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::ReversePostorder;
17 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
18 use rustc_middle::mir::*;
19 use rustc_middle::ty::cast::CastTy;
20 use rustc_middle::ty::subst::InternalSubsts;
21 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
24 use rustc_index::vec::{Idx, IndexVec};
27 use std::{cmp, iter, mem};
29 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
30 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstCx};
31 use crate::transform::MirPass;
33 /// A `MirPass` for promotion.
35 /// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
36 /// `'static` lifetime.
38 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
39 /// newly created `Constant`.
41 pub struct PromoteTemps<'tcx> {
42 pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
45 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
46 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
47 // There's not really any point in promoting errorful MIR.
49 // This does not include MIR that failed const-checking, which we still try to promote.
50 if body.return_ty().references_error() {
51 tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
55 if body.source.promoted.is_some() {
59 let mut rpo = traversal::reverse_postorder(body);
60 let ccx = ConstCx::new(tcx, body);
61 let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
63 let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates);
65 let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
66 self.promoted_fragments.set(promoted);
70 /// State of a temporary during collection and promotion.
71 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
73 /// No references to this temp.
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 },
79 /// Any other combination of assignments/uses.
81 /// This temp was part of an rvalue which got extracted
82 /// during promotion and needs cleanup.
87 pub fn is_promotable(&self) -> bool {
88 debug!("is_promotable: self={:?}", self);
89 matches!(self, TempState::Defined { .. })
93 /// A "root candidate" for promotion, which will become the
94 /// returned value in a promoted MIR, unless it's a subset
95 /// of a larger candidate.
96 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
98 /// Borrow of a constant temporary, candidate for lifetime extension.
103 fn source_info(&self, body: &Body<'_>) -> SourceInfo {
105 Candidate::Ref(location) => *body.source_info(*location),
110 struct Collector<'a, 'tcx> {
111 ccx: &'a ConstCx<'a, 'tcx>,
112 temps: IndexVec<Local, TempState>,
113 candidates: Vec<Candidate>,
116 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
117 fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
118 debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
119 // We're only interested in temporaries and the return place
120 match self.ccx.body.local_kind(index) {
121 LocalKind::Temp | LocalKind::ReturnPointer => {}
122 LocalKind::Arg | LocalKind::Var => return,
125 // Ignore drops, if the temp gets promoted,
126 // then it's constant and thus drop is noop.
127 // Non-uses are also irrelevant.
128 if context.is_drop() || !context.is_use() {
130 "visit_local: context.is_drop={:?} context.is_use={:?}",
137 let temp = &mut self.temps[index];
138 debug!("visit_local: temp={:?}", temp);
139 if *temp == TempState::Undefined {
141 PlaceContext::MutatingUse(MutatingUseContext::Store)
142 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
143 *temp = TempState::Defined { location, uses: 0 };
146 _ => { /* mark as unpromotable below */ }
148 } else if let TempState::Defined { ref mut uses, .. } = *temp {
149 // We always allow borrows, even mutable ones, as we need
150 // to promote mutable borrows of some ZSTs e.g., `&mut []`.
151 let allowed_use = match context {
152 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
153 | PlaceContext::NonMutatingUse(_) => true,
154 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
156 debug!("visit_local: allowed_use={:?}", allowed_use);
161 /* mark as unpromotable below */
163 *temp = TempState::Unpromotable;
166 fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
167 self.super_rvalue(rvalue, location);
171 self.candidates.push(Candidate::Ref(location));
178 pub fn collect_temps_and_candidates(
179 ccx: &ConstCx<'mir, 'tcx>,
180 rpo: &mut ReversePostorder<'_, 'tcx>,
181 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
182 let mut collector = Collector {
183 temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
187 for (bb, data) in rpo {
188 collector.visit_basic_block_data(bb, data);
190 (collector.temps, collector.candidates)
193 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
195 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
196 struct Validator<'a, 'tcx> {
197 ccx: &'a ConstCx<'a, 'tcx>,
198 temps: &'a IndexVec<Local, TempState>,
201 impl std::ops::Deref for Validator<'a, 'tcx> {
202 type Target = ConstCx<'a, 'tcx>;
204 fn deref(&self) -> &Self::Target {
211 impl<'tcx> Validator<'_, 'tcx> {
212 fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
214 Candidate::Ref(loc) => {
215 let statement = &self.body[loc.block].statements[loc.statement_index];
216 match &statement.kind {
217 StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
218 // We can only promote interior borrows of promotable temps (non-temps
219 // don't get promoted anyway).
