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
//! See docs in build/expr/mod.rs

use crate::build::expr::category::{Category, RvalueFunc};
use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder, NeedsTemporary};
use rustc_ast::InlineAsmOptions;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir as hir;
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use rustc_middle::ty::CanonicalUserTypeAnnotation;
use rustc_span::source_map::Spanned;
use std::iter;

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Compile `expr`, storing the result into `destination`, which
    /// is assumed to be uninitialized.
    #[instrument(level = "debug", skip(self))]
    pub(crate) fn expr_into_dest(
        &mut self,
        destination: Place<'tcx>,
        mut block: BasicBlock,
        expr_id: ExprId,
    ) -> BlockAnd<()> {
        // since we frequently have to reference `self` from within a
        // closure, where `self` would be shadowed, it's easier to
        // just use the name `this` uniformly
        let this = self;
        let expr = &this.thir[expr_id];
        let expr_span = expr.span;
        let source_info = this.source_info(expr_span);

        let expr_is_block_or_scope =
            matches!(expr.kind, ExprKind::Block { .. } | ExprKind::Scope { .. });

        if !expr_is_block_or_scope {
            this.block_context.push(BlockFrame::SubExpr);
        }

        let block_and = match expr.kind {
            ExprKind::Scope { region_scope, lint_level, value } => {
                let region_scope = (region_scope, source_info);
                ensure_sufficient_stack(|| {
                    this.in_scope(region_scope, lint_level, |this| {
                        this.expr_into_dest(destination, block, value)
                    })
                })
            }
            ExprKind::Block { block: ast_block } => {
                this.ast_block(destination, block, ast_block, source_info)
            }
            ExprKind::Match { scrutinee, ref arms, .. } => this.match_expr(
                destination,
                block,
                scrutinee,
                arms,
                expr_span,
                this.thir[scrutinee].span,
            ),
            ExprKind::If { cond, then, else_opt, if_then_scope } => {
                let then_span = this.thir[then].span;
                let then_source_info = this.source_info(then_span);
                let condition_scope = this.local_scope();

                let then_and_else_blocks = this.in_scope(
                    (if_then_scope, then_source_info),
                    LintLevel::Inherited,
                    |this| {
                        // FIXME: Does this need extra logic to handle let-chains?
                        let source_info = if this.is_let(cond) {
                            let variable_scope =
                                this.new_source_scope(then_span, LintLevel::Inherited);
                            this.source_scope = variable_scope;
                            SourceInfo { span: then_span, scope: variable_scope }
                        } else {
                            this.source_info(then_span)
                        };

                        // Lower the condition, and have it branch into `then` and `else` blocks.
                        let (then_block, else_block) =
                            this.in_if_then_scope(condition_scope, then_span, |this| {
                                let then_blk = unpack!(this.then_else_break(
                                    block,
                                    cond,
                                    Some(condition_scope), // Temp scope
                                    source_info,
                                    true, // Declare `let` bindings normally
                                ));

                                // Lower the `then` arm into its block.
                                this.expr_into_dest(destination, then_blk, then)
                            });

                        // Pack `(then_block, else_block)` into `BlockAnd<BasicBlock>`.
                        then_block.and(else_block)
                    },
                );

                // Unpack `BlockAnd<BasicBlock>` into `(then_blk, else_blk)`.
                let (then_blk, mut else_blk);
                else_blk = unpack!(then_blk = then_and_else_blocks);

                // If there is an `else` arm, lower it into `else_blk`.
                if let Some(else_expr) = else_opt {
                    unpack!(else_blk = this.expr_into_dest(destination, else_blk, else_expr));
                } else {
                    // There is no `else` arm, so we know both arms have type `()`.
                    // Generate the implicit `else {}` by assigning unit.
                    let correct_si = this.source_info(expr_span.shrink_to_hi());
                    this.cfg.push_assign_unit(else_blk, correct_si, destination, this.tcx);
                }

