compiler/rustc_const_eval/src/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::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};
  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::{is_lang_special_const_fn, qualifs, ConstCx};
  30 use crate::transform::MirPass;
  31
  32 /// A `MirPass` for promotion.
  33 ///
  34 /// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
  35 /// `'static` lifetime.
  36 ///
  37 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  38 /// newly created `Constant`.
  39 #[derive(Default)]
  40 pub struct PromoteTemps<'tcx> {
  41     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  42 }
  43
  44 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  45     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  46         // There's not really any point in promoting errorful MIR.
  47         //
  48         // This does not include MIR that failed const-checking, which we still try to promote.
  49         if body.return_ty().references_error() {
  50             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  51             return;
  52         }
  53
  54         if body.source.promoted.is_some() {
  55             return;
  56         }
  57
  58         let mut rpo = traversal::reverse_postorder(body);
  59         let ccx = ConstCx::new(tcx, body);
  60         let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  61
  62         let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates);
  63
  64         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
  65         self.promoted_fragments.set(promoted);
  66     }
  67 }
  68
  69 /// State of a temporary during collection and promotion.
  70 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  71 pub enum TempState {
  72     /// No references to this temp.
  73     Undefined,
  74     /// One direct assignment and any number of direct uses.
  75     /// A borrow of this temp is promotable if the assigned
  76     /// value is qualified as constant.
  77     Defined { location: Location, uses: usize },
  78     /// Any other combination of assignments/uses.
  79     Unpromotable,
  80     /// This temp was part of an rvalue which got extracted
  81     /// during promotion and needs cleanup.
  82     PromotedOut,
  83 }
  84
  85 impl TempState {
  86     pub fn is_promotable(&self) -> bool {
  87         debug!("is_promotable: self={:?}", self);
  88         matches!(self, TempState::Defined { .. })
  89     }
  90 }
  91
  92 /// A "root candidate" for promotion, which will become the
  93 /// returned value in a promoted MIR, unless it's a subset
  94 /// of a larger candidate.
  95 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  96 pub enum Candidate {
  97     /// Borrow of a constant temporary, candidate for lifetime extension.
  98     Ref(Location),
  99 }
 100
 101 impl Candidate {
 102     fn source_info(&self, body: &Body<'_>) -> SourceInfo {
 103         match self {
 104             Candidate::Ref(location) => *body.source_info(*location),
 105         }
 106     }
 107 }
 108
 109 struct Collector<'a, 'tcx> {
 110     ccx: &'a ConstCx<'a, 'tcx>,
 111     temps: IndexVec<Local, TempState>,
 112     candidates: Vec<Candidate>,
 113 }
 114
 115 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 116     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 117         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 118         // We're only interested in temporaries and the return place
 119         match self.ccx.body.local_kind(index) {
 120             LocalKind::Temp | LocalKind::ReturnPointer => {}
 121             LocalKind::Arg | LocalKind::Var => return,
 122         }
 123
 124         // Ignore drops, if the temp gets promoted,
 125         // then it's constant and thus drop is noop.
 126         // Non-uses are also irrelevant.
 127         if context.is_drop() || !context.is_use() {
 128             debug!(
 129                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 130                 context.is_drop(),
 131                 context.is_use(),
 132             );
 133             return;
 134         }
 135
 136         let temp = &mut self.temps[index];
 137         debug!("visit_local: temp={:?}", temp);
 138         if *temp == TempState::Undefined {
 139             match context {
 140                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 141                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 142                     *temp = TempState::Defined { location, uses: 0 };
 143                     return;
 144                 }
 145                 _ => { /* mark as unpromotable below */ }
 146             }
 147         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 148             // We always allow borrows, even mutable ones, as we need
 149             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 150             let allowed_use = match context {
 151                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 152                 | PlaceContext::NonMutatingUse(_) => true,
 153                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 154             };
 155             debug!("visit_local: allowed_use={:?}", allowed_use);
 156             if allowed_use {
 157                 *uses += 1;
 158                 return;
 159             }
 160             /* mark as unpromotable below */
 161         }
 162         *temp = TempState::Unpromotable;
 163     }
 164
 165     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 166         self.super_rvalue(rvalue, location);
 167
 168         match *rvalue {
 169             Rvalue::Ref(..) => {
 170                 self.candidates.push(Candidate::Ref(location));
 171             }
 172             _ => {}
 173         }
 174     }
 175 }
 176
 177 pub fn collect_temps_and_candidates(
 178     ccx: &ConstCx<'mir, 'tcx>,
 179     rpo: &mut ReversePostorder<'_, 'tcx>,
 180 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 181     let mut collector = Collector {
 182         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
