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
use crate::build::expr::as_place::{PlaceBase, PlaceBuilder};
use crate::build::matches::{Binding, Candidate, FlatPat, MatchPair, TestCase};
use crate::build::Builder;
use rustc_data_structures::fx::FxIndexMap;
use rustc_middle::mir::*;
use rustc_middle::thir::{self, *};
use rustc_middle::ty::TypeVisitableExt;
use rustc_middle::ty::{self, Ty};
use rustc_span::Span;

impl<'a, 'tcx> Builder<'a, 'tcx> {
    pub(crate) fn field_match_pairs<'pat>(
        &mut self,
        place: PlaceBuilder<'tcx>,
        subpatterns: &'pat [FieldPat<'tcx>],
    ) -> Vec<MatchPair<'pat, 'tcx>> {
        subpatterns
            .iter()
            .map(|fieldpat| {
                let place =
                    place.clone_project(PlaceElem::Field(fieldpat.field, fieldpat.pattern.ty));
                MatchPair::new(place, &fieldpat.pattern, self)
            })
            .collect()
    }

    pub(crate) fn prefix_slice_suffix<'pat>(
        &mut self,
        match_pairs: &mut Vec<MatchPair<'pat, 'tcx>>,
        place: &PlaceBuilder<'tcx>,
        prefix: &'pat [Box<Pat<'tcx>>],
        opt_slice: &'pat Option<Box<Pat<'tcx>>>,
        suffix: &'pat [Box<Pat<'tcx>>],
    ) {
        let tcx = self.tcx;
        let (min_length, exact_size) = if let Some(place_resolved) = place.try_to_place(self) {
            match place_resolved.ty(&self.local_decls, tcx).ty.kind() {
                ty::Array(_, length) => (length.eval_target_usize(tcx, self.param_env), true),
                _ => ((prefix.len() + suffix.len()).try_into().unwrap(), false),
            }
        } else {
            ((prefix.len() + suffix.len()).try_into().unwrap(), false)
        };

        match_pairs.extend(prefix.iter().enumerate().map(|(idx, subpattern)| {
            let elem =
                ProjectionElem::ConstantIndex { offset: idx as u64, min_length, from_end: false };
            MatchPair::new(place.clone_project(elem), subpattern, self)
        }));

        if let Some(subslice_pat) = opt_slice {
            let suffix_len = suffix.len() as u64;
            let subslice = place.clone_project(PlaceElem::Subslice {
                from: prefix.len() as u64,
                to: if exact_size { min_length - suffix_len } else { suffix_len },
                from_end: !exact_size,
            });
            match_pairs.push(MatchPair::new(subslice, subslice_pat, self));
        }

        match_pairs.extend(suffix.iter().rev().enumerate().map(|(idx, subpattern)| {
            let end_offset = (idx + 1) as u64;
            let elem = ProjectionElem::ConstantIndex {
                offset: if exact_size { min_length - end_offset } else { end_offset },
                min_length,
                from_end: !exact_size,
            };
            let place = place.clone_project(elem);
            MatchPair::new(place, subpattern, self)
        }));
    }

    /// Creates a false edge to `imaginary_target` and a real edge to
    /// real_target. If `imaginary_target` is none, or is the same as the real
    /// target, a Goto is generated instead to simplify the generated MIR.
    pub(crate) fn false_edges(
        &mut self,
        from_block: BasicBlock,
        real_target: BasicBlock,
        imaginary_target: Option<BasicBlock>,
        source_info: SourceInfo,
    ) {
        match imaginary_target {
            Some(target) if target != real_target => {
                self.cfg.terminate(
                    from_block,
                    source_info,
                    TerminatorKind::FalseEdge { real_target, imaginary_target: target },
                );
            }
            _ => self.cfg.goto(from_block, source_info, real_target),
        }
    }
}

impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
    pub(in crate::build) fn new(
        mut place_builder: PlaceBuilder<'tcx>,
        pattern: &'pat Pat<'tcx>,
        cx: &mut Builder<'_, 'tcx>,
    ) -> MatchPair<'pat, 'tcx> {
        // Force the place type to the pattern's type.
        // FIXME(oli-obk): can we use this to simplify slice/array pattern hacks?
        if let Some(resolved) = place_builder.resolve_upvar(cx) {
            place_builder = resolved;
        }

