rustc_mir_transform/
promote_consts.rs

1//! A pass that promotes borrows of constant rvalues.
2//!
3//! The rvalues considered constant are trees of temps, each with exactly one
4//! initialization, and holding a constant value with no interior mutability.
5//! They are placed into a new MIR constant body in `promoted` and the borrow
6//! rvalue is replaced with a `Literal::Promoted` using the index into
7//! `promoted` of that constant MIR.
8//!
9//! This pass assumes that every use is dominated by an initialization and can
10//! otherwise silence errors, if move analysis runs after promotion on broken
11//! MIR.
12
13use std::assert_matches::assert_matches;
14use std::cell::Cell;
15use std::{cmp, iter, mem};
16
17use either::{Left, Right};
18use rustc_const_eval::check_consts::{ConstCx, qualifs};
19use rustc_data_structures::fx::FxHashSet;
20use rustc_hir as hir;
21use rustc_index::{IndexSlice, IndexVec};
22use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
23use rustc_middle::mir::*;
24use rustc_middle::ty::{self, GenericArgs, List, Ty, TyCtxt, TypeVisitableExt};
25use rustc_middle::{bug, mir, span_bug};
26use rustc_span::Span;
27use rustc_span::source_map::Spanned;
28use tracing::{debug, instrument};
29
30/// A `MirPass` for promotion.
31///
32/// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
33/// `'static` lifetime.
34///
35/// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
36/// newly created `Constant`.
37#[derive(Default)]
38pub(super) struct PromoteTemps<'tcx> {
39    // Must use `Cell` because `run_pass` takes `&self`, not `&mut self`.
40    pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
41}
42
43impl<'tcx> crate::MirPass<'tcx> for PromoteTemps<'tcx> {
44    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
45        // There's not really any point in promoting errorful MIR.
46        //
47        // This does not include MIR that failed const-checking, which we still try to promote.
48        if let Err(_) = body.return_ty().error_reported() {
49            debug!("PromoteTemps: MIR had errors");
50            return;
51        }
52        if body.source.promoted.is_some() {
53            return;
54        }
55
56        let ccx = ConstCx::new(tcx, body);
57        let (mut temps, all_candidates) = collect_temps_and_candidates(&ccx);
58
59        let promotable_candidates = validate_candidates(&ccx, &mut temps, all_candidates);
60
61        let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
62        self.promoted_fragments.set(promoted);
63    }
64
65    fn is_required(&self) -> bool {
66        true
67    }
68}
69
70/// State of a temporary during collection and promotion.
71#[derive(Copy, Clone, PartialEq, Eq, Debug)]
72enum TempState {
73    /// No references to this temp.
74    Undefined,
75    /// One direct assignment and any number of direct uses.
76    /// A borrow of this temp is promotable if the assigned
77    /// value is qualified as constant.
78    Defined { location: Location, uses: usize, valid: Result<(), ()> },
79    /// Any other combination of assignments/uses.
80    Unpromotable,
81    /// This temp was part of an rvalue which got extracted
82    /// during promotion and needs cleanup.
83    PromotedOut,
84}
85
86/// A "root candidate" for promotion, which will become the
87/// returned value in a promoted MIR, unless it's a subset
88/// of a larger candidate.
89#[derive(Copy, Clone, PartialEq, Eq, Debug)]
90struct Candidate {
91    location: Location,
92}
93
94struct Collector<'a, 'tcx> {
95    ccx: &'a ConstCx<'a, 'tcx>,
96    temps: IndexVec<Local, TempState>,
97    candidates: Vec<Candidate>,
98}
99
100impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
101    #[instrument(level = "debug", skip(self))]
102    fn visit_local(&mut self, index: Local, context: PlaceContext, location: Location) {
103        // We're only interested in temporaries and the return place
104        match self.ccx.body.local_kind(index) {
105            LocalKind::Arg => return,
106            LocalKind::Temp if self.ccx.body.local_decls[index].is_user_variable() => return,
107            LocalKind::ReturnPointer | LocalKind::Temp => {}
108        }
109
110        // Ignore drops, if the temp gets promoted,
111        // then it's constant and thus drop is noop.
112        // Non-uses are also irrelevant.
113        if context.is_drop() || !context.is_use() {
114            debug!(is_drop = context.is_drop(), is_use = context.is_use());
115            return;
116        }
117
118        let temp = &mut self.temps[index];
119        debug!(?temp);
120        *temp = match *temp {
121            TempState::Undefined => match context {
122                PlaceContext::MutatingUse(MutatingUseContext::Store | MutatingUseContext::Call) => {
123                    TempState::Defined { location, uses: 0, valid: Err(()) }
124                }
125                _ => TempState::Unpromotable,
126            },
127            TempState::Defined { ref mut uses, .. } => {
128                // We always allow borrows, even mutable ones, as we need
129                // to promote mutable borrows of some ZSTs e.g., `&mut []`.