220 self.validate_local(place.local)?;
222 // The reference operation itself must be promotable.
223 // (Needs to come after `validate_local` to avoid ICEs.)
224 self.validate_ref(*kind, place)?;
226 // We do not check all the projections (they do not get promoted anyway),
227 // but we do stay away from promoting anything involving a dereference.
228 if place.projection.contains(&ProjectionElem::Deref) {
229 return Err(Unpromotable);
232 // We cannot promote things that need dropping, since the promoted value
233 // would not get dropped.
234 if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
235 return Err(Unpromotable);
246 // FIXME(eddyb) maybe cache this?
247 fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
248 if let TempState::Defined { location: loc, .. } = self.temps[local] {
249 let num_stmts = self.body[loc.block].statements.len();
251 if loc.statement_index < num_stmts {
252 let statement = &self.body[loc.block].statements[loc.statement_index];
253 match &statement.kind {
254 StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
256 &mut |l| self.qualif_local::<Q>(l),
261 statement.source_info.span,
262 "{:?} is not an assignment",
268 let terminator = self.body[loc.block].terminator();
269 match &terminator.kind {
270 TerminatorKind::Call { .. } => {
271 let return_ty = self.body.local_decls[local].ty;
272 Q::in_any_value_of_ty(&self.ccx, return_ty)
275 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
280 let span = self.body.local_decls[local].source_info.span;
281 span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
285 // FIXME(eddyb) maybe cache this?
286 fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
287 if let TempState::Defined { location: loc, .. } = self.temps[local] {
288 let block = &self.body[loc.block];
289 let num_stmts = block.statements.len();
291 if loc.statement_index < num_stmts {
292 let statement = &block.statements[loc.statement_index];
293 match &statement.kind {
294 StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
297 statement.source_info.span,
298 "{:?} is not an assignment",
304 let terminator = block.terminator();
305 match &terminator.kind {
306 TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
307 TerminatorKind::Yield { .. } => Err(Unpromotable),
309 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
318 fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
319 match place.last_projection() {
320 None => self.validate_local(place.local),
321 Some((place_base, elem)) => {
322 // Validate topmost projection, then recurse.
324 ProjectionElem::Deref => {
325 let mut promotable = false;
326 // We need to make sure this is a `Deref` of a local with no further projections.
327 // Discussion can be found at
328 // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
329 if let Some(local) = place_base.as_local() {
330 // This is a special treatment for cases like *&STATIC where STATIC is a
331 // global static variable.
332 // This pattern is generated only when global static variables are directly
333 // accessed and is qualified for promotion safely.
334 if let TempState::Defined { location, .. } = self.temps[local] {
335 let def_stmt = self.body[location.block]
337 .get(location.statement_index);
338 if let Some(Statement {
340 StatementKind::Assign(box (
342 Rvalue::Use(Operand::Constant(c)),
347 if let Some(did) = c.check_static_ptr(self.tcx) {
348 // Evaluating a promoted may not read statics except if it got
349 // promoted from a static (this is a CTFE check). So we
350 // can only promote static accesses inside statics.
351 if let Some(hir::ConstContext::Static(..)) = self.const_kind
353 if !self.tcx.is_thread_local_static(did) {
362 return Err(Unpromotable);
365 ProjectionElem::Downcast(..) => {
366 return Err(Unpromotable);
369 ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
371 ProjectionElem::Index(local) => {
372 let mut promotable = false;
373 // Only accept if we can predict the index and are indexing an array.