                // The `then` and `else` arms have been lowered into their respective
                // blocks, so make both of them meet up in a new block.
                let join_block = this.cfg.start_new_block();
                this.cfg.goto(then_blk, source_info, join_block);
                this.cfg.goto(else_blk, source_info, join_block);
                join_block.unit()
            }
            ExprKind::Let { .. } => {
                // After desugaring, `let` expressions should only appear inside `if`
                // expressions or `match` guards, possibly nested within a let-chain.
                // In both cases they are specifically handled by the lowerings of
                // those expressions, so this case is currently unreachable.
                span_bug!(expr_span, "unexpected let expression outside of if or match-guard");
            }
            ExprKind::NeverToAny { source } => {
                let source_expr = &this.thir[source];
                let is_call =
                    matches!(source_expr.kind, ExprKind::Call { .. } | ExprKind::InlineAsm { .. });

                // (#66975) Source could be a const of type `!`, so has to
                // exist in the generated MIR.
                unpack!(
                    block = this.as_temp(block, Some(this.local_scope()), source, Mutability::Mut)
                );

                // This is an optimization. If the expression was a call then we already have an
                // unreachable block. Don't bother to terminate it and create a new one.
                if is_call {
                    block.unit()
                } else {
                    this.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
                    let end_block = this.cfg.start_new_block();
                    end_block.unit()
                }
            }
            ExprKind::LogicalOp { op, lhs, rhs } => {
                let condition_scope = this.local_scope();
                let source_info = this.source_info(expr.span);
                // We first evaluate the left-hand side of the predicate ...
                let (then_block, else_block) =
                    this.in_if_then_scope(condition_scope, expr.span, |this| {
                        this.then_else_break(
                            block,
                            lhs,
                            Some(condition_scope), // Temp scope
                            source_info,
                            // This flag controls how inner `let` expressions are lowered,
                            // but either way there shouldn't be any of those in here.
                            true,
                        )
                    });
                let (short_circuit, continuation, constant) = match op {
                    LogicalOp::And => (else_block, then_block, false),
                    LogicalOp::Or => (then_block, else_block, true),
                };
                // At this point, the control flow splits into a short-circuiting path
                // and a continuation path.
                // - If the operator is `&&`, passing `lhs` leads to continuation of evaluation on `rhs`;
                //   failing it leads to the short-circuting path which assigns `false` to the place.
                // - If the operator is `||`, failing `lhs` leads to continuation of evaluation on `rhs`;
                //   passing it leads to the short-circuting path which assigns `true` to the place.
                this.cfg.push_assign_constant(
                    short_circuit,
                    source_info,
                    destination,
                    ConstOperand {
                        span: expr.span,
                        user_ty: None,
                        const_: Const::from_bool(this.tcx, constant),
                    },
                );
                let rhs = unpack!(this.expr_into_dest(destination, continuation, rhs));
                let target = this.cfg.start_new_block();
                this.cfg.goto(rhs, source_info, target);
                this.cfg.goto(short_circuit, source_info, target);
                target.unit()
            }
            ExprKind::Loop { body } => {
                // [block]
                //    |
                //   [loop_block] -> [body_block] -/eval. body/-> [body_block_end]
                //    |        ^                                         |
                // false link  |                                         |
                //    |        +-----------------------------------------+
                //    +-> [diverge_cleanup]
                // The false link is required to make sure borrowck considers unwinds through the
                // body, even when the exact code in the body cannot unwind

                let loop_block = this.cfg.start_new_block();

                // Start the loop.
                this.cfg.goto(block, source_info, loop_block);

                this.in_breakable_scope(Some(loop_block), destination, expr_span, move |this| {
                    // conduct the test, if necessary
                    let body_block = this.cfg.start_new_block();
                    this.cfg.terminate(
                        loop_block,
                        source_info,
                        TerminatorKind::FalseUnwind {
                            real_target: body_block,
                            unwind: UnwindAction::Continue,
                        },
                    );
                    this.diverge_from(loop_block);

                    // The “return” value of the loop body must always be a unit. We therefore
                    // introduce a unit temporary as the destination for the loop body.
                    let tmp = this.get_unit_temp();
                    // Execute the body, branching back to the test.
                    let body_block_end = unpack!(this.expr_into_dest(tmp, body_block, body));
                    this.cfg.goto(body_block_end, source_info, loop_block);

