1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
//! See docs in build/expr/mod.rs

use crate::build::expr::category::Category;
use crate::build::ForGuard::{OutsideGuard, RefWithinGuard};
use crate::build::{BlockAnd, BlockAndExtension, Builder, Capture, CaptureMap};
use rustc_hir::def_id::LocalDefId;
use rustc_middle::hir::place::Projection as HirProjection;
use rustc_middle::hir::place::ProjectionKind as HirProjectionKind;
use rustc_middle::middle::region;
use rustc_middle::mir::AssertKind::BoundsCheck;
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use rustc_middle::ty::AdtDef;
use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, Variance};
use rustc_span::Span;
use rustc_target::abi::{FieldIdx, VariantIdx, FIRST_VARIANT};

use std::assert_matches::assert_matches;
use std::iter;

/// The "outermost" place that holds this value.
#[derive(Copy, Clone, Debug, PartialEq)]
pub(crate) enum PlaceBase {
    /// Denotes the start of a `Place`.
    Local(Local),

    /// When building place for an expression within a closure, the place might start off a
    /// captured path. When `capture_disjoint_fields` is enabled, we might not know the capture
    /// index (within the desugared closure) of the captured path until most of the projections
    /// are applied. We use `PlaceBase::Upvar` to keep track of the root variable off of which the
    /// captured path starts, the closure the capture belongs to and the trait the closure
    /// implements.
    ///
    /// Once we have figured out the capture index, we can convert the place builder to start from
    /// `PlaceBase::Local`.
    ///
    /// Consider the following example
    /// ```rust
    /// let t = (((10, 10), 10), 10);
    ///
    /// let c = || {
    ///     println!("{}", t.0.0.0);
    /// };
    /// ```
    /// Here the THIR expression for `t.0.0.0` will be something like
    ///
    /// ```ignore (illustrative)
    /// * Field(0)
    ///     * Field(0)
    ///         * Field(0)
    ///             * UpvarRef(t)
    /// ```
    ///
    /// When `capture_disjoint_fields` is enabled, `t.0.0.0` is captured and we won't be able to
    /// figure out that it is captured until all the `Field` projections are applied.
    Upvar {
        /// HirId of the upvar
        var_hir_id: LocalVarId,
        /// DefId of the closure
        closure_def_id: LocalDefId,
    },
}

/// `PlaceBuilder` is used to create places during MIR construction. It allows you to "build up" a
/// place by pushing more and more projections onto the end, and then convert the final set into a
/// place using the `to_place` method.
///
/// This is used internally when building a place for an expression like `a.b.c`. The fields `b`
/// and `c` can be progressively pushed onto the place builder that is created when converting `a`.
#[derive(Clone, Debug, PartialEq)]
pub(in crate::build) struct PlaceBuilder<'tcx> {
    base: PlaceBase,
    projection: Vec<PlaceElem<'tcx>>,
}

/// Given a list of MIR projections, convert them to list of HIR ProjectionKind.
/// The projections are truncated to represent a path that might be captured by a
/// closure/coroutine. This implies the vector returned from this function doesn't contain
/// ProjectionElems `Downcast`, `ConstantIndex`, `Index`, or `Subslice` because those will never be
/// part of a path that is captured by a closure. We stop applying projections once we see the first
/// projection that isn't captured by a closure.
fn convert_to_hir_projections_and_truncate_for_capture(
    mir_projections: &[PlaceElem<'_>],
) -> Vec<HirProjectionKind> {
    let mut hir_projections = Vec::new();
    let mut variant = None;

