compiler/rustc_mir/src/transform/promote_consts.rs

   1 //! A pass that promotes borrows of constant rvalues.
   2 //!
   3 //! The rvalues considered constant are trees of temps,
   4 //! each with exactly one initialization, and holding
   5 //! a constant value with no interior mutability.
   6 //! They are placed into a new MIR constant body in
   7 //! `promoted` and the borrow rvalue is replaced with
   8 //! a `Literal::Promoted` using the index into `promoted`
   9 //! of that constant MIR.
  10 //!
  11 //! This pass assumes that every use is dominated by an
  12 //! initialization and can otherwise silence errors, if
  13 //! move analysis runs after promotion on broken MIR.
  14
  15 use rustc_hir as hir;
  16 use rustc_middle::mir::traversal::ReversePostorder;
  17 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  18 use rustc_middle::mir::*;
  19 use rustc_middle::ty::cast::CastTy;
  20 use rustc_middle::ty::subst::InternalSubsts;
  21 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
  22 use rustc_span::Span;
  23
  24 use rustc_index::vec::{Idx, IndexVec};
  25
  26 use std::cell::Cell;
  27 use std::{cmp, iter, mem};
  28
  29 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
  30 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstCx};
  31 use crate::transform::MirPass;
  32
  33 /// A `MirPass` for promotion.
  34 ///
  35 /// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
  36 /// `'static` lifetime.
  37 ///
  38 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  39 /// newly created `Constant`.
  40 #[derive(Default)]
  41 pub struct PromoteTemps<'tcx> {
  42     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  43 }
  44
  45 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  46     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  47         // There's not really any point in promoting errorful MIR.
  48         //
  49         // This does not include MIR that failed const-checking, which we still try to promote.
  50         if body.return_ty().references_error() {
  51             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  52             return;
  53         }
  54
  55         if body.source.promoted.is_some() {
  56             return;
  57         }
  58
  59         let mut rpo = traversal::reverse_postorder(body);
  60         let ccx = ConstCx::new(tcx, body);
  61         let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  62
  63         let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates);
  64
  65         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
  66         self.promoted_fragments.set(promoted);
  67     }
  68 }
  69
  70 /// State of a temporary during collection and promotion.
  71 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  72 pub enum TempState {
  73     /// No references to this temp.
  74     Undefined,
  75     /// One direct assignment and any number of direct uses.
  76     /// A borrow of this temp is promotable if the assigned
  77     /// value is qualified as constant.
  78     Defined { location: Location, uses: usize },
  79     /// Any other combination of assignments/uses.
  80     Unpromotable,
  81     /// This temp was part of an rvalue which got extracted
  82     /// during promotion and needs cleanup.
  83     PromotedOut,
  84 }
  85
  86 impl TempState {
  87     pub fn is_promotable(&self) -> bool {
  88         debug!("is_promotable: self={:?}", self);
  89         matches!(self, TempState::Defined { .. })
  90     }
  91 }
  92
  93 /// A "root candidate" for promotion, which will become the
  94 /// returned value in a promoted MIR, unless it's a subset
  95 /// of a larger candidate.
  96 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  97 pub enum Candidate {
  98     /// Borrow of a constant temporary, candidate for lifetime extension.
  99     Ref(Location),
 100 }
 101
 102 impl Candidate {
 103     fn source_info(&self, body: &Body<'_>) -> SourceInfo {
 104         match self {
 105             Candidate::Ref(location) => *body.source_info(*location),
 106         }
 107     }
 108 }
 109
 110 struct Collector<'a, 'tcx> {
 111     ccx: &'a ConstCx<'a, 'tcx>,
 112     temps: IndexVec<Local, TempState>,
 113     candidates: Vec<Candidate>,
 114 }
 115
 116 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 117     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 118         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 119         // We're only interested in temporaries and the return place
 120         match self.ccx.body.local_kind(index) {
 121             LocalKind::Temp | LocalKind::ReturnPointer => {}
 122             LocalKind::Arg | LocalKind::Var => return,
 123         }
 124
 125         // Ignore drops, if the temp gets promoted,
 126         // then it's constant and thus drop is noop.
 127         // Non-uses are also irrelevant.