        // Only add the OpaqueCast projection if the given place is an opaque type and the
        // expected type from the pattern is not.
        let may_need_cast = match place_builder.base() {
            PlaceBase::Local(local) => {
                let ty =
                    Place::ty_from(local, place_builder.projection(), &cx.local_decls, cx.tcx).ty;
                ty != pattern.ty && ty.has_opaque_types()
            }
            _ => true,
        };
        if may_need_cast {
            place_builder = place_builder.project(ProjectionElem::OpaqueCast(pattern.ty));
        }

        let place = place_builder.try_to_place(cx);
        let default_irrefutable = || TestCase::Irrefutable { binding: None, ascription: None };
        let mut subpairs = Vec::new();
        let test_case = match pattern.kind {
            PatKind::Wild | PatKind::Error(_) => default_irrefutable(),

            PatKind::Or { ref pats } => TestCase::Or {
                pats: pats.iter().map(|pat| FlatPat::new(place_builder.clone(), pat, cx)).collect(),
            },

            PatKind::Range(ref range) => {
                if range.is_full_range(cx.tcx) == Some(true) {
                    default_irrefutable()
                } else {
                    TestCase::Range(range)
                }
            }

            PatKind::Constant { value } => TestCase::Constant { value },

            PatKind::AscribeUserType {
                ascription: thir::Ascription { ref annotation, variance },
                ref subpattern,
                ..
            } => {
                // Apply the type ascription to the value at `match_pair.place`
                let ascription = place.map(|source| super::Ascription {
                    annotation: annotation.clone(),
                    source,
                    variance,
                });

                subpairs.push(MatchPair::new(place_builder, subpattern, cx));
                TestCase::Irrefutable { ascription, binding: None }
            }

            PatKind::Binding { mode, var, ref subpattern, .. } => {
                let binding = place.map(|source| super::Binding {
                    span: pattern.span,
                    source,
                    var_id: var,
                    binding_mode: mode,
                });

                if let Some(subpattern) = subpattern.as_ref() {
                    // this is the `x @ P` case; have to keep matching against `P` now
                    subpairs.push(MatchPair::new(place_builder, subpattern, cx));
                }
                TestCase::Irrefutable { ascription: None, binding }
            }

            PatKind::InlineConstant { subpattern: ref pattern, def, .. } => {
                // Apply a type ascription for the inline constant to the value at `match_pair.place`
                let ascription = place.map(|source| {
                    let span = pattern.span;
                    let parent_id = cx.tcx.typeck_root_def_id(cx.def_id.to_def_id());
                    let args = ty::InlineConstArgs::new(
                        cx.tcx,
                        ty::InlineConstArgsParts {
                            parent_args: ty::GenericArgs::identity_for_item(cx.tcx, parent_id),
                            ty: cx.infcx.next_ty_var(span),
                        },
                    )
                    .args;
                    let user_ty = cx.infcx.canonicalize_user_type_annotation(ty::UserType::TypeOf(
                        def.to_def_id(),
                        ty::UserArgs { args, user_self_ty: None },
                    ));
                    let annotation = ty::CanonicalUserTypeAnnotation {
                        inferred_ty: pattern.ty,
                        span,
                        user_ty: Box::new(user_ty),
                    };
                    super::Ascription { annotation, source, variance: ty::Contravariant }
                });

                subpairs.push(MatchPair::new(place_builder, pattern, cx));
                TestCase::Irrefutable { ascription, binding: None }
            }

            PatKind::Array { ref prefix, ref slice, ref suffix } => {
                cx.prefix_slice_suffix(&mut subpairs, &place_builder, prefix, slice, suffix);
                default_irrefutable()
            }
            PatKind::Slice { ref prefix, ref slice, ref suffix } => {
                cx.prefix_slice_suffix(&mut subpairs, &place_builder, prefix, slice, suffix);

                if prefix.is_empty() && slice.is_some() && suffix.is_empty() {
                    default_irrefutable()
                } else {
                    TestCase::Slice {
                        len: prefix.len() + suffix.len(),
                        variable_length: slice.is_some(),
                    }
                }
            }

            PatKind::Variant { adt_def, variant_index, args, ref subpatterns } => {
                let downcast_place = place_builder.downcast(adt_def, variant_index); // `(x as Variant)`
                subpairs = cx.field_match_pairs(downcast_place, subpatterns);

                let irrefutable = adt_def.variants().iter_enumerated().all(|(i, v)| {
                    i == variant_index || {
                        (cx.tcx.features().exhaustive_patterns
                            || cx.tcx.features().min_exhaustive_patterns)
                            && !v
                                .inhabited_predicate(cx.tcx, adt_def)
                                .instantiate(cx.tcx, args)
                                .apply_ignore_module(cx.tcx, cx.param_env)
                    }
                }) && (adt_def.did().is_local()
                    || !adt_def.is_variant_list_non_exhaustive());
                if irrefutable {
                    default_irrefutable()
                } else {
                    TestCase::Variant { adt_def, variant_index }
                }
            }