    for mir_projection in mir_projections {
        let hir_projection = match mir_projection {
            ProjectionElem::Deref => HirProjectionKind::Deref,
            ProjectionElem::Field(field, _) => {
                let variant = variant.unwrap_or(FIRST_VARIANT);
                HirProjectionKind::Field(*field, variant)
            }
            ProjectionElem::Downcast(.., idx) => {
                // We don't expect to see multi-variant enums here, as earlier
                // phases will have truncated them already. However, there can
                // still be downcasts, thanks to single-variant enums.
                // We keep track of VariantIdx so we can use this information
                // if the next ProjectionElem is a Field.
                variant = Some(*idx);
                continue;
            }
            // These do not affect anything, they just make sure we know the right type.
            ProjectionElem::OpaqueCast(_) | ProjectionElem::Subtype(..) => continue,
            ProjectionElem::Index(..)
            | ProjectionElem::ConstantIndex { .. }
            | ProjectionElem::Subslice { .. } => {
                // We don't capture array-access projections.
                // We can stop here as arrays are captured completely.
                break;
            }
        };
        variant = None;
        hir_projections.push(hir_projection);
    }

    hir_projections
}

/// Return true if the `proj_possible_ancestor` represents an ancestor path
/// to `proj_capture` or `proj_possible_ancestor` is same as `proj_capture`,
/// assuming they both start off of the same root variable.
///
/// **Note:** It's the caller's responsibility to ensure that both lists of projections
///           start off of the same root variable.
///
/// Eg: 1. `foo.x` which is represented using `projections=[Field(x)]` is an ancestor of
///        `foo.x.y` which is represented using `projections=[Field(x), Field(y)]`.
///        Note both `foo.x` and `foo.x.y` start off of the same root variable `foo`.
///     2. Since we only look at the projections here function will return `bar.x` as an a valid
///        ancestor of `foo.x.y`. It's the caller's responsibility to ensure that both projections
///        list are being applied to the same root variable.
fn is_ancestor_or_same_capture(
    proj_possible_ancestor: &[HirProjectionKind],
    proj_capture: &[HirProjectionKind],
) -> bool {
    // We want to make sure `is_ancestor_or_same_capture("x.0.0", "x.0")` to return false.
    // Therefore we can't just check if all projections are same in the zipped iterator below.
    if proj_possible_ancestor.len() > proj_capture.len() {
        return false;
    }

    iter::zip(proj_possible_ancestor, proj_capture).all(|(a, b)| a == b)
}

/// Given a closure, returns the index of a capture within the desugared closure struct and the
/// `ty::CapturedPlace` which is the ancestor of the Place represented using the `var_hir_id`
/// and `projection`.
///
/// Note there will be at most one ancestor for any given Place.
///
/// Returns None, when the ancestor is not found.
fn find_capture_matching_projections<'a, 'tcx>(
    upvars: &'a CaptureMap<'tcx>,
    var_hir_id: LocalVarId,
    projections: &[PlaceElem<'tcx>],
) -> Option<(usize, &'a Capture<'tcx>)> {
    let hir_projections = convert_to_hir_projections_and_truncate_for_capture(projections);

    upvars.get_by_key_enumerated(var_hir_id.0).find(|(_, capture)| {
        let possible_ancestor_proj_kinds: Vec<_> =
            capture.captured_place.place.projections.iter().map(|proj| proj.kind).collect();
        is_ancestor_or_same_capture(&possible_ancestor_proj_kinds, &hir_projections)
    })
}

/// Takes an upvar place and tries to resolve it into a `PlaceBuilder`
/// with `PlaceBase::Local`
#[instrument(level = "trace", skip(cx), ret)]
fn to_upvars_resolved_place_builder<'tcx>(
    cx: &Builder<'_, 'tcx>,
    var_hir_id: LocalVarId,
    closure_def_id: LocalDefId,
    projection: &[PlaceElem<'tcx>],
) -> Option<PlaceBuilder<'tcx>> {
    let Some((capture_index, capture)) =
        find_capture_matching_projections(&cx.upvars, var_hir_id, projection)
    else {
        let closure_span = cx.tcx.def_span(closure_def_id);
        if !enable_precise_capture(closure_span) {
            bug!(
                "No associated capture found for {:?}[{:#?}] even though \
                    capture_disjoint_fields isn't enabled",
                var_hir_id,
                projection
            )
        } else {
            debug!("No associated capture found for {:?}[{:#?}]", var_hir_id, projection,);
        }
        return None;
    };