 128         if context.is_drop() || !context.is_use() {
 129             debug!(
 130                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 131                 context.is_drop(),
 132                 context.is_use(),
 133             );
 134             return;
 135         }
 136
 137         let temp = &mut self.temps[index];
 138         debug!("visit_local: temp={:?}", temp);
 139         if *temp == TempState::Undefined {
 140             match context {
 141                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 142                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 143                     *temp = TempState::Defined { location, uses: 0 };
 144                     return;
 145                 }
 146                 _ => { /* mark as unpromotable below */ }
 147             }
 148         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 149             // We always allow borrows, even mutable ones, as we need
 150             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 151             let allowed_use = match context {
 152                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 153                 | PlaceContext::NonMutatingUse(_) => true,
 154                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 155             };
 156             debug!("visit_local: allowed_use={:?}", allowed_use);
 157             if allowed_use {
 158                 *uses += 1;
 159                 return;
 160             }
 161             /* mark as unpromotable below */
 162         }
 163         *temp = TempState::Unpromotable;
 164     }
 165
 166     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 167         self.super_rvalue(rvalue, location);
 168
 169         match *rvalue {
 170             Rvalue::Ref(..) => {
 171                 self.candidates.push(Candidate::Ref(location));
 172             }
 173             _ => {}
 174         }
 175     }
 176 }
 177
 178 pub fn collect_temps_and_candidates(
 179     ccx: &ConstCx<'mir, 'tcx>,
 180     rpo: &mut ReversePostorder<'_, 'tcx>,
 181 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 182     let mut collector = Collector {
 183         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
 184         candidates: vec![],
 185         ccx,
 186     };
 187     for (bb, data) in rpo {
 188         collector.visit_basic_block_data(bb, data);
 189     }
 190     (collector.temps, collector.candidates)
 191 }
 192
 193 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 194 ///
 195 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 196 struct Validator<'a, 'tcx> {
 197     ccx: &'a ConstCx<'a, 'tcx>,
 198     temps: &'a IndexVec<Local, TempState>,
 199 }
 200
 201 impl std::ops::Deref for Validator<'a, 'tcx> {
 202     type Target = ConstCx<'a, 'tcx>;
 203
 204     fn deref(&self) -> &Self::Target {
 205         &self.ccx
 206     }
 207 }
 208
 209 struct Unpromotable;
 210
 211 impl<'tcx> Validator<'_, 'tcx> {
 212     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 213         match candidate {
 214             Candidate::Ref(loc) => {
 215                 let statement = &self.body[loc.block].statements[loc.statement_index];
 216                 match &statement.kind {
 217                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 218                         // We can only promote interior borrows of promotable temps (non-temps
 219                         // don't get promoted anyway).
 220                         self.validate_local(place.local)?;
 221
 222                         // The reference operation itself must be promotable.
 223                         // (Needs to come after `validate_local` to avoid ICEs.)
 224                         self.validate_ref(*kind, place)?;
 225
 226                         // We do not check all the projections (they do not get promoted anyway),
 227                         // but we do stay away from promoting anything involving a dereference.
 228                         if place.projection.contains(&ProjectionElem::Deref) {
 229                             return Err(Unpromotable);
 230                         }
 231
 232                         // We cannot promote things that need dropping, since the promoted value
 233                         // would not get dropped.
 234                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 235                             return Err(Unpromotable);
 236                         }
 237
 238                         Ok(())
 239                     }
 240                     _ => bug!(),
 241                 }
 242             }
 243         }
 244     }
 245
 246     // FIXME(eddyb) maybe cache this?
 247     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 248         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 249             let num_stmts = self.body[loc.block].statements.len();
 250
 251             if loc.statement_index < num_stmts {
 252                 let statement = &self.body[loc.block].statements[loc.statement_index];
 253                 match &statement.kind {
 254                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 255                         &self.ccx,
 256                         &mut |l| self.qualif_local::<Q>(l),
 257                         rhs,
 258                     ),
 259                     _ => {
 260                         span_bug!(
 261                             statement.source_info.span,
 262                             "{:?} is not an assignment",
 263                             statement
 264                         );
 265                     }
 266                 }
 267             } else {
 268                 let terminator = self.body[loc.block].terminator();
 269                 match &terminator.kind {
 270                     TerminatorKind::Call { .. } => {
 271                         let return_ty = self.body.local_decls[local].ty;
 272                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 273                     }
 274                     kind => {
 275                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 276                     }
 277                 }
 278             }
 279         } else {
 280             let span = self.body.local_decls[local].source_info.span;
 281             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 282         }
 283     }
 284
 285     // FIXME(eddyb) maybe cache this?
