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 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 +416,72 @@ pub fn to_mutbl_lossy(self) -> hir::Mutability {
     }
 }
 
+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> {
+    std::iter::from_coroutine(move || {
+        let mut child_captures = child_captures.into_iter().enumerate().peekable();
+
+        // 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.
+        for (parent_field_idx, parent_capture) in parent_captures.into_iter().enumerate() {
+            // 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;
+
+            // A parent matches a child if 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.
+            while child_captures.peek().map_or(false, |(_, child_capture)| {
+                child_prefix_matches_parent_projections(parent_capture, child_capture)
+            }) {
+                let (child_field_idx, child_capture) = child_captures.next().unwrap();
+                // This analysis only makes sense if the parent capture is a
+                // prefix of the child capture.
+                assert!(
+                    child_capture.place.projections.len() >= parent_capture.place.projections.len(),
+                    "parent capture ({parent_capture:#?}) expected to be prefix of \
+                    child capture ({child_capture:#?})"
+                );
+
+                yield for_each(
+                    (parent_field_idx, parent_capture),
+                    (child_field_idx, child_capture),
+                );
+
+                field_used_at_least_once = true;
+            }
+
+            // 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?"
+            );
+        }
+        assert_eq!(child_captures.next(), None, "leftover child captures?");
+    })
+}
+
+fn child_prefix_matches_parent_projections(
+    parent_capture: &ty::CapturedPlace<'_>,
+    child_capture: &ty::CapturedPlace<'_>,
+) -> bool {
+    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");
+    };
+
+    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)
+}
+
 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::*;
 
 pub use self::closure::{
-    is_ancestor_or_same_capture, place_to_string_for_capture, BorrowKind, CaptureInfo,
+    analyze_coroutine_closure_captures, is_ancestor_or_same_capture, place_to_string_for_capture,
-    CapturedPlace, ClosureTypeInfo, MinCaptureInformationMap, MinCaptureList,
+    BorrowKind, CaptureInfo, CapturedPlace, ClosureTypeInfo, MinCaptureInformationMap,
-    RootVariableMinCaptureList, UpvarCapture, UpvarId, UpvarPath, CAPTURE_STRUCT_LOCAL,
+    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};
@@ -124,44 +124,10 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
             .tuple_fields()
             .len();
 
-        let mut field_remapping = UnordMap::default();
+        let field_remapping: UnordMap<_, _> = ty::analyze_coroutine_closure_captures(
-
+            tcx.closure_captures(parent_def_id).iter().copied(),
-        let mut child_captures = tcx
+            tcx.closure_captures(coroutine_def_id).iter().skip(num_args).copied(),
-            .closure_captures(coroutine_def_id)
+            |(parent_field_idx, parent_capture), (child_field_idx, child_capture)| {
-            .iter()
-            .copied()
-            // By construction we capture all the args first.
-            .skip(num_args)
-            .enumerate()
-            .peekable();
-
-        // 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.
-        for (parent_field_idx, parent_capture) in
-            tcx.closure_captures(parent_def_id).iter().copied().enumerate()
-        {
-            // 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;
-
-            // A parent matches a child if 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.
-            while child_captures.peek().map_or(false, |(_, child_capture)| {
-                child_prefix_matches_parent_projections(parent_capture, child_capture)
-            }) {
-                let (child_field_idx, child_capture) = child_captures.next().unwrap();
-
-                // This analysis only makes sense if the parent capture is a
-                // prefix of the child capture.
-                assert!(
-                    child_capture.place.projections.len() >= parent_capture.place.projections.len(),
-                    "parent capture ({parent_capture:#?}) expected to be prefix of \
-                    child capture ({child_capture:#?})"
-                );
-
                 // Store this set of additional projections (fields and derefs).
                 // We need to re-apply them later.
                 let child_precise_captures =
@@ -192,7 +158,7 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
                     ),
                 };
 
-                field_remapping.insert(
+                (
                     FieldIdx::from_usize(child_field_idx + num_args),
                     (
                         FieldIdx::from_usize(parent_field_idx + num_args),
@@ -200,18 +166,10 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
                         needs_deref,
                         child_precise_captures,
                     ),
-                );
+                )
-
+            },
-                field_used_at_least_once = true;
+        )
-            }
+        .collect();
-
-            // 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?"
-            );
-        }
-        assert_eq!(child_captures.next(), None, "leftover child captures?");
 
         if coroutine_kind == ty::ClosureKind::FnOnce {
             assert_eq!(field_remapping.len(), tcx.closure_captures(parent_def_id).len());
@@ -241,22 +199,6 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
     }
 }
 
-fn child_prefix_matches_parent_projections(
-    parent_capture: &ty::CapturedPlace<'_>,
-    child_capture: &ty::CapturedPlace<'_>,
-) -> bool {
-    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");
-    };
-
-    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)
-}
-
 struct MakeByMoveBody<'tcx> {
     tcx: TyCtxt<'tcx>,
     field_remapping: UnordMap<FieldIdx, (FieldIdx, Ty<'tcx>, bool, &'tcx [Projection<'tcx>])>,