130                let allowed_use = match context {
131                    PlaceContext::MutatingUse(MutatingUseContext::Borrow)
132                    | PlaceContext::NonMutatingUse(_) => true,
133                    PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
134                };
135                debug!(?allowed_use);
136                if allowed_use {
137                    *uses += 1;
138                    return;
139                }
140                TempState::Unpromotable
141            }
142            TempState::Unpromotable | TempState::PromotedOut => TempState::Unpromotable,
143        };
144        debug!(?temp);
145    }
146
147    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
148        self.super_rvalue(rvalue, location);
149
150        if let Rvalue::Ref(..) = *rvalue {
151            self.candidates.push(Candidate { location });
152        }
153    }
154}
155
156fn collect_temps_and_candidates<'tcx>(
157    ccx: &ConstCx<'_, 'tcx>,
158) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
159    let mut collector = Collector {
160        temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
161        candidates: vec![],
162        ccx,
163    };
164    for (bb, data) in traversal::reverse_postorder(ccx.body) {
165        collector.visit_basic_block_data(bb, data);
166    }
167    (collector.temps, collector.candidates)
168}
169
170/// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
171///
172/// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
173struct Validator<'a, 'tcx> {
174    ccx: &'a ConstCx<'a, 'tcx>,
175    temps: &'a mut IndexSlice<Local, TempState>,
176    /// For backwards compatibility, we are promoting function calls in `const`/`static`
177    /// initializers. But we want to avoid evaluating code that might panic and that otherwise would
178    /// not have been evaluated, so we only promote such calls in basic blocks that are guaranteed
179    /// to execute. In other words, we only promote such calls in basic blocks that are definitely
180    /// not dead code. Here we cache the result of computing that set of basic blocks.
181    promotion_safe_blocks: Option<FxHashSet<BasicBlock>>,
182}
183
184impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> {
185    type Target = ConstCx<'a, 'tcx>;
186
187    fn deref(&self) -> &Self::Target {
188        self.ccx
189    }
190}
191
192struct Unpromotable;
193
194impl<'tcx> Validator<'_, 'tcx> {
195    fn validate_candidate(&mut self, candidate: Candidate) -> Result<(), Unpromotable> {
196        let Left(statement) = self.body.stmt_at(candidate.location) else { bug!() };
197        let Some((_, Rvalue::Ref(_, kind, place))) = statement.kind.as_assign() else { bug!() };
198
199        // We can only promote interior borrows of promotable temps (non-temps
200        // don't get promoted anyway).
201        self.validate_local(place.local)?;
202
203        // The reference operation itself must be promotable.
204        // (Needs to come after `validate_local` to avoid ICEs.)
205        self.validate_ref(*kind, place)?;
206
207        // We do not check all the projections (they do not get promoted anyway),
208        // but we do stay away from promoting anything involving a dereference.
209        if place.projection.contains(&ProjectionElem::Deref) {
210            return Err(Unpromotable);
211        }
212
213        Ok(())
214    }
215
216    // FIXME(eddyb) maybe cache this?
217    fn qualif_local<Q: qualifs::Qualif>(&mut self, local: Local) -> bool {
218        let TempState::Defined { location: loc, .. } = self.temps[local] else {
219            return false;
220        };
221
222        let stmt_or_term = self.body.stmt_at(loc);
223        match stmt_or_term {
224            Left(statement) => {
225                let Some((_, rhs)) = statement.kind.as_assign() else {
226                    span_bug!(statement.source_info.span, "{:?} is not an assignment", statement)
227                };
228                qualifs::in_rvalue::<Q, _>(self.ccx, &mut |l| self.qualif_local::<Q>(l), rhs)
229            }
230            Right(terminator) => {
231                assert_matches!(terminator.kind, TerminatorKind::Call { .. });
232                let return_ty = self.body.local_decls[local].ty;
233                Q::in_any_value_of_ty(self.ccx, return_ty)
234            }
235        }
236    }
237
238    fn validate_local(&mut self, local: Local) -> Result<(), Unpromotable> {
239        let TempState::Defined { location: loc, uses, valid } = self.temps[local] else {
240            return Err(Unpromotable);
241        };
242
243        // We cannot promote things that need dropping, since the promoted value would not get
244        // dropped.
