Auto merge of #150187 - RalfJung:visitor-in-mem-order, r=nnethercote

interpreter/visitor: always iterate in in-memory order

Now that this is directly available in the type, there's no reason to ever iterate in any other order.

Author: bors

compiler/rustc_const_eval/src/interpret/visitor.rs

@@ -4,7 +4,6 @@
 use std::num::NonZero;
 
 use rustc_abi::{FieldIdx, FieldsShape, VariantIdx, Variants};
-use rustc_index::{Idx as _, IndexVec};
 use rustc_middle::mir::interpret::InterpResult;
 use rustc_middle::ty::{self, Ty};
 use tracing::trace;
@@ -24,20 +23,6 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
         self.ecx().read_discriminant(&v.to_op(self.ecx())?)
     }
 
-    /// This function provides the chance to reorder the order in which fields are visited for
-    /// `FieldsShape::Aggregate`.
-    ///
-    /// The default means we iterate in source declaration order; alternatively this can use
-    /// `in_memory_order` to iterate in memory order.
-    #[inline(always)]
-    fn aggregate_field_iter(
-        in_memory_order: &IndexVec<u32, FieldIdx>,
-    ) -> impl Iterator<Item = FieldIdx> {
-        // Allow the optimizer to elide the bounds checking when creating each index.
-        let _ = FieldIdx::new(in_memory_order.len());
-        (0..in_memory_order.len()).map(FieldIdx::new)
-    }
-
     // Recursive actions, ready to be overloaded.
     /// Visits the given value, dispatching as appropriate to more specialized visitors.
     #[inline(always)]
@@ -171,7 +156,7 @@ pub trait ValueVisitor<'tcx, M: Machine<'tcx>>: Sized {
                 self.visit_union(v, fields)?;
             }
             FieldsShape::Arbitrary { in_memory_order, .. } => {
-                for idx in Self::aggregate_field_iter(in_memory_order) {
+                for idx in in_memory_order.iter().copied() {
                     let field = self.ecx().project_field(v, idx)?;
                     self.visit_field(v, idx.as_usize(), &field)?;
                 }

src/tools/miri/src/helpers.rs

@@ -10,7 +10,6 @@ use rustc_data_structures::fx::{FxBuildHasher, FxHashSet};
 use rustc_hir::Safety;
 use rustc_hir::def::{DefKind, Namespace};
 use rustc_hir::def_id::{CRATE_DEF_INDEX, CrateNum, DefId, LOCAL_CRATE};
-use rustc_index::IndexVec;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::middle::dependency_format::Linkage;
 use rustc_middle::middle::exported_symbols::ExportedSymbol;
@@ -583,12 +582,6 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 self.ecx
             }
 
-            fn aggregate_field_iter(
-                in_memory_order: &IndexVec<u32, FieldIdx>,
-            ) -> impl Iterator<Item = FieldIdx> {
-                in_memory_order.iter().copied()
-            }
-
             // Hook to detect `UnsafeCell`.
             fn visit_value(&mut self, v: &MPlaceTy<'tcx>) -> InterpResult<'tcx> {
                 trace!("UnsafeCellVisitor: {:?} {:?}", *v, v.layout.ty);