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::ReversePostorderIter;
  17 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  18 use rustc_middle::mir::*;
  19 use rustc_middle::ty::subst::InternalSubsts;
  20 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
  21 use rustc_span::Span;
  22
  23 use rustc_index::vec::{Idx, IndexVec};
  24
  25 use std::cell::Cell;
  26 use std::{cmp, iter, mem};
  27
  28 use crate::transform::check_consts::{qualifs, ConstCx};
  29
  30 /// A `MirPass` for promotion.
  31 ///
  32 /// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
  33 /// `'static` lifetime.
  34 ///
  35 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  36 /// newly created `Constant`.
  37 #[derive(Default)]
  38 pub struct PromoteTemps<'tcx> {
  39     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  40 }
  41
  42 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  43     fn phase_change(&self) -> Option<MirPhase> {
  44         Some(MirPhase::ConstsPromoted)
  45     }
  46
  47     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  48         // There's not really any point in promoting errorful MIR.
  49         //
  50         // This does not include MIR that failed const-checking, which we still try to promote.
  51         if body.return_ty().references_error() {
  52             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  53             return;
  54         }
  55
  56         if body.source.promoted.is_some() {
  57             return;
  58         }
  59
  60         let mut rpo = traversal::reverse_postorder(body);
  61         let ccx = ConstCx::new(tcx, body);
  62         let (mut temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  63
  64         let promotable_candidates = validate_candidates(&ccx, &mut temps, &all_candidates);
  65
  66         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
  67         self.promoted_fragments.set(promoted);
  68     }
  69 }
  70
  71 /// State of a temporary during collection and promotion.
  72 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  73 pub enum TempState {
  74     /// No references to this temp.
  75     Undefined,
  76     /// One direct assignment and any number of direct uses.
  77     /// A borrow of this temp is promotable if the assigned
  78     /// value is qualified as constant.
  79     Defined { location: Location, uses: usize, valid: Result<(), ()> },
  80     /// Any other combination of assignments/uses.
  81     Unpromotable,
  82     /// This temp was part of an rvalue which got extracted
  83     /// during promotion and needs cleanup.
  84     PromotedOut,
  85 }
  86
  87 impl TempState {
  88     pub fn is_promotable(&self) -> bool {
  89         debug!("is_promotable: self={:?}", self);
  90         matches!(self, TempState::Defined { .. })
  91     }
  92 }
  93
  94 /// A "root candidate" for promotion, which will become the
  95 /// returned value in a promoted MIR, unless it's a subset
  96 /// of a larger candidate.
  97 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  98 pub struct Candidate {
  99     location: Location,
 100 }
 101
 102 struct Collector<'a, 'tcx> {
 103     ccx: &'a ConstCx<'a, 'tcx>,
 104     temps: IndexVec<Local, TempState>,
 105     candidates: Vec<Candidate>,
 106 }
 107
 108 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 109     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 110         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 111         // We're only interested in temporaries and the return place
 112         match self.ccx.body.local_kind(index) {
 113             LocalKind::Temp | LocalKind::ReturnPointer => {}
 114             LocalKind::Arg | LocalKind::Var => return,
 115         }
 116
 117         // Ignore drops, if the temp gets promoted,
 118         // then it's constant and thus drop is noop.
 119         // Non-uses are also irrelevant.