 183         candidates: vec![],
 184         ccx,
 185     };
 186     for (bb, data) in rpo {
 187         collector.visit_basic_block_data(bb, data);
 188     }
 189     (collector.temps, collector.candidates)
 190 }
 191
 192 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 193 ///
 194 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 195 struct Validator<'a, 'tcx> {
 196     ccx: &'a ConstCx<'a, 'tcx>,
 197     temps: &'a IndexVec<Local, TempState>,
 198 }
 199
 200 impl std::ops::Deref for Validator<'a, 'tcx> {
 201     type Target = ConstCx<'a, 'tcx>;
 202
 203     fn deref(&self) -> &Self::Target {
 204         &self.ccx
 205     }
 206 }
 207
 208 struct Unpromotable;
 209
 210 impl<'tcx> Validator<'_, 'tcx> {
 211     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 212         match candidate {
 213             Candidate::Ref(loc) => {
 214                 let statement = &self.body[loc.block].statements[loc.statement_index];
 215                 match &statement.kind {
 216                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 217                         // We can only promote interior borrows of promotable temps (non-temps
 218                         // don't get promoted anyway).
 219                         self.validate_local(place.local)?;
 220
 221                         // The reference operation itself must be promotable.
 222                         // (Needs to come after `validate_local` to avoid ICEs.)
 223                         self.validate_ref(*kind, place)?;
 224
 225                         // We do not check all the projections (they do not get promoted anyway),
 226                         // but we do stay away from promoting anything involving a dereference.
 227                         if place.projection.contains(&ProjectionElem::Deref) {
 228                             return Err(Unpromotable);
 229                         }
 230
 231                         // We cannot promote things that need dropping, since the promoted value
 232                         // would not get dropped.
 233                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 234                             return Err(Unpromotable);
 235                         }
 236
 237                         Ok(())
 238                     }
 239                     _ => bug!(),
 240                 }
 241             }
 242         }
 243     }
 244
 245     // FIXME(eddyb) maybe cache this?
 246     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 247         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 248             let num_stmts = self.body[loc.block].statements.len();
 249
 250             if loc.statement_index < num_stmts {
 251                 let statement = &self.body[loc.block].statements[loc.statement_index];
 252                 match &statement.kind {
 253                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 254                         &self.ccx,
 255                         &mut |l| self.qualif_local::<Q>(l),
 256                         rhs,
 257                     ),
 258                     _ => {
 259                         span_bug!(
 260                             statement.source_info.span,
 261                             "{:?} is not an assignment",
 262                             statement
 263                         );
 264                     }
 265                 }
 266             } else {
 267                 let terminator = self.body[loc.block].terminator();
 268                 match &terminator.kind {
 269                     TerminatorKind::Call { .. } => {
 270                         let return_ty = self.body.local_decls[local].ty;
 271                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 272                     }
 273                     kind => {
 274                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 275                     }
 276                 }
 277             }
 278         } else {
 279             let span = self.body.local_decls[local].source_info.span;
 280             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 281         }
 282     }
 283
 284     // FIXME(eddyb) maybe cache this?
 285     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 286         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 287             let block = &self.body[loc.block];
 288             let num_stmts = block.statements.len();
 289
 290             if loc.statement_index < num_stmts {
 291                 let statement = &block.statements[loc.statement_index];
 292                 match &statement.kind {
 293                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 294                     _ => {
 295                         span_bug!(
 296                             statement.source_info.span,
 297                             "{:?} is not an assignment",
 298                             statement
 299                         );
 300                     }
 301                 }
 302             } else {
 303                 let terminator = block.terminator();
 304                 match &terminator.kind {
 305                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 306                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 307                     kind => {
 308                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 309                     }
 310                 }
 311             }
 312         } else {
 313             Err(Unpromotable)
 314         }
 315     }
 316
 317     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 318         match place.last_projection() {
 319             None => self.validate_local(place.local),
 320             Some((place_base, elem)) => {
 321                 // Validate topmost projection, then recurse.
 322                 match elem {
 323                     ProjectionElem::Deref => {
 324                         let mut promotable = false;
 325                         // We need to make sure this is a `Deref` of a local with no further projections.