 286     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 287         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 288             let block = &self.body[loc.block];
 289             let num_stmts = block.statements.len();
 290
 291             if loc.statement_index < num_stmts {
 292                 let statement = &block.statements[loc.statement_index];
 293                 match &statement.kind {
 294                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 295                     _ => {
 296                         span_bug!(
 297                             statement.source_info.span,
 298                             "{:?} is not an assignment",
 299                             statement
 300                         );
 301                     }
 302                 }
 303             } else {
 304                 let terminator = block.terminator();
 305                 match &terminator.kind {
 306                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 307                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 308                     kind => {
 309                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 310                     }
 311                 }
 312             }
 313         } else {
 314             Err(Unpromotable)
 315         }
 316     }
 317
 318     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 319         match place.last_projection() {
 320             None => self.validate_local(place.local),
 321             Some((place_base, elem)) => {
 322                 // Validate topmost projection, then recurse.
 323                 match elem {
 324                     ProjectionElem::Deref => {
 325                         let mut promotable = false;
 326                         // We need to make sure this is a `Deref` of a local with no further projections.
 327                         // Discussion can be found at
 328                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 329                         if let Some(local) = place_base.as_local() {
 330                             // This is a special treatment for cases like *&STATIC where STATIC is a
 331                             // global static variable.
 332                             // This pattern is generated only when global static variables are directly
 333                             // accessed and is qualified for promotion safely.
 334                             if let TempState::Defined { location, .. } = self.temps[local] {
 335                                 let def_stmt = self.body[location.block]
 336                                     .statements
 337                                     .get(location.statement_index);
 338                                 if let Some(Statement {
 339                                     kind:
 340                                         StatementKind::Assign(box (
 341                                             _,
 342                                             Rvalue::Use(Operand::Constant(c)),
 343                                         )),
 344                                     ..
 345                                 }) = def_stmt
 346                                 {
 347                                     if let Some(did) = c.check_static_ptr(self.tcx) {
 348                                         // Evaluating a promoted may not read statics except if it got
 349                                         // promoted from a static (this is a CTFE check). So we
 350                                         // can only promote static accesses inside statics.
 351                                         if let Some(hir::ConstContext::Static(..)) = self.const_kind
 352                                         {
 353                                             if !self.tcx.is_thread_local_static(did) {
 354                                                 promotable = true;
 355                                             }
 356                                         }
 357                                     }
 358                                 }
 359                             }
 360                         }
 361                         if !promotable {
 362                             return Err(Unpromotable);
 363                         }
 364                     }
 365                     ProjectionElem::Downcast(..) => {
 366                         return Err(Unpromotable);
 367                     }
 368
 369                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 370
 371                     ProjectionElem::Index(local) => {
 372                         let mut promotable = false;
 373                         // Only accept if we can predict the index and are indexing an array.
 374                         let val =
 375                             if let TempState::Defined { location: loc, .. } = self.temps[local] {
 376                                 let block = &self.body[loc.block];
 377                                 if loc.statement_index < block.statements.len() {
 378                                     let statement = &block.statements[loc.statement_index];
 379                                     match &statement.kind {
 380                                         StatementKind::Assign(box (
 381                                             _,
 382                                             Rvalue::Use(Operand::Constant(c)),
 383                                         )) => c.literal.try_eval_usize(self.tcx, self.param_env),
 384                                         _ => None,
 385                                     }
 386                                 } else {
 387                                     None
 388                                 }
 389                             } else {
 390                                 None
 391                             };
 392                         if let Some(idx) = val {
 393                             // Determine the type of the thing we are indexing.
 394                             let ty = place_base.ty(self.body, self.tcx).ty;
 395                             match ty.kind() {
 396                                 ty::Array(_, len) => {
 397                                     // It's an array; determine its length.
