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.
15 use rustc_ast
::LitKind
;
17 use rustc_hir
::def_id
::DefId
;
18 use rustc_middle
::mir
::traversal
::ReversePostorder
;
19 use rustc_middle
::mir
::visit
::{MutVisitor, MutatingUseContext, PlaceContext, Visitor}
;
20 use rustc_middle
::mir
::*;
21 use rustc_middle
::ty
::cast
::CastTy
;
22 use rustc_middle
::ty
::subst
::InternalSubsts
;
23 use rustc_middle
::ty
::{self, List, TyCtxt, TypeFoldable}
;
24 use rustc_span
::symbol
::sym
;
25 use rustc_span
::{Span, DUMMY_SP}
;
27 use rustc_index
::vec
::{Idx, IndexVec}
;
28 use rustc_target
::spec
::abi
::Abi
;
31 use std
::{cmp, iter, mem}
;
33 use crate::const_eval
::{is_const_fn, is_unstable_const_fn}
;
34 use crate::transform
::check_consts
::{is_lang_panic_fn, qualifs, ConstCx}
;
35 use crate::transform
::{MirPass, MirSource}
;
37 /// A `MirPass` for promotion.
39 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
40 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
42 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
43 /// newly created `Constant`.
45 pub struct PromoteTemps
<'tcx
> {
46 pub promoted_fragments
: Cell
<IndexVec
<Promoted
, Body
<'tcx
>>>,
49 impl<'tcx
> MirPass
<'tcx
> for PromoteTemps
<'tcx
> {
50 fn run_pass(&self, tcx
: TyCtxt
<'tcx
>, src
: MirSource
<'tcx
>, body
: &mut Body
<'tcx
>) {
51 // There's not really any point in promoting errorful MIR.
53 // This does not include MIR that failed const-checking, which we still try to promote.
54 if body
.return_ty().references_error() {
55 tcx
.sess
.delay_span_bug(body
.span
, "PromoteTemps: MIR had errors");
59 if src
.promoted
.is_some() {
63 let def
= src
.with_opt_param().expect_local();
65 let mut rpo
= traversal
::reverse_postorder(body
);
66 let ccx
= ConstCx
::new(tcx
, def
.did
, body
);
67 let (temps
, all_candidates
) = collect_temps_and_candidates(&ccx
, &mut rpo
);
69 let promotable_candidates
= validate_candidates(&ccx
, &temps
, &all_candidates
);
71 let promoted
= promote_candidates(def
.to_global(), body
, tcx
, temps
, promotable_candidates
);
72 self.promoted_fragments
.set(promoted
);
76 /// State of a temporary during collection and promotion.
77 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
79 /// No references to this temp.
81 /// One direct assignment and any number of direct uses.
82 /// A borrow of this temp is promotable if the assigned
83 /// value is qualified as constant.
84 Defined { location: Location, uses: usize }
,
85 /// Any other combination of assignments/uses.
87 /// This temp was part of an rvalue which got extracted
88 /// during promotion and needs cleanup.
93 pub fn is_promotable(&self) -> bool
{
94 debug
!("is_promotable: self={:?}", self);
95 matches
!(self, TempState
::Defined { .. }
)
99 /// A "root candidate" for promotion, which will become the
100 /// returned value in a promoted MIR, unless it's a subset
101 /// of a larger candidate.
102 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
104 /// Borrow of a constant temporary, candidate for lifetime extension.
107 /// Promotion of the `x` in `[x; 32]`.
110 /// Currently applied to function calls where the callee has the unstable
111 /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
112 /// intrinsic. The intrinsic requires the arguments are indeed constant and
113 /// the attribute currently provides the semantic requirement that arguments
114 /// must be constant.
115 Argument { bb: BasicBlock, index: usize }
,
117 /// `const` operand in asm!.
118 InlineAsm { bb: BasicBlock, index: usize }
,
122 /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
123 fn forces_explicit_promotion(&self) -> bool
{
125 Candidate
::Ref(_
) | Candidate
::Repeat(_
) => false,
126 Candidate
::Argument { .. }
| Candidate
::InlineAsm { .. }
=> true,
131 fn args_required_const(tcx
: TyCtxt
<'_
>, def_id
: DefId
) -> Option
<Vec
<usize>> {
132 let attrs
= tcx
.get_attrs(def_id
);
133 let attr
= attrs
.iter().find(|a
| tcx
.sess
.check_name(a
, sym
::rustc_args_required_const
))?
;
134 let mut ret
= vec
![];
135 for meta
in attr
.meta_item_list()?
{
136 match meta
.literal()?
.kind
{
137 LitKind
::Int(a
, _
) => {
138 ret
.push(a
as usize);
140 _
=> bug
!("invalid arg index"),
146 struct Collector
<'a
, 'tcx
> {
147 ccx
: &'a ConstCx
<'a
, 'tcx
>,
148 temps
: IndexVec
<Local
, TempState
>,
149 candidates
: Vec
<Candidate
>,
152 impl<'tcx
> Visitor
<'tcx
> for Collector
<'_
, 'tcx
> {
153 fn visit_local(&mut self, &index
: &Local
, context
: PlaceContext
, location
: Location
) {
154 debug
!("visit_local: index={:?} context={:?} location={:?}", index
, context
, location
);
155 // We're only interested in temporaries and the return place
156 match self.ccx
.body
.local_kind(index
) {
157 LocalKind
::Temp
| LocalKind
::ReturnPointer
=> {}
158 LocalKind
::Arg
| LocalKind
::Var
=> return,
161 // Ignore drops, if the temp gets promoted,
162 // then it's constant and thus drop is noop.
