Use a helper to zip together parent and child captures for coroutine-…

…closures

compiler/rustc_middle/src/ty/closure.rs

@@ -6,6 +6,7 @@ use crate::{mir, ty};
 use std::fmt::Write;
 
 use crate::query::Providers;
+use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
@@ -415,6 +416,52 @@ impl BorrowKind {
     }
 }
 
+pub fn analyze_coroutine_closure_captures<'a, 'tcx: 'a, T>(
+    parent_captures: impl IntoIterator<Item = &'a CapturedPlace<'tcx>>,
+    child_captures: impl IntoIterator<Item = &'a CapturedPlace<'tcx>>,
+    mut for_each: impl FnMut((usize, &'a CapturedPlace<'tcx>), (usize, &'a CapturedPlace<'tcx>)) -> T,
+) -> impl Iterator<Item = T> + Captures<'a> + Captures<'tcx> {
+    let mut parent_captures = parent_captures.into_iter().enumerate().peekable();
+    // Make sure we use every field at least once, b/c why are we capturing something
+    // if it's not used in the inner coroutine.
+    let mut field_used_at_least_once = false;
+    child_captures.into_iter().enumerate().map(move |(child_field_idx, child_capture)| {
+        loop {
+            let Some(&(parent_field_idx, parent_capture)) = parent_captures.peek() else {
+                bug!("we ran out of parent captures!")
+            };
+
+            let HirPlaceBase::Upvar(parent_base) = parent_capture.place.base else {
+                bug!("expected capture to be an upvar");
+            };
+            let HirPlaceBase::Upvar(child_base) = child_capture.place.base else {
+                bug!("expected capture to be an upvar");
+            };
+
+            if parent_base.var_path.hir_id != child_base.var_path.hir_id
+                || !std::iter::zip(
+                    &child_capture.place.projections,
+                    &parent_capture.place.projections,
+                )
+                .all(|(child, parent)| child.kind == parent.kind)
+            {
+                // Make sure the field was used at least once.
+                assert!(
+                    field_used_at_least_once,
+                    "we captured {parent_capture:#?} but it was not used in the child coroutine?"
+                );
+                field_used_at_least_once = false;
+                // Skip this field.
+                let _ = parent_captures.next().unwrap();
+                continue;
+            }
+
+            field_used_at_least_once = true;
+            break for_each((parent_field_idx, parent_capture), (child_field_idx, child_capture));
+        }
+    })
+}
+
 pub fn provide(providers: &mut Providers) {
     *providers = Providers { closure_typeinfo, ..*providers }
 }

compiler/rustc_middle/src/ty/mod.rs

@@ -77,9 +77,10 @@ pub use rustc_type_ir::ConstKind::{
 pub use rustc_type_ir::*;
 
 pub use self::closure::{
-    is_ancestor_or_same_capture, place_to_string_for_capture, BorrowKind, CaptureInfo,
-    CapturedPlace, ClosureTypeInfo, MinCaptureInformationMap, MinCaptureList,
-    RootVariableMinCaptureList, UpvarCapture, UpvarId, UpvarPath, CAPTURE_STRUCT_LOCAL,
+    analyze_coroutine_closure_captures, is_ancestor_or_same_capture, place_to_string_for_capture,
+    BorrowKind, CaptureInfo, CapturedPlace, ClosureTypeInfo, MinCaptureInformationMap,
+    MinCaptureList, RootVariableMinCaptureList, UpvarCapture, UpvarId, UpvarPath,
+    CAPTURE_STRUCT_LOCAL,
 };
 pub use self::consts::{
     Const, ConstData, ConstInt, ConstKind, Expr, ScalarInt, UnevaluatedConst, ValTree,

compiler/rustc_mir_transform/src/coroutine/by_move_body.rs

@@ -71,7 +71,7 @@
 
 use rustc_data_structures::unord::UnordMap;
 use rustc_hir as hir;
-use rustc_middle::hir::place::{PlaceBase, Projection, ProjectionKind};
+use rustc_middle::hir::place::{Projection, ProjectionKind};
 use rustc_middle::mir::visit::MutVisitor;
 use rustc_middle::mir::{self, dump_mir, MirPass};
 use rustc_middle::ty::{self, InstanceDef, Ty, TyCtxt, TypeVisitableExt};
@@ -124,62 +124,10 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
             .tuple_fields()
             .len();
 
