1 //! A pass that promotes borrows of constant rvalues.
3 //! The rvalues considered constant are trees of temps,
4 //! each with exactly one initialization, and holding
5 //! a constant value with no interior mutability.
6 //! They are placed into a new MIR constant body in
7 //! `promoted` and the borrow rvalue is replaced with
8 //! a `Literal::Promoted` using the index into `promoted`
9 //! of that constant MIR.
11 //! This pass assumes that every use is dominated by an
12 //! initialization and can otherwise silence errors, if
13 //! move analysis runs after promotion on broken MIR.
16 use rustc_middle::mir::traversal::ReversePostorderIter;
17 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
18 use rustc_middle::mir::*;
19 use rustc_middle::ty::subst::InternalSubsts;
20 use rustc_middle::ty::{self, List, TyCtxt, TypeVisitable};
23 use rustc_index::vec::{Idx, IndexVec};
26 use std::{cmp, iter, mem};
28 use crate::transform::check_consts::{qualifs, ConstCx};
30 /// A `MirPass` for promotion.
32 /// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
33 /// `'static` lifetime.
35 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
36 /// newly created `Constant`.
38 pub struct PromoteTemps<'tcx> {
39 pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
42 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
43 fn phase_change(&self) -> Option<MirPhase> {
44 Some(MirPhase::ConstsPromoted)
47 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
48 // There's not really any point in promoting errorful MIR.
50 // This does not include MIR that failed const-checking, which we still try to promote.
51 if body.return_ty().references_error() {
52 tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
56 if body.source.promoted.is_some() {
60 let mut rpo = traversal::reverse_postorder(body);
61 let ccx = ConstCx::new(tcx, body);
62 let (mut temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
64 let promotable_candidates = validate_candidates(&ccx, &mut temps, &all_candidates);
66 let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
67 self.promoted_fragments.set(promoted);
71 /// State of a temporary during collection and promotion.
72 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
74 /// No references to this temp.
76 /// One direct assignment and any number of direct uses.
77 /// A borrow of this temp is promotable if the assigned
78 /// value is qualified as constant.
79 Defined { location: Location, uses: usize, valid: Result<(), ()> },
80 /// Any other combination of assignments/uses.
82 /// This temp was part of an rvalue which got extracted
83 /// during promotion and needs cleanup.
88 pub fn is_promotable(&self) -> bool {
89 debug!("is_promotable: self={:?}", self);
90 matches!(self, TempState::Defined { .. })
94 /// A "root candidate" for promotion, which will become the
95 /// returned value in a promoted MIR, unless it's a subset
96 /// of a larger candidate.
97 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
98 pub struct Candidate {
102 struct Collector<'a, 'tcx> {
103 ccx: &'a ConstCx<'a, 'tcx>,
104 temps: IndexVec<Local, TempState>,
105 candidates: Vec<Candidate>,
108 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
109 fn visit_local(&mut self, index: Local, context: PlaceContext, location: Location) {
110 debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
111 // We're only interested in temporaries and the return place
112 match self.ccx.body.local_kind(index) {
113 LocalKind::Temp | LocalKind::ReturnPointer => {}
114 LocalKind::Arg | LocalKind::Var => return,
117 // Ignore drops, if the temp gets promoted,
118 // then it's constant and thus drop is noop.
119 // Non-uses are also irrelevant.
120 if context.is_drop() || !context.is_use() {
122 "visit_local: context.is_drop={:?} context.is_use={:?}",
129 let temp = &mut self.temps[index];
130 debug!("visit_local: temp={:?}", temp);
131 if *temp == TempState::Undefined {
133 PlaceContext::MutatingUse(MutatingUseContext::Store)
134 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
135 *temp = TempState::Defined { location, uses: 0, valid: Err(()) };
138 _ => { /* mark as unpromotable below */ }
140 } else if let TempState::Defined { ref mut uses, .. } = *temp {
141 // We always allow borrows, even mutable ones, as we need
142 // to promote mutable borrows of some ZSTs e.g., `&mut []`.