    // Access the capture by accessing the field within the Closure struct.
    let capture_info = &cx.upvars[capture_index];

    let mut upvar_resolved_place_builder = PlaceBuilder::from(capture_info.use_place);

    // We used some of the projections to build the capture itself,
    // now we apply the remaining to the upvar resolved place.
    trace!(?capture.captured_place, ?projection);
    let remaining_projections = strip_prefix(
        capture.captured_place.place.base_ty,
        projection,
        &capture.captured_place.place.projections,
    );
    upvar_resolved_place_builder.projection.extend(remaining_projections);

    Some(upvar_resolved_place_builder)
}

/// Returns projections remaining after stripping an initial prefix of HIR
/// projections.
///
/// Supports only HIR projection kinds that represent a path that might be
/// captured by a closure or a coroutine, i.e., an `Index` or a `Subslice`
/// projection kinds are unsupported.
fn strip_prefix<'a, 'tcx>(
    mut base_ty: Ty<'tcx>,
    projections: &'a [PlaceElem<'tcx>],
    prefix_projections: &[HirProjection<'tcx>],
) -> impl Iterator<Item = PlaceElem<'tcx>> + 'a {
    let mut iter = projections
        .iter()
        .copied()
        // Filter out opaque casts, they are unnecessary in the prefix.
        .filter(|elem| !matches!(elem, ProjectionElem::OpaqueCast(..)));
    for projection in prefix_projections {
        match projection.kind {
            HirProjectionKind::Deref => {
                assert_matches!(iter.next(), Some(ProjectionElem::Deref));
            }
            HirProjectionKind::Field(..) => {
                if base_ty.is_enum() {
                    assert_matches!(iter.next(), Some(ProjectionElem::Downcast(..)));
                }
                assert_matches!(iter.next(), Some(ProjectionElem::Field(..)));
            }
            HirProjectionKind::OpaqueCast => {
                assert_matches!(iter.next(), Some(ProjectionElem::OpaqueCast(..)));
            }
            HirProjectionKind::Index | HirProjectionKind::Subslice => {
                bug!("unexpected projection kind: {:?}", projection);
            }
        }
        base_ty = projection.ty;
    }
    iter
}

impl<'tcx> PlaceBuilder<'tcx> {
    pub(in crate::build) fn to_place(&self, cx: &Builder<'_, 'tcx>) -> Place<'tcx> {
        self.try_to_place(cx).unwrap()
    }

    /// Creates a `Place` or returns `None` if an upvar cannot be resolved
    pub(in crate::build) fn try_to_place(&self, cx: &Builder<'_, 'tcx>) -> Option<Place<'tcx>> {
        let resolved = self.resolve_upvar(cx);
        let builder = resolved.as_ref().unwrap_or(self);
        let PlaceBase::Local(local) = builder.base else { return None };
        let projection = cx.tcx.mk_place_elems(&builder.projection);
        Some(Place { local, projection })
    }

    /// Attempts to resolve the `PlaceBuilder`.
    /// Returns `None` if this is not an upvar.
    ///
    /// Upvars resolve may fail for a `PlaceBuilder` when attempting to
    /// resolve a disjoint field whose root variable is not captured
    /// (destructured assignments) or when attempting to resolve a root
    /// variable (discriminant matching with only wildcard arm) that is
    /// not captured. This can happen because the final mir that will be
    /// generated doesn't require a read for this place. Failures will only
    /// happen inside closures.
    pub(in crate::build) fn resolve_upvar(
        &self,
        cx: &Builder<'_, 'tcx>,
    ) -> Option<PlaceBuilder<'tcx>> {
        let PlaceBase::Upvar { var_hir_id, closure_def_id } = self.base else {
            return None;
        };
        to_upvars_resolved_place_builder(cx, var_hir_id, closure_def_id, &self.projection)
    }