 326                         // Discussion can be found at
 327                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 328                         if let Some(local) = place_base.as_local() {
 329                             // This is a special treatment for cases like *&STATIC where STATIC is a
 330                             // global static variable.
 331                             // This pattern is generated only when global static variables are directly
 332                             // accessed and is qualified for promotion safely.
 333                             if let TempState::Defined { location, .. } = self.temps[local] {
 334                                 let def_stmt = self.body[location.block]
 335                                     .statements
 336                                     .get(location.statement_index);
 337                                 if let Some(Statement {
 338                                     kind:
 339                                         StatementKind::Assign(box (
 340                                             _,
 341                                             Rvalue::Use(Operand::Constant(c)),
 342                                         )),
 343                                     ..
 344                                 }) = def_stmt
 345                                 {
 346                                     if let Some(did) = c.check_static_ptr(self.tcx) {
 347                                         // Evaluating a promoted may not read statics except if it got
 348                                         // promoted from a static (this is a CTFE check). So we
 349                                         // can only promote static accesses inside statics.
 350                                         if let Some(hir::ConstContext::Static(..)) = self.const_kind
 351                                         {
 352                                             if !self.tcx.is_thread_local_static(did) {
 353                                                 promotable = true;
 354                                             }
 355                                         }
 356                                     }
 357                                 }
 358                             }
 359                         }
 360                         if !promotable {
 361                             return Err(Unpromotable);
 362                         }
 363                     }
 364                     ProjectionElem::Downcast(..) => {
 365                         return Err(Unpromotable);
 366                     }
 367
 368                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 369
 370                     ProjectionElem::Index(local) => {
 371                         let mut promotable = false;
 372                         // Only accept if we can predict the index and are indexing an array.
 373                         let val =
 374                             if let TempState::Defined { location: loc, .. } = self.temps[local] {
 375                                 let block = &self.body[loc.block];
 376                                 if loc.statement_index < block.statements.len() {
 377                                     let statement = &block.statements[loc.statement_index];
 378                                     match &statement.kind {
 379                                         StatementKind::Assign(box (
 380                                             _,
 381                                             Rvalue::Use(Operand::Constant(c)),
 382                                         )) => c.literal.try_eval_usize(self.tcx, self.param_env),
 383                                         _ => None,
 384                                     }
 385                                 } else {
 386                                     None
 387                                 }
 388                             } else {
 389                                 None
 390                             };
 391                         if let Some(idx) = val {
 392                             // Determine the type of the thing we are indexing.
 393                             let ty = place_base.ty(self.body, self.tcx).ty;
 394                             match ty.kind() {
 395                                 ty::Array(_, len) => {
 396                                     // It's an array; determine its length.
 397                                     if let Some(len) = len.try_eval_usize(self.tcx, self.param_env)
 398                                     {
 399                                         // If the index is in-bounds, go ahead.
 400                                         if idx < len {
 401                                             promotable = true;
 402                                         }
 403                                     }
 404                                 }
 405                                 _ => {}
 406                             }
 407                         }
 408                         if !promotable {
 409                             return Err(Unpromotable);
 410                         }
 411
 412                         self.validate_local(local)?;
 413                     }
 414
 415                     ProjectionElem::Field(..) => {
 416                         let base_ty = place_base.ty(self.body, self.tcx).ty;
 417                         if base_ty.is_union() {
 418                             // No promotion of union field accesses.
 419                             return Err(Unpromotable);
 420                         }
 421                     }
 422                 }
 423
 424                 self.validate_place(place_base)
 425             }
 426         }
 427     }
 428
 429     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 430         match operand {
 431             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 432
 433             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 434             // `validate_rvalue` upon access.
 435             Operand::Constant(c) => {
 436                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 437                     // Only allow statics (not consts) to refer to other statics.
 438                     // FIXME(eddyb) does this matter at all for promotion?
 439                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 440                     // and in `const` this cannot occur anyway. The only concern is that we might
 441                     // promote even `let x = &STATIC` which would be useless, but this applies to
 442                     // promotion inside statics as well.
 443                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 444                     if !is_static {
 445                         return Err(Unpromotable);
 446                     }
 447
 448                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 449                     if is_thread_local {
 450                         return Err(Unpromotable);
 451                     }
 452                 }
 453
 454                 Ok(())
 455             }
 456         }
 457     }
 458
 459     fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
 460         match kind {
 461             // Reject these borrow types just to be safe.
