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, TypeVisitable};
  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             Rvalue::CopyForDeref(place) => {
 498                 let op = &Operand::Copy(*place);
 499                 self.validate_operand(op)?
 500             }
 501
 502             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 503                 self.validate_place(place.as_ref())?
 504             }
 505
 506             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 507
 508             // ptr-to-int casts are not possible in consts and thus not promotable
 509             Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => return Err(Unpromotable),
 510
 511             // all other casts including int-to-ptr casts are fine, they just use the integer value
 512             // at pointer type.
 513             Rvalue::Cast(_, operand, _) => {
 514                 self.validate_operand(operand)?;
 515             }
 516
 517             Rvalue::NullaryOp(op, _) => match op {
 518                 NullOp::SizeOf => {}
 519                 NullOp::AlignOf => {}
 520             },
 521
 522             Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable),
 523
 524             Rvalue::UnaryOp(op, operand) => {
 525                 match op {
 526                     // These operations can never fail.
 527                     UnOp::Neg | UnOp::Not => {}
 528                 }
 529
 530                 self.validate_operand(operand)?;
 531             }
 532
 533             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 534                 let op = *op;
 535                 let lhs_ty = lhs.ty(self.body, self.tcx);
 536
 537                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 538                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 539                     assert!(matches!(
 540                         op,
 541                         BinOp::Eq
 542                             | BinOp::Ne
 543                             | BinOp::Le
 544                             | BinOp::Lt
 545                             | BinOp::Ge
 546                             | BinOp::Gt
 547                             | BinOp::Offset
 548                     ));
 549                     return Err(Unpromotable);
 550                 }
 551
 552                 match op {
 553                     BinOp::Div | BinOp::Rem => {
 554                         if lhs_ty.is_integral() {
 555                             // Integer division: the RHS must be a non-zero const.
 556                             let const_val = match rhs {
 557                                 Operand::Constant(c) => {
 558                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 559                                 }
 560                                 _ => None,
 561                             };
 562                             match const_val {
 563                                 Some(x) if x != 0 => {}        // okay
 564                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 565                             }
 566                         }
 567                     }
 568                     // The remaining operations can never fail.
 569                     BinOp::Eq
 570                     | BinOp::Ne
 571                     | BinOp::Le
 572                     | BinOp::Lt
 573                     | BinOp::Ge
 574                     | BinOp::Gt
 575                     | BinOp::Offset
 576                     | BinOp::Add
 577                     | BinOp::Sub
 578                     | BinOp::Mul
 579                     | BinOp::BitXor
 580                     | BinOp::BitAnd
 581                     | BinOp::BitOr
 582                     | BinOp::Shl
 583                     | BinOp::Shr => {}
 584                 }
 585
 586                 self.validate_operand(lhs)?;
 587                 self.validate_operand(rhs)?;
 588             }
 589
 590             Rvalue::AddressOf(_, place) => {
 591                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 592                 // no problem, only using it is.
 593                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 594                 {
 595                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 596                     if let ty::Ref(..) = base_ty.kind() {
 597                         return self.validate_place(place_base);
 598                     }
 599                 }
 600                 return Err(Unpromotable);
 601             }
 602
 603             Rvalue::Ref(_, kind, place) => {
 604                 // Special-case reborrows to be more like a copy of the reference.
 605                 let mut place_simplified = place.as_ref();
 606                 if let Some((place_base, ProjectionElem::Deref)) =
 607                     place_simplified.last_projection()
 608                 {
 609                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 610                     if let ty::Ref(..) = base_ty.kind() {
 611                         place_simplified = place_base;
 612                     }
 613                 }
 614
 615                 self.validate_place(place_simplified)?;
 616
 617                 // Check that the reference is fine (using the original place!).
 618                 // (Needs to come after `validate_place` to avoid ICEs.)
 619                 self.validate_ref(*kind, place)?;
 620             }
 621
 622             Rvalue::Aggregate(_, operands) => {
 623                 for o in operands {
 624                     self.validate_operand(o)?;
 625                 }
 626             }
 627         }
 628
 629         Ok(())
 630     }
 631
 632     fn validate_call(
 633         &mut self,
 634         callee: &Operand<'tcx>,
 635         args: &[Operand<'tcx>],
 636     ) -> Result<(), Unpromotable> {
 637         let fn_ty = callee.ty(self.body, self.tcx);
 638
 639         // Inside const/static items, we promote all (eligible) function calls.
