(op).discard_err()?;
@@ -890,7 +890,8 @@ fn try_write_constant<'tcx>(
 
         ty::Tuple(elem_tys) => {
             for (i, elem) in elem_tys.iter().enumerate() {
-                let Some(field) = map.apply(place, TrackElem::Field(FieldIdx::from_usize(i))) else {
+                let i = FieldIdx::from_usize(i);
+                let Some(field) = map.apply(place, TrackElem::Field(i)) else {
                     throw_machine_stop_str!("missing field in tuple")
                 };
                 let field_dest = ecx.project_field(dest, i)?;
@@ -928,7 +929,7 @@ fn try_write_constant<'tcx>(
                 let Some(field) = map.apply(variant_place, TrackElem::Field(i)) else {
                     throw_machine_stop_str!("missing field in ADT")
                 };
-                let field_dest = ecx.project_field(&variant_dest, i.as_usize())?;
+                let field_dest = ecx.project_field(&variant_dest, i)?;
                 try_write_constant(ecx, &field_dest, field, ty, state, map)?;
             }
             ecx.write_discriminant(variant_idx, dest)?;
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index a91d46ec406..92c30d239b5 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -438,8 +438,10 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                         dest.clone()
                     };
                     for (field_index, op) in fields.into_iter().enumerate() {
-                        let field_dest =
-                            self.ecx.project_field(&variant_dest, field_index).discard_err()?;
+                        let field_dest = self
+                            .ecx
+                            .project_field(&variant_dest, FieldIdx::from_usize(field_index))
+                            .discard_err()?;
                         self.ecx.copy_op(op, &field_dest).discard_err()?;
                     }
                     self.ecx
@@ -1583,7 +1585,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             // We needed to check the variant to avoid trying to read the tag
             // field from an enum where no fields have variants, since that tag
             // field isn't in the `Aggregate` from which we're getting values.
-            Some((FieldIdx::from_usize(field_idx), field_layout.ty))
+            Some((field_idx, field_layout.ty))
         } else if let ty::Adt(adt, args) = ty.kind()
             && adt.is_struct()
             && adt.repr().transparent()
diff --git a/compiler/rustc_mir_transform/src/jump_threading.rs b/compiler/rustc_mir_transform/src/jump_threading.rs
index 31b361ec1a9..48db536c122 100644
--- a/compiler/rustc_mir_transform/src/jump_threading.rs
+++ b/compiler/rustc_mir_transform/src/jump_threading.rs
@@ -388,7 +388,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             lhs,
             constant,
             &mut |elem, op| match elem {
-                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).discard_err(),
+                TrackElem::Field(idx) => self.ecx.project_field(op, idx).discard_err(),
                 TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).discard_err(),
                 TrackElem::Discriminant => {
                     let variant = self.ecx.read_discriminant(op).discard_err()?;
diff --git a/src/tools/miri/src/eval.rs b/src/tools/miri/src/eval.rs
index 8fe034d2582..5880e5fbc37 100644
--- a/src/tools/miri/src/eval.rs
+++ b/src/tools/miri/src/eval.rs
@@ -359,8 +359,8 @@ pub fn create_ecx<'tcx>(
         let argvs_layout =
             ecx.layout_of(Ty::new_array(tcx, u8_ptr_type, u64::try_from(argvs.len()).unwrap()))?;
         let argvs_place = ecx.allocate(argvs_layout, MiriMemoryKind::Machine.into())?;
-        for (idx, arg) in argvs.into_iter().enumerate() {
-            let place = ecx.project_field(&argvs_place, idx)?;
+        for (arg, idx) in argvs.into_iter().zip(0..) {
+            let place = ecx.project_index(&argvs_place, idx)?;
             ecx.write_immediate(arg, &place)?;
         }
         ecx.mark_immutable(&argvs_place);
@@ -389,8 +389,8 @@ pub fn create_ecx<'tcx>(
                 ecx.allocate(ecx.layout_of(cmd_type)?, MiriMemoryKind::Machine.into())?;
             ecx.machine.cmd_line = Some(cmd_place.ptr());
             // Store the UTF-16 string. We just allocated so we know the bounds are fine.
-            for (idx, &c) in cmd_utf16.iter().enumerate() {
-                let place = ecx.project_field(&cmd_place, idx)?;
+            for (&c, idx) in cmd_utf16.iter().zip(0..) {
+                let place = ecx.project_index(&cmd_place, idx)?;
                 ecx.write_scalar(Scalar::from_u16(c), &place)?;
             }
             ecx.mark_immutable(&cmd_place);
diff --git a/src/tools/miri/src/helpers.rs b/src/tools/miri/src/helpers.rs
index 20ea239b7e5..4edecc864dd 100644
--- a/src/tools/miri/src/helpers.rs
+++ b/src/tools/miri/src/helpers.rs
@@ -326,7 +326,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     ) -> InterpResult<'tcx, Option<P>> {
         let this = self.eval_context_ref();
         let adt = base.layout().ty.ty_adt_def().unwrap();
-        for (idx, field) in adt.non_enum_variant().fields.iter().enumerate() {
+        for (idx, field) in adt.non_enum_variant().fields.iter_enumerated() {
             if field.name.as_str() == name {
                 return interp_ok(Some(this.project_field(base, idx)?));
             }
@@ -376,6 +376,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     ) -> InterpResult<'tcx> {
         let this = self.eval_context_mut();
         for (idx, &val) in values.iter().enumerate() {
+            let idx = FieldIdx::from_usize(idx);
             let field = this.project_field(dest, idx)?;
             this.write_int(val, &field)?;
         }
@@ -763,10 +764,10 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     /// `EINVAL` in this case.
     fn read_timespec(&mut self, tp: &MPlaceTy<'tcx>) -> InterpResult<'tcx, Option<Duration>> {
         let this = self.eval_context_mut();
-        let seconds_place = this.project_field(tp, 0)?;
+        let seconds_place = this.project_field(tp, FieldIdx::ZERO)?;
         let seconds_scalar = this.read_scalar(&seconds_place)?;
         let seconds = seconds_scalar.to_target_isize(this)?;
-        let nanoseconds_place = this.project_field(tp, 1)?;
+        let nanoseconds_place = this.project_field(tp, FieldIdx::ONE)?;
         let nanoseconds_scalar = this.read_scalar(&nanoseconds_place)?;
         let nanoseconds = nanoseconds_scalar.to_target_isize(this)?;
 