 462             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
 463             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 464
 465             BorrowKind::Shared => {
 466                 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
 467                 if has_mut_interior {
 468                     return Err(Unpromotable);
 469                 }
 470             }
 471
 472             BorrowKind::Mut { .. } => {
 473                 let ty = place.ty(self.body, self.tcx).ty;
 474
 475                 // In theory, any zero-sized value could be borrowed
 476                 // mutably without consequences. However, only &mut []
 477                 // is allowed right now.
 478                 if let ty::Array(_, len) = ty.kind() {
 479                     match len.try_eval_usize(self.tcx, self.param_env) {
 480                         Some(0) => {}
 481                         _ => return Err(Unpromotable),
 482                     }
 483                 } else {
 484                     return Err(Unpromotable);
 485                 }
 486             }
 487         }
 488
 489         Ok(())
 490     }
 491
 492     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 493         match rvalue {
 494             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
 495                 self.validate_operand(operand)?;
 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             Rvalue::Cast(kind, operand, cast_ty) => {
 505                 if matches!(kind, CastKind::Misc) {
 506                     let operand_ty = operand.ty(self.body, self.tcx);
 507                     let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 508                     let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 509                     if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 510                         // ptr-to-int casts are not possible in consts and thus not promotable
 511                         return Err(Unpromotable);
 512                     }
 513                     // int-to-ptr casts are fine, they just use the integer value at pointer type.
 514                 }
 515
 516                 self.validate_operand(operand)?;
 517             }
 518
 519             Rvalue::NullaryOp(op, _) => match op {
 520                 NullOp::Box => return Err(Unpromotable),
 521                 NullOp::SizeOf => {}
 522                 NullOp::AlignOf => {}
 523             },
 524
 525             Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable),
 526
 527             Rvalue::UnaryOp(op, operand) => {
 528                 match op {
 529                     // These operations can never fail.
 530                     UnOp::Neg | UnOp::Not => {}
 531                 }
 532
 533                 self.validate_operand(operand)?;
 534             }
 535
 536             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 537                 let op = *op;
 538                 let lhs_ty = lhs.ty(self.body, self.tcx);
 539
 540                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 541                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 542                     assert!(matches!(
 543                         op,
 544                         BinOp::Eq
 545                             | BinOp::Ne
 546                             | BinOp::Le
 547                             | BinOp::Lt
 548                             | BinOp::Ge
 549                             | BinOp::Gt
 550                             | BinOp::Offset
 551                     ));
 552                     return Err(Unpromotable);
 553                 }
 554
 555                 match op {
 556                     BinOp::Div | BinOp::Rem => {
 557                         if lhs_ty.is_integral() {
 558                             // Integer division: the RHS must be a non-zero const.
 559                             let const_val = match rhs {
 560                                 Operand::Constant(c) => {
 561                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 562                                 }
 563                                 _ => None,
 564                             };
 565                             match const_val {
 566                                 Some(x) if x != 0 => {}        // okay
 567                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 568                             }
 569                         }
 570                     }
 571                     // The remaining operations can never fail.
 572                     BinOp::Eq
 573                     | BinOp::Ne
 574                     | BinOp::Le
 575                     | BinOp::Lt
 576                     | BinOp::Ge
 577                     | BinOp::Gt
 578                     | BinOp::Offset
 579                     | BinOp::Add
 580                     | BinOp::Sub
 581                     | BinOp::Mul
 582                     | BinOp::BitXor
 583                     | BinOp::BitAnd
 584                     | BinOp::BitOr
 585                     | BinOp::Shl
 586                     | BinOp::Shr => {}
 587                 }
 588
 589                 self.validate_operand(lhs)?;
 590                 self.validate_operand(rhs)?;
 591             }
 592
 593             Rvalue::AddressOf(_, place) => {
 594                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 595                 // no problem, only using it is.
 596                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 597                 {
 598                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 599                     if let ty::Ref(..) = base_ty.kind() {
 600                         return self.validate_place(place_base);
 601                     }
 602                 }
 603                 return Err(Unpromotable);
 604             }
 605
 606             Rvalue::Ref(_, kind, place) => {
 607                 // Special-case reborrows to be more like a copy of the reference.