375 if let TempState::Defined { location: loc, .. } = self.temps[local] {
376 let block = &self.body[loc.block];
377 if loc.statement_index < block.statements.len() {
378 let statement = &block.statements[loc.statement_index];
379 match &statement.kind {
380 StatementKind::Assign(box (
382 Rvalue::Use(Operand::Constant(c)),
383 )) => c.literal.try_eval_usize(self.tcx, self.param_env),
392 if let Some(idx) = val {
393 // Determine the type of the thing we are indexing.
394 let ty = place_base.ty(self.body, self.tcx).ty;
396 ty::Array(_, len) => {
397 // It's an array; determine its length.
398 if let Some(len) = len.try_eval_usize(self.tcx, self.param_env)
400 // If the index is in-bounds, go ahead.
410 return Err(Unpromotable);
413 self.validate_local(local)?;
416 ProjectionElem::Field(..) => {
417 let base_ty = place_base.ty(self.body, self.tcx).ty;
418 if base_ty.is_union() {
419 // No promotion of union field accesses.
420 return Err(Unpromotable);
425 self.validate_place(place_base)
430 fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
432 Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
434 // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
435 // `validate_rvalue` upon access.
436 Operand::Constant(c) => {
437 if let Some(def_id) = c.check_static_ptr(self.tcx) {
438 // Only allow statics (not consts) to refer to other statics.
439 // FIXME(eddyb) does this matter at all for promotion?
440 // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
441 // and in `const` this cannot occur anyway. The only concern is that we might
442 // promote even `let x = &STATIC` which would be useless, but this applies to
443 // promotion inside statics as well.
444 let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
446 return Err(Unpromotable);
449 let is_thread_local = self.tcx.is_thread_local_static(def_id);
451 return Err(Unpromotable);
460 fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
462 // Reject these borrow types just to be safe.
463 // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
464 BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
466 BorrowKind::Shared => {
467 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
468 if has_mut_interior {
469 return Err(Unpromotable);
473 BorrowKind::Mut { .. } => {
474 let ty = place.ty(self.body, self.tcx).ty;
476 // In theory, any zero-sized value could be borrowed
477 // mutably without consequences. However, only &mut []
478 // is allowed right now.
479 if let ty::Array(_, len) = ty.kind() {
480 match len.try_eval_usize(self.tcx, self.param_env) {
482 _ => return Err(Unpromotable),
485 return Err(Unpromotable);
493 fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
495 Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
496 self.validate_operand(operand)?;
499 Rvalue::Discriminant(place) | Rvalue::Len(place) => {
500 self.validate_place(place.as_ref())?
503 Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
505 Rvalue::Cast(kind, operand, cast_ty) => {
506 if matches!(kind, CastKind::Misc) {
507 let operand_ty = operand.ty(self.body, self.tcx);
508 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
509 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
510 if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
511 // ptr-to-int casts are not possible in consts and thus not promotable
512 return Err(Unpromotable);
514 // int-to-ptr casts are fine, they just use the integer value at pointer type.
517 self.validate_operand(operand)?;
520 Rvalue::NullaryOp(op, _) => match op {
521 NullOp::Box => return Err(Unpromotable),
525 Rvalue::UnaryOp(op, operand) => {
527 // These operations can never fail.
528 UnOp::Neg | UnOp::Not => {}
531 self.validate_operand(operand)?;
534 Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
536 let lhs_ty = lhs.ty(self.body, self.tcx);
538 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
539 // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
550 return Err(Unpromotable);
554 BinOp::Div | BinOp::Rem => {
555 if lhs_ty.is_integral() {
556 // Integer division: the RHS must be a non-zero const.