163 // Non-uses are also irrelevant.
164 if context
.is_drop() || !context
.is_use() {
166 "visit_local: context.is_drop={:?} context.is_use={:?}",
173 let temp
= &mut self.temps
[index
];
174 debug
!("visit_local: temp={:?}", temp
);
175 if *temp
== TempState
::Undefined
{
177 PlaceContext
::MutatingUse(MutatingUseContext
::Store
)
178 | PlaceContext
::MutatingUse(MutatingUseContext
::Call
) => {
179 *temp
= TempState
::Defined { location, uses: 0 }
;
182 _
=> { /* mark as unpromotable below */ }
184 } else if let TempState
::Defined { ref mut uses, .. }
= *temp
{
185 // We always allow borrows, even mutable ones, as we need
186 // to promote mutable borrows of some ZSTs e.g., `&mut []`.
187 let allowed_use
= match context
{
188 PlaceContext
::MutatingUse(MutatingUseContext
::Borrow
)
189 | PlaceContext
::NonMutatingUse(_
) => true,
190 PlaceContext
::MutatingUse(_
) | PlaceContext
::NonUse(_
) => false,
192 debug
!("visit_local: allowed_use={:?}", allowed_use
);
197 /* mark as unpromotable below */
199 *temp
= TempState
::Unpromotable
;
202 fn visit_rvalue(&mut self, rvalue
: &Rvalue
<'tcx
>, location
: Location
) {
203 self.super_rvalue(rvalue
, location
);
207 self.candidates
.push(Candidate
::Ref(location
));
209 Rvalue
::Repeat(..) if self.ccx
.tcx
.features().const_in_array_repeat_expressions
=> {
210 // FIXME(#49147) only promote the element when it isn't `Copy`
211 // (so that code that can copy it at runtime is unaffected).
212 self.candidates
.push(Candidate
::Repeat(location
));
218 fn visit_terminator(&mut self, terminator
: &Terminator
<'tcx
>, location
: Location
) {
219 self.super_terminator(terminator
, location
);
221 match terminator
.kind
{
222 TerminatorKind
::Call { ref func, .. }
=> {
223 if let ty
::FnDef(def_id
, _
) = *func
.ty(self.ccx
.body
, self.ccx
.tcx
).kind() {
224 let fn_sig
= self.ccx
.tcx
.fn_sig(def_id
);
225 if let Abi
::RustIntrinsic
| Abi
::PlatformIntrinsic
= fn_sig
.abi() {
226 let name
= self.ccx
.tcx
.item_name(def_id
);
227 // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
228 if name
.as_str().starts_with("simd_shuffle") {
230 .push(Candidate
::Argument { bb: location.block, index: 2 }
);
232 return; // Don't double count `simd_shuffle` candidates
236 if let Some(constant_args
) = args_required_const(self.ccx
.tcx
, def_id
) {
237 for index
in constant_args
{
238 self.candidates
.push(Candidate
::Argument { bb: location.block, index }
);
243 TerminatorKind
::InlineAsm { ref operands, .. }
=> {
244 for (index
, op
) in operands
.iter().enumerate() {
245 if let InlineAsmOperand
::Const { .. }
= op
{
246 self.candidates
.push(Candidate
::InlineAsm { bb: location.block, index }
)
255 pub fn collect_temps_and_candidates(
256 ccx
: &ConstCx
<'mir
, 'tcx
>,
257 rpo
: &mut ReversePostorder
<'_
, 'tcx
>,
258 ) -> (IndexVec
<Local
, TempState
>, Vec
<Candidate
>) {
259 let mut collector
= Collector
{
260 temps
: IndexVec
::from_elem(TempState
::Undefined
, &ccx
.body
.local_decls
),
264 for (bb
, data
) in rpo
{
265 collector
.visit_basic_block_data(bb
, data
);
267 (collector
.temps
, collector
.candidates
)
270 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
272 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
273 struct Validator
<'a
, 'tcx
> {
274 ccx
: &'a ConstCx
<'a
, 'tcx
>,
275 temps
: &'a IndexVec
<Local
, TempState
>,
277 /// Explicit promotion happens e.g. for constant arguments declared via
278 /// `rustc_args_required_const`.
279 /// Implicit promotion has almost the same rules, except that disallows `const fn`
280 /// except for those marked `#[rustc_promotable]`. This is to avoid changing
281 /// a legitimate run-time operation into a failing compile-time operation
282 /// e.g. due to addresses being compared inside the function.
286 impl std
::ops
::Deref
for Validator
<'a
, 'tcx
> {
287 type Target
= ConstCx
<'a
, 'tcx
>;
289 fn deref(&self) -> &Self::Target
{
296 impl<'tcx
> Validator
<'_
, 'tcx
> {
297 /// Determines if this code could be executed at runtime and thus is subject to codegen.
298 /// That means even unused constants need to be evaluated.
300 /// `const_kind` should not be used in this file other than through this method!