 640         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 641         let promote_all_const_fn = matches!(
 642             self.const_kind,
 643             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 644         );
 645         if !promote_all_const_fn {
 646             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 647                 // Never promote runtime `const fn` calls of
 648                 // functions without `#[rustc_promotable]`.
 649                 if !self.tcx.is_promotable_const_fn(def_id) {
 650                     return Err(Unpromotable);
 651                 }
 652             }
 653         }
 654
 655         let is_const_fn = match *fn_ty.kind() {
 656             ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
 657             _ => false,
 658         };
 659         if !is_const_fn {
 660             return Err(Unpromotable);
 661         }
 662
 663         self.validate_operand(callee)?;
 664         for arg in args {
 665             self.validate_operand(arg)?;
 666         }
 667
 668         Ok(())
 669     }
 670 }
 671
 672 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 673 pub fn validate_candidates(
 674     ccx: &ConstCx<'_, '_>,
 675     temps: &mut IndexVec<Local, TempState>,
 676     candidates: &[Candidate],
 677 ) -> Vec<Candidate> {
 678     let mut validator = Validator { ccx, temps };
 679
 680     candidates
 681         .iter()
 682         .copied()
 683         .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
 684         .collect()
 685 }
 686
 687 struct Promoter<'a, 'tcx> {
 688     tcx: TyCtxt<'tcx>,
 689     source: &'a mut Body<'tcx>,
 690     promoted: Body<'tcx>,
 691     temps: &'a mut IndexVec<Local, TempState>,
 692     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 693
 694     /// If true, all nested temps are also kept in the
 695     /// source MIR, not moved to the promoted MIR.
 696     keep_original: bool,
 697 }
 698
 699 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 700     fn new_block(&mut self) -> BasicBlock {
 701         let span = self.promoted.span;
 702         self.promoted.basic_blocks_mut().push(BasicBlockData {
 703             statements: vec![],
 704             terminator: Some(Terminator {
 705                 source_info: SourceInfo::outermost(span),
 706                 kind: TerminatorKind::Return,
 707             }),
 708             is_cleanup: false,
 709         })
 710     }
 711
 712     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 713         let last = self.promoted.basic_blocks().last().unwrap();
 714         let data = &mut self.promoted[last];
 715         data.statements.push(Statement {
 716             source_info: SourceInfo::outermost(span),
 717             kind: StatementKind::Assign(Box::new((Place::from(dest), rvalue))),
 718         });
 719     }
 720
 721     fn is_temp_kind(&self, local: Local) -> bool {
 722         self.source.local_kind(local) == LocalKind::Temp
 723     }
 724
 725     /// Copies the initialization of this temp to the
 726     /// promoted MIR, recursing through temps.
 727     fn promote_temp(&mut self, temp: Local) -> Local {
 728         let old_keep_original = self.keep_original;
 729         let loc = match self.temps[temp] {
 730             TempState::Defined { location, uses, .. } if uses > 0 => {
 731                 if uses > 1 {
 732                     self.keep_original = true;
 733                 }
 734                 location
 735             }
 736             state => {
 737                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 738             }
 739         };
 740         if !self.keep_original {
 741             self.temps[temp] = TempState::PromotedOut;
 742         }
 743
 744         let num_stmts = self.source[loc.block].statements.len();
 745         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 746             self.source.local_decls[temp].ty,
 747             self.source.local_decls[temp].source_info.span,
 748         ));
 749
 750         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 751
 752         // First, take the Rvalue or Call out of the source MIR,
 753         // or duplicate it, depending on keep_original.