    pub(crate) fn base(&self) -> PlaceBase {
        self.base
    }

    pub(crate) fn projection(&self) -> &[PlaceElem<'tcx>] {
        &self.projection
    }

    pub(crate) fn field(self, f: FieldIdx, ty: Ty<'tcx>) -> Self {
        self.project(PlaceElem::Field(f, ty))
    }

    pub(crate) fn deref(self) -> Self {
        self.project(PlaceElem::Deref)
    }

    pub(crate) fn downcast(self, adt_def: AdtDef<'tcx>, variant_index: VariantIdx) -> Self {
        self.project(PlaceElem::Downcast(Some(adt_def.variant(variant_index).name), variant_index))
    }

    fn index(self, index: Local) -> Self {
        self.project(PlaceElem::Index(index))
    }

    pub(crate) fn project(mut self, elem: PlaceElem<'tcx>) -> Self {
        self.projection.push(elem);
        self
    }

    /// Same as `.clone().project(..)` but more efficient
    pub(crate) fn clone_project(&self, elem: PlaceElem<'tcx>) -> Self {
        Self {
            base: self.base,
            projection: Vec::from_iter(self.projection.iter().copied().chain([elem])),
        }
    }
}

impl<'tcx> From<Local> for PlaceBuilder<'tcx> {
    fn from(local: Local) -> Self {
        Self { base: PlaceBase::Local(local), projection: Vec::new() }
    }
}

impl<'tcx> From<PlaceBase> for PlaceBuilder<'tcx> {
    fn from(base: PlaceBase) -> Self {
        Self { base, projection: Vec::new() }
    }
}

impl<'tcx> From<Place<'tcx>> for PlaceBuilder<'tcx> {
    fn from(p: Place<'tcx>) -> Self {
        Self { base: PlaceBase::Local(p.local), projection: p.projection.to_vec() }
    }
}

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Compile `expr`, yielding a place that we can move from etc.
    ///
    /// WARNING: Any user code might:
    /// * Invalidate any slice bounds checks performed.
    /// * Change the address that this `Place` refers to.
    /// * Modify the memory that this place refers to.
    /// * Invalidate the memory that this place refers to, this will be caught
    ///   by borrow checking.
    ///
    /// Extra care is needed if any user code is allowed to run between calling
    /// this method and using it, as is the case for `match` and index
    /// expressions.
    pub(crate) fn as_place(
        &mut self,
        mut block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<Place<'tcx>> {
        let place_builder = unpack!(block = self.as_place_builder(block, expr_id));
        block.and(place_builder.to_place(self))
    }

    /// This is used when constructing a compound `Place`, so that we can avoid creating
    /// intermediate `Place` values until we know the full set of projections.
    pub(crate) fn as_place_builder(
        &mut self,
        block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        self.expr_as_place(block, expr_id, Mutability::Mut, None)
    }

    /// Compile `expr`, yielding a place that we can move from etc.
    /// Mutability note: The caller of this method promises only to read from the resulting
    /// place. The place itself may or may not be mutable:
    /// * If this expr is a place expr like a.b, then we will return that place.
    /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
    pub(crate) fn as_read_only_place(
        &mut self,
        mut block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<Place<'tcx>> {
        let place_builder = unpack!(block = self.as_read_only_place_builder(block, expr_id));
        block.and(place_builder.to_place(self))
    }