-        let mut field_remapping = UnordMap::default();
-
-        // One parent capture may correspond to several child captures if we end up
-        // refining the set of captures via edition-2021 precise captures. We want to
-        // match up any number of child captures with one parent capture, so we keep
-        // peeking off this `Peekable` until the child doesn't match anymore.
-        let mut parent_captures =
-            tcx.closure_captures(parent_def_id).iter().copied().enumerate().peekable();
-        // Make sure we use every field at least once, b/c why are we capturing something
-        // if it's not used in the inner coroutine.
-        let mut field_used_at_least_once = false;
-
-        for (child_field_idx, child_capture) in tcx
-            .closure_captures(coroutine_def_id)
-            .iter()
-            .copied()
-            // By construction we capture all the args first.
-            .skip(num_args)
-            .enumerate()
-        {
-            loop {
-                let Some(&(parent_field_idx, parent_capture)) = parent_captures.peek() else {
-                    bug!("we ran out of parent captures!")
-                };
-
-                let PlaceBase::Upvar(parent_base) = parent_capture.place.base else {
-                    bug!("expected capture to be an upvar");
-                };
-                let PlaceBase::Upvar(child_base) = child_capture.place.base else {
-                    bug!("expected capture to be an upvar");
-                };
-
-                assert!(
-                    child_capture.place.projections.len() >= parent_capture.place.projections.len()
-                );
-                // A parent matches a child they share the same prefix of projections.
-                // The child may have more, if it is capturing sub-fields out of
-                // something that is captured by-move in the parent closure.
-                if parent_base.var_path.hir_id != child_base.var_path.hir_id
-                    || !std::iter::zip(
-                        &child_capture.place.projections,
-                        &parent_capture.place.projections,
-                    )
-                    .all(|(child, parent)| child.kind == parent.kind)
-                {
-                    // Make sure the field was used at least once.
-                    assert!(
-                        field_used_at_least_once,
-                        "we captured {parent_capture:#?} but it was not used in the child coroutine?"
-                    );
-                    field_used_at_least_once = false;
-                    // Skip this field.
-                    let _ = parent_captures.next().unwrap();
-                    continue;
-                }
-
+        let field_remapping: UnordMap<_, _> = ty::analyze_coroutine_closure_captures(
+            tcx.closure_captures(parent_def_id).iter().copied(),
+            tcx.closure_captures(coroutine_def_id).iter().skip(num_args).copied(),
+            |(parent_field_idx, parent_capture), (child_field_idx, child_capture)| {
                 // Store this set of additional projections (fields and derefs).
                 // We need to re-apply them later.
                 let child_precise_captures =
@@ -210,26 +158,18 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
                     ),
                 };
 
-                field_remapping.insert(
+                (
                     FieldIdx::from_usize(child_field_idx + num_args),
                     (
                         FieldIdx::from_usize(parent_field_idx + num_args),
                         parent_capture_ty,
                         needs_deref,
                         child_precise_captures,
                     ),
-                );
-
-                field_used_at_least_once = true;
-                break;
-            }
-        }
-
-        // Pop the last parent capture
-        if field_used_at_least_once {
-            let _ = parent_captures.next().unwrap();
-        }
-        assert_eq!(parent_captures.next(), None, "leftover parent captures?");
+                )
+            },
+        )
+        .collect();
 
         if coroutine_kind == ty::ClosureKind::FnOnce {
             assert_eq!(field_remapping.len(), tcx.closure_captures(parent_def_id).len());