143 let allowed_use = match context {
144 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
145 | PlaceContext::NonMutatingUse(_) => true,
146 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
148 debug!("visit_local: allowed_use={:?}", allowed_use);
153 /* mark as unpromotable below */
155 *temp = TempState::Unpromotable;
158 fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
159 self.super_rvalue(rvalue, location);
163 self.candidates.push(Candidate { location });
170 pub fn collect_temps_and_candidates<'tcx>(
171 ccx: &ConstCx<'_, 'tcx>,
172 rpo: &mut ReversePostorderIter<'_, 'tcx>,
173 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
174 let mut collector = Collector {
175 temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
179 for (bb, data) in rpo {
180 collector.visit_basic_block_data(bb, data);
182 (collector.temps, collector.candidates)
185 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
187 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
188 struct Validator<'a, 'tcx> {
189 ccx: &'a ConstCx<'a, 'tcx>,
190 temps: &'a mut IndexVec<Local, TempState>,
193 impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> {
194 type Target = ConstCx<'a, 'tcx>;
196 fn deref(&self) -> &Self::Target {
203 impl<'tcx> Validator<'_, 'tcx> {
204 fn validate_candidate(&mut self, candidate: Candidate) -> Result<(), Unpromotable> {
205 let loc = candidate.location;
206 let statement = &self.body[loc.block].statements[loc.statement_index];
207 match &statement.kind {
208 StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
209 // We can only promote interior borrows of promotable temps (non-temps
210 // don't get promoted anyway).
211 self.validate_local(place.local)?;
213 // The reference operation itself must be promotable.
214 // (Needs to come after `validate_local` to avoid ICEs.)
215 self.validate_ref(*kind, place)?;
217 // We do not check all the projections (they do not get promoted anyway),
218 // but we do stay away from promoting anything involving a dereference.
219 if place.projection.contains(&ProjectionElem::Deref) {
220 return Err(Unpromotable);
223 // We cannot promote things that need dropping, since the promoted value
224 // would not get dropped.
225 if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
226 return Err(Unpromotable);
235 // FIXME(eddyb) maybe cache this?
236 fn qualif_local<Q: qualifs::Qualif>(&mut self, local: Local) -> bool {
237 if let TempState::Defined { location: loc, .. } = self.temps[local] {
238 let num_stmts = self.body[loc.block].statements.len();
240 if loc.statement_index < num_stmts {
241 let statement = &self.body[loc.block].statements[loc.statement_index];
242 match &statement.kind {
243 StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
245 &mut |l| self.qualif_local::<Q>(l),
250 statement.source_info.span,
251 "{:?} is not an assignment",
257 let terminator = self.body[loc.block].terminator();
258 match &terminator.kind {
259 TerminatorKind::Call { .. } => {
260 let return_ty = self.body.local_decls[local].ty;
261 Q::in_any_value_of_ty(&self.ccx, return_ty)
264 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
269 let span = self.body.local_decls[local].source_info.span;
270 span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
274 fn validate_local(&mut self, local: Local) -> Result<(), Unpromotable> {
275 if let TempState::Defined { location: loc, uses, valid } = self.temps[local] {
278 let block = &self.body[loc.block];
279 let num_stmts = block.statements.len();
281 if loc.statement_index < num_stmts {
282 let statement = &block.statements[loc.statement_index];
283 match &statement.kind {
284 StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
287 statement.source_info.span,
288 "{:?} is not an assignment",
294 let terminator = block.terminator();
295 match &terminator.kind {
296 TerminatorKind::Call { func, args, .. } => {
297 self.validate_call(func, args)
299 TerminatorKind::Yield { .. } => Err(Unpromotable),
301 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
306 self.temps[local] = match ok {
307 Ok(()) => TempState::Defined { location: loc, uses, valid: Ok(()) },
308 Err(_) => TempState::Unpromotable,
317 fn validate_place(&mut self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
318 match place.last_projection() {
319 None => self.validate_local(place.local),
320 Some((place_base, elem)) => {
321 // Validate topmost projection, then recurse.
323 ProjectionElem::Deref => {
324 let mut promotable = false;
325 // We need to make sure this is a `Deref` of a local with no further projections.