    /// This is used when constructing a compound `Place`, so that we can avoid creating
    /// intermediate `Place` values until we know the full set of projections.
    /// Mutability note: The caller of this method promises only to read from the resulting
    /// place. The place itself may or may not be mutable:
    /// * If this expr is a place expr like a.b, then we will return that place.
    /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
    fn as_read_only_place_builder(
        &mut self,
        block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        self.expr_as_place(block, expr_id, Mutability::Not, None)
    }

    fn expr_as_place(
        &mut self,
        mut block: BasicBlock,
        expr_id: ExprId,
        mutability: Mutability,
        fake_borrow_temps: Option<&mut Vec<Local>>,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        let expr = &self.thir[expr_id];
        debug!("expr_as_place(block={:?}, expr={:?}, mutability={:?})", block, expr, mutability);

        let this = self;
        let expr_span = expr.span;
        let source_info = this.source_info(expr_span);
        match expr.kind {
            ExprKind::Scope { region_scope, lint_level, value } => {
                this.in_scope((region_scope, source_info), lint_level, |this| {
                    this.expr_as_place(block, value, mutability, fake_borrow_temps)
                })
            }
            ExprKind::Field { lhs, variant_index, name } => {
                let lhs_expr = &this.thir[lhs];
                let mut place_builder =
                    unpack!(block = this.expr_as_place(block, lhs, mutability, fake_borrow_temps,));
                if let ty::Adt(adt_def, _) = lhs_expr.ty.kind() {
                    if adt_def.is_enum() {
                        place_builder = place_builder.downcast(*adt_def, variant_index);
                    }
                }
                block.and(place_builder.field(name, expr.ty))
            }
            ExprKind::Deref { arg } => {
                let place_builder =
                    unpack!(block = this.expr_as_place(block, arg, mutability, fake_borrow_temps,));
                block.and(place_builder.deref())
            }
            ExprKind::Index { lhs, index } => this.lower_index_expression(
                block,
                lhs,
                index,
                mutability,
                fake_borrow_temps,
                expr.temp_lifetime,
                expr_span,
                source_info,
            ),
            ExprKind::UpvarRef { closure_def_id, var_hir_id } => {
                this.lower_captured_upvar(block, closure_def_id.expect_local(), var_hir_id)
            }

            ExprKind::VarRef { id } => {
                let place_builder = if this.is_bound_var_in_guard(id) {
                    let index = this.var_local_id(id, RefWithinGuard);
                    PlaceBuilder::from(index).deref()
                } else {
                    let index = this.var_local_id(id, OutsideGuard);
                    PlaceBuilder::from(index)
                };
                block.and(place_builder)
            }

            ExprKind::PlaceTypeAscription { source, ref user_ty } => {
                let place_builder = unpack!(
                    block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
                );
                if let Some(user_ty) = user_ty {
                    let annotation_index =
                        this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                            span: source_info.span,
                            user_ty: user_ty.clone(),
                            inferred_ty: expr.ty,
                        });

                    let place = place_builder.to_place(this);
                    this.cfg.push(
                        block,
                        Statement {
                            source_info,
                            kind: StatementKind::AscribeUserType(
                                Box::new((
                                    place,
                                    UserTypeProjection { base: annotation_index, projs: vec![] },
                                )),
                                Variance::Invariant,
                            ),
                        },
                    );
                }
                block.and(place_builder)
            }
            ExprKind::ValueTypeAscription { source, ref user_ty } => {
                let source_expr = &this.thir[source];
                let temp = unpack!(
                    block = this.as_temp(block, source_expr.temp_lifetime, source, mutability)
                );
                if let Some(user_ty) = user_ty {
                    let annotation_index =
                        this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                            span: source_info.span,
                            user_ty: user_ty.clone(),
                            inferred_ty: expr.ty,
                        });
                    this.cfg.push(
                        block,
                        Statement {
                            source_info,
                            kind: StatementKind::AscribeUserType(
                                Box::new((
                                    Place::from(temp),
                                    UserTypeProjection { base: annotation_index, projs: vec![] },
                                )),
                                Variance::Invariant,
                            ),
                        },
                    );
                }
                block.and(PlaceBuilder::from(temp))
            }