diff --git a/src/tools/miri/src/shims/backtrace.rs b/src/tools/miri/src/shims/backtrace.rs
index ab11553df63..8606735c913 100644
--- a/src/tools/miri/src/shims/backtrace.rs
+++ b/src/tools/miri/src/shims/backtrace.rs
@@ -1,4 +1,4 @@
-use rustc_abi::{CanonAbi, Size};
+use rustc_abi::{CanonAbi, FieldIdx, Size};
 use rustc_middle::ty::layout::LayoutOf as _;
 use rustc_middle::ty::{self, Instance, Ty};
 use rustc_span::{BytePos, Loc, Symbol, hygiene};
@@ -159,23 +159,23 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             1 => {
                 this.write_scalar(
                     Scalar::from_target_usize(name.len().to_u64(), this),
-                    &this.project_field(dest, 0)?,
+                    &this.project_field(dest, FieldIdx::from_u32(0))?,
                 )?;
                 this.write_scalar(
                     Scalar::from_target_usize(filename.len().to_u64(), this),
-                    &this.project_field(dest, 1)?,
+                    &this.project_field(dest, FieldIdx::from_u32(1))?,
                 )?;
             }
             _ => throw_unsup_format!("unknown `miri_resolve_frame` flags {}", flags),
         }
 
-        this.write_scalar(Scalar::from_u32(lineno), &this.project_field(dest, 2)?)?;
-        this.write_scalar(Scalar::from_u32(colno), &this.project_field(dest, 3)?)?;
+        this.write_scalar(Scalar::from_u32(lineno), &this.project_field(dest, FieldIdx::from_u32(2))?)?;
+        this.write_scalar(Scalar::from_u32(colno), &this.project_field(dest, FieldIdx::from_u32(3))?)?;
 
         // Support a 4-field struct for now - this is deprecated
         // and slated for removal.
         if num_fields == 5 {
-            this.write_pointer(fn_ptr, &this.project_field(dest, 4)?)?;
+            this.write_pointer(fn_ptr, &this.project_field(dest, FieldIdx::from_u32(4))?)?;
         }
 
         interp_ok(())
diff --git a/src/tools/miri/src/shims/unix/env.rs b/src/tools/miri/src/shims/unix/env.rs
index aebb5757aec..62ac7ee3806 100644
--- a/src/tools/miri/src/shims/unix/env.rs
+++ b/src/tools/miri/src/shims/unix/env.rs
@@ -2,8 +2,9 @@ use std::ffi::{OsStr, OsString};
 use std::io::ErrorKind;
 use std::{env, mem};
 
-use rustc_abi::Size;
+use rustc_abi::{FieldIdx, Size};
 use rustc_data_structures::fx::FxHashMap;
+use rustc_index::IndexVec;
 use rustc_middle::ty::Ty;
 use rustc_middle::ty::layout::LayoutOf;
 