 754         if loc.statement_index < num_stmts {
 755             let (mut rvalue, source_info) = {
 756                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 757                 let StatementKind::Assign(box (_, ref mut rhs)) = statement.kind else {
 758                     span_bug!(
 759                         statement.source_info.span,
 760                         "{:?} is not an assignment",
 761                         statement
 762                     );
 763                 };
 764
 765                 (
 766                     if self.keep_original {
 767                         rhs.clone()
 768                     } else {
 769                         let unit = Rvalue::Use(Operand::Constant(Box::new(Constant {
 770                             span: statement.source_info.span,
 771                             user_ty: None,
 772                             literal: ConstantKind::zero_sized(self.tcx.types.unit),
 773                         })));
 774                         mem::replace(rhs, unit)
 775                     },
 776                     statement.source_info,
 777                 )
 778             };
 779
 780             self.visit_rvalue(&mut rvalue, loc);
 781             self.assign(new_temp, rvalue, source_info.span);
 782         } else {
 783             let terminator = if self.keep_original {
 784                 self.source[loc.block].terminator().clone()
 785             } else {
 786                 let terminator = self.source[loc.block].terminator_mut();
 787                 let target = match terminator.kind {
 788                     TerminatorKind::Call { target: Some(target), .. } => target,
 789                     ref kind => {
 790                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 791                     }
 792                 };
 793                 Terminator {
 794                     source_info: terminator.source_info,
 795                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 796                 }
 797             };
 798
 799             match terminator.kind {
 800                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 801                     self.visit_operand(&mut func, loc);
 802                     for arg in &mut args {
 803                         self.visit_operand(arg, loc);
 804                     }
 805
 806                     let last = self.promoted.basic_blocks().last().unwrap();
 807                     let new_target = self.new_block();
 808
 809                     *self.promoted[last].terminator_mut() = Terminator {
 810                         kind: TerminatorKind::Call {
 811                             func,
 812                             args,
 813                             cleanup: None,
 814                             destination: Place::from(new_temp),
 815                             target: Some(new_target),
 816                             from_hir_call,
 817                             fn_span,
 818                         },
 819                         source_info: SourceInfo::outermost(terminator.source_info.span),
 820                         ..terminator
 821                     };
 822                 }
 823                 ref kind => {
 824                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 825                 }
 826             };
 827         };
 828
 829         self.keep_original = old_keep_original;
 830         new_temp
 831     }
 832
 833     fn promote_candidate(mut self, candidate: Candidate, next_promoted_id: usize) -> Body<'tcx> {
 834         let def = self.source.source.with_opt_param();
 835         let mut rvalue = {
 836             let promoted = &mut self.promoted;
 837             let promoted_id = Promoted::new(next_promoted_id);
 838             let tcx = self.tcx;
 839             let mut promoted_operand = |ty, span| {
 840                 promoted.span = span;
 841                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 842                 let _const = tcx.mk_const(ty::ConstS {
 843                     ty,
 844                     kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
 845                         def,
 846                         substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
 847                             if let ty::GenericParamDefKind::Lifetime = param.kind {
 848                                 tcx.lifetimes.re_erased.into()
 849                             } else {
 850                                 tcx.mk_param_from_def(param)
 851                             }
 852                         }),
 853                         promoted: Some(promoted_id),
 854                     }),
 855                 });
 856
 857                 Operand::Constant(Box::new(Constant {
 858                     span,
 859                     user_ty: None,
 860                     literal: ConstantKind::from_const(_const, tcx),
 861                 }))
 862             };
 863             let blocks = self.source.basic_blocks.as_mut();
 864             let local_decls = &mut self.source.local_decls;
 865             let loc = candidate.location;
 866             let statement = &mut blocks[loc.block].statements[loc.statement_index];
 867             match statement.kind {
 868                 StatementKind::Assign(box (
 869                     _,
 870                     Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 871                 )) => {
 872                     // Use the underlying local for this (necessarily interior) borrow.
 873                     let ty = local_decls[place.local].ty;
 874                     let span = statement.source_info.span;
 875
 876                     let ref_ty = tcx.mk_ref(
 877                         tcx.lifetimes.re_erased,
 878                         ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 879                     );
 880
 881                     *region = tcx.lifetimes.re_erased;
 882
 883                     let mut projection = vec![PlaceElem::Deref];
 884                     projection.extend(place.projection);
 885                     place.projection = tcx.intern_place_elems(&projection);
 886
 887                     // Create a temp to hold the promoted reference.
 888                     // This is because `*r` requires `r` to be a local,
 889                     // otherwise we would use the `promoted` directly.
 890                     let mut promoted_ref = LocalDecl::new(ref_ty, span);
 891                     promoted_ref.source_info = statement.source_info;
 892                     let promoted_ref = local_decls.push(promoted_ref);
 893                     assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 894
 895                     let promoted_ref_statement = Statement {
 896                         source_info: statement.source_info,
 897                         kind: StatementKind::Assign(Box::new((
 898                             Place::from(promoted_ref),
 899                             Rvalue::Use(promoted_operand(ref_ty, span)),
 900                         ))),
 901                     };
 902                     self.extra_statements.push((loc, promoted_ref_statement));
 903
 904                     Rvalue::Ref(
 905                         tcx.lifetimes.re_erased,
 906                         borrow_kind,
 907                         Place {
 908                             local: mem::replace(&mut place.local, promoted_ref),
 909                             projection: List::empty(),
 910                         },
 911                     )
 912                 }
 913                 _ => bug!(),
 914             }
 915         };
 916
 917         assert_eq!(self.new_block(), START_BLOCK);
 918         self.visit_rvalue(
 919             &mut rvalue,
 920             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 921         );
 922
 923         let span = self.promoted.span;
 924         self.assign(RETURN_PLACE, rvalue, span);
 925         self.promoted
 926     }
 927 }
 928
 929 /// Replaces all temporaries with their promoted counterparts.