 608                 let mut place_simplified = place.as_ref();
 609                 if let Some((place_base, ProjectionElem::Deref)) =
 610                     place_simplified.last_projection()
 611                 {
 612                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 613                     if let ty::Ref(..) = base_ty.kind() {
 614                         place_simplified = place_base;
 615                     }
 616                 }
 617
 618                 self.validate_place(place_simplified)?;
 619
 620                 // Check that the reference is fine (using the original place!).
 621                 // (Needs to come after `validate_place` to avoid ICEs.)
 622                 self.validate_ref(*kind, place)?;
 623             }
 624
 625             Rvalue::Aggregate(_, operands) => {
 626                 for o in operands {
 627                     self.validate_operand(o)?;
 628                 }
 629             }
 630         }
 631
 632         Ok(())
 633     }
 634
 635     fn validate_call(
 636         &self,
 637         callee: &Operand<'tcx>,
 638         args: &[Operand<'tcx>],
 639     ) -> Result<(), Unpromotable> {
 640         let fn_ty = callee.ty(self.body, self.tcx);
 641
 642         // Inside const/static items, we promote all (eligible) function calls.
 643         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 644         let promote_all_const_fn = matches!(
 645             self.const_kind,
 646             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 647         );
 648         if !promote_all_const_fn {
 649             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 650                 // Never promote runtime `const fn` calls of
 651                 // functions without `#[rustc_promotable]`.
 652                 if !self.tcx.is_promotable_const_fn(def_id) {
 653                     return Err(Unpromotable);
 654                 }
 655             }
 656         }
 657
 658         let is_const_fn = match *fn_ty.kind() {
 659             ty::FnDef(def_id, _) => {
 660                 self.tcx.is_const_fn_raw(def_id) || is_lang_special_const_fn(self.tcx, def_id)
 661             }
 662             _ => false,
 663         };
 664         if !is_const_fn {
 665             return Err(Unpromotable);
 666         }
 667
 668         self.validate_operand(callee)?;
 669         for arg in args {
 670             self.validate_operand(arg)?;
 671         }
 672
 673         Ok(())
 674     }
 675 }
 676
 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 };
 684
 685     candidates
 686         .iter()
 687         .copied()
 688         .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
 689         .collect()
 690 }
 691
 692 struct Promoter<'a, 'tcx> {
 693     tcx: TyCtxt<'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>)>,
 698
 699     /// If true, all nested temps are also kept in the
 700     /// source MIR, not moved to the promoted MIR.
 701     keep_original: bool,
 702 }
 703
 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 {
 708             statements: vec![],
 709             terminator: Some(Terminator {
 710                 source_info: SourceInfo::outermost(span),
 711                 kind: TerminatorKind::Return,
 712             }),
 713             is_cleanup: false,
 714         })
 715     }
 716
 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::new((Place::from(dest), rvalue))),
 723         });
 724     }
 725
 726     fn is_temp_kind(&self, local: Local) -> bool {
 727         self.source.local_kind(local) == LocalKind::Temp
 728     }
 729
 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 => {
 736                 if uses > 1 {
 737                     self.keep_original = true;
 738                 }
 739                 location
 740             }
 741             state => {
 742                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 743             }
 744         };
 745         if !self.keep_original {
 746             self.temps[temp] = TempState::PromotedOut;
 747         }
 748
 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,
 753         ));
 754
 755         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 756
 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,
 764                     _ => {
 765                         span_bug!(
 766                             statement.source_info.span,
 767                             "{:?} is not an assignment",
 768                             statement
 769                         );
 770                     }
 771                 };
 772
 773                 (
 774                     if self.keep_original {
 775                         rhs.clone()
 776                     } else {
 777                         let unit = Rvalue::Use(Operand::Constant(Box::new(Constant {
 778                             span: statement.source_info.span,
 779                             user_ty: None,
 780                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 781                         })));
 782                         mem::replace(rhs, unit)
 783                     },
 784                     statement.source_info,
 785                 )
 786             };
 787
 788             self.visit_rvalue(&mut rvalue, loc);
 789             self.assign(new_temp, rvalue, source_info.span);
 790         } else {
 791             let terminator = if self.keep_original {
 792                 self.source[loc.block].terminator().clone()
 793             } else {
 794                 let terminator = self.source[loc.block].terminator_mut();
 795                 let target = match terminator.kind {
 796                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 797                     ref kind => {
 798                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 799                     }
 800                 };
 801                 Terminator {
 802                     source_info: terminator.source_info,
 803                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 804                 }
 805             };
 806
 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);
 812                     }
 813
 814                     let last = self.promoted.basic_blocks().last().unwrap();