557 let const_val = match rhs {
558 Operand::Constant(c) => {
559 c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
564 Some(x) if x != 0 => {} // okay
565 _ => return Err(Unpromotable), // value not known or 0 -- not okay
569 // The remaining operations can never fail.
587 self.validate_operand(lhs)?;
588 self.validate_operand(rhs)?;
591 Rvalue::AddressOf(_, place) => {
592 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
593 // no problem, only using it is.
594 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
596 let base_ty = place_base.ty(self.body, self.tcx).ty;
597 if let ty::Ref(..) = base_ty.kind() {
598 return self.validate_place(place_base);
601 return Err(Unpromotable);
604 Rvalue::Ref(_, kind, place) => {
605 // Special-case reborrows to be more like a copy of the reference.
606 let mut place_simplified = place.as_ref();
607 if let Some((place_base, ProjectionElem::Deref)) =
608 place_simplified.last_projection()
610 let base_ty = place_base.ty(self.body, self.tcx).ty;
611 if let ty::Ref(..) = base_ty.kind() {
612 place_simplified = place_base;
616 self.validate_place(place_simplified)?;
618 // Check that the reference is fine (using the original place!).
619 // (Needs to come after `validate_place` to avoid ICEs.)
620 self.validate_ref(*kind, place)?;
623 Rvalue::Aggregate(_, operands) => {
625 self.validate_operand(o)?;
635 callee: &Operand<'tcx>,
636 args: &[Operand<'tcx>],
637 ) -> Result<(), Unpromotable> {
638 let fn_ty = callee.ty(self.body, self.tcx);
640 // Inside const/static items, we promote all (eligible) function calls.
641 // Everywhere else, we require `#[rustc_promotable]` on the callee.
642 let promote_all_const_fn = matches!(
644 Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
646 if !promote_all_const_fn {
647 if let ty::FnDef(def_id, _) = *fn_ty.kind() {
648 // Never promote runtime `const fn` calls of
649 // functions without `#[rustc_promotable]`.
650 if !self.tcx.is_promotable_const_fn(def_id) {
651 return Err(Unpromotable);
656 let is_const_fn = match *fn_ty.kind() {
657 ty::FnDef(def_id, _) => {
658 is_const_fn(self.tcx, def_id)
659 || is_unstable_const_fn(self.tcx, def_id).is_some()
660 || is_lang_panic_fn(self.tcx, def_id)
665 return Err(Unpromotable);
668 self.validate_operand(callee)?;
670 self.validate_operand(arg)?;
677 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
678 pub fn validate_candidates(
679 ccx: &ConstCx<'_, '_>,
680 temps: &IndexVec<Local, TempState>,
681 candidates: &[Candidate],
682 ) -> Vec<Candidate> {
683 let validator = Validator { ccx, temps };
688 .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
692 struct Promoter<'a, 'tcx> {
694 source: &'a mut Body<'tcx>,
695 promoted: Body<'tcx>,
696 temps: &'a mut IndexVec<Local, TempState>,
697 extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
699 /// If true, all nested temps are also kept in the
700 /// source MIR, not moved to the promoted MIR.
704 impl<'a, 'tcx> Promoter<'a, 'tcx> {
705 fn new_block(&mut self) -> BasicBlock {
706 let span = self.promoted.span;
707 self.promoted.basic_blocks_mut().push(BasicBlockData {
709 terminator: Some(Terminator {
710 source_info: SourceInfo::outermost(span),
711 kind: TerminatorKind::Return,
717 fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
718 let last = self.promoted.basic_blocks().last().unwrap();
719 let data = &mut self.promoted[last];
720 data.statements.push(Statement {
721 source_info: SourceInfo::outermost(span),
722 kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
726 fn is_temp_kind(&self, local: Local) -> bool {
727 self.source.local_kind(local) == LocalKind::Temp
730 /// Copies the initialization of this temp to the
731 /// promoted MIR, recursing through temps.