326 // Discussion can be found at
327 // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
328 if let Some(local) = place_base.as_local() {
329 // This is a special treatment for cases like *&STATIC where STATIC is a
330 // global static variable.
331 // This pattern is generated only when global static variables are directly
332 // accessed and is qualified for promotion safely.
333 if let TempState::Defined { location, .. } = self.temps[local] {
334 let def_stmt = self.body[location.block]
336 .get(location.statement_index);
337 if let Some(Statement {
339 StatementKind::Assign(box (
341 Rvalue::Use(Operand::Constant(c)),
346 if let Some(did) = c.check_static_ptr(self.tcx) {
347 // Evaluating a promoted may not read statics except if it got
348 // promoted from a static (this is a CTFE check). So we
349 // can only promote static accesses inside statics.
350 if let Some(hir::ConstContext::Static(..)) = self.const_kind
352 if !self.tcx.is_thread_local_static(did) {
361 return Err(Unpromotable);
364 ProjectionElem::OpaqueCast(..) | ProjectionElem::Downcast(..) => {
365 return Err(Unpromotable);
368 ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
370 ProjectionElem::Index(local) => {
371 let mut promotable = false;
372 // Only accept if we can predict the index and are indexing an array.
374 if let TempState::Defined { location: loc, .. } = self.temps[local] {
375 let block = &self.body[loc.block];
376 if loc.statement_index < block.statements.len() {
377 let statement = &block.statements[loc.statement_index];
378 match &statement.kind {
379 StatementKind::Assign(box (
381 Rvalue::Use(Operand::Constant(c)),
382 )) => c.literal.try_eval_usize(self.tcx, self.param_env),
391 if let Some(idx) = val {
392 // Determine the type of the thing we are indexing.
393 let ty = place_base.ty(self.body, self.tcx).ty;
395 ty::Array(_, len) => {
396 // It's an array; determine its length.
397 if let Some(len) = len.try_eval_usize(self.tcx, self.param_env)
399 // If the index is in-bounds, go ahead.
409 return Err(Unpromotable);
412 self.validate_local(local)?;
415 ProjectionElem::Field(..) => {
416 let base_ty = place_base.ty(self.body, self.tcx).ty;
417 if base_ty.is_union() {
418 // No promotion of union field accesses.
419 return Err(Unpromotable);
424 self.validate_place(place_base)
429 fn validate_operand(&mut self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
431 Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
433 // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
434 // `validate_rvalue` upon access.
435 Operand::Constant(c) => {
436 if let Some(def_id) = c.check_static_ptr(self.tcx) {
437 // Only allow statics (not consts) to refer to other statics.
438 // FIXME(eddyb) does this matter at all for promotion?
439 // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
440 // and in `const` this cannot occur anyway. The only concern is that we might
441 // promote even `let x = &STATIC` which would be useless, but this applies to
442 // promotion inside statics as well.
443 let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
445 return Err(Unpromotable);
448 let is_thread_local = self.tcx.is_thread_local_static(def_id);
450 return Err(Unpromotable);
459 fn validate_ref(&mut self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
461 // Reject these borrow types just to be safe.
462 // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
463 BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
465 BorrowKind::Shared => {
466 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
467 if has_mut_interior {
468 return Err(Unpromotable);
472 BorrowKind::Mut { .. } => {
473 let ty = place.ty(self.body, self.tcx).ty;
475 // In theory, any zero-sized value could be borrowed
476 // mutably without consequences. However, only &mut []
477 // is allowed right now.
478 if let ty::Array(_, len) = ty.kind() {
479 match len.try_eval_usize(self.tcx, self.param_env) {
481 _ => return Err(Unpromotable),
484 return Err(Unpromotable);
492 fn validate_rvalue(&mut self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
494 Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
495 self.validate_operand(operand)?;
497 Rvalue::CopyForDeref(place) => {
498 let op = &Operand::Copy(*place);
499 self.validate_operand(op)?
502 Rvalue::Discriminant(place) | Rvalue::Len(place) => {
503 self.validate_place(place.as_ref())?