245        if self.qualif_local::<qualifs::NeedsDrop>(local) {
246            return Err(Unpromotable);
247        }
248
249        if valid.is_ok() {
250            return Ok(());
251        }
252
253        let ok = {
254            let stmt_or_term = self.body.stmt_at(loc);
255            match stmt_or_term {
256                Left(statement) => {
257                    let Some((_, rhs)) = statement.kind.as_assign() else {
258                        span_bug!(
259                            statement.source_info.span,
260                            "{:?} is not an assignment",
261                            statement
262                        )
263                    };
264                    self.validate_rvalue(rhs)
265                }
266                Right(terminator) => match &terminator.kind {
267                    TerminatorKind::Call { func, args, .. } => {
268                        self.validate_call(func, args, loc.block)
269                    }
270                    TerminatorKind::Yield { .. } => Err(Unpromotable),
271                    kind => {
272                        span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
273                    }
274                },
275            }
276        };
277
278        self.temps[local] = match ok {
279            Ok(()) => TempState::Defined { location: loc, uses, valid: Ok(()) },
280            Err(_) => TempState::Unpromotable,
281        };
282
283        ok
284    }
285
286    fn validate_place(&mut self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
287        let Some((place_base, elem)) = place.last_projection() else {
288            return self.validate_local(place.local);
289        };
290
291        // Validate topmost projection, then recurse.
292        match elem {
293            // Recurse directly.
294            ProjectionElem::ConstantIndex { .. }
295            | ProjectionElem::Subtype(_)
296            | ProjectionElem::Subslice { .. }
297            | ProjectionElem::UnwrapUnsafeBinder(_) => {}
298
299            // Never recurse.
300            ProjectionElem::OpaqueCast(..) | ProjectionElem::Downcast(..) => {
301                return Err(Unpromotable);
302            }
303
304            ProjectionElem::Deref => {
305                // When a static is used by-value, that gets desugared to `*STATIC_ADDR`,
306                // and we need to be able to promote this. So check if this deref matches
307                // that specific pattern.
308
309                // We need to make sure this is a `Deref` of a local with no further projections.
310                // Discussion can be found at
311                // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
312                if let Some(local) = place_base.as_local()
313                    && let TempState::Defined { location, .. } = self.temps[local]
314                    && let Left(def_stmt) = self.body.stmt_at(location)
315                    && let Some((_, Rvalue::Use(Operand::Constant(c)))) = def_stmt.kind.as_assign()
316                    && let Some(did) = c.check_static_ptr(self.tcx)
317                    // Evaluating a promoted may not read statics except if it got
318                    // promoted from a static (this is a CTFE check). So we
319                    // can only promote static accesses inside statics.
320                    && let Some(hir::ConstContext::Static(..)) = self.const_kind
321                    && !self.tcx.is_thread_local_static(did)
322                {
323                    // Recurse.
324                } else {
325                    return Err(Unpromotable);
326                }
327            }
328            ProjectionElem::Index(local) => {
329                // Only accept if we can predict the index and are indexing an array.
330                if let TempState::Defined { location: loc, .. } = self.temps[local]
331                    && let Left(statement) =  self.body.stmt_at(loc)
332                    && let Some((_, Rvalue::Use(Operand::Constant(c)))) = statement.kind.as_assign()
333                    && let Some(idx) = c.const_.try_eval_target_usize(self.tcx, self.typing_env)
334                    // Determine the type of the thing we are indexing.
335                    && let ty::Array(_, len) = place_base.ty(self.body, self.tcx).ty.kind()
336                    // It's an array; determine its length.
337                    && let Some(len) = len.try_to_target_usize(self.tcx)
338                    // If the index is in-bounds, go ahead.
339                    && idx < len
340                {
341                    self.validate_local(local)?;
342                    // Recurse.
343                } else {
344                    return Err(Unpromotable);
345                }
346            }
347
348            ProjectionElem::Field(..) => {
349                let base_ty = place_base.ty(self.body, self.tcx).ty;
350                if base_ty.is_union() {
351                    // No promotion of union field accesses.
352                    return Err(Unpromotable);
353                }
354            }
355        }
356
357        self.validate_place(place_base)
358    }
359
360    fn validate_operand(&mut self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
361        match operand {
362            Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
363
364            // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
365            // `validate_rvalue` upon access.
366            Operand::Constant(c) => {
367                if let Some(def_id) = c.check_static_ptr(self.tcx) {
368                    // Only allow statics (not consts) to refer to other statics.
369                    // FIXME(eddyb) does this matter at all for promotion?
370                    // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
371                    // and in `const` this cannot occur anyway. The only concern is that we might
372                    // promote even `let x = &STATIC` which would be useless, but this applies to
373                    // promotion inside statics as well.
374                    let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
375                    if !is_static {
376                        return Err(Unpromotable);
377                    }
378
379                    let is_thread_local = self.tcx.is_thread_local_static(def_id);
380                    if is_thread_local {
381                        return Err(Unpromotable);
382                    }
383                }
384
385                Ok(())
386            }
387        }
388    }
389
390    fn validate_ref(&mut self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
391        match kind {
392            // Reject these borrow types just to be safe.