 398                                     if let Some(len) = len.try_eval_usize(self.tcx, self.param_env)
 399                                     {
 400                                         // If the index is in-bounds, go ahead.
 401                                         if idx < len {
 402                                             promotable = true;
 403                                         }
 404                                     }
 405                                 }
 406                                 _ => {}
 407                             }
 408                         }
 409                         if !promotable {
 410                             return Err(Unpromotable);
 411                         }
 412
 413                         self.validate_local(local)?;
 414                     }
 415
 416                     ProjectionElem::Field(..) => {
 417                         let base_ty = place_base.ty(self.body, self.tcx).ty;
 418                         if base_ty.is_union() {
 419                             // No promotion of union field accesses.
 420                             return Err(Unpromotable);
 421                         }
 422                     }
 423                 }
 424
 425                 self.validate_place(place_base)
 426             }
 427         }
 428     }
 429
 430     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 431         match operand {
 432             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 433
 434             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 435             // `validate_rvalue` upon access.
 436             Operand::Constant(c) => {
 437                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 438                     // Only allow statics (not consts) to refer to other statics.
 439                     // FIXME(eddyb) does this matter at all for promotion?
 440                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 441                     // and in `const` this cannot occur anyway. The only concern is that we might
 442                     // promote even `let x = &STATIC` which would be useless, but this applies to
 443                     // promotion inside statics as well.
 444                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 445                     if !is_static {
 446                         return Err(Unpromotable);
 447                     }
 448
 449                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 450                     if is_thread_local {
 451                         return Err(Unpromotable);
 452                     }
 453                 }
 454
 455                 Ok(())
 456             }
 457         }
 458     }
 459
 460     fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
 461         match kind {
 462             // Reject these borrow types just to be safe.
 463             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
 464             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 465
 466             BorrowKind::Shared => {
 467                 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
 468                 if has_mut_interior {
 469                     return Err(Unpromotable);
 470                 }
 471             }
 472
 473             BorrowKind::Mut { .. } => {
 474                 let ty = place.ty(self.body, self.tcx).ty;
 475
 476                 // In theory, any zero-sized value could be borrowed
 477                 // mutably without consequences. However, only &mut []
 478                 // is allowed right now.
 479                 if let ty::Array(_, len) = ty.kind() {
 480                     match len.try_eval_usize(self.tcx, self.param_env) {
 481                         Some(0) => {}
 482                         _ => return Err(Unpromotable),
 483                     }
 484                 } else {
 485                     return Err(Unpromotable);
 486                 }
 487             }
 488         }
 489
 490         Ok(())
 491     }
 492
 493     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 494         match rvalue {
 495             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
 496                 self.validate_operand(operand)?;
 497             }
 498
 499             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 500                 self.validate_place(place.as_ref())?
 501             }
 502
 503             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 504
 505             Rvalue::Cast(kind, operand, cast_ty) => {
 506                 if matches!(kind, CastKind::Misc) {
 507                     let operand_ty = operand.ty(self.body, self.tcx);
 508                     let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 509                     let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 510                     if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 511                         // ptr-to-int casts are not possible in consts and thus not promotable
 512                         return Err(Unpromotable);
 513                     }
 514                     // int-to-ptr casts are fine, they just use the integer value at pointer type.
 515                 }
 516
 517                 self.validate_operand(operand)?;
 518             }
 519
 520             Rvalue::NullaryOp(op, _) => match op {
 521                 NullOp::Box => return Err(Unpromotable),
 522                 NullOp::SizeOf => {}
 523             },
 524
 525             Rvalue::UnaryOp(op, operand) => {
 526                 match op {
 527                     // These operations can never fail.
 528                     UnOp::Neg | UnOp::Not => {}
 529                 }
 530
 531                 self.validate_operand(operand)?;
 532             }
 533
 534             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 535                 let op = *op;
 536                 let lhs_ty = lhs.ty(self.body, self.tcx);
 537
 538                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 539                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 540                     assert!(matches!(
 541                         op,
 542                         BinOp::Eq
 543                             | BinOp::Ne
 544                             | BinOp::Le
 545                             | BinOp::Lt
 546                             | BinOp::Ge
 547                             | BinOp::Gt
 548                             | BinOp::Offset
 549                     ));
 550                     return Err(Unpromotable);
 551                 }
 552
 553                 match op {
 554                     BinOp::Div | BinOp::Rem => {
 555                         if lhs_ty.is_integral() {
 556                             // Integer division: the RHS must be a non-zero const.