506 Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
508 // ptr-to-int casts are not possible in consts and thus not promotable
509 Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => return Err(Unpromotable),
511 // all other casts including int-to-ptr casts are fine, they just use the integer value
513 Rvalue::Cast(_, operand, _) => {
514 self.validate_operand(operand)?;
517 Rvalue::NullaryOp(op, _) => match op {
519 NullOp::AlignOf => {}
522 Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable),
524 Rvalue::UnaryOp(op, operand) => {
526 // These operations can never fail.
527 UnOp::Neg | UnOp::Not => {}
530 self.validate_operand(operand)?;
533 Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
535 let lhs_ty = lhs.ty(self.body, self.tcx);
537 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
538 // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
549 return Err(Unpromotable);
553 BinOp::Div | BinOp::Rem => {
554 if lhs_ty.is_integral() {
555 // Integer division: the RHS must be a non-zero const.
556 let const_val = match rhs {
557 Operand::Constant(c) => {
558 c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
563 Some(x) if x != 0 => {} // okay
564 _ => return Err(Unpromotable), // value not known or 0 -- not okay
568 // The remaining operations can never fail.
586 self.validate_operand(lhs)?;
587 self.validate_operand(rhs)?;
590 Rvalue::AddressOf(_, place) => {
591 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
592 // no problem, only using it is.
593 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
595 let base_ty = place_base.ty(self.body, self.tcx).ty;
596 if let ty::Ref(..) = base_ty.kind() {
597 return self.validate_place(place_base);
600 return Err(Unpromotable);
603 Rvalue::Ref(_, kind, place) => {
604 // Special-case reborrows to be more like a copy of the reference.
605 let mut place_simplified = place.as_ref();
606 if let Some((place_base, ProjectionElem::Deref)) =
607 place_simplified.last_projection()
609 let base_ty = place_base.ty(self.body, self.tcx).ty;
610 if let ty::Ref(..) = base_ty.kind() {
611 place_simplified = place_base;
615 self.validate_place(place_simplified)?;
617 // Check that the reference is fine (using the original place!).
618 // (Needs to come after `validate_place` to avoid ICEs.)
619 self.validate_ref(*kind, place)?;
622 Rvalue::Aggregate(_, operands) => {
624 self.validate_operand(o)?;
634 callee: &Operand<'tcx>,
635 args: &[Operand<'tcx>],
636 ) -> Result<(), Unpromotable> {
637 let fn_ty = callee.ty(self.body, self.tcx);
639 // Inside const/static items, we promote all (eligible) function calls.
640 // Everywhere else, we require `#[rustc_promotable]` on the callee.
641 let promote_all_const_fn = matches!(
643 Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
645 if !promote_all_const_fn {
646 if let ty::FnDef(def_id, _) = *fn_ty.kind() {
647 // Never promote runtime `const fn` calls of
648 // functions without `#[rustc_promotable]`.
649 if !self.tcx.is_promotable_const_fn(def_id) {
650 return Err(Unpromotable);
655 let is_const_fn = match *fn_ty.kind() {
656 ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
660 return Err(Unpromotable);
663 self.validate_operand(callee)?;
665 self.validate_operand(arg)?;
672 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
673 pub fn validate_candidates(
674 ccx: &ConstCx<'_, '_>,
675 temps: &mut IndexVec<Local, TempState>,
676 candidates: &[Candidate],
677 ) -> Vec<Candidate> {
678 let mut validator = Validator { ccx, temps };
683 .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
687 struct Promoter<'a, 'tcx> {
689 source: &'a mut Body<'tcx>,
690 promoted: Body<'tcx>,
691 temps: &'a mut IndexVec<Local, TempState>,
692 extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
694 /// If true, all nested temps are also kept in the
695 /// source MIR, not moved to the promoted MIR.
699 impl<'a, 'tcx> Promoter<'a, 'tcx> {
700 fn new_block(&mut self) -> BasicBlock {
701 let span = self.promoted.span;
702 self.promoted.basic_blocks_mut().push(BasicBlockData {
704 terminator: Some(Terminator {
705 source_info: SourceInfo::outermost(span),
706 kind: TerminatorKind::Return,
712 fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
713 let last = self.promoted.basic_blocks().last().unwrap();
714 let data = &mut self.promoted[last];
715 data.statements.push(Statement {
716 source_info: SourceInfo::outermost(span),
717 kind: StatementKind::Assign(Box::new((Place::from(dest), rvalue))),
721 fn is_temp_kind(&self, local: Local) -> bool {
722 self.source.local_kind(local) == LocalKind::Temp
725 /// Copies the initialization of this temp to the
726 /// promoted MIR, recursing through temps.