732 fn promote_temp(&mut self, temp: Local) -> Local {
733 let old_keep_original = self.keep_original;
734 let loc = match self.temps[temp] {
735 TempState::Defined { location, uses } if uses > 0 => {
737 self.keep_original = true;
742 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
745 if !self.keep_original {
746 self.temps[temp] = TempState::PromotedOut;
749 let num_stmts = self.source[loc.block].statements.len();
750 let new_temp = self.promoted.local_decls.push(LocalDecl::new(
751 self.source.local_decls[temp].ty,
752 self.source.local_decls[temp].source_info.span,
755 debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
757 // First, take the Rvalue or Call out of the source MIR,
758 // or duplicate it, depending on keep_original.
759 if loc.statement_index < num_stmts {
760 let (mut rvalue, source_info) = {
761 let statement = &mut self.source[loc.block].statements[loc.statement_index];
762 let rhs = match statement.kind {
763 StatementKind::Assign(box (_, ref mut rhs)) => rhs,
766 statement.source_info.span,
767 "{:?} is not an assignment",
774 if self.keep_original {
777 let unit = Rvalue::Use(Operand::Constant(box Constant {
778 span: statement.source_info.span,
780 literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
782 mem::replace(rhs, unit)
784 statement.source_info,
788 self.visit_rvalue(&mut rvalue, loc);
789 self.assign(new_temp, rvalue, source_info.span);
791 let terminator = if self.keep_original {
792 self.source[loc.block].terminator().clone()
794 let terminator = self.source[loc.block].terminator_mut();
795 let target = match terminator.kind {
796 TerminatorKind::Call { destination: Some((_, target)), .. } => target,
798 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
802 source_info: terminator.source_info,
803 kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
807 match terminator.kind {
808 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
809 self.visit_operand(&mut func, loc);
810 for arg in &mut args {
811 self.visit_operand(arg, loc);
814 let last = self.promoted.basic_blocks().last().unwrap();
815 let new_target = self.new_block();
817 *self.promoted[last].terminator_mut() = Terminator {
818 kind: TerminatorKind::Call {
822 destination: Some((Place::from(new_temp), new_target)),
826 source_info: SourceInfo::outermost(terminator.source_info.span),
831 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
836 self.keep_original = old_keep_original;
840 fn promote_candidate(
842 candidate: Candidate,
843 next_promoted_id: usize,
844 ) -> Option<Body<'tcx>> {
845 let def = self.source.source.with_opt_param();
847 let promoted = &mut self.promoted;
848 let promoted_id = Promoted::new(next_promoted_id);
850 let mut promoted_operand = |ty, span| {
851 promoted.span = span;
852 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
854 Operand::Constant(Box::new(Constant {
858 .mk_const(ty::Const {
860 val: ty::ConstKind::Unevaluated(ty::Unevaluated {
862 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
863 if let ty::GenericParamDefKind::Lifetime = param.kind {
864 tcx.lifetimes.re_erased.into()
866 tcx.mk_param_from_def(param)
869 promoted: Some(promoted_id),
875 let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
877 Candidate::Ref(loc) => {
878 let statement = &mut blocks[loc.block].statements[loc.statement_index];
879 match statement.kind {
880 StatementKind::Assign(box (
882 Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
884 // Use the underlying local for this (necessarily interior) borrow.
885 let ty = local_decls.local_decls()[place.local].ty;
886 let span = statement.source_info.span;
888 let ref_ty = tcx.mk_ref(
889 tcx.lifetimes.re_erased,
890 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
893 *region = tcx.lifetimes.re_erased;
895 let mut projection = vec![PlaceElem::Deref];
896 projection.extend(place.projection);
897 place.projection = tcx.intern_place_elems(&projection);
899 // Create a temp to hold the promoted reference.
900 // This is because `*r` requires `r` to be a local,
901 // otherwise we would use the `promoted` directly.