            PatKind::Leaf { ref subpatterns } => {
                subpairs = cx.field_match_pairs(place_builder, subpatterns);
                default_irrefutable()
            }

            PatKind::Deref { ref subpattern } => {
                subpairs.push(MatchPair::new(place_builder.deref(), subpattern, cx));
                default_irrefutable()
            }

            PatKind::DerefPattern { ref subpattern, mutability } => {
                // Create a new temporary for each deref pattern.
                // FIXME(deref_patterns): dedup temporaries to avoid multiple `deref()` calls?
                let temp = cx.temp(
                    Ty::new_ref(cx.tcx, cx.tcx.lifetimes.re_erased, subpattern.ty, mutability),
                    pattern.span,
                );
                subpairs.push(MatchPair::new(PlaceBuilder::from(temp).deref(), subpattern, cx));
                TestCase::Deref { temp, mutability }
            }

            PatKind::Never => TestCase::Never,
        };

        MatchPair { place, test_case, subpairs, pattern }
    }
}

pub(super) struct FakeBorrowCollector<'a, 'b, 'tcx> {
    cx: &'a mut Builder<'b, 'tcx>,
    /// Base of the scrutinee place. Used to distinguish bindings inside the scrutinee place from
    /// bindings inside deref patterns.
    scrutinee_base: PlaceBase,
    /// Store for each place the kind of borrow to take. In case of conflicts, we take the strongest
    /// borrow (i.e. Deep > Shallow).
    /// Invariant: for any place in `fake_borrows`, all the prefixes of this place that are
    /// dereferences are also borrowed with the same of stronger borrow kind.
    fake_borrows: FxIndexMap<Place<'tcx>, FakeBorrowKind>,
}

/// Determine the set of places that have to be stable across match guards.
///
/// Returns a list of places that need a fake borrow along with a local to store it.
///
/// Match exhaustiveness checking is not able to handle the case where the place being matched on is
/// mutated in the guards. We add "fake borrows" to the guards that prevent any mutation of the
/// place being matched. There are a some subtleties:
///
/// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared reference, the borrow
///    isn't even tracked. As such we have to add fake borrows of any prefixes of a place.
/// 2. We don't want `match x { (Some(_), _) => (), .. }` to conflict with mutable borrows of `x.1`, so we
///    only add fake borrows for places which are bound or tested by the match.
/// 3. We don't want `match x { Some(_) => (), .. }` to conflict with mutable borrows of `(x as
///    Some).0`, so the borrows are a special shallow borrow that only affects the place and not its
///    projections.
///    ```rust
///    let mut x = (Some(0), true);
///    match x {
///        (Some(_), false) => {}
///        _ if { if let Some(ref mut y) = x.0 { *y += 1 }; true } => {}
///        _ => {}
///    }
///    ```
/// 4. The fake borrows may be of places in inactive variants, e.g. here we need to fake borrow `x`
///    and `(x as Some).0`, but when we reach the guard `x` may not be `Some`.
///    ```rust
///    let mut x = (Some(Some(0)), true);
///    match x {
///        (Some(Some(_)), false) => {}
///        _ if { if let Some(Some(ref mut y)) = x.0 { *y += 1 }; true } => {}
///        _ => {}
///    }
///    ```
///    So it would be UB to generate code for the fake borrows. They therefore have to be removed by
///    a MIR pass run after borrow checking.
pub(super) fn collect_fake_borrows<'tcx>(
    cx: &mut Builder<'_, 'tcx>,
    candidates: &[&mut Candidate<'_, 'tcx>],
    temp_span: Span,
    scrutinee_base: PlaceBase,
) -> Vec<(Place<'tcx>, Local, FakeBorrowKind)> {
    let mut collector =
        FakeBorrowCollector { cx, scrutinee_base, fake_borrows: FxIndexMap::default() };
    for candidate in candidates.iter() {
        collector.visit_candidate(candidate);
    }
    let fake_borrows = collector.fake_borrows;
    debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
    let tcx = cx.tcx;
    fake_borrows
        .iter()
        .map(|(matched_place, borrow_kind)| {
            let fake_borrow_deref_ty = matched_place.ty(&cx.local_decls, tcx).ty;
            let fake_borrow_ty =
                Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, fake_borrow_deref_ty);
            let mut fake_borrow_temp = LocalDecl::new(fake_borrow_ty, temp_span);
            fake_borrow_temp.local_info = ClearCrossCrate::Set(Box::new(LocalInfo::FakeBorrow));
            let fake_borrow_temp = cx.local_decls.push(fake_borrow_temp);
            (*matched_place, fake_borrow_temp, *borrow_kind)
        })
        .collect()
}

impl<'a, 'b, 'tcx> FakeBorrowCollector<'a, 'b, 'tcx> {
    // Fake borrow this place and its dereference prefixes.
    fn fake_borrow(&mut self, place: Place<'tcx>, kind: FakeBorrowKind) {
        if self.fake_borrows.get(&place).is_some_and(|k| *k >= kind) {
            return;
        }
        self.fake_borrows.insert(place, kind);
        // Also fake borrow the prefixes of any fake borrow.
        self.fake_borrow_deref_prefixes(place, kind);
    }

