transform/promote_consts.rs

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