393            // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a
394            // usecase.
395            BorrowKind::Fake(_) | BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => {
396                return Err(Unpromotable);
397            }
398
399            BorrowKind::Shared => {
400                let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
401                if has_mut_interior {
402                    return Err(Unpromotable);
403                }
404            }
405
406            // FIXME: consider changing this to only promote &mut [] for default borrows,
407            // also forbidding two phase borrows
408            BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow } => {
409                let ty = place.ty(self.body, self.tcx).ty;
410
411                // In theory, any zero-sized value could be borrowed
412                // mutably without consequences. However, only &mut []
413                // is allowed right now.
414                if let ty::Array(_, len) = ty.kind() {
415                    match len.try_to_target_usize(self.tcx) {
416                        Some(0) => {}
417                        _ => return Err(Unpromotable),
418                    }
419                } else {
420                    return Err(Unpromotable);
421                }
422            }
423        }
424
425        Ok(())
426    }
427
428    fn validate_rvalue(&mut self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
429        match rvalue {
430            Rvalue::Use(operand)
431            | Rvalue::Repeat(operand, _)
432            | Rvalue::WrapUnsafeBinder(operand, _) => {
433                self.validate_operand(operand)?;
434            }
435            Rvalue::CopyForDeref(place) => {
436                let op = &Operand::Copy(*place);
437                self.validate_operand(op)?
438            }
439
440            Rvalue::Discriminant(place) => self.validate_place(place.as_ref())?,
441
442            Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
443
444            // ptr-to-int casts are not possible in consts and thus not promotable
445            Rvalue::Cast(CastKind::PointerExposeProvenance, _, _) => return Err(Unpromotable),
446
447            // all other casts including int-to-ptr casts are fine, they just use the integer value
448            // at pointer type.
449            Rvalue::Cast(_, operand, _) => {
450                self.validate_operand(operand)?;
451            }
452
453            Rvalue::NullaryOp(op, _) => match op {
454                NullOp::SizeOf => {}
455                NullOp::AlignOf => {}
456                NullOp::OffsetOf(_) => {}
457                NullOp::UbChecks => {}
458                NullOp::ContractChecks => {}
459            },
460
461            Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable),
462
463            Rvalue::UnaryOp(op, operand) => {
464                match op {
465                    // These operations can never fail.
466                    UnOp::Neg | UnOp::Not | UnOp::PtrMetadata => {}
467                }
468
469                self.validate_operand(operand)?;
470            }
471
472            Rvalue::BinaryOp(op, box (lhs, rhs)) => {
473                let op = *op;
474                let lhs_ty = lhs.ty(self.body, self.tcx);
475
476                if let ty::RawPtr(_, _) | ty::FnPtr(..) = lhs_ty.kind() {
477                    // Raw and fn pointer operations are not allowed inside consts and thus not
478                    // promotable.
479                    assert_matches!(
480                        op,
481                        BinOp::Eq
482                            | BinOp::Ne
483                            | BinOp::Le
484                            | BinOp::Lt
485                            | BinOp::Ge
486                            | BinOp::Gt
487                            | BinOp::Offset
488                    );
489                    return Err(Unpromotable);
490                }
491
492                match op {
493                    BinOp::Div | BinOp::Rem => {
494                        if lhs_ty.is_integral() {
495                            let sz = lhs_ty.primitive_size(self.tcx);
496                            // Integer division: the RHS must be a non-zero const.
497                            let rhs_val = match rhs {
498                                Operand::Constant(c) => {
499                                    c.const_.try_eval_scalar_int(self.tcx, self.typing_env)
500                                }
501                                _ => None,
502                            };
503                            match rhs_val.map(|x| x.to_uint(sz)) {
504                                // for the zero test, int vs uint does not matter
505                                Some(x) if x != 0 => {}        // okay
506                                _ => return Err(Unpromotable), // value not known or 0 -- not okay
507                            }
508                            // Furthermore, for signed division, we also have to exclude `int::MIN /
509                            // -1`.
510                            if lhs_ty.is_signed() {
511                                match rhs_val.map(|x| x.to_int(sz)) {
512                                    Some(-1) | None => {
513                                        // The RHS is -1 or unknown, so we have to be careful.
514                                        // But is the LHS int::MIN?
515                                        let lhs_val = match lhs {
516                                            Operand::Constant(c) => c
517                                                .const_
518                                                .try_eval_scalar_int(self.tcx, self.typing_env),
519                                            _ => None,
520                                        };
521                                        let lhs_min = sz.signed_int_min();
522                                        match lhs_val.map(|x| x.to_int(sz)) {
523                                            // okay
524                                            Some(x) if x != lhs_min => {}
525
526                                            // value not known or int::MIN -- not okay
527                                            _ => return Err(Unpromotable),
528                                        }
529                                    }
530                                    _ => {}
531                                }
532                            }
533                        }
534                    }
535                    // The remaining operations can never fail.