301 fn maybe_runtime(&self) -> bool
{
302 match self.const_kind
{
303 None
| Some(hir
::ConstContext
::ConstFn
) => true,
304 Some(hir
::ConstContext
::Static(_
) | hir
::ConstContext
::Const
) => false,
308 fn validate_candidate(&self, candidate
: Candidate
) -> Result
<(), Unpromotable
> {
310 Candidate
::Ref(loc
) => {
311 assert
!(!self.explicit
);
313 let statement
= &self.body
[loc
.block
].statements
[loc
.statement_index
];
314 match &statement
.kind
{
315 StatementKind
::Assign(box (_
, Rvalue
::Ref(_
, kind
, place
))) => {
317 BorrowKind
::Shared
| BorrowKind
::Mut { .. }
=> {}
319 // FIXME(eddyb) these aren't promoted here but *could*
320 // be promoted as part of a larger value because
321 // `validate_rvalue` doesn't check them, need to
322 // figure out what is the intended behavior.
323 BorrowKind
::Shallow
| BorrowKind
::Unique
=> return Err(Unpromotable
),
326 // We can only promote interior borrows of promotable temps (non-temps
327 // don't get promoted anyway).
328 self.validate_local(place
.local
)?
;
330 if place
.projection
.contains(&ProjectionElem
::Deref
) {
331 return Err(Unpromotable
);
334 let mut has_mut_interior
=
335 self.qualif_local
::<qualifs
::HasMutInterior
>(place
.local
);
336 // HACK(eddyb) this should compute the same thing as
337 // `<HasMutInterior as Qualif>::in_projection` from
338 // `check_consts::qualifs` but without recursion.
339 if has_mut_interior
{
340 // This allows borrowing fields which don't have
341 // `HasMutInterior`, from a type that does, e.g.:
342 // `let _: &'static _ = &(Cell::new(1), 2).1;`
343 let mut place_projection
= &place
.projection
[..];
344 // FIXME(eddyb) use a forward loop instead of a reverse one.
345 while let &[ref proj_base @
.., elem
] = place_projection
{
346 // FIXME(eddyb) this is probably excessive, with
347 // the exception of `union` member accesses.
349 Place
::ty_from(place
.local
, proj_base
, self.body
, self.tcx
)
350 .projection_ty(self.tcx
, elem
)
352 if ty
.is_freeze(self.tcx
.at(DUMMY_SP
), self.param_env
) {
353 has_mut_interior
= false;
357 place_projection
= proj_base
;
361 // FIXME(eddyb) this duplicates part of `validate_rvalue`.
362 if has_mut_interior
{
363 return Err(Unpromotable
);
365 if self.qualif_local
::<qualifs
::NeedsDrop
>(place
.local
) {
366 return Err(Unpromotable
);
369 if let BorrowKind
::Mut { .. }
= kind
{
370 let ty
= place
.ty(self.body
, self.tcx
).ty
;
372 // In theory, any zero-sized value could be borrowed
373 // mutably without consequences. However, only &mut []
374 // is allowed right now.
375 if let ty
::Array(_
, len
) = ty
.kind() {
376 match len
.try_eval_usize(self.tcx
, self.param_env
) {
378 _
=> return Err(Unpromotable
),
381 return Err(Unpromotable
);
390 Candidate
::Repeat(loc
) => {
391 assert
!(!self.explicit
);
393 let statement
= &self.body
[loc
.block
].statements
[loc
.statement_index
];
394 match &statement
.kind
{
395 StatementKind
::Assign(box (_
, Rvalue
::Repeat(ref operand
, _
))) => {
396 if !self.tcx
.features().const_in_array_repeat_expressions
{
397 return Err(Unpromotable
);
400 self.validate_operand(operand
)
405 Candidate
::Argument { bb, index }
=> {
406 assert
!(self.explicit
);
408 let terminator
= self.body
[bb
].terminator();
409 match &terminator
.kind
{
410 TerminatorKind
::Call { args, .. }
=> self.validate_operand(&args
[index
]),
414 Candidate
::InlineAsm { bb, index }
=> {
415 assert
!(self.explicit
);
417 let terminator
= self.body
[bb
].terminator();
418 match &terminator
.kind
{
419 TerminatorKind
::InlineAsm { operands, .. }
=> match &operands
[index
] {
420 InlineAsmOperand
::Const { value }
=> self.validate_operand(value
),
429 // FIXME(eddyb) maybe cache this?
430 fn qualif_local
<Q
: qualifs
::Qualif
>(&self, local
: Local
) -> bool
{
431 if let TempState
::Defined { location: loc, .. }
= self.temps
[local
] {
432 let num_stmts
= self.body
[loc
.block
].statements
.len();
434 if loc
.statement_index
< num_stmts
{
435 let statement
= &self.body
[loc
.block
].statements
[loc
.statement_index
];
436 match &statement
.kind
{
437 StatementKind
::Assign(box (_
, rhs
)) => qualifs
::in_rvalue
::<Q
, _
>(
439 &mut |l
| self.qualif_local
::<Q
>(l
),
444 statement
.source_info
.span
,
445 "{:?} is not an assignment",
451 let terminator
= self.body
[loc
.block
].terminator();
452 match &terminator
.kind
{
453 TerminatorKind
::Call { .. }
=> {
454 let return_ty
= self.body
.local_decls
[local
].ty
;
455 Q
::in_any_value_of_ty(&self.ccx
, return_ty
)
458 span_bug
!(terminator
.source_info
.span
, "{:?} not promotable", kind
);
463 let span
= self.body
.local_decls
[local
].source_info
.span
;
464 span_bug
!(span
, "{:?} not promotable, qualif_local shouldn't have been called", local
);
468 // FIXME(eddyb) maybe cache this?