                    // Loops are only exited by `break` expressions.
                    None
                })
            }
            ExprKind::Call { ty: _, fun, ref args, from_hir_call, fn_span } => {
                let fun = unpack!(block = this.as_local_operand(block, fun));
                let args: Vec<_> = args
                    .into_iter()
                    .copied()
                    .map(|arg| Spanned {
                        node: unpack!(block = this.as_local_call_operand(block, arg)),
                        span: this.thir.exprs[arg].span,
                    })
                    .collect();

                let success = this.cfg.start_new_block();

                this.record_operands_moved(&args);

                debug!("expr_into_dest: fn_span={:?}", fn_span);

                this.cfg.terminate(
                    block,
                    source_info,
                    TerminatorKind::Call {
                        func: fun,
                        args,
                        unwind: UnwindAction::Continue,
                        destination,
                        // The presence or absence of a return edge affects control-flow sensitive
                        // MIR checks and ultimately whether code is accepted or not. We can only
                        // omit the return edge if a return type is visibly uninhabited to a module
                        // that makes the call.
                        target: expr
                            .ty
                            .is_inhabited_from(this.tcx, this.parent_module, this.param_env)
                            .then_some(success),
                        call_source: if from_hir_call {
                            CallSource::Normal
                        } else {
                            CallSource::OverloadedOperator
                        },
                        fn_span,
                    },
                );
                this.diverge_from(block);
                success.unit()
            }
            ExprKind::Use { source } => this.expr_into_dest(destination, block, source),
            ExprKind::Borrow { arg, borrow_kind } => {
                // We don't do this in `as_rvalue` because we use `as_place`
                // for borrow expressions, so we cannot create an `RValue` that
                // remains valid across user code. `as_rvalue` is usually called
                // by this method anyway, so this shouldn't cause too many
                // unnecessary temporaries.
                let arg_place = match borrow_kind {
                    BorrowKind::Shared => {
                        unpack!(block = this.as_read_only_place(block, arg))
                    }
                    _ => unpack!(block = this.as_place(block, arg)),
                };
                let borrow = Rvalue::Ref(this.tcx.lifetimes.re_erased, borrow_kind, arg_place);
                this.cfg.push_assign(block, source_info, destination, borrow);
                block.unit()
            }
            ExprKind::AddressOf { mutability, arg } => {
                let place = match mutability {
                    hir::Mutability::Not => this.as_read_only_place(block, arg),
                    hir::Mutability::Mut => this.as_place(block, arg),
                };
                let address_of = Rvalue::AddressOf(mutability, unpack!(block = place));
                this.cfg.push_assign(block, source_info, destination, address_of);
                block.unit()
            }
            ExprKind::Adt(box AdtExpr {
                adt_def,
                variant_index,
                args,
                ref user_ty,
                ref fields,
                ref base,
            }) => {
                // See the notes for `ExprKind::Array` in `as_rvalue` and for
                // `ExprKind::Borrow` above.
                let is_union = adt_def.is_union();
                let active_field_index = is_union.then(|| fields[0].name);

                let scope = this.local_scope();

                // first process the set of fields that were provided
                // (evaluating them in order given by user)
                let fields_map: FxHashMap<_, _> = fields
                    .into_iter()
                    .map(|f| {
                        (
                            f.name,
                            unpack!(
                                block = this.as_operand(
                                    block,
                                    Some(scope),
                                    f.expr,
                                    LocalInfo::AggregateTemp,
                                    NeedsTemporary::Maybe,
                                )
                            ),
                        )
                    })
                    .collect();

                let field_names = adt_def.variant(variant_index).fields.indices();

                let fields = if let Some(FruInfo { base, field_types }) = base {
                    let place_builder = unpack!(block = this.as_place_builder(block, *base));