536                    BinOp::Eq
537                    | BinOp::Ne
538                    | BinOp::Le
539                    | BinOp::Lt
540                    | BinOp::Ge
541                    | BinOp::Gt
542                    | BinOp::Cmp
543                    | BinOp::Offset
544                    | BinOp::Add
545                    | BinOp::AddUnchecked
546                    | BinOp::AddWithOverflow
547                    | BinOp::Sub
548                    | BinOp::SubUnchecked
549                    | BinOp::SubWithOverflow
550                    | BinOp::Mul
551                    | BinOp::MulUnchecked
552                    | BinOp::MulWithOverflow
553                    | BinOp::BitXor
554                    | BinOp::BitAnd
555                    | BinOp::BitOr
556                    | BinOp::Shl
557                    | BinOp::ShlUnchecked
558                    | BinOp::Shr
559                    | BinOp::ShrUnchecked => {}
560                }
561
562                self.validate_operand(lhs)?;
563                self.validate_operand(rhs)?;
564            }
565
566            Rvalue::RawPtr(_, place) => {
567                // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
568                // no problem, only using it is.
569                if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
570                {
571                    let base_ty = place_base.ty(self.body, self.tcx).ty;
572                    if let ty::Ref(..) = base_ty.kind() {
573                        return self.validate_place(place_base);
574                    }
575                }
576                return Err(Unpromotable);
577            }
578
579            Rvalue::Ref(_, kind, place) => {
580                // Special-case reborrows to be more like a copy of the reference.
581                let mut place_simplified = place.as_ref();
582                if let Some((place_base, ProjectionElem::Deref)) =
583                    place_simplified.last_projection()
584                {
585                    let base_ty = place_base.ty(self.body, self.tcx).ty;
586                    if let ty::Ref(..) = base_ty.kind() {
587                        place_simplified = place_base;
588                    }
589                }
590
591                self.validate_place(place_simplified)?;
592
593                // Check that the reference is fine (using the original place!).
594                // (Needs to come after `validate_place` to avoid ICEs.)
595                self.validate_ref(*kind, place)?;
596            }
597
598            Rvalue::Aggregate(_, operands) => {
599                for o in operands {
600                    self.validate_operand(o)?;
601                }
602            }
603        }
604
605        Ok(())
606    }
607
608    /// Computes the sets of blocks of this MIR that are definitely going to be executed
609    /// if the function returns successfully. That makes it safe to promote calls in them
610    /// that might fail.
611    fn promotion_safe_blocks(body: &mir::Body<'tcx>) -> FxHashSet<BasicBlock> {
612        let mut safe_blocks = FxHashSet::default();
613        let mut safe_block = START_BLOCK;
614        loop {
615            safe_blocks.insert(safe_block);
616            // Let's see if we can find another safe block.
617            safe_block = match body.basic_blocks[safe_block].terminator().kind {
618                TerminatorKind::Goto { target } => target,
619                TerminatorKind::Call { target: Some(target), .. }
620                | TerminatorKind::Drop { target, .. } => {
621                    // This calls a function or the destructor. `target` does not get executed if
622                    // the callee loops or panics. But in both cases the const already fails to
623                    // evaluate, so we are fine considering `target` a safe block for promotion.
624                    target
625                }
626                TerminatorKind::Assert { target, .. } => {
627                    // Similar to above, we only consider successful execution.
628                    target
629                }
630                _ => {
631                    // No next safe block.
632                    break;
633                }
634            };
635        }
636        safe_blocks
637    }
638
639    /// Returns whether the block is "safe" for promotion, which means it cannot be dead code.
640    /// We use this to avoid promoting operations that can fail in dead code.
641    fn is_promotion_safe_block(&mut self, block: BasicBlock) -> bool {
642        let body = self.body;
643        let safe_blocks =
644            self.promotion_safe_blocks.get_or_insert_with(|| Self::promotion_safe_blocks(body));
645        safe_blocks.contains(&block)
646    }
647
648    fn validate_call(
649        &mut self,
650        callee: &Operand<'tcx>,
651        args: &[Spanned<Operand<'tcx>>],
652        block: BasicBlock,
653    ) -> Result<(), Unpromotable> {
654        // Validate the operands. If they fail, there's no question -- we cannot promote.
655        self.validate_operand(callee)?;
656        for arg in args {
657            self.validate_operand(&arg.node)?;
658        }
659
660        // Functions marked `#[rustc_promotable]` are explicitly allowed to be promoted, so we can
661        // accept them at this point.