469 fn validate_local(&self, local
: Local
) -> Result
<(), Unpromotable
> {
470 if let TempState
::Defined { location: loc, .. }
= self.temps
[local
] {
471 let num_stmts
= self.body
[loc
.block
].statements
.len();
473 if loc
.statement_index
< num_stmts
{
474 let statement
= &self.body
[loc
.block
].statements
[loc
.statement_index
];
475 match &statement
.kind
{
476 StatementKind
::Assign(box (_
, rhs
)) => self.validate_rvalue(rhs
),
479 statement
.source_info
.span
,
480 "{:?} is not an assignment",
486 let terminator
= self.body
[loc
.block
].terminator();
487 match &terminator
.kind
{
488 TerminatorKind
::Call { func, args, .. }
=> self.validate_call(func
, args
),
489 TerminatorKind
::Yield { .. }
=> Err(Unpromotable
),
491 span_bug
!(terminator
.source_info
.span
, "{:?} not promotable", kind
);
500 fn validate_place(&self, place
: PlaceRef
<'tcx
>) -> Result
<(), Unpromotable
> {
502 PlaceRef { local, projection: [] }
=> self.validate_local(local
),
503 PlaceRef { local, projection: [proj_base @ .., elem] }
=> {
504 // Validate topmost projection, then recurse.
506 ProjectionElem
::Deref
=> {
507 let mut promotable
= false;
508 // This is a special treatment for cases like *&STATIC where STATIC is a
509 // global static variable.
510 // This pattern is generated only when global static variables are directly
511 // accessed and is qualified for promotion safely.
512 if let TempState
::Defined { location, .. }
= self.temps
[local
] {
514 self.body
[location
.block
].statements
.get(location
.statement_index
);
515 if let Some(Statement
{
517 StatementKind
::Assign(box (_
, Rvalue
::Use(Operand
::Constant(c
)))),
521 if let Some(did
) = c
.check_static_ptr(self.tcx
) {
522 // Evaluating a promoted may not read statics except if it got
523 // promoted from a static (this is a CTFE check). So we
524 // can only promote static accesses inside statics.
525 if let Some(hir
::ConstContext
::Static(..)) = self.const_kind
{
526 // The `is_empty` predicate is introduced to exclude the case
527 // where the projection operations are [ .field, * ].
528 // The reason is because promotion will be illegal if field
529 // accesses precede the dereferencing.
530 // Discussion can be found at
531 // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
532 // There may be opportunity for generalization, but this needs to be
534 if proj_base
.is_empty()
535 && !self.tcx
.is_thread_local_static(did
)
544 return Err(Unpromotable
);
547 ProjectionElem
::Downcast(..) => {
548 return Err(Unpromotable
);
551 ProjectionElem
::ConstantIndex { .. }
| ProjectionElem
::Subslice { .. }
=> {}
553 ProjectionElem
::Index(local
) => {
554 self.validate_local(local
)?
;
557 ProjectionElem
::Field(..) => {
558 if self.maybe_runtime() {
560 Place
::ty_from(place
.local
, proj_base
, self.body
, self.tcx
).ty
;
561 if let Some(def
) = base_ty
.ty_adt_def() {
562 // No promotion of union field accesses.
564 return Err(Unpromotable
);
571 self.validate_place(PlaceRef { local: place.local, projection: proj_base }
)
576 fn validate_operand(&self, operand
: &Operand
<'tcx
>) -> Result
<(), Unpromotable
> {
578 Operand
::Copy(place
) | Operand
::Move(place
) => self.validate_place(place
.as_ref()),
580 // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
581 // `validate_rvalue` upon access.
582 Operand
::Constant(c
) => {
583 if let Some(def_id
) = c
.check_static_ptr(self.tcx
) {
584 // Only allow statics (not consts) to refer to other statics.
585 // FIXME(eddyb) does this matter at all for promotion?
586 // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
587 // and in `const` this cannot occur anyway. The only concern is that we might
588 // promote even `let x = &STATIC` which would be useless, but this applies to
589 // promotion inside statics as well.
590 let is_static
= matches
!(self.const_kind
, Some(hir
::ConstContext
::Static(_
)));
592 return Err(Unpromotable
);
595 let is_thread_local
= self.tcx
.is_thread_local_static(def_id
);
597 return Err(Unpromotable
);
606 fn validate_rvalue(&self, rvalue
: &Rvalue
<'tcx
>) -> Result
<(), Unpromotable
> {
608 Rvalue
::Cast(CastKind
::Misc
, ref operand
, cast_ty
) => {
609 let operand_ty
= operand
.ty(self.body
, self.tcx
);
610 let cast_in
= CastTy
::from_ty(operand_ty
).expect("bad input type for cast");
611 let cast_out
= CastTy
::from_ty(cast_ty
).expect("bad output type for cast");
612 if let (CastTy
::Ptr(_
) | CastTy
::FnPtr
, CastTy
::Int(_
)) = (cast_in
, cast_out
) {
613 // ptr-to-int casts are not possible in consts and thus not promotable
614 return Err(Unpromotable
);
618 Rvalue
::BinaryOp(op
, ref lhs
, _
) => {
619 if let ty
::RawPtr(_
) | ty
::FnPtr(..) = lhs
.ty(self.body
, self.tcx
).kind() {
627 || op
== BinOp
::Offset
630 // raw pointer operations are not allowed inside consts and thus not promotable
631 return Err(Unpromotable
);
635 Rvalue
::NullaryOp(NullOp
::Box
, _
) => return Err(Unpromotable
),
637 // FIXME(RalfJung): the rest is *implicitly considered promotable*... that seems dangerous.