            ExprKind::Array { .. }
            | ExprKind::Tuple { .. }
            | ExprKind::Adt { .. }
            | ExprKind::Closure { .. }
            | ExprKind::Unary { .. }
            | ExprKind::Binary { .. }
            | ExprKind::LogicalOp { .. }
            | ExprKind::Box { .. }
            | ExprKind::Cast { .. }
            | ExprKind::Use { .. }
            | ExprKind::NeverToAny { .. }
            | ExprKind::PointerCoercion { .. }
            | ExprKind::Repeat { .. }
            | ExprKind::Borrow { .. }
            | ExprKind::AddressOf { .. }
            | ExprKind::Match { .. }
            | ExprKind::If { .. }
            | ExprKind::Loop { .. }
            | ExprKind::Block { .. }
            | ExprKind::Let { .. }
            | ExprKind::Assign { .. }
            | ExprKind::AssignOp { .. }
            | ExprKind::Break { .. }
            | ExprKind::Continue { .. }
            | ExprKind::Return { .. }
            | ExprKind::Become { .. }
            | ExprKind::Literal { .. }
            | ExprKind::NamedConst { .. }
            | ExprKind::NonHirLiteral { .. }
            | ExprKind::ZstLiteral { .. }
            | ExprKind::ConstParam { .. }
            | ExprKind::ConstBlock { .. }
            | ExprKind::StaticRef { .. }
            | ExprKind::InlineAsm { .. }
            | ExprKind::OffsetOf { .. }
            | ExprKind::Yield { .. }
            | ExprKind::ThreadLocalRef(_)
            | ExprKind::Call { .. } => {
                // these are not places, so we need to make a temporary.
                debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
                let temp =
                    unpack!(block = this.as_temp(block, expr.temp_lifetime, expr_id, mutability));
                block.and(PlaceBuilder::from(temp))
            }
        }
    }

    /// Lower a captured upvar. Note we might not know the actual capture index,
    /// so we create a place starting from `PlaceBase::Upvar`, which will be resolved
    /// once all projections that allow us to identify a capture have been applied.
    fn lower_captured_upvar(
        &mut self,
        block: BasicBlock,
        closure_def_id: LocalDefId,
        var_hir_id: LocalVarId,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        block.and(PlaceBuilder::from(PlaceBase::Upvar { var_hir_id, closure_def_id }))
    }

    /// Lower an index expression
    ///
    /// This has two complications;
    ///
    /// * We need to do a bounds check.
    /// * We need to ensure that the bounds check can't be invalidated using an
    ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
    ///   that this is the case.
    fn lower_index_expression(
        &mut self,
        mut block: BasicBlock,
        base: ExprId,
        index: ExprId,
        mutability: Mutability,
        fake_borrow_temps: Option<&mut Vec<Local>>,
        temp_lifetime: Option<region::Scope>,
        expr_span: Span,
        source_info: SourceInfo,
    ) -> BlockAnd<PlaceBuilder<'tcx>> {
        let base_fake_borrow_temps = &mut Vec::new();
        let is_outermost_index = fake_borrow_temps.is_none();
        let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);

        let base_place =
            unpack!(block = self.expr_as_place(block, base, mutability, Some(fake_borrow_temps),));

        // Making this a *fresh* temporary means we do not have to worry about
        // the index changing later: Nothing will ever change this temporary.
        // The "retagging" transformation (for Stacked Borrows) relies on this.
        let idx = unpack!(block = self.as_temp(block, temp_lifetime, index, Mutability::Not));

        block = self.bounds_check(block, &base_place, idx, expr_span, source_info);

        if is_outermost_index {
            self.read_fake_borrows(block, fake_borrow_temps, source_info)
        } else {
            self.add_fake_borrows_of_base(
                base_place.to_place(self),
                block,
                fake_borrow_temps,
                expr_span,
                source_info,
            );
        }

        block.and(base_place.index(idx))
    }

    fn bounds_check(
        &mut self,
        block: BasicBlock,
        slice: &PlaceBuilder<'tcx>,
        index: Local,
        expr_span: Span,
        source_info: SourceInfo,
    ) -> BasicBlock {
        let usize_ty = self.tcx.types.usize;
        let bool_ty = self.tcx.types.bool;
        // bounds check:
        let len = self.temp(usize_ty, expr_span);
        let lt = self.temp(bool_ty, expr_span);