    // Fake borrow the prefixes of this place that are dereferences.
    fn fake_borrow_deref_prefixes(&mut self, place: Place<'tcx>, kind: FakeBorrowKind) {
        for (place_ref, elem) in place.as_ref().iter_projections().rev() {
            if let ProjectionElem::Deref = elem {
                // Insert a shallow borrow after a deref. For other projections the borrow of
                // `place_ref` will conflict with any mutation of `place.base`.
                let place = place_ref.to_place(self.cx.tcx);
                if self.fake_borrows.get(&place).is_some_and(|k| *k >= kind) {
                    return;
                }
                self.fake_borrows.insert(place, kind);
            }
        }
    }

    fn visit_candidate(&mut self, candidate: &Candidate<'_, 'tcx>) {
        for binding in &candidate.extra_data.bindings {
            self.visit_binding(binding);
        }
        for match_pair in &candidate.match_pairs {
            self.visit_match_pair(match_pair);
        }
    }

    fn visit_flat_pat(&mut self, flat_pat: &FlatPat<'_, 'tcx>) {
        for binding in &flat_pat.extra_data.bindings {
            self.visit_binding(binding);
        }
        for match_pair in &flat_pat.match_pairs {
            self.visit_match_pair(match_pair);
        }
    }

    fn visit_match_pair(&mut self, match_pair: &MatchPair<'_, 'tcx>) {
        if let TestCase::Or { pats, .. } = &match_pair.test_case {
            for flat_pat in pats.iter() {
                self.visit_flat_pat(flat_pat)
            }
        } else if matches!(match_pair.test_case, TestCase::Deref { .. }) {
            // The subpairs of a deref pattern are all places relative to the deref temporary, so we
            // don't fake borrow them. Problem is, if we only shallowly fake-borrowed
            // `match_pair.place`, this would allow:
            // ```
            // let mut b = Box::new(false);
            // match b {
            //     deref!(true) => {} // not reached because `*b == false`
            //     _ if { *b = true; false } => {} // not reached because the guard is `false`
            //     deref!(false) => {} // not reached because the guard changed it
            //     // UB because we reached the unreachable.
            // }
            // ```
            // Hence we fake borrow using a deep borrow.
            if let Some(place) = match_pair.place {
                self.fake_borrow(place, FakeBorrowKind::Deep);
            }
        } else {
            // Insert a Shallow borrow of any place that is switched on.
            if let Some(place) = match_pair.place {
                self.fake_borrow(place, FakeBorrowKind::Shallow);
            }

            for subpair in &match_pair.subpairs {
                self.visit_match_pair(subpair);
            }
        }
    }

    fn visit_binding(&mut self, Binding { source, .. }: &Binding<'tcx>) {
        if let PlaceBase::Local(l) = self.scrutinee_base
            && l != source.local
        {
            // The base of this place is a temporary created for deref patterns. We don't emit fake
            // borrows for these as they are not initialized in all branches.
            return;
        }

        // Insert a borrows of prefixes of places that are bound and are
        // behind a dereference projection.
        //
        // These borrows are taken to avoid situations like the following:
        //
        // match x[10] {
        //     _ if { x = &[0]; false } => (),
        //     y => (), // Out of bounds array access!
        // }
        //
        // match *x {
        //     // y is bound by reference in the guard and then by copy in the
        //     // arm, so y is 2 in the arm!
        //     y if { y == 1 && (x = &2) == () } => y,
        //     _ => 3,
        // }
        //
        // We don't just fake borrow the whole place because this is allowed:
        // match u {
        //     _ if { u = true; false } => (),
        //     x => (),
        // }
        self.fake_borrow_deref_prefixes(*source, FakeBorrowKind::Shallow);
    }
}

#[must_use]
pub fn ref_pat_borrow_kind(ref_mutability: Mutability) -> BorrowKind {
    match ref_mutability {
        Mutability::Mut => BorrowKind::Mut { kind: MutBorrowKind::Default },
        Mutability::Not => BorrowKind::Shared,
    }
}