 557                             let const_val = match rhs {
 558                                 Operand::Constant(c) => {
 559                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 560                                 }
 561                                 _ => None,
 562                             };
 563                             match const_val {
 564                                 Some(x) if x != 0 => {}        // okay
 565                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 566                             }
 567                         }
 568                     }
 569                     // The remaining operations can never fail.
 570                     BinOp::Eq
 571                     | BinOp::Ne
 572                     | BinOp::Le
 573                     | BinOp::Lt
 574                     | BinOp::Ge
 575                     | BinOp::Gt
 576                     | BinOp::Offset
 577                     | BinOp::Add
 578                     | BinOp::Sub
 579                     | BinOp::Mul
 580                     | BinOp::BitXor
 581                     | BinOp::BitAnd
 582                     | BinOp::BitOr
 583                     | BinOp::Shl
 584                     | BinOp::Shr => {}
 585                 }
 586
 587                 self.validate_operand(lhs)?;
 588                 self.validate_operand(rhs)?;
 589             }
 590
 591             Rvalue::AddressOf(_, place) => {
 592                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 593                 // no problem, only using it is.
 594                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 595                 {
 596                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 597                     if let ty::Ref(..) = base_ty.kind() {
 598                         return self.validate_place(place_base);
 599                     }
 600                 }
 601                 return Err(Unpromotable);
 602             }
 603
 604             Rvalue::Ref(_, kind, place) => {
 605                 // Special-case reborrows to be more like a copy of the reference.
 606                 let mut place_simplified = place.as_ref();
 607                 if let Some((place_base, ProjectionElem::Deref)) =
 608                     place_simplified.last_projection()
 609                 {
 610                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 611                     if let ty::Ref(..) = base_ty.kind() {
 612                         place_simplified = place_base;
 613                     }
 614                 }
 615
 616                 self.validate_place(place_simplified)?;
 617
 618                 // Check that the reference is fine (using the original place!).
 619                 // (Needs to come after `validate_place` to avoid ICEs.)
 620                 self.validate_ref(*kind, place)?;
 621             }
 622
 623             Rvalue::Aggregate(_, operands) => {
 624                 for o in operands {
 625                     self.validate_operand(o)?;
 626                 }
 627             }
 628         }
 629
 630         Ok(())
 631     }
 632
 633     fn validate_call(
 634         &self,
 635         callee: &Operand<'tcx>,
 636         args: &[Operand<'tcx>],
 637     ) -> Result<(), Unpromotable> {
 638         let fn_ty = callee.ty(self.body, self.tcx);
 639
 640         // Inside const/static items, we promote all (eligible) function calls.
 641         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 642         let promote_all_const_fn = matches!(
 643             self.const_kind,
 644             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 645         );
 646         if !promote_all_const_fn {
 647             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 648                 // Never promote runtime `const fn` calls of
 649                 // functions without `#[rustc_promotable]`.
 650                 if !self.tcx.is_promotable_const_fn(def_id) {
 651                     return Err(Unpromotable);
 652                 }
 653             }
 654         }
 655
 656         let is_const_fn = match *fn_ty.kind() {
 657             ty::FnDef(def_id, _) => {
 658                 is_const_fn(self.tcx, def_id)
 659                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 660                     || is_lang_panic_fn(self.tcx, def_id)
 661             }
 662             _ => false,
 663         };
 664         if !is_const_fn {
 665             return Err(Unpromotable);
 666         }
 667
 668         self.validate_operand(callee)?;
 669         for arg in args {
 670             self.validate_operand(arg)?;
 671         }
 672
 673         Ok(())
 674     }
 675 }
 676
 677 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 678 pub fn validate_candidates(
 679     ccx: &ConstCx<'_, '_>,
 680     temps: &IndexVec<Local, TempState>,
 681     candidates: &[Candidate],
 682 ) -> Vec<Candidate> {
 683     let validator = Validator { ccx, temps };
 684
 685     candidates
 686         .iter()
 687         .copied()
 688         .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
 689         .collect()
 690 }
 691
 692 struct Promoter<'a, 'tcx> {
 693     tcx: TyCtxt<'tcx>,
 694     source: &'a mut Body<'tcx>,
 695     promoted: Body<'tcx>,
 696     temps: &'a mut IndexVec<Local, TempState>,
 697     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 698
 699     /// If true, all nested temps are also kept in the
 700     /// source MIR, not moved to the promoted MIR.