727 fn promote_temp(&mut self, temp: Local) -> Local {
728 let old_keep_original = self.keep_original;
729 let loc = match self.temps[temp] {
730 TempState::Defined { location, uses, .. } if uses > 0 => {
732 self.keep_original = true;
737 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
740 if !self.keep_original {
741 self.temps[temp] = TempState::PromotedOut;
744 let num_stmts = self.source[loc.block].statements.len();
745 let new_temp = self.promoted.local_decls.push(LocalDecl::new(
746 self.source.local_decls[temp].ty,
747 self.source.local_decls[temp].source_info.span,
750 debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
752 // First, take the Rvalue or Call out of the source MIR,
753 // or duplicate it, depending on keep_original.
754 if loc.statement_index < num_stmts {
755 let (mut rvalue, source_info) = {
756 let statement = &mut self.source[loc.block].statements[loc.statement_index];
757 let StatementKind::Assign(box (_, ref mut rhs)) = statement.kind else {
759 statement.source_info.span,
760 "{:?} is not an assignment",
766 if self.keep_original {
769 let unit = Rvalue::Use(Operand::Constant(Box::new(Constant {
770 span: statement.source_info.span,
772 literal: ConstantKind::zero_sized(self.tcx.types.unit),
774 mem::replace(rhs, unit)
776 statement.source_info,
780 self.visit_rvalue(&mut rvalue, loc);
781 self.assign(new_temp, rvalue, source_info.span);
783 let terminator = if self.keep_original {
784 self.source[loc.block].terminator().clone()
786 let terminator = self.source[loc.block].terminator_mut();
787 let target = match terminator.kind {
788 TerminatorKind::Call { target: Some(target), .. } => target,
790 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
794 source_info: terminator.source_info,
795 kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
799 match terminator.kind {
800 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
801 self.visit_operand(&mut func, loc);
802 for arg in &mut args {
803 self.visit_operand(arg, loc);
806 let last = self.promoted.basic_blocks().last().unwrap();
807 let new_target = self.new_block();
809 *self.promoted[last].terminator_mut() = Terminator {
810 kind: TerminatorKind::Call {
814 destination: Place::from(new_temp),
815 target: Some(new_target),
819 source_info: SourceInfo::outermost(terminator.source_info.span),
824 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
829 self.keep_original = old_keep_original;
833 fn promote_candidate(mut self, candidate: Candidate, next_promoted_id: usize) -> Body<'tcx> {
834 let def = self.source.source.with_opt_param();
836 let promoted = &mut self.promoted;
837 let promoted_id = Promoted::new(next_promoted_id);
839 let mut promoted_operand = |ty, span| {
840 promoted.span = span;
841 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
842 let _const = tcx.mk_const(ty::ConstS {
844 kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
846 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
847 if let ty::GenericParamDefKind::Lifetime = param.kind {
848 tcx.lifetimes.re_erased.into()
850 tcx.mk_param_from_def(param)
853 promoted: Some(promoted_id),
857 Operand::Constant(Box::new(Constant {
860 literal: ConstantKind::from_const(_const, tcx),
863 let blocks = self.source.basic_blocks.as_mut();
864 let local_decls = &mut self.source.local_decls;
865 let loc = candidate.location;
866 let statement = &mut blocks[loc.block].statements[loc.statement_index];
867 match statement.kind {
868 StatementKind::Assign(box (
870 Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
872 // Use the underlying local for this (necessarily interior) borrow.
873 let ty = local_decls[place.local].ty;
874 let span = statement.source_info.span;
876 let ref_ty = tcx.mk_ref(
877 tcx.lifetimes.re_erased,
878 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
881 *region = tcx.lifetimes.re_erased;
883 let mut projection = vec![PlaceElem::Deref];
884 projection.extend(place.projection);
885 place.projection = tcx.intern_place_elems(&projection);
887 // Create a temp to hold the promoted reference.