 120         if context.is_drop() || !context.is_use() {
 121             debug!(
 122                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 123                 context.is_drop(),
 124                 context.is_use(),
 125             );
 126             return;
 127         }
 128
 129         let temp = &mut self.temps[index];
 130         debug!("visit_local: temp={:?}", temp);
 131         if *temp == TempState::Undefined {
 132             match context {
 133                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 134                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 135                     *temp = TempState::Defined { location, uses: 0, valid: Err(()) };
 136                     return;
 137                 }
 138                 _ => { /* mark as unpromotable below */ }
 139             }
 140         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 141             // We always allow borrows, even mutable ones, as we need
 142             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 143             let allowed_use = match context {
 144                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 145                 | PlaceContext::NonMutatingUse(_) => true,
 146                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 147             };
 148             debug!("visit_local: allowed_use={:?}", allowed_use);
 149             if allowed_use {
 150                 *uses += 1;
 151                 return;
 152             }
 153             /* mark as unpromotable below */
 154         }
 155         *temp = TempState::Unpromotable;
 156     }
 157
 158     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 159         self.super_rvalue(rvalue, location);
 160
 161         match *rvalue {
 162             Rvalue::Ref(..) => {
 163                 self.candidates.push(Candidate { location });
 164             }
 165             _ => {}
 166         }
 167     }
 168 }
 169
 170 pub fn collect_temps_and_candidates<'tcx>(
 171     ccx: &ConstCx<'_, 'tcx>,
 172     rpo: &mut ReversePostorderIter<'_, 'tcx>,
 173 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 174     let mut collector = Collector {
 175         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
 176         candidates: vec![],
 177         ccx,
 178     };
 179     for (bb, data) in rpo {
 180         collector.visit_basic_block_data(bb, data);
 181     }
 182     (collector.temps, collector.candidates)
 183 }
 184
 185 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 186 ///
 187 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 188 struct Validator<'a, 'tcx> {
 189     ccx: &'a ConstCx<'a, 'tcx>,
 190     temps: &'a mut IndexVec<Local, TempState>,
 191 }
 192
 193 impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> {
 194     type Target = ConstCx<'a, 'tcx>;
 195
 196     fn deref(&self) -> &Self::Target {
 197         &self.ccx
 198     }
 199 }
 200
 201 struct Unpromotable;
 202
 203 impl<'tcx> Validator<'_, 'tcx> {
 204     fn validate_candidate(&mut self, candidate: Candidate) -> Result<(), Unpromotable> {
 205         let loc = candidate.location;
 206         let statement = &self.body[loc.block].statements[loc.statement_index];
 207         match &statement.kind {
 208             StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 209                 // We can only promote interior borrows of promotable temps (non-temps
 210                 // don't get promoted anyway).
 211                 self.validate_local(place.local)?;
 212
 213                 // The reference operation itself must be promotable.
 214                 // (Needs to come after `validate_local` to avoid ICEs.)
 215                 self.validate_ref(*kind, place)?;
 216
 217                 // We do not check all the projections (they do not get promoted anyway),
 218                 // but we do stay away from promoting anything involving a dereference.
 219                 if place.projection.contains(&ProjectionElem::Deref) {
 220                     return Err(Unpromotable);
 221                 }
 222
 223                 // We cannot promote things that need dropping, since the promoted value
 224                 // would not get dropped.
 225                 if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 226                     return Err(Unpromotable);
 227                 }
 228
 229                 Ok(())
 230             }
 231             _ => bug!(),
 232         }
 233     }
 234
 235     // FIXME(eddyb) maybe cache this?
 236     fn qualif_local<Q: qualifs::Qualif>(&mut self, local: Local) -> bool {
 237         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 238             let num_stmts = self.body[loc.block].statements.len();
 239
 240             if loc.statement_index < num_stmts {
 241                 let statement = &self.body[loc.block].statements[loc.statement_index];
 242                 match &statement.kind {
 243                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 244                         &self.ccx,
 245                         &mut |l| self.qualif_local::<Q>(l),
 246                         rhs,
 247                     ),
 248                     _ => {
 249                         span_bug!(
 250                             statement.source_info.span,
 251                             "{:?} is not an assignment",
 252                             statement
 253                         );
 254                     }
 255                 }
 256             } else {
 257                 let terminator = self.body[loc.block].terminator();
 258                 match &terminator.kind {
 259                     TerminatorKind::Call { .. } => {
 260                         let return_ty = self.body.local_decls[local].ty;
 261                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 262                     }
 263                     kind => {
 264                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 265                     }
 266                 }
 267             }
 268         } else {
 269             let span = self.body.local_decls[local].source_info.span;
 270             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 271         }
 272     }
 273
 274     fn validate_local(&mut self, local: Local) -> Result<(), Unpromotable> {
 275         if let TempState::Defined { location: loc, uses, valid } = self.temps[local] {
 276             valid.or_else(|_| {
 277                 let ok = {
 278                     let block = &self.body[loc.block];
 279                     let num_stmts = block.statements.len();
 280
 281                     if loc.statement_index < num_stmts {
 282                         let statement = &block.statements[loc.statement_index];
 283                         match &statement.kind {
 284                             StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 285                             _ => {
 286                                 span_bug!(
 287                                     statement.source_info.span,
 288                                     "{:?} is not an assignment",
 289                                     statement
 290                                 );
 291                             }
 292                         }
 293                     } else {
 294                         let terminator = block.terminator();
 295                         match &terminator.kind {
 296                             TerminatorKind::Call { func, args, .. } => {
 297                                 self.validate_call(func, args)
 298                             }
 299                             TerminatorKind::Yield { .. } => Err(Unpromotable),
 300                             kind => {
 301                                 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 302                             }
 303                         }
 304                     }
 305                 };
 306                 self.temps[local] = match ok {
 307                     Ok(()) => TempState::Defined { location: loc, uses, valid: Ok(()) },
 308                     Err(_) => TempState::Unpromotable,
 309                 };
 310                 ok
 311             })
 312         } else {
 313             Err(Unpromotable)
 314         }
 315     }
 316
 317     fn validate_place(&mut self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 318         match place.last_projection() {
 319             None => self.validate_local(place.local),
 320             Some((place_base, elem)) => {
 321                 // Validate topmost projection, then recurse.