 701     keep_original: bool,
 702 }
 703
 704 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 705     fn new_block(&mut self) -> BasicBlock {
 706         let span = self.promoted.span;
 707         self.promoted.basic_blocks_mut().push(BasicBlockData {
 708             statements: vec![],
 709             terminator: Some(Terminator {
 710                 source_info: SourceInfo::outermost(span),
 711                 kind: TerminatorKind::Return,
 712             }),
 713             is_cleanup: false,
 714         })
 715     }
 716
 717     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 718         let last = self.promoted.basic_blocks().last().unwrap();
 719         let data = &mut self.promoted[last];
 720         data.statements.push(Statement {
 721             source_info: SourceInfo::outermost(span),
 722             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 723         });
 724     }
 725
 726     fn is_temp_kind(&self, local: Local) -> bool {
 727         self.source.local_kind(local) == LocalKind::Temp
 728     }
 729
 730     /// Copies the initialization of this temp to the
 731     /// promoted MIR, recursing through temps.
 732     fn promote_temp(&mut self, temp: Local) -> Local {
 733         let old_keep_original = self.keep_original;
 734         let loc = match self.temps[temp] {
 735             TempState::Defined { location, uses } if uses > 0 => {
 736                 if uses > 1 {
 737                     self.keep_original = true;
 738                 }
 739                 location
 740             }
 741             state => {
 742                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 743             }
 744         };
 745         if !self.keep_original {
 746             self.temps[temp] = TempState::PromotedOut;
 747         }
 748
 749         let num_stmts = self.source[loc.block].statements.len();
 750         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 751             self.source.local_decls[temp].ty,
 752             self.source.local_decls[temp].source_info.span,
 753         ));
 754
 755         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 756
 757         // First, take the Rvalue or Call out of the source MIR,
 758         // or duplicate it, depending on keep_original.
 759         if loc.statement_index < num_stmts {
 760             let (mut rvalue, source_info) = {
 761                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 762                 let rhs = match statement.kind {
 763                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 764                     _ => {
 765                         span_bug!(
 766                             statement.source_info.span,
 767                             "{:?} is not an assignment",
 768                             statement
 769                         );
 770                     }
 771                 };
 772
 773                 (
 774                     if self.keep_original {
 775                         rhs.clone()
 776                     } else {
 777                         let unit = Rvalue::Use(Operand::Constant(box Constant {
 778                             span: statement.source_info.span,
 779                             user_ty: None,
 780                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 781                         }));
 782                         mem::replace(rhs, unit)
 783                     },
 784                     statement.source_info,
 785                 )
 786             };
 787
 788             self.visit_rvalue(&mut rvalue, loc);
 789             self.assign(new_temp, rvalue, source_info.span);
 790         } else {
 791             let terminator = if self.keep_original {
 792                 self.source[loc.block].terminator().clone()
 793             } else {
 794                 let terminator = self.source[loc.block].terminator_mut();
 795                 let target = match terminator.kind {
 796                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 797                     ref kind => {
 798                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 799                     }
 800                 };
 801                 Terminator {
 802                     source_info: terminator.source_info,
 803                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 804                 }
 805             };
 806
 807             match terminator.kind {
 808                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 809                     self.visit_operand(&mut func, loc);
 810                     for arg in &mut args {
 811                         self.visit_operand(arg, loc);
 812                     }
 813
 814                     let last = self.promoted.basic_blocks().last().unwrap();
 815                     let new_target = self.new_block();
 816
 817                     *self.promoted[last].terminator_mut() = Terminator {
 818                         kind: TerminatorKind::Call {
 819                             func,
 820                             args,
 821                             cleanup: None,
 822                             destination: Some((Place::from(new_temp), new_target)),
 823                             from_hir_call,
 824                             fn_span,
 825                         },
 826                         source_info: SourceInfo::outermost(terminator.source_info.span),