888 // This is because `*r` requires `r` to be a local,
889 // otherwise we would use the `promoted` directly.
890 let mut promoted_ref = LocalDecl::new(ref_ty, span);
891 promoted_ref.source_info = statement.source_info;
892 let promoted_ref = local_decls.push(promoted_ref);
893 assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
895 let promoted_ref_statement = Statement {
896 source_info: statement.source_info,
897 kind: StatementKind::Assign(Box::new((
898 Place::from(promoted_ref),
899 Rvalue::Use(promoted_operand(ref_ty, span)),
902 self.extra_statements.push((loc, promoted_ref_statement));
905 tcx.lifetimes.re_erased,
908 local: mem::replace(&mut place.local, promoted_ref),
909 projection: List::empty(),
917 assert_eq!(self.new_block(), START_BLOCK);
920 Location { block: BasicBlock::new(0), statement_index: usize::MAX },
923 let span = self.promoted.span;
924 self.assign(RETURN_PLACE, rvalue, span);
929 /// Replaces all temporaries with their promoted counterparts.
930 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
931 fn tcx(&self) -> TyCtxt<'tcx> {
935 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
936 if self.is_temp_kind(*local) {
937 *local = self.promote_temp(*local);
942 pub fn promote_candidates<'tcx>(
943 body: &mut Body<'tcx>,
945 mut temps: IndexVec<Local, TempState>,
946 candidates: Vec<Candidate>,
947 ) -> IndexVec<Promoted, Body<'tcx>> {
948 // Visit candidates in reverse, in case they're nested.
949 debug!("promote_candidates({:?})", candidates);
951 let mut promotions = IndexVec::new();
953 let mut extra_statements = vec![];
954 for candidate in candidates.into_iter().rev() {
955 let Location { block, statement_index } = candidate.location;
956 if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
958 if let Some(local) = place.as_local() {
959 if temps[local] == TempState::PromotedOut {
966 // Declare return place local so that `mir::Body::new` doesn't complain.
967 let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
969 let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
970 scope.parent_scope = None;
972 let promoted = Body::new(
973 body.source, // `promoted` gets filled in below
975 IndexVec::from_elem_n(scope, 1),
981 body.generator_kind(),
982 body.tainted_by_errors,
985 let promoter = Promoter {
990 extra_statements: &mut extra_statements,
991 keep_original: false,
994 let mut promoted = promoter.promote_candidate(candidate, promotions.len());
995 promoted.source.promoted = Some(promotions.next_index());
996 promotions.push(promoted);
999 // Insert each of `extra_statements` before its indicated location, which
1000 // has to be done in reverse location order, to not invalidate the rest.
1001 extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1002 for (loc, statement) in extra_statements {
1003 body[loc.block].statements.insert(loc.statement_index, statement);
1006 // Eliminate assignments to, and drops of promoted temps.
1007 let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1008 for block in body.basic_blocks_mut() {
1009 block.statements.retain(|statement| match &statement.kind {
1010 StatementKind::Assign(box (place, _)) => {
1011 if let Some(index) = place.as_local() {
1017 StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1022 let terminator = block.terminator_mut();
1023 if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1024 if let Some(index) = place.as_local() {
1025 if promoted(index) {
1026 terminator.kind = TerminatorKind::Goto { target: *target };
1035 /// This function returns `true` if the function being called in the array
1036 /// repeat expression is a `const` function.
1037 pub fn is_const_fn_in_array_repeat_expression<'tcx>(
1038 ccx: &ConstCx<'_, 'tcx>,
1039 place: &Place<'tcx>,
1042 match place.as_local() {
1043 // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1044 Some(local) if body.local_decls[local].is_user_variable() => return false,
1045 None => return false,
1049 for block in body.basic_blocks() {
1050 if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1053 if let Operand::Constant(box Constant { literal, .. }) = func {
1054 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1055 if destination == place {
1056 if ccx.tcx.is_const_fn(def_id) {