 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(&mut 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(&mut 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(&mut 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             // ptr-to-int casts are not possible in consts and thus not promotable
 505             Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => return Err(Unpromotable),
 506
 507             // all ohter 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: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 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
 839                 Operand::Constant(Box::new(Constant {
 840                     span,
 841                     user_ty: None,
 842                     literal: tcx
 843                         .mk_const(ty::ConstS {
 844                             ty,
 845                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 846                                 def,
 847                                 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
 848                                     if let ty::GenericParamDefKind::Lifetime = param.kind {
 849                                         tcx.lifetimes.re_erased.into()
 850                                     } else {
 851                                         tcx.mk_param_from_def(param)
 852                                     }
 853                                 }),
 854                                 promoted: Some(promoted_id),
 855                             }),
 856                         })
 857                         .into(),
 858                 }))
 859             };
 860             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 861             let loc = candidate.location;
 862             let statement = &mut blocks[loc.block].statements[loc.statement_index];
 863             match statement.kind {
 864                 StatementKind::Assign(box (
 865                     _,
 866                     Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 867                 )) => {
 868                     // Use the underlying local for this (necessarily interior) borrow.
 869                     let ty = local_decls.local_decls()[place.local].ty;
 870                     let span = statement.source_info.span;
 871
 872                     let ref_ty = tcx.mk_ref(
 873                         tcx.lifetimes.re_erased,
 874                         ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 875                     );
 876
 877                     *region = tcx.lifetimes.re_erased;
 878
 879                     let mut projection = vec![PlaceElem::Deref];
 880                     projection.extend(place.projection);
 881                     place.projection = tcx.intern_place_elems(&projection);
 882
 883                     // Create a temp to hold the promoted reference.
 884                     // This is because `*r` requires `r` to be a local,
 885                     // otherwise we would use the `promoted` directly.
 886                     let mut promoted_ref = LocalDecl::new(ref_ty, span);
 887                     promoted_ref.source_info = statement.source_info;
 888                     let promoted_ref = local_decls.push(promoted_ref);
 889                     assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 890
 891                     let promoted_ref_statement = Statement {
 892                         source_info: statement.source_info,
 893                         kind: StatementKind::Assign(Box::new((
 894                             Place::from(promoted_ref),
 895                             Rvalue::Use(promoted_operand(ref_ty, span)),
 896                         ))),
 897                     };
 898                     self.extra_statements.push((loc, promoted_ref_statement));
 899
 900                     Rvalue::Ref(
 901                         tcx.lifetimes.re_erased,
 902                         borrow_kind,
 903                         Place {
 904                             local: mem::replace(&mut place.local, promoted_ref),
 905                             projection: List::empty(),
 906                         },
 907                     )
 908                 }
 909                 _ => bug!(),
 910             }
 911         };
 912
 913         assert_eq!(self.new_block(), START_BLOCK);
 914         self.visit_rvalue(
 915             &mut rvalue,
 916             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 917         );
 918
 919         let span = self.promoted.span;
 920         self.assign(RETURN_PLACE, rvalue, span);
 921         self.promoted
 922     }
 923 }
 924
 925 /// Replaces all temporaries with their promoted counterparts.