642 Rvalue
::ThreadLocalRef(_
) => Err(Unpromotable
),
644 Rvalue
::NullaryOp(..) => Ok(()),
646 Rvalue
::Discriminant(place
) | Rvalue
::Len(place
) => self.validate_place(place
.as_ref()),
649 | Rvalue
::Repeat(operand
, _
)
650 | Rvalue
::UnaryOp(_
, operand
)
651 | Rvalue
::Cast(_
, operand
, _
) => self.validate_operand(operand
),
653 Rvalue
::BinaryOp(_
, lhs
, rhs
) | Rvalue
::CheckedBinaryOp(_
, lhs
, rhs
) => {
654 self.validate_operand(lhs
)?
;
655 self.validate_operand(rhs
)
658 Rvalue
::AddressOf(_
, place
) => {
659 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
660 // no problem, only using it is.
661 if let [proj_base @
.., ProjectionElem
::Deref
] = place
.projection
.as_ref() {
662 let base_ty
= Place
::ty_from(place
.local
, proj_base
, self.body
, self.tcx
).ty
;
663 if let ty
::Ref(..) = base_ty
.kind() {
664 return self.validate_place(PlaceRef
{
666 projection
: proj_base
,
673 Rvalue
::Ref(_
, kind
, place
) => {
674 if let BorrowKind
::Mut { .. }
= kind
{
675 let ty
= place
.ty(self.body
, self.tcx
).ty
;
677 // In theory, any zero-sized value could be borrowed
678 // mutably without consequences. However, only &mut []
679 // is allowed right now.
680 if let ty
::Array(_
, len
) = ty
.kind() {
681 match len
.try_eval_usize(self.tcx
, self.param_env
) {
683 _
=> return Err(Unpromotable
),
686 return Err(Unpromotable
);
690 // Special-case reborrows to be more like a copy of the reference.
691 let mut place
= place
.as_ref();
692 if let [proj_base @
.., ProjectionElem
::Deref
] = &place
.projection
{
693 let base_ty
= Place
::ty_from(place
.local
, proj_base
, self.body
, self.tcx
).ty
;
694 if let ty
::Ref(..) = base_ty
.kind() {
695 place
= PlaceRef { local: place.local, projection: proj_base }
;
699 self.validate_place(place
)?
;
701 // HACK(eddyb) this should compute the same thing as
702 // `<HasMutInterior as Qualif>::in_projection` from
703 // `check_consts::qualifs` but without recursion.
704 let mut has_mut_interior
=
705 self.qualif_local
::<qualifs
::HasMutInterior
>(place
.local
);
706 if has_mut_interior
{
707 let mut place_projection
= place
.projection
;
708 // FIXME(eddyb) use a forward loop instead of a reverse one.
709 while let &[ref proj_base @
.., elem
] = place_projection
{
710 // FIXME(eddyb) this is probably excessive, with
711 // the exception of `union` member accesses.
712 let ty
= Place
::ty_from(place
.local
, proj_base
, self.body
, self.tcx
)
713 .projection_ty(self.tcx
, elem
)
715 if ty
.is_freeze(self.tcx
.at(DUMMY_SP
), self.param_env
) {
716 has_mut_interior
= false;
720 place_projection
= proj_base
;
723 if has_mut_interior
{
724 return Err(Unpromotable
);
730 Rvalue
::Aggregate(_
, ref operands
) => {
732 self.validate_operand(o
)?
;
742 callee
: &Operand
<'tcx
>,
743 args
: &[Operand
<'tcx
>],
744 ) -> Result
<(), Unpromotable
> {
745 let fn_ty
= callee
.ty(self.body
, self.tcx
);
747 if !self.explicit
&& self.maybe_runtime() {
748 if let ty
::FnDef(def_id
, _
) = *fn_ty
.kind() {
749 // Never promote runtime `const fn` calls of
750 // functions without `#[rustc_promotable]`.
751 if !self.tcx
.is_promotable_const_fn(def_id
) {
752 return Err(Unpromotable
);
757 let is_const_fn
= match *fn_ty
.kind() {
758 ty
::FnDef(def_id
, _
) => {
759 is_const_fn(self.tcx
, def_id
)
760 || is_unstable_const_fn(self.tcx
, def_id
).is_some()
761 || is_lang_panic_fn(self.tcx
, self.def_id
.to_def_id())
766 return Err(Unpromotable
);
769 self.validate_operand(callee
)?
;
771 self.validate_operand(arg
)?