 930 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 931     fn tcx(&self) -> TyCtxt<'tcx> {
 932         self.tcx
 933     }
 934
 935     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 936         if self.is_temp_kind(*local) {
 937             *local = self.promote_temp(*local);
 938         }
 939     }
 940 }
 941
 942 pub fn promote_candidates<'tcx>(
 943     body: &mut Body<'tcx>,
 944     tcx: TyCtxt<'tcx>,
 945     mut temps: IndexVec<Local, TempState>,
 946     candidates: Vec<Candidate>,
 947 ) -> IndexVec<Promoted, Body<'tcx>> {
 948     // Visit candidates in reverse, in case they're nested.
 949     debug!("promote_candidates({:?})", candidates);
 950
 951     let mut promotions = IndexVec::new();
 952
 953     let mut extra_statements = vec![];
 954     for candidate in candidates.into_iter().rev() {
 955         let Location { block, statement_index } = candidate.location;
 956         if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
 957         {
 958             if let Some(local) = place.as_local() {
 959                 if temps[local] == TempState::PromotedOut {
 960                     // Already promoted.
 961                     continue;
 962                 }
 963             }
 964         }
 965
 966         // Declare return place local so that `mir::Body::new` doesn't complain.
 967         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 968
 969         let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
 970         scope.parent_scope = None;
 971
 972         let promoted = Body::new(
 973             body.source, // `promoted` gets filled in below
 974             IndexVec::new(),
 975             IndexVec::from_elem_n(scope, 1),
 976             initial_locals,
 977             IndexVec::new(),
 978             0,
 979             vec![],
 980             body.span,
 981             body.generator_kind(),
 982             body.tainted_by_errors,
 983         );
 984
 985         let promoter = Promoter {
 986             promoted,
 987             tcx,
 988             source: body,
 989             temps: &mut temps,
 990             extra_statements: &mut extra_statements,
 991             keep_original: false,
 992         };
 993
 994         let mut promoted = promoter.promote_candidate(candidate, promotions.len());
 995         promoted.source.promoted = Some(promotions.next_index());
 996         promotions.push(promoted);
 997     }
 998
 999     // Insert each of `extra_statements` before its indicated location, which
1000     // has to be done in reverse location order, to not invalidate the rest.
1001     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1002     for (loc, statement) in extra_statements {
1003         body[loc.block].statements.insert(loc.statement_index, statement);
1004     }
1005
1006     // Eliminate assignments to, and drops of promoted temps.
1007     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1008     for block in body.basic_blocks_mut() {
1009         block.statements.retain(|statement| match &statement.kind {
1010             StatementKind::Assign(box (place, _)) => {
1011                 if let Some(index) = place.as_local() {
1012                     !promoted(index)
1013                 } else {
1014                     true
1015                 }
1016             }
1017             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1018                 !promoted(*index)
1019             }
1020             _ => true,
1021         });
1022         let terminator = block.terminator_mut();
1023         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1024             if let Some(index) = place.as_local() {
1025                 if promoted(index) {
1026                     terminator.kind = TerminatorKind::Goto { target: *target };
1027                 }
1028             }
1029         }
1030     }
1031
1032     promotions
1033 }
1034
1035 /// This function returns `true` if the function being called in the array
1036 /// repeat expression is a `const` function.
1037 pub fn is_const_fn_in_array_repeat_expression<'tcx>(
1038     ccx: &ConstCx<'_, 'tcx>,
1039     place: &Place<'tcx>,
1040     body: &Body<'tcx>,
1041 ) -> bool {
1042     match place.as_local() {
1043         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1044         Some(local) if body.local_decls[local].is_user_variable() => return false,
1045         None => return false,
1046         _ => {}
1047     }
1048
1049     for block in body.basic_blocks() {
1050         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1051             &block.terminator
1052         {
1053             if let Operand::Constant(box Constant { literal, .. }) = func {
1054                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1055                     if destination == place {
1056                         if ccx.tcx.is_const_fn(def_id) {
1057                             return true;
1058                         }
1059                     }
1060                 }
1061             }
1062         }
1063     }
1064
1065     false
1066 }