662        let fn_ty = callee.ty(self.body, self.tcx);
663        if let ty::FnDef(def_id, _) = *fn_ty.kind() {
664            if self.tcx.is_promotable_const_fn(def_id) {
665                return Ok(());
666            }
667        }
668
669        // Ideally, we'd stop here and reject the rest.
670        // But for backward compatibility, we have to accept some promotion in const/static
671        // initializers. Inline consts are explicitly excluded, they are more recent so we have no
672        // backwards compatibility reason to allow more promotion inside of them.
673        let promote_all_fn = matches!(
674            self.const_kind,
675            Some(hir::ConstContext::Static(_) | hir::ConstContext::Const { inline: false })
676        );
677        if !promote_all_fn {
678            return Err(Unpromotable);
679        }
680        // Make sure the callee is a `const fn`.
681        let is_const_fn = match *fn_ty.kind() {
682            ty::FnDef(def_id, _) => self.tcx.is_const_fn(def_id),
683            _ => false,
684        };
685        if !is_const_fn {
686            return Err(Unpromotable);
687        }
688        // The problem is, this may promote calls to functions that panic.
689        // We don't want to introduce compilation errors if there's a panic in a call in dead code.
690        // So we ensure that this is not dead code.
691        if !self.is_promotion_safe_block(block) {
692            return Err(Unpromotable);
693        }
694        // This passed all checks, so let's accept.
695        Ok(())
696    }
697}
698
699fn validate_candidates(
700    ccx: &ConstCx<'_, '_>,
701    temps: &mut IndexSlice<Local, TempState>,
702    mut candidates: Vec<Candidate>,
703) -> Vec<Candidate> {
704    let mut validator = Validator { ccx, temps, promotion_safe_blocks: None };
705
706    candidates.retain(|&candidate| validator.validate_candidate(candidate).is_ok());
707    candidates
708}
709
710struct Promoter<'a, 'tcx> {
711    tcx: TyCtxt<'tcx>,
712    source: &'a mut Body<'tcx>,
713    promoted: Body<'tcx>,
714    temps: &'a mut IndexVec<Local, TempState>,
715    extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
716
717    /// Used to assemble the required_consts list while building the promoted.
718    required_consts: Vec<ConstOperand<'tcx>>,
719
720    /// If true, all nested temps are also kept in the
721    /// source MIR, not moved to the promoted MIR.
722    keep_original: bool,
723
724    /// If true, add the new const (the promoted) to the required_consts of the parent MIR.
725    /// This is initially false and then set by the visitor when it encounters a `Call` terminator.
726    add_to_required: bool,
727}
728
729impl<'a, 'tcx> Promoter<'a, 'tcx> {
730    fn new_block(&mut self) -> BasicBlock {
731        let span = self.promoted.span;
732        self.promoted.basic_blocks_mut().push(BasicBlockData::new(
733            Some(Terminator {
734                source_info: SourceInfo::outermost(span),
735                kind: TerminatorKind::Return,
736            }),
737            false,
738        ))
739    }
740
741    fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
742        let last = self.promoted.basic_blocks.last_index().unwrap();
743        let data = &mut self.promoted[last];
744        data.statements.push(Statement::new(
745            SourceInfo::outermost(span),
746            StatementKind::Assign(Box::new((Place::from(dest), rvalue))),
747        ));
748    }
749
750    fn is_temp_kind(&self, local: Local) -> bool {
751        self.source.local_kind(local) == LocalKind::Temp
752    }
753
754    /// Copies the initialization of this temp to the
755    /// promoted MIR, recursing through temps.
756    fn promote_temp(&mut self, temp: Local) -> Local {
757        let old_keep_original = self.keep_original;
758        let loc = match self.temps[temp] {
759            TempState::Defined { location, uses, .. } if uses > 0 => {
760                if uses > 1 {
761                    self.keep_original = true;
762                }
763                location
764            }
765            state => {
766                span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
767            }
768        };
769        if !self.keep_original {
770            self.temps[temp] = TempState::PromotedOut;
771        }
772
773        let num_stmts = self.source[loc.block].statements.len();
774        let new_temp = self.promoted.local_decls.push(LocalDecl::new(
775            self.source.local_decls[temp].ty,
776            self.source.local_decls[temp].source_info.span,
777        ));
778
779        debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
780
781        // First, take the Rvalue or Call out of the source MIR,
782        // or duplicate it, depending on keep_original.