 926 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 927     fn tcx(&self) -> TyCtxt<'tcx> {
 928         self.tcx
 929     }
 930
 931     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 932         if self.is_temp_kind(*local) {
 933             *local = self.promote_temp(*local);
 934         }
 935     }
 936 }
 937
 938 pub fn promote_candidates<'tcx>(
 939     body: &mut Body<'tcx>,
 940     tcx: TyCtxt<'tcx>,
 941     mut temps: IndexVec<Local, TempState>,
 942     candidates: Vec<Candidate>,
 943 ) -> IndexVec<Promoted, Body<'tcx>> {
 944     // Visit candidates in reverse, in case they're nested.
 945     debug!("promote_candidates({:?})", candidates);
 946
 947     let mut promotions = IndexVec::new();
 948
 949     let mut extra_statements = vec![];
 950     for candidate in candidates.into_iter().rev() {
 951         let Location { block, statement_index } = candidate.location;
 952         if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
 953         {
 954             if let Some(local) = place.as_local() {
 955                 if temps[local] == TempState::PromotedOut {
 956                     // Already promoted.
 957                     continue;
 958                 }
 959             }
 960         }
 961
 962         // Declare return place local so that `mir::Body::new` doesn't complain.
 963         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 964
 965         let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
 966         scope.parent_scope = None;
 967
 968         let promoted = Body::new(
 969             body.source, // `promoted` gets filled in below
 970             IndexVec::new(),
 971             IndexVec::from_elem_n(scope, 1),
 972             initial_locals,
 973             IndexVec::new(),
 974             0,
 975             vec![],
 976             body.span,
 977             body.generator_kind(),
 978             body.tainted_by_errors,
 979         );
 980
 981         let promoter = Promoter {
 982             promoted,
 983             tcx,
 984             source: body,
 985             temps: &mut temps,
 986             extra_statements: &mut extra_statements,
 987             keep_original: false,
 988         };
 989
 990         let mut promoted = promoter.promote_candidate(candidate, promotions.len());
 991         promoted.source.promoted = Some(promotions.next_index());
 992         promotions.push(promoted);
 993     }
 994
 995     // Insert each of `extra_statements` before its indicated location, which
 996     // has to be done in reverse location order, to not invalidate the rest.
 997     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
 998     for (loc, statement) in extra_statements {
 999         body[loc.block].statements.insert(loc.statement_index, statement);
1000     }
1001
1002     // Eliminate assignments to, and drops of promoted temps.
1003     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1004     for block in body.basic_blocks_mut() {
1005         block.statements.retain(|statement| match &statement.kind {
1006             StatementKind::Assign(box (place, _)) => {
1007                 if let Some(index) = place.as_local() {
1008                     !promoted(index)
1009                 } else {
1010                     true
1011                 }
1012             }
1013             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1014                 !promoted(*index)
1015             }
1016             _ => true,
1017         });
1018         let terminator = block.terminator_mut();
1019         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1020             if let Some(index) = place.as_local() {
1021                 if promoted(index) {
1022                     terminator.kind = TerminatorKind::Goto { target: *target };
1023                 }
1024             }
1025         }
1026     }
1027
1028     promotions
1029 }
1030
1031 /// This function returns `true` if the function being called in the array
1032 /// repeat expression is a `const` function.
1033 pub fn is_const_fn_in_array_repeat_expression<'tcx>(
1034     ccx: &ConstCx<'_, 'tcx>,
1035     place: &Place<'tcx>,
1036     body: &Body<'tcx>,
1037 ) -> bool {
1038     match place.as_local() {
1039         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1040         Some(local) if body.local_decls[local].is_user_variable() => return false,
1041         None => return false,
1042         _ => {}
1043     }
1044
1045     for block in body.basic_blocks() {
1046         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1047             &block.terminator
1048         {
1049             if let Operand::Constant(box Constant { literal, .. }) = func {
1050                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1051                     if destination == place {
1052                         if ccx.tcx.is_const_fn(def_id) {
1053                             return true;
1054                         }
1055                     }
1056                 }
1057             }
1058         }
1059     }
1060
1061     false
1062 }