;
778 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
779 pub fn validate_candidates(
780 ccx
: &ConstCx
<'_
, '_
>,
781 temps
: &IndexVec
<Local
, TempState
>,
782 candidates
: &[Candidate
],
783 ) -> Vec
<Candidate
> {
784 let mut validator
= Validator { ccx, temps, explicit: false }
;
789 .filter(|&candidate
| {
790 validator
.explicit
= candidate
.forces_explicit_promotion();
792 // FIXME(eddyb) also emit the errors for shuffle indices
793 // and `#[rustc_args_required_const]` arguments here.
795 let is_promotable
= validator
.validate_candidate(candidate
).is_ok();
797 // If we use explicit validation, we carry the risk of turning a legitimate run-time
798 // operation into a failing compile-time operation. Make sure that does not happen
799 // by asserting that there is no possible run-time behavior here in case promotion
801 if validator
.explicit
&& !is_promotable
{
802 ccx
.tcx
.sess
.delay_span_bug(
804 "Explicit promotion requested, but failed to promote",
809 Candidate
::Argument { bb, index }
| Candidate
::InlineAsm { bb, index }
812 let span
= ccx
.body
[bb
].terminator().source_info
.span
;
813 let msg
= format
!("argument {} is required to be a constant", index
+ 1);
814 ccx
.tcx
.sess
.span_err(span
, &msg
);
824 struct Promoter
<'a
, 'tcx
> {
826 source
: &'a
mut Body
<'tcx
>,
827 promoted
: Body
<'tcx
>,
828 temps
: &'a
mut IndexVec
<Local
, TempState
>,
829 extra_statements
: &'a
mut Vec
<(Location
, Statement
<'tcx
>)>,
831 /// If true, all nested temps are also kept in the
832 /// source MIR, not moved to the promoted MIR.
836 impl<'a
, 'tcx
> Promoter
<'a
, 'tcx
> {
837 fn new_block(&mut self) -> BasicBlock
{
838 let span
= self.promoted
.span
;
839 self.promoted
.basic_blocks_mut().push(BasicBlockData
{
841 terminator
: Some(Terminator
{
842 source_info
: SourceInfo
::outermost(span
),
843 kind
: TerminatorKind
::Return
,
849 fn assign(&mut self, dest
: Local
, rvalue
: Rvalue
<'tcx
>, span
: Span
) {
850 let last
= self.promoted
.basic_blocks().last().unwrap();
851 let data
= &mut self.promoted
[last
];
852 data
.statements
.push(Statement
{
853 source_info
: SourceInfo
::outermost(span
),
854 kind
: StatementKind
::Assign(box (Place
::from(dest
), rvalue
)),
858 fn is_temp_kind(&self, local
: Local
) -> bool
{
859 self.source
.local_kind(local
) == LocalKind
::Temp
862 /// Copies the initialization of this temp to the
863 /// promoted MIR, recursing through temps.
864 fn promote_temp(&mut self, temp
: Local
) -> Local
{
865 let old_keep_original
= self.keep_original
;
866 let loc
= match self.temps
[temp
] {
867 TempState
::Defined { location, uses }
if uses
> 0 => {
869 self.keep_original
= true;
874 span_bug
!(self.promoted
.span
, "{:?} not promotable: {:?}", temp
, state
);
877 if !self.keep_original
{
878 self.temps
[temp
] = TempState
::PromotedOut
;
881 let num_stmts
= self.source
[loc
.block
].statements
.len();
882 let new_temp
= self.promoted
.local_decls
.push(LocalDecl
::new(
883 self.source
.local_decls
[temp
].ty
,
884 self.source
.local_decls
[temp
].source_info
.span
,
887 debug
!("promote({:?} @ {:?}/{:?}, {:?})", temp
, loc
, num_stmts
, self.keep_original
);
889 // First, take the Rvalue or Call out of the source MIR,
890 // or duplicate it, depending on keep_original.