902 let mut promoted_ref = LocalDecl::new(ref_ty, span);
903 promoted_ref.source_info = statement.source_info;
904 let promoted_ref = local_decls.push(promoted_ref);
905 assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
907 let promoted_ref_statement = Statement {
908 source_info: statement.source_info,
909 kind: StatementKind::Assign(Box::new((
910 Place::from(promoted_ref),
911 Rvalue::Use(promoted_operand(ref_ty, span)),
914 self.extra_statements.push((loc, promoted_ref_statement));
917 tcx.lifetimes.re_erased,
920 local: mem::replace(&mut place.local, promoted_ref),
921 projection: List::empty(),
931 assert_eq!(self.new_block(), START_BLOCK);
934 Location { block: BasicBlock::new(0), statement_index: usize::MAX },
937 let span = self.promoted.span;
938 self.assign(RETURN_PLACE, rvalue, span);
943 /// Replaces all temporaries with their promoted counterparts.
944 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
945 fn tcx(&self) -> TyCtxt<'tcx> {
949 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
950 if self.is_temp_kind(*local) {
951 *local = self.promote_temp(*local);
956 pub fn promote_candidates<'tcx>(
957 body: &mut Body<'tcx>,
959 mut temps: IndexVec<Local, TempState>,
960 candidates: Vec<Candidate>,
961 ) -> IndexVec<Promoted, Body<'tcx>> {
962 // Visit candidates in reverse, in case they're nested.
963 debug!("promote_candidates({:?})", candidates);
965 let mut promotions = IndexVec::new();
967 let mut extra_statements = vec![];
968 for candidate in candidates.into_iter().rev() {
970 Candidate::Ref(Location { block, statement_index }) => {
971 if let StatementKind::Assign(box (place, _)) =
972 &body[block].statements[statement_index].kind
974 if let Some(local) = place.as_local() {
975 if temps[local] == TempState::PromotedOut {
984 // Declare return place local so that `mir::Body::new` doesn't complain.
985 let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
987 let mut scope = body.source_scopes[candidate.source_info(body).scope].clone();
988 scope.parent_scope = None;
990 let promoted = Body::new(
991 body.source, // `promoted` gets filled in below
993 IndexVec::from_elem_n(scope, 1),
999 body.generator_kind(),
1002 let promoter = Promoter {
1007 extra_statements: &mut extra_statements,
1008 keep_original: false,
1011 //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1012 if let Some(mut promoted) = promoter.promote_candidate(candidate, promotions.len()) {
1013 promoted.source.promoted = Some(promotions.next_index());
1014 promotions.push(promoted);
1018 // Insert each of `extra_statements` before its indicated location, which
1019 // has to be done in reverse location order, to not invalidate the rest.
1020 extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1021 for (loc, statement) in extra_statements {
1022 body[loc.block].statements.insert(loc.statement_index, statement);
1025 // Eliminate assignments to, and drops of promoted temps.
1026 let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1027 for block in body.basic_blocks_mut() {
1028 block.statements.retain(|statement| match &statement.kind {
1029 StatementKind::Assign(box (place, _)) => {
1030 if let Some(index) = place.as_local() {
1036 StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1041 let terminator = block.terminator_mut();
1042 if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1043 if let Some(index) = place.as_local() {
1044 if promoted(index) {
1045 terminator.kind = TerminatorKind::Goto { target: *target };
1054 /// This function returns `true` if the function being called in the array
1055 /// repeat expression is a `const` function.
1056 crate fn is_const_fn_in_array_repeat_expression<'tcx>(
1057 ccx: &ConstCx<'_, 'tcx>,
1058 place: &Place<'tcx>,
1061 match place.as_local() {
1062 // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1063 Some(local) if body.local_decls[local].is_user_variable() => return false,
1064 None => return false,
1068 for block in body.basic_blocks() {
1069 if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1072 if let Operand::Constant(box Constant { literal, .. }) = func {
1073 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1074 if let Some((destination_place, _)) = destination {
1075 if destination_place == place {
1076 if is_const_fn(ccx.tcx, def_id) {