783        if loc.statement_index < num_stmts {
784            let (mut rvalue, source_info) = {
785                let statement = &mut self.source[loc.block].statements[loc.statement_index];
786                let StatementKind::Assign(box (_, rhs)) = &mut statement.kind else {
787                    span_bug!(statement.source_info.span, "{:?} is not an assignment", statement);
788                };
789
790                (
791                    if self.keep_original {
792                        rhs.clone()
793                    } else {
794                        let unit = Rvalue::Use(Operand::Constant(Box::new(ConstOperand {
795                            span: statement.source_info.span,
796                            user_ty: None,
797                            const_: Const::zero_sized(self.tcx.types.unit),
798                        })));
799                        mem::replace(rhs, unit)
800                    },
801                    statement.source_info,
802                )
803            };
804
805            self.visit_rvalue(&mut rvalue, loc);
806            self.assign(new_temp, rvalue, source_info.span);
807        } else {
808            let terminator = if self.keep_original {
809                self.source[loc.block].terminator().clone()
810            } else {
811                let terminator = self.source[loc.block].terminator_mut();
812                let target = match &terminator.kind {
813                    TerminatorKind::Call { target: Some(target), .. } => *target,
814                    kind => {
815                        span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
816                    }
817                };
818                Terminator {
819                    source_info: terminator.source_info,
820                    kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
821                }
822            };
823
824            match terminator.kind {
825                TerminatorKind::Call {
826                    mut func, mut args, call_source: desugar, fn_span, ..
827                } => {
828                    // This promoted involves a function call, so it may fail to evaluate. Let's
829                    // make sure it is added to `required_consts` so that failure cannot get lost.
830                    self.add_to_required = true;
831
832                    self.visit_operand(&mut func, loc);
833                    for arg in &mut args {
834                        self.visit_operand(&mut arg.node, loc);
835                    }
836
837                    let last = self.promoted.basic_blocks.last_index().unwrap();
838                    let new_target = self.new_block();
839
840                    *self.promoted[last].terminator_mut() = Terminator {
841                        kind: TerminatorKind::Call {
842                            func,
843                            args,
844                            unwind: UnwindAction::Continue,
845                            destination: Place::from(new_temp),
846                            target: Some(new_target),
847                            call_source: desugar,
848                            fn_span,
849                        },
850                        source_info: SourceInfo::outermost(terminator.source_info.span),
851                        ..terminator
852                    };
853                }
854                kind => {
855                    span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
856                }
857            };
858        };
859
860        self.keep_original = old_keep_original;
861        new_temp
862    }
863
864    fn promote_candidate(
865        mut self,
866        candidate: Candidate,
867        next_promoted_index: Promoted,
868    ) -> Body<'tcx> {
869        let def = self.source.source.def_id();
870        let (mut rvalue, promoted_op) = {
871            let promoted = &mut self.promoted;
872            let tcx = self.tcx;
873            let mut promoted_operand = |ty, span| {
874                promoted.span = span;
875                promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
876                let args =
877                    tcx.erase_and_anonymize_regions(GenericArgs::identity_for_item(tcx, def));
878                let uneval =
879                    mir::UnevaluatedConst { def, args, promoted: Some(next_promoted_index) };
880
881                ConstOperand { span, user_ty: None, const_: Const::Unevaluated(uneval, ty) }
882            };
883
884            let blocks = self.source.basic_blocks.as_mut();
885            let local_decls = &mut self.source.local_decls;
886            let loc = candidate.location;
887            let statement = &mut blocks[loc.block].statements[loc.statement_index];
888            let StatementKind::Assign(box (_, Rvalue::Ref(region, borrow_kind, place))) =
889                &mut statement.kind
890            else {
891                bug!()
892            };
893
894            // Use the underlying local for this (necessarily interior) borrow.
895            debug_assert!(region.is_erased());
896            let ty = local_decls[place.local].ty;
897            let span = statement.source_info.span;
898
899            let ref_ty =
900                Ty::new_ref(tcx, tcx.lifetimes.re_erased, ty, borrow_kind.to_mutbl_lossy());
901
902            let mut projection = vec![PlaceElem::Deref];
903            projection.extend(place.projection);
904            place.projection = tcx.mk_place_elems(&projection);
905
906            // Create a temp to hold the promoted reference.
907            // This is because `*r` requires `r` to be a local,
908            // otherwise we would use the `promoted` directly.