        // len = len(slice)
        self.cfg.push_assign(block, source_info, len, Rvalue::Len(slice.to_place(self)));
        // lt = idx < len
        self.cfg.push_assign(
            block,
            source_info,
            lt,
            Rvalue::BinaryOp(
                BinOp::Lt,
                Box::new((Operand::Copy(Place::from(index)), Operand::Copy(len))),
            ),
        );
        let msg = BoundsCheck { len: Operand::Move(len), index: Operand::Copy(Place::from(index)) };
        // assert!(lt, "...")
        self.assert(block, Operand::Move(lt), true, msg, expr_span)
    }

    fn add_fake_borrows_of_base(
        &mut self,
        base_place: Place<'tcx>,
        block: BasicBlock,
        fake_borrow_temps: &mut Vec<Local>,
        expr_span: Span,
        source_info: SourceInfo,
    ) {
        let tcx = self.tcx;

        let place_ty = base_place.ty(&self.local_decls, tcx);
        if let ty::Slice(_) = place_ty.ty.kind() {
            // We need to create fake borrows to ensure that the bounds
            // check that we just did stays valid. Since we can't assign to
            // unsized values, we only need to ensure that none of the
            // pointers in the base place are modified.
            for (base_place, elem) in base_place.iter_projections().rev() {
                match elem {
                    ProjectionElem::Deref => {
                        let fake_borrow_deref_ty = base_place.ty(&self.local_decls, tcx).ty;
                        let fake_borrow_ty =
                            Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, fake_borrow_deref_ty);
                        let fake_borrow_temp =
                            self.local_decls.push(LocalDecl::new(fake_borrow_ty, expr_span));
                        let projection = tcx.mk_place_elems(base_place.projection);
                        self.cfg.push_assign(
                            block,
                            source_info,
                            fake_borrow_temp.into(),
                            Rvalue::Ref(
                                tcx.lifetimes.re_erased,
                                BorrowKind::Fake,
                                Place { local: base_place.local, projection },
                            ),
                        );
                        fake_borrow_temps.push(fake_borrow_temp);
                    }
                    ProjectionElem::Index(_) => {
                        let index_ty = base_place.ty(&self.local_decls, tcx);
                        match index_ty.ty.kind() {
                            // The previous index expression has already
                            // done any index expressions needed here.
                            ty::Slice(_) => break,
                            ty::Array(..) => (),
                            _ => bug!("unexpected index base"),
                        }
                    }
                    ProjectionElem::Field(..)
                    | ProjectionElem::Downcast(..)
                    | ProjectionElem::OpaqueCast(..)
                    | ProjectionElem::Subtype(..)
                    | ProjectionElem::ConstantIndex { .. }
                    | ProjectionElem::Subslice { .. } => (),
                }
            }
        }
    }

    fn read_fake_borrows(
        &mut self,
        bb: BasicBlock,
        fake_borrow_temps: &mut Vec<Local>,
        source_info: SourceInfo,
    ) {
        // All indexes have been evaluated now, read all of the
        // fake borrows so that they are live across those index
        // expressions.
        for temp in fake_borrow_temps {
            self.cfg.push_fake_read(bb, source_info, FakeReadCause::ForIndex, Place::from(*temp));
        }
    }
}

/// Precise capture is enabled if user is using Rust Edition 2021 or higher.
fn enable_precise_capture(closure_span: Span) -> bool {
    closure_span.at_least_rust_2021()
}