 815                     let new_target = self.new_block();
 816
 817                     *self.promoted[last].terminator_mut() = Terminator {
 818                         kind: TerminatorKind::Call {
 819                             func,
 820                             args,
 821                             cleanup: None,
 822                             destination: Some((Place::from(new_temp), new_target)),
 823                             from_hir_call,
 824                             fn_span,
 825                         },
 826                         source_info: SourceInfo::outermost(terminator.source_info.span),
 827                         ..terminator
 828                     };
 829                 }
 830                 ref kind => {
 831                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 832                 }
 833             };
 834         };
 835
 836         self.keep_original = old_keep_original;
 837         new_temp
 838     }
 839
 840     fn promote_candidate(
 841         mut self,
 842         candidate: Candidate,
 843         next_promoted_id: usize,
 844     ) -> Option<Body<'tcx>> {
 845         let def = self.source.source.with_opt_param();
 846         let mut rvalue = {
 847             let promoted = &mut self.promoted;
 848             let promoted_id = Promoted::new(next_promoted_id);
 849             let tcx = self.tcx;
 850             let mut promoted_operand = |ty, span| {
 851                 promoted.span = span;
 852                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 853
 854                 Operand::Constant(Box::new(Constant {
 855                     span,
 856                     user_ty: None,
 857                     literal: tcx
 858                         .mk_const(ty::Const {
 859                             ty,
 860                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 861                                 def,
 862                                 substs_: Some(InternalSubsts::for_item(
 863                                     tcx,
 864                                     def.did,
 865                                     |param, _| {
 866                                         if let ty::GenericParamDefKind::Lifetime = param.kind {
 867                                             tcx.lifetimes.re_erased.into()
 868                                         } else {
 869                                             tcx.mk_param_from_def(param)
 870                                         }
 871                                     },
 872                                 )),
 873                                 promoted: Some(promoted_id),
 874                             }),
 875                         })
 876                         .into(),
 877                 }))
 878             };
 879             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 880             match candidate {
 881                 Candidate::Ref(loc) => {
 882                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
 883                     match statement.kind {
 884                         StatementKind::Assign(box (
 885                             _,
 886                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 887                         )) => {
 888                             // Use the underlying local for this (necessarily interior) borrow.
 889                             let ty = local_decls.local_decls()[place.local].ty;
 890                             let span = statement.source_info.span;
 891
 892                             let ref_ty = tcx.mk_ref(
 893                                 tcx.lifetimes.re_erased,
 894                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 895                             );
 896
 897                             *region = tcx.lifetimes.re_erased;
 898
 899                             let mut projection = vec![PlaceElem::Deref];
 900                             projection.extend(place.projection);
 901                             place.projection = tcx.intern_place_elems(&projection);
 902
 903                             // Create a temp to hold the promoted reference.
 904                             // This is because `*r` requires `r` to be a local,
 905                             // otherwise we would use the `promoted` directly.
 906                             let mut promoted_ref = LocalDecl::new(ref_ty, span);
 907                             promoted_ref.source_info = statement.source_info;
 908                             let promoted_ref = local_decls.push(promoted_ref);
 909                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 910
 911                             let promoted_ref_statement = Statement {
 912                                 source_info: statement.source_info,
 913                                 kind: StatementKind::Assign(Box::new((
 914                                     Place::from(promoted_ref),
 915                                     Rvalue::Use(promoted_operand(ref_ty, span)),
 916                                 ))),
 917                             };
 918                             self.extra_statements.push((loc, promoted_ref_statement));
 919
 920                             Rvalue::Ref(
 921                                 tcx.lifetimes.re_erased,
 922                                 borrow_kind,
 923                                 Place {
 924                                     local: mem::replace(&mut place.local, promoted_ref),
 925                                     projection: List::empty(),
 926                                 },
 927                             )
 928                         }
 929                         _ => bug!(),
 930                     }
 931                 }
 932             }
 933         };
 934
 935         assert_eq!(self.new_block(), START_BLOCK);
 936         self.visit_rvalue(
 937             &mut rvalue,
 938             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 939         );
 940
 941         let span = self.promoted.span;
 942         self.assign(RETURN_PLACE, rvalue, span);
 943         Some(self.promoted)
 944     }
 945 }
 946
 947 /// Replaces all temporaries with their promoted counterparts.