891 if loc
.statement_index
< num_stmts
{
892 let (mut rvalue
, source_info
) = {
893 let statement
= &mut self.source
[loc
.block
].statements
[loc
.statement_index
];
894 let rhs
= match statement
.kind
{
895 StatementKind
::Assign(box (_
, ref mut rhs
)) => rhs
,
898 statement
.source_info
.span
,
899 "{:?} is not an assignment",
906 if self.keep_original
{
909 let unit
= Rvalue
::Use(Operand
::Constant(box Constant
{
910 span
: statement
.source_info
.span
,
912 literal
: ty
::Const
::zero_sized(self.tcx
, self.tcx
.types
.unit
),
914 mem
::replace(rhs
, unit
)
916 statement
.source_info
,
920 self.visit_rvalue(&mut rvalue
, loc
);
921 self.assign(new_temp
, rvalue
, source_info
.span
);
923 let terminator
= if self.keep_original
{
924 self.source
[loc
.block
].terminator().clone()
926 let terminator
= self.source
[loc
.block
].terminator_mut();
927 let target
= match terminator
.kind
{
928 TerminatorKind
::Call { destination: Some((_, target)), .. }
=> target
,
930 span_bug
!(terminator
.source_info
.span
, "{:?} not promotable", kind
);
934 source_info
: terminator
.source_info
,
935 kind
: mem
::replace(&mut terminator
.kind
, TerminatorKind
::Goto { target }
),
939 match terminator
.kind
{
940 TerminatorKind
::Call { mut func, mut args, from_hir_call, fn_span, .. }
=> {
941 self.visit_operand(&mut func
, loc
);
942 for arg
in &mut args
{
943 self.visit_operand(arg
, loc
);
946 let last
= self.promoted
.basic_blocks().last().unwrap();
947 let new_target
= self.new_block();
949 *self.promoted
[last
].terminator_mut() = Terminator
{
950 kind
: TerminatorKind
::Call
{
954 destination
: Some((Place
::from(new_temp
), new_target
)),
962 span_bug
!(terminator
.source_info
.span
, "{:?} not promotable", kind
);
967 self.keep_original
= old_keep_original
;
971 fn promote_candidate(
973 def
: ty
::WithOptConstParam
<DefId
>,
974 candidate
: Candidate
,
975 next_promoted_id
: usize,
976 ) -> Option
<Body
<'tcx
>> {
978 let promoted
= &mut self.promoted
;
979 let promoted_id
= Promoted
::new(next_promoted_id
);
981 let mut promoted_operand
= |ty
, span
| {
982 promoted
.span
= span
;
983 promoted
.local_decls
[RETURN_PLACE
] = LocalDecl
::new(ty
, span
);
985 Operand
::Constant(Box
::new(Constant
{
988 literal
: tcx
.mk_const(ty
::Const
{
990 val
: ty
::ConstKind
::Unevaluated(
992 InternalSubsts
::for_item(tcx
, def
.did
, |param
, _
| {
993 if let ty
::GenericParamDefKind
::Lifetime
= param
.kind
{
994 tcx
.lifetimes
.re_erased
.into()
996 tcx
.mk_param_from_def(param
)
1004 let (blocks
, local_decls
) = self.source
.basic_blocks_and_local_decls_mut();
1006 Candidate
::Ref(loc
) => {
1007 let statement
= &mut blocks
[loc
.block
].statements
[loc
.statement_index
];
1008 match statement
.kind
{
1009 StatementKind
::Assign(box (
1011 Rvalue
::Ref(ref mut region
, borrow_kind
, ref mut place
),
1013 // Use the underlying local for this (necessarily interior) borrow.
1014 let ty
= local_decls
.local_decls()[place
.local
].ty
;
1015 let span
= statement
.source_info
.span
;
1017 let ref_ty
= tcx
.mk_ref(
1018 tcx
.lifetimes
.re_erased
,
1019 ty
::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() }
,
1022 *region
= tcx
.lifetimes
.re_erased
;
1024 let mut projection
= vec
![PlaceElem
::Deref
];
1025 projection
.extend(place
.projection
);
1026 place
.projection
= tcx
.intern_place_elems(&projection
);
1028 // Create a temp to hold the promoted reference.
1029 // This is because `*r` requires `r` to be a local,
1030 // otherwise we would use the `promoted` directly.
1031 let mut promoted_ref
= LocalDecl
::new(ref_ty
, span
);
1032 promoted_ref
.source_info
= statement
.source_info
;
1033 let promoted_ref
= local_decls
.push(promoted_ref
);
1034 assert_eq
!(self.temps
.push(TempState
::Unpromotable
), promoted_ref
);
1036 let promoted_ref_statement
= Statement
{
1037 source_info
: statement
.source_info
,
1038 kind
: StatementKind
::Assign(Box
::new((
1039 Place
::from(promoted_ref
),
1040 Rvalue
::Use(promoted_operand(ref_ty
, span
)),
1043 self.extra_statements
.push((loc
, promoted_ref_statement
));
1046 tcx
.lifetimes
.re_erased
,
1049 local
: mem
::replace(&mut place
.local
, promoted_ref
),
1050 projection
: List
::empty(),
1057 Candidate
::Repeat(loc
) => {
1058 let statement
= &mut blocks
[loc
.block
].statements
[loc
.statement_index
];
1059 match statement
.kind
{
1060 StatementKind
::Assign(box (_
, Rvalue
::Repeat(ref mut operand
, _
))) => {
1061 let ty
= operand
.ty(local_decls
, self.tcx
);
1062 let span
= statement
.source_info
.span
;
1064 Rvalue
::Use(mem
::replace(operand
, promoted_operand(ty
, span
)))
1069 Candidate
::Argument { bb, index }
=> {
1070 let terminator
= blocks
[bb
].terminator_mut();
1071 match terminator
.kind
{
1072 TerminatorKind
::Call { ref mut args, .. }
=> {
1073 let ty
= args
[index
].ty(local_decls
, self.tcx
);
1074 let span
= terminator
.source_info
.span
;
1076 Rvalue
::Use(mem
::replace(&mut args
[index
], promoted_operand(ty
, span
)))
1078 // We expected a `TerminatorKind::Call` for which we'd like to promote an
1079 // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
1080 // we are seeing a `Goto`. That means that the `promote_temps` method
1081 // already promoted this call away entirely. This case occurs when calling
1082 // a function requiring a constant argument and as that constant value
1083 // providing a value whose computation contains another call to a function
1084 // requiring a constant argument.