 827                         ..terminator
 828                     };
 829                 }
 830                 ref kind => {
 831                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 832                 }
 833             };
 834         };
 835
 836         self.keep_original = old_keep_original;
 837         new_temp
 838     }
 839
 840     fn promote_candidate(
 841         mut self,
 842         candidate: Candidate,
 843         next_promoted_id: usize,
 844     ) -> Option<Body<'tcx>> {
 845         let def = self.source.source.with_opt_param();
 846         let mut rvalue = {
 847             let promoted = &mut self.promoted;
 848             let promoted_id = Promoted::new(next_promoted_id);
 849             let tcx = self.tcx;
 850             let mut promoted_operand = |ty, span| {
 851                 promoted.span = span;
 852                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 853
 854                 Operand::Constant(Box::new(Constant {
 855                     span,
 856                     user_ty: None,
 857                     literal: tcx
 858                         .mk_const(ty::Const {
 859                             ty,
 860                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 861                                 def,
 862                                 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
 863                                     if let ty::GenericParamDefKind::Lifetime = param.kind {
 864                                         tcx.lifetimes.re_erased.into()
 865                                     } else {
 866                                         tcx.mk_param_from_def(param)
 867                                     }
 868                                 }),
 869                                 promoted: Some(promoted_id),
 870                             }),
 871                         })
 872                         .into(),
 873                 }))
 874             };
 875             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 876             match candidate {
 877                 Candidate::Ref(loc) => {
 878                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
 879                     match statement.kind {
 880                         StatementKind::Assign(box (
 881                             _,
 882                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 883                         )) => {
 884                             // Use the underlying local for this (necessarily interior) borrow.
 885                             let ty = local_decls.local_decls()[place.local].ty;
 886                             let span = statement.source_info.span;
 887
 888                             let ref_ty = tcx.mk_ref(
 889                                 tcx.lifetimes.re_erased,
 890                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 891                             );
 892
 893                             *region = tcx.lifetimes.re_erased;
 894
 895                             let mut projection = vec![PlaceElem::Deref];
 896                             projection.extend(place.projection);
 897                             place.projection = tcx.intern_place_elems(&projection);
 898
 899                             // Create a temp to hold the promoted reference.
 900                             // This is because `*r` requires `r` to be a local,
 901                             // otherwise we would use the `promoted` directly.
 902                             let mut promoted_ref = LocalDecl::new(ref_ty, span);
 903                             promoted_ref.source_info = statement.source_info;
 904                             let promoted_ref = local_decls.push(promoted_ref);
 905                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 906
 907                             let promoted_ref_statement = Statement {
 908                                 source_info: statement.source_info,
 909                                 kind: StatementKind::Assign(Box::new((
 910                                     Place::from(promoted_ref),
 911                                     Rvalue::Use(promoted_operand(ref_ty, span)),
 912                                 ))),
 913                             };
 914                             self.extra_statements.push((loc, promoted_ref_statement));
 915
 916                             Rvalue::Ref(
 917                                 tcx.lifetimes.re_erased,
 918                                 borrow_kind,
 919                                 Place {
 920                                     local: mem::replace(&mut place.local, promoted_ref),
 921                                     projection: List::empty(),
 922                                 },
 923                             )
 924                         }
 925                         _ => bug!(),
 926                     }
 927                 }
 928             }
 929         };
 930
 931         assert_eq!(self.new_block(), START_BLOCK);
 932         self.visit_rvalue(
 933             &mut rvalue,
 934             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 935         );
 936
 937         let span = self.promoted.span;
 938         self.assign(RETURN_PLACE, rvalue, span);
 939         Some(self.promoted)
 940     }
 941 }
 942
 943 /// Replaces all temporaries with their promoted counterparts.
