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 let Some(def) = base_ty.ty_adt_def() {
 419                             // No promotion of union field accesses.
 420                             if def.is_union() {
 421                                 return Err(Unpromotable);
 422                             }
 423                         }
 424                     }
 425                 }
 426
 427                 self.validate_place(place_base)
 428             }
 429         }
 430     }
 431
 432     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 433         match operand {
 434             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 435
 436             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 437             // `validate_rvalue` upon access.
 438             Operand::Constant(c) => {
 439                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 440                     // Only allow statics (not consts) to refer to other statics.
 441                     // FIXME(eddyb) does this matter at all for promotion?
 442                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 443                     // and in `const` this cannot occur anyway. The only concern is that we might
 444                     // promote even `let x = &STATIC` which would be useless, but this applies to
 445                     // promotion inside statics as well.
 446                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 447                     if !is_static {
 448                         return Err(Unpromotable);
 449                     }
 450
 451                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 452                     if is_thread_local {
 453                         return Err(Unpromotable);
 454                     }
 455                 }
 456
 457                 Ok(())
 458             }
 459         }
 460     }
 461
 462     fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
 463         match kind {
 464             // Reject these borrow types just to be safe.
 465             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
 466             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 467
 468             BorrowKind::Shared => {
 469                 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
 470                 if has_mut_interior {
 471                     return Err(Unpromotable);
 472                 }
 473             }
 474
 475             BorrowKind::Mut { .. } => {
 476                 let ty = place.ty(self.body, self.tcx).ty;
 477
 478                 // In theory, any zero-sized value could be borrowed
 479                 // mutably without consequences. However, only &mut []
 480                 // is allowed right now.
 481                 if let ty::Array(_, len) = ty.kind() {
 482                     match len.try_eval_usize(self.tcx, self.param_env) {
 483                         Some(0) => {}
 484                         _ => return Err(Unpromotable),
 485                     }
 486                 } else {
 487                     return Err(Unpromotable);
 488                 }
 489             }
 490         }
 491
 492         Ok(())
 493     }
 494
 495     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 496         match rvalue {
 497             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
 498                 self.validate_operand(operand)?;
 499             }
 500
 501             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 502                 self.validate_place(place.as_ref())?
 503             }
 504
 505             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 506
 507             Rvalue::Cast(kind, operand, cast_ty) => {
 508                 if matches!(kind, CastKind::Misc) {
 509                     let operand_ty = operand.ty(self.body, self.tcx);
 510                     let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 511                     let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 512                     if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 513                         // ptr-to-int casts are not possible in consts and thus not promotable
 514                         return Err(Unpromotable);
 515                     }
 516                     // int-to-ptr casts are fine, they just use the integer value at pointer type.
 517                 }
 518
 519                 self.validate_operand(operand)?;
 520             }
 521
 522             Rvalue::NullaryOp(op, _) => match op {
 523                 NullOp::Box => return Err(Unpromotable),
 524                 NullOp::SizeOf => {}
 525             },
 526
 527             Rvalue::UnaryOp(op, operand) => {
 528                 match op {
 529                     // These operations can never fail.
 530                     UnOp::Neg | UnOp::Not => {}
 531                 }
 532
 533                 self.validate_operand(operand)?;
 534             }
 535
 536             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 537                 let op = *op;
 538                 let lhs_ty = lhs.ty(self.body, self.tcx);
 539
 540                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 541                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 542                     assert!(matches!(
 543                         op,
 544                         BinOp::Eq
 545                             | BinOp::Ne
 546                             | BinOp::Le
 547                             | BinOp::Lt
 548                             | BinOp::Ge
 549                             | BinOp::Gt
 550                             | BinOp::Offset
 551                     ));
 552                     return Err(Unpromotable);
 553                 }
 554
 555                 match op {
 556                     BinOp::Div | BinOp::Rem => {
 557                         if lhs_ty.is_integral() {
 558                             // Integer division: the RHS must be a non-zero const.
 559                             let const_val = match rhs {
 560                                 Operand::Constant(c) => {
 561                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 562                                 }
 563                                 _ => None,
 564                             };
 565                             match const_val {
 566                                 Some(x) if x != 0 => {}        // okay
 567                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 568                             }
 569                         }
 570                     }
 571                     // The remaining operations can never fail.
 572                     BinOp::Eq
 573                     | BinOp::Ne
 574                     | BinOp::Le
 575                     | BinOp::Lt
 576                     | BinOp::Ge
 577                     | BinOp::Gt
 578                     | BinOp::Offset
 579                     | BinOp::Add
 580                     | BinOp::Sub
 581                     | BinOp::Mul
 582                     | BinOp::BitXor
 583                     | BinOp::BitAnd
 584                     | BinOp::BitOr
 585                     | BinOp::Shl
 586                     | BinOp::Shr => {}
 587                 }
 588
 589                 self.validate_operand(lhs)?;
 590                 self.validate_operand(rhs)?;
 591             }
 592
 593             Rvalue::AddressOf(_, place) => {
 594                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 595                 // no problem, only using it is.
 596                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 597                 