1085 TerminatorKind
::Goto { .. }
=> return None
,
1089 Candidate
::InlineAsm { bb, index }
=> {
1090 let terminator
= blocks
[bb
].terminator_mut();
1091 match terminator
.kind
{
1092 TerminatorKind
::InlineAsm { ref mut operands, .. }
=> {
1093 match &mut operands
[index
] {
1094 InlineAsmOperand
::Const { ref mut value }
=> {
1095 let ty
= value
.ty(local_decls
, self.tcx
);
1096 let span
= terminator
.source_info
.span
;
1098 Rvalue
::Use(mem
::replace(value
, promoted_operand(ty
, span
)))
1110 assert_eq
!(self.new_block(), START_BLOCK
);
1113 Location { block: BasicBlock::new(0), statement_index: usize::MAX }
,
1116 let span
= self.promoted
.span
;
1117 self.assign(RETURN_PLACE
, rvalue
, span
);
1122 /// Replaces all temporaries with their promoted counterparts.
1123 impl<'a
, 'tcx
> MutVisitor
<'tcx
> for Promoter
<'a
, 'tcx
> {
1124 fn tcx(&self) -> TyCtxt
<'tcx
> {
1128 fn visit_local(&mut self, local
: &mut Local
, _
: PlaceContext
, _
: Location
) {
1129 if self.is_temp_kind(*local
) {
1130 *local
= self.promote_temp(*local
);
1135 pub fn promote_candidates
<'tcx
>(
1136 def
: ty
::WithOptConstParam
<DefId
>,
1137 body
: &mut Body
<'tcx
>,
1139 mut temps
: IndexVec
<Local
, TempState
>,
1140 candidates
: Vec
<Candidate
>,
1141 ) -> IndexVec
<Promoted
, Body
<'tcx
>> {
1142 // Visit candidates in reverse, in case they're nested.
1143 debug
!("promote_candidates({:?})", candidates
);
1145 let mut promotions
= IndexVec
::new();
1147 let mut extra_statements
= vec
![];
1148 for candidate
in candidates
.into_iter().rev() {
1150 Candidate
::Repeat(Location { block, statement_index }
)
1151 | Candidate
::Ref(Location { block, statement_index }
) => {
1152 if let StatementKind
::Assign(box (place
, _
)) =
1153 &body
[block
].statements
[statement_index
].kind
1155 if let Some(local
) = place
.as_local() {
1156 if temps
[local
] == TempState
::PromotedOut
{
1157 // Already promoted.
1163 Candidate
::Argument { .. }
| Candidate
::InlineAsm { .. }
=> {}
1166 // Declare return place local so that `mir::Body::new` doesn't complain.
1167 let initial_locals
= iter
::once(LocalDecl
::new(tcx
.types
.never
, body
.span
)).collect();
1169 let mut promoted
= Body
::new(
1171 // FIXME: maybe try to filter this to avoid blowing up
1173 body
.source_scopes
.clone(),
1179 body
.generator_kind
,
1181 promoted
.ignore_interior_mut_in_const_validation
= true;
1183 let promoter
= Promoter
{
1188 extra_statements
: &mut extra_statements
,
1189 keep_original
: false,
1192 //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1193 if let Some(promoted
) = promoter
.promote_candidate(def
, candidate
, promotions
.len()) {
1194 promotions
.push(promoted
);
1198 // Insert each of `extra_statements` before its indicated location, which
1199 // has to be done in reverse location order, to not invalidate the rest.
1200 extra_statements
.sort_by_key(|&(loc
, _
)| cmp
::Reverse(loc
));
1201 for (loc
, statement
) in extra_statements
{
1202 body
[loc
.block
].statements
.insert(loc
.statement_index
, statement
);
1205 // Eliminate assignments to, and drops of promoted temps.
1206 let promoted
= |index
: Local
| temps
[index
] == TempState
::PromotedOut
;
1207 for block
in body
.basic_blocks_mut() {
1208 block
.statements
.retain(|statement
| match &statement
.kind
{
1209 StatementKind
::Assign(box (place
, _
)) => {
1210 if let Some(index
) = place
.as_local() {
1216 StatementKind
::StorageLive(index
) | StatementKind
::StorageDead(index
) => {
1221 let terminator
= block
.terminator_mut();
1222 if let TerminatorKind
::Drop { place, target, .. }
= &terminator
.kind
{
1223 if let Some(index
) = place
.as_local() {
1224 if promoted(index
) {
1225 terminator
.kind
= TerminatorKind
::Goto { target: *target }
;
1234 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1235 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1236 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1238 crate fn should_suggest_const_in_array_repeat_expressions_attribute
<'tcx
>(
1239 ccx
: &ConstCx
<'_
, 'tcx
>,
1240 operand
: &Operand
<'tcx
>,
1242 let mut rpo
= traversal
::reverse_postorder(&ccx
.body
);
1243 let (temps
, _
) = collect_temps_and_candidates(&ccx
, &mut rpo
);
1244 let validator
= Validator { ccx, temps: &temps, explicit: false }
;
1246 let should_promote
= validator
.validate_operand(operand
).is_ok();
1247 let feature_flag
= validator
.ccx
.tcx
.features().const_in_array_repeat_expressions
;
1249 "should_suggest_const_in_array_repeat_expressions_flag: def_id={:?} \
1250 should_promote={:?} feature_flag={:?}",
1251 validator
.ccx
.def_id
, should_promote
, feature_flag
1253 should_promote
&& !feature_flag