                    // MIR does not natively support FRU, so for each
                    // base-supplied field, generate an operand that
                    // reads it from the base.
                    iter::zip(field_names, &**field_types)
                        .map(|(n, ty)| match fields_map.get(&n) {
                            Some(v) => v.clone(),
                            None => {
                                let place = place_builder.clone_project(PlaceElem::Field(n, *ty));
                                this.consume_by_copy_or_move(place.to_place(this))
                            }
                        })
                        .collect()
                } else {
                    field_names.filter_map(|n| fields_map.get(&n).cloned()).collect()
                };

                let inferred_ty = expr.ty;
                let user_ty = user_ty.as_ref().map(|user_ty| {
                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                        span: source_info.span,
                        user_ty: user_ty.clone(),
                        inferred_ty,
                    })
                });
                let adt = Box::new(AggregateKind::Adt(
                    adt_def.did(),
                    variant_index,
                    args,
                    user_ty,
                    active_field_index,
                ));
                this.cfg.push_assign(
                    block,
                    source_info,
                    destination,
                    Rvalue::Aggregate(adt, fields),
                );
                block.unit()
            }
            ExprKind::InlineAsm(box InlineAsmExpr {
                template,
                ref operands,
                options,
                line_spans,
            }) => {
                use rustc_middle::{mir, thir};

                let destination_block = this.cfg.start_new_block();
                let mut targets = if options.contains(InlineAsmOptions::NORETURN) {
                    vec![]
                } else {
                    vec![destination_block]
                };

                let operands = operands
                    .into_iter()
                    .map(|op| match *op {
                        thir::InlineAsmOperand::In { reg, expr } => mir::InlineAsmOperand::In {
                            reg,
                            value: unpack!(block = this.as_local_operand(block, expr)),
                        },
                        thir::InlineAsmOperand::Out { reg, late, expr } => {
                            mir::InlineAsmOperand::Out {
                                reg,
                                late,
                                place: expr.map(|expr| unpack!(block = this.as_place(block, expr))),
                            }
                        }
                        thir::InlineAsmOperand::InOut { reg, late, expr } => {
                            let place = unpack!(block = this.as_place(block, expr));
                            mir::InlineAsmOperand::InOut {
                                reg,
                                late,
                                // This works because asm operands must be Copy
                                in_value: Operand::Copy(place),
                                out_place: Some(place),
                            }
                        }
                        thir::InlineAsmOperand::SplitInOut { reg, late, in_expr, out_expr } => {
                            mir::InlineAsmOperand::InOut {
                                reg,
                                late,
                                in_value: unpack!(block = this.as_local_operand(block, in_expr)),
                                out_place: out_expr.map(|out_expr| {
                                    unpack!(block = this.as_place(block, out_expr))
                                }),
                            }
                        }
                        thir::InlineAsmOperand::Const { value, span } => {
                            mir::InlineAsmOperand::Const {
                                value: Box::new(ConstOperand {
                                    span,
                                    user_ty: None,
                                    const_: value,
                                }),
                            }
                        }
                        thir::InlineAsmOperand::SymFn { value, span } => {
                            mir::InlineAsmOperand::SymFn {
                                value: Box::new(ConstOperand {
                                    span,
                                    user_ty: None,
                                    const_: value,
                                }),
                            }
                        }
                        thir::InlineAsmOperand::SymStatic { def_id } => {
                            mir::InlineAsmOperand::SymStatic { def_id }
                        }
                        thir::InlineAsmOperand::Label { block } => {
                            let target = this.cfg.start_new_block();
                            let target_index = targets.len();
                            targets.push(target);

                            let tmp = this.get_unit_temp();
                            let target = unpack!(this.ast_block(tmp, target, block, source_info));
                            this.cfg.terminate(
                                target,
                                source_info,
                                TerminatorKind::Goto { target: destination_block },
                            );

                            mir::InlineAsmOperand::Label { target_index }
                        }
                    })
                    .collect();

                if !expr.ty.is_never() {
                    this.cfg.push_assign_unit(block, source_info, destination, this.tcx);
                }

                this.cfg.terminate(
                    block,
                    source_info,
                    TerminatorKind::InlineAsm {
                        template,
                        operands,
                        options,
                        line_spans,
                        targets,
                        unwind: if options.contains(InlineAsmOptions::MAY_UNWIND) {
                            UnwindAction::Continue
                        } else {
                            UnwindAction::Unreachable
                        },
                    },
                );
                if options.contains(InlineAsmOptions::MAY_UNWIND) {
                    this.diverge_from(block);
                }
                destination_block.unit()
            }