 944 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 945     fn tcx(&self) -> TyCtxt<'tcx> {
 946         self.tcx
 947     }
 948
 949     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 950         if self.is_temp_kind(*local) {
 951             *local = self.promote_temp(*local);
 952         }
 953     }
 954 }
 955
 956 pub fn promote_candidates<'tcx>(
 957     body: &mut Body<'tcx>,
 958     tcx: TyCtxt<'tcx>,
 959     mut temps: IndexVec<Local, TempState>,
 960     candidates: Vec<Candidate>,
 961 ) -> IndexVec<Promoted, Body<'tcx>> {
 962     // Visit candidates in reverse, in case they're nested.
 963     debug!("promote_candidates({:?})", candidates);
 964
 965     let mut promotions = IndexVec::new();
 966
 967     let mut extra_statements = vec![];
 968     for candidate in candidates.into_iter().rev() {
 969         match candidate {
 970             Candidate::Ref(Location { block, statement_index }) => {
 971                 if let StatementKind::Assign(box (place, _)) =
 972                     &body[block].statements[statement_index].kind
 973                 {
 974                     if let Some(local) = place.as_local() {
 975                         if temps[local] == TempState::PromotedOut {
 976                             // Already promoted.
 977                             continue;
 978                         }
 979                     }
 980                 }
 981             }
 982         }
 983
 984         // Declare return place local so that `mir::Body::new` doesn't complain.
 985         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 986
 987         let mut scope = body.source_scopes[candidate.source_info(body).scope].clone();
 988         scope.parent_scope = None;
 989
 990         let promoted = Body::new(
 991             body.source, // `promoted` gets filled in below
 992             IndexVec::new(),
 993             IndexVec::from_elem_n(scope, 1),
 994             initial_locals,
 995             IndexVec::new(),
 996             0,
 997             vec![],
 998             body.span,
 999             body.generator_kind(),
1000         );
1001
1002         let promoter = Promoter {
1003             promoted,
1004             tcx,
1005             source: body,
1006             temps: &mut temps,
1007             extra_statements: &mut extra_statements,
1008             keep_original: false,
1009         };
1010
1011         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1012         if let Some(mut promoted) = promoter.promote_candidate(candidate, promotions.len()) {
1013             promoted.source.promoted = Some(promotions.next_index());
1014             promotions.push(promoted);
1015         }
1016     }
1017
1018     // Insert each of `extra_statements` before its indicated location, which
1019     // has to be done in reverse location order, to not invalidate the rest.
1020     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1021     for (loc, statement) in extra_statements {
1022         body[loc.block].statements.insert(loc.statement_index, statement);
1023     }
1024
1025     // Eliminate assignments to, and drops of promoted temps.
1026     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1027     for block in body.basic_blocks_mut() {
1028         block.statements.retain(|statement| match &statement.kind {
1029             StatementKind::Assign(box (place, _)) => {
1030                 if let Some(index) = place.as_local() {
1031                     !promoted(index)
1032                 } else {
1033                     true
1034                 }
1035             }
1036             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1037                 !promoted(*index)
1038             }
1039             _ => true,
1040         });
1041         let terminator = block.terminator_mut();
1042         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1043             if let Some(index) = place.as_local() {
1044                 if promoted(index) {
1045                     terminator.kind = TerminatorKind::Goto { target: *target };
1046                 }
1047             }
1048         }
1049     }
1050
1051     promotions
1052 }
1053
1054 /// This function returns `true` if the function being called in the array
1055 /// repeat expression is a `const` function.
1056 crate fn is_const_fn_in_array_repeat_expression<'tcx>(
1057     ccx: &ConstCx<'_, 'tcx>,
1058     place: &Place<'tcx>,
1059     body: &Body<'tcx>,
1060 ) -> bool {
1061     match place.as_local() {
1062         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1063         Some(local) if body.local_decls[local].is_user_variable() => return false,
1064         None => return false,
1065         _ => {}
1066     }
1067
1068     for block in body.basic_blocks() {
1069         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1070             &block.terminator
1071         {
1072             if let Operand::Constant(box Constant { literal, .. }) = func {
1073                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1074                     if let Some((destination_place, _)) = destination {
1075                         if destination_place == place {
1076                             if is_const_fn(ccx.tcx, def_id) {
1077                                 return true;
1078                             }
1079                         }
1080                     }
1081                 }
1082             }
1083         }
1084     }
1085
1086     false
1087 }