909            let mut promoted_ref = LocalDecl::new(ref_ty, span);
910            promoted_ref.source_info = statement.source_info;
911            let promoted_ref = local_decls.push(promoted_ref);
912            assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
913
914            let promoted_operand = promoted_operand(ref_ty, span);
915            let promoted_ref_statement = Statement::new(
916                statement.source_info,
917                StatementKind::Assign(Box::new((
918                    Place::from(promoted_ref),
919                    Rvalue::Use(Operand::Constant(Box::new(promoted_operand))),
920                ))),
921            );
922            self.extra_statements.push((loc, promoted_ref_statement));
923
924            (
925                Rvalue::Ref(
926                    tcx.lifetimes.re_erased,
927                    *borrow_kind,
928                    Place {
929                        local: mem::replace(&mut place.local, promoted_ref),
930                        projection: List::empty(),
931                    },
932                ),
933                promoted_operand,
934            )
935        };
936
937        assert_eq!(self.new_block(), START_BLOCK);
938        self.visit_rvalue(
939            &mut rvalue,
940            Location { block: START_BLOCK, statement_index: usize::MAX },
941        );
942
943        let span = self.promoted.span;
944        self.assign(RETURN_PLACE, rvalue, span);
945
946        // Now that we did promotion, we know whether we'll want to add this to `required_consts` of
947        // the surrounding MIR body.
948        if self.add_to_required {
949            self.source.required_consts.as_mut().unwrap().push(promoted_op);
950        }
951
952        self.promoted.set_required_consts(self.required_consts);
953
954        self.promoted
955    }
956}
957
958/// Replaces all temporaries with their promoted counterparts.
959impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
960    fn tcx(&self) -> TyCtxt<'tcx> {
961        self.tcx
962    }
963
964    fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
965        if self.is_temp_kind(*local) {
966            *local = self.promote_temp(*local);
967        }
968    }
969
970    fn visit_const_operand(&mut self, constant: &mut ConstOperand<'tcx>, _location: Location) {
971        if constant.const_.is_required_const() {
972            self.required_consts.push(*constant);
973        }
974
975        // Skipping `super_constant` as the visitor is otherwise only looking for locals.
976    }
977}
978
979fn promote_candidates<'tcx>(
980    body: &mut Body<'tcx>,
981    tcx: TyCtxt<'tcx>,
982    mut temps: IndexVec<Local, TempState>,
983    candidates: Vec<Candidate>,
984) -> IndexVec<Promoted, Body<'tcx>> {
985    // Visit candidates in reverse, in case they're nested.
986    debug!(promote_candidates = ?candidates);
987
988    // eagerly fail fast
989    if candidates.is_empty() {
990        return IndexVec::new();
991    }
992
993    let mut promotions = IndexVec::new();
994
995    let mut extra_statements = vec![];
996    for candidate in candidates.into_iter().rev() {
997        let Location { block, statement_index } = candidate.location;
998        if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
999            && let Some(local) = place.as_local()
1000        {
1001            if temps[local] == TempState::PromotedOut {
1002                // Already promoted.
1003                continue;
1004            }
1005        }
1006
1007        // Declare return place local so that `mir::Body::new` doesn't complain.
1008        let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
1009
1010        let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
1011        scope.parent_scope = None;
1012
1013        let mut promoted = Body::new(
1014            body.source, // `promoted` gets filled in below
1015            IndexVec::new(),
1016            IndexVec::from_elem_n(scope, 1),
1017            initial_locals,
1018            IndexVec::new(),
1019            0,
1020            vec![],
1021            body.span,
1022            None,
1023            body.tainted_by_errors,
1024        );
1025        promoted.phase = MirPhase::Analysis(AnalysisPhase::Initial);
1026
1027        let promoter = Promoter {
1028            promoted,
1029            tcx,
1030            source: body,
1031            temps: &mut temps,
1032            extra_statements: &mut extra_statements,
1033            keep_original: false,
1034            add_to_required: false,
1035            required_consts: Vec::new(),
1036        };
1037
1038        let mut promoted = promoter.promote_candidate(candidate, promotions.next_index());
1039        promoted.source.promoted = Some(promotions.next_index());
1040        promotions.push(promoted);
1041    }
1042
1043    // Insert each of `extra_statements` before its indicated location, which
1044    // has to be done in reverse location order, to not invalidate the rest.
1045    extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1046    for (loc, statement) in extra_statements {
1047        body[loc.block].statements.insert(loc.statement_index, statement);
1048    }
1049
1050    // Eliminate assignments to, and drops of promoted temps.
1051    let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1052    for block in body.basic_blocks_mut() {
1053        block.statements.retain(|statement| match &statement.kind {
1054            StatementKind::Assign(box (place, _)) => {
1055                if let Some(index) = place.as_local() {
1056                    !promoted(index)
1057                } else {
1058                    true
1059                }
1060            }
1061            StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1062                !promoted(*index)
1063            }
1064            _ => true,
1065        });
1066        let terminator = block.terminator_mut();
1067        if let TerminatorKind::Drop { place, target, .. } = &terminator.kind
1068            && let Some(index) = place.as_local()
1069        {
1070            if promoted(index) {
1071                terminator.kind = TerminatorKind::Goto { target: *target };
1072            }
1073        }
1074    }
1075
1076    promotions
1077}