@@ -118,7 +119,7 @@ fn alloc_env_var<'tcx>(
 /// Allocates an `environ` block with the given list of pointers.
 fn alloc_environ_block<'tcx>(
     ecx: &mut InterpCx<'tcx, MiriMachine<'tcx>>,
-    mut vars: Vec<Pointer>,
+    mut vars: IndexVec<FieldIdx, Pointer>,
 ) -> InterpResult<'tcx, Pointer> {
     // Add trailing null.
     vars.push(Pointer::null());
@@ -129,7 +130,7 @@ fn alloc_environ_block<'tcx>(
         u64::try_from(vars.len()).unwrap(),
     ))?;
     let vars_place = ecx.allocate(vars_layout, MiriMemoryKind::Runtime.into())?;
-    for (idx, var) in vars.into_iter().enumerate() {
+    for (idx, var) in vars.into_iter_enumerated() {
         let place = ecx.project_field(&vars_place, idx)?;
         ecx.write_pointer(var, &place)?;
     }
diff --git a/src/tools/miri/src/shims/unix/freebsd/sync.rs b/src/tools/miri/src/shims/unix/freebsd/sync.rs
index 54650f35b2c..f4e7d9e58f9 100644
--- a/src/tools/miri/src/shims/unix/freebsd/sync.rs
+++ b/src/tools/miri/src/shims/unix/freebsd/sync.rs
@@ -2,6 +2,8 @@
 
 use core::time::Duration;
 
+use rustc_abi::FieldIdx;
+
 use crate::concurrency::sync::FutexRef;
 use crate::*;
 
@@ -214,18 +216,18 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         // Only flag allowed is UMTX_ABSTIME.
         let abs_time = this.eval_libc_u32("UMTX_ABSTIME");
 
-        let timespec_place = this.project_field(ut, 0)?;
+        let timespec_place = this.project_field(ut, FieldIdx::from_u32(0))?;
         // Inner `timespec` must still be valid.
         let duration = match this.read_timespec(&timespec_place)? {
             Some(dur) => dur,
             None => return interp_ok(None),
         };
 
-        let flags_place = this.project_field(ut, 1)?;
+        let flags_place = this.project_field(ut, FieldIdx::from_u32(1))?;
         let flags = this.read_scalar(&flags_place)?.to_u32()?;
         let abs_time_flag = flags == abs_time;
 
-        let clock_id_place = this.project_field(ut, 2)?;
+        let clock_id_place = this.project_field(ut, FieldIdx::from_u32(2))?;
         let clock_id = this.read_scalar(&clock_id_place)?.to_i32()?;
         let timeout_clock = this.translate_umtx_time_clock_id(clock_id)?;
 
diff --git a/src/tools/miri/src/shims/unix/linux_like/epoll.rs b/src/tools/miri/src/shims/unix/linux_like/epoll.rs
index b489595b4cd..f971fb10b19 100644
--- a/src/tools/miri/src/shims/unix/linux_like/epoll.rs
+++ b/src/tools/miri/src/shims/unix/linux_like/epoll.rs
@@ -4,6 +4,8 @@ use std::io;
 use std::rc::{Rc, Weak};
 use std::time::Duration;
 
+use rustc_abi::FieldIdx;
+
 use crate::concurrency::VClock;
 use crate::shims::files::{
     DynFileDescriptionRef, FdId, FileDescription, FileDescriptionRef, WeakFileDescriptionRef,
@@ -284,8 +286,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         if op == epoll_ctl_add || op == epoll_ctl_mod {
             // Read event bitmask and data from epoll_event passed by caller.
-            let mut events = this.read_scalar(&this.project_field(&event, 0)?)?.to_u32()?;
-            let data = this.read_scalar(&this.project_field(&event, 1)?)?.to_u64()?;
+            let mut events = this.read_scalar(&this.project_field(&event, FieldIdx::ZERO)?)?.to_u32()?;
+            let data = this.read_scalar(&this.project_field(&event, FieldIdx::ONE)?)?.to_u64()?;
 
             // Unset the flag we support to discover if any unsupported flags are used.
             let mut flags = events;
diff --git a/src/tools/miri/src/shims/x86/mod.rs b/src/tools/miri/src/shims/x86/mod.rs
index 7dee8ddd23c..1e82f521249 100644
--- a/src/tools/miri/src/shims/x86/mod.rs
+++ b/src/tools/miri/src/shims/x86/mod.rs
@@ -1,4 +1,4 @@
-use rustc_abi::{CanonAbi, Size};
+use rustc_abi::{CanonAbi, FieldIdx, Size};
 use rustc_apfloat::Float;
 use rustc_apfloat::ieee::Single;
 use rustc_middle::ty::Ty;
@@ -54,8 +54,8 @@ pub(super) trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 };
 
                 let (sum, cb_out) = carrying_add(this, cb_in, a, b, op)?;
-                this.write_scalar(cb_out, &this.project_field(dest, 0)?)?;
-                this.write_immediate(*sum, &this.project_field(dest, 1)?)?;
+                this.write_scalar(cb_out, &this.project_field(dest, FieldIdx::ZERO)?)?;
+                this.write_immediate(*sum, &this.project_field(dest, FieldIdx::ONE)?)?;
             }
 
             // Used to implement the `_addcarryx_u{32, 64}` functions. They are semantically identical with the `_addcarry_u{32, 64}` functions,