 948 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 949     fn tcx(&self) -> TyCtxt<'tcx> {
 950         self.tcx
 951     }
 952
 953     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 954         if self.is_temp_kind(*local) {
 955             *local = self.promote_temp(*local);
 956         }
 957     }
 958 }
 959
 960 pub fn promote_candidates<'tcx>(
 961     body: &mut Body<'tcx>,
 962     tcx: TyCtxt<'tcx>,
 963     mut temps: IndexVec<Local, TempState>,
 964     candidates: Vec<Candidate>,
 965 ) -> IndexVec<Promoted, Body<'tcx>> {
 966     // Visit candidates in reverse, in case they're nested.
 967     debug!("promote_candidates({:?})", candidates);
 968
 969     let mut promotions = IndexVec::new();
 970
 971     let mut extra_statements = vec![];
 972     for candidate in candidates.into_iter().rev() {
 973         match candidate {
 974             Candidate::Ref(Location { block, statement_index }) => {
 975                 if let StatementKind::Assign(box (place, _)) =
 976                     &body[block].statements[statement_index].kind
 977                 {
 978                     if let Some(local) = place.as_local() {
 979                         if temps[local] == TempState::PromotedOut {
 980                             // Already promoted.
 981                             continue;
 982                         }
 983                     }
 984                 }
 985             }
 986         }
 987
 988         // Declare return place local so that `mir::Body::new` doesn't complain.
 989         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 990
 991         let mut scope = body.source_scopes[candidate.source_info(body).scope].clone();
 992         scope.parent_scope = None;
 993
 994         let promoted = Body::new(
 995             tcx,
 996             body.source, // `promoted` gets filled in below
 997             IndexVec::new(),
 998             IndexVec::from_elem_n(scope, 1),
 999             initial_locals,
1000             IndexVec::new(),
1001             0,
1002             vec![],
1003             body.span,
1004             body.generator_kind(),
1005         );
1006
1007         let promoter = Promoter {
1008             promoted,
1009             tcx,
1010             source: body,
1011             temps: &mut temps,
1012             extra_statements: &mut extra_statements,
1013             keep_original: false,
1014         };
1015
1016         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1017         if let Some(mut promoted) = promoter.promote_candidate(candidate, promotions.len()) {
1018             promoted.source.promoted = Some(promotions.next_index());
1019             promotions.push(promoted);
1020         }
1021     }
1022
1023     // Insert each of `extra_statements` before its indicated location, which
1024     // has to be done in reverse location order, to not invalidate the rest.
1025     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1026     for (loc, statement) in extra_statements {
1027         body[loc.block].statements.insert(loc.statement_index, statement);
1028     }
1029
1030     // Eliminate assignments to, and drops of promoted temps.
1031     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1032     for block in body.basic_blocks_mut() {
1033         block.statements.retain(|statement| match &statement.kind {
1034             StatementKind::Assign(box (place, _)) => {
1035                 if let Some(index) = place.as_local() {
1036                     !promoted(index)
1037                 } else {
1038                     true
1039                 }
1040             }
1041             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1042                 !promoted(*index)
1043             }
1044             _ => true,
1045         });
1046         let terminator = block.terminator_mut();
1047         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1048             if let Some(index) = place.as_local() {
1049                 if promoted(index) {
1050                     terminator.kind = TerminatorKind::Goto { target: *target };
1051                 }
1052             }
1053         }
1054     }
1055
1056     promotions
1057 }
1058
1059 /// This function returns `true` if the function being called in the array
1060 /// repeat expression is a `const` function.
1061 pub fn is_const_fn_in_array_repeat_expression<'tcx>(
1062     ccx: &ConstCx<'_, 'tcx>,
1063     place: &Place<'tcx>,
1064     body: &Body<'tcx>,
1065 ) -> bool {
1066     match place.as_local() {
1067         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1068         Some(local) if body.local_decls[local].is_user_variable() => return false,
1069         None => return false,
1070         _ => {}
1071     }
1072
1073     for block in body.basic_blocks() {
1074         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1075             &block.terminator
1076         {
1077             if let Operand::Constant(box Constant { literal, .. }) = func {
1078                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1079                     if let Some((destination_place, _)) = destination {
1080                         if destination_place == place {
1081                             if ccx.tcx.is_const_fn(def_id) {
1082                                 return true;
1083                             }
1084                         }
1085                     }
1086                 }
1087             }
1088         }
1089     }
1090
1091     false
1092 }