            // These cases don't actually need a destination
            ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
                unpack!(block = this.stmt_expr(block, expr_id, None));
                this.cfg.push_assign_unit(block, source_info, destination, this.tcx);
                block.unit()
            }

            ExprKind::Continue { .. }
            | ExprKind::Break { .. }
            | ExprKind::Return { .. }
            | ExprKind::Become { .. } => {
                unpack!(block = this.stmt_expr(block, expr_id, None));
                // No assign, as these have type `!`.
                block.unit()
            }

            // Avoid creating a temporary
            ExprKind::VarRef { .. }
            | ExprKind::UpvarRef { .. }
            | ExprKind::PlaceTypeAscription { .. }
            | ExprKind::ValueTypeAscription { .. } => {
                debug_assert!(Category::of(&expr.kind) == Some(Category::Place));

                let place = unpack!(block = this.as_place(block, expr_id));
                let rvalue = Rvalue::Use(this.consume_by_copy_or_move(place));
                this.cfg.push_assign(block, source_info, destination, rvalue);
                block.unit()
            }
            ExprKind::Index { .. } | ExprKind::Deref { .. } | ExprKind::Field { .. } => {
                debug_assert_eq!(Category::of(&expr.kind), Some(Category::Place));

                // Create a "fake" temporary variable so that we check that the
                // value is Sized. Usually, this is caught in type checking, but
                // in the case of box expr there is no such check.
                if !destination.projection.is_empty() {
                    this.local_decls.push(LocalDecl::new(expr.ty, expr.span));
                }

                let place = unpack!(block = this.as_place(block, expr_id));
                let rvalue = Rvalue::Use(this.consume_by_copy_or_move(place));
                this.cfg.push_assign(block, source_info, destination, rvalue);
                block.unit()
            }

            ExprKind::Yield { value } => {
                let scope = this.local_scope();
                let value = unpack!(
                    block = this.as_operand(
                        block,
                        Some(scope),
                        value,
                        LocalInfo::Boring,
                        NeedsTemporary::No
                    )
                );
                let resume = this.cfg.start_new_block();
                this.cfg.terminate(
                    block,
                    source_info,
                    TerminatorKind::Yield { value, resume, resume_arg: destination, drop: None },
                );
                this.coroutine_drop_cleanup(block);
                resume.unit()
            }

            // these are the cases that are more naturally handled by some other mode
            ExprKind::Unary { .. }
            | ExprKind::Binary { .. }
            | ExprKind::Box { .. }
            | ExprKind::Cast { .. }
            | ExprKind::PointerCoercion { .. }
            | ExprKind::Repeat { .. }
            | ExprKind::Array { .. }
            | ExprKind::Tuple { .. }
            | ExprKind::Closure { .. }
            | ExprKind::ConstBlock { .. }
            | ExprKind::Literal { .. }
            | ExprKind::NamedConst { .. }
            | ExprKind::NonHirLiteral { .. }
            | ExprKind::ZstLiteral { .. }
            | ExprKind::ConstParam { .. }
            | ExprKind::ThreadLocalRef(_)
            | ExprKind::StaticRef { .. }
            | ExprKind::OffsetOf { .. } => {
                debug_assert!(match Category::of(&expr.kind).unwrap() {
                    // should be handled above
                    Category::Rvalue(RvalueFunc::Into) => false,

                    // must be handled above or else we get an
                    // infinite loop in the builder; see
                    // e.g., `ExprKind::VarRef` above
                    Category::Place => false,

                    _ => true,
                });

                let rvalue = unpack!(block = this.as_local_rvalue(block, expr_id));
                this.cfg.push_assign(block, source_info, destination, rvalue);
                block.unit()
            }
        };

        if !expr_is_block_or_scope {
            let popped = this.block_context.pop();
            assert!(popped.is_some());
        }

        block_and
    }

    fn is_let(&self, expr: ExprId) -> bool {
        match self.thir[expr].kind {
            ExprKind::Let { .. } => true,
            ExprKind::Scope { value, .. } => self.is_let(value),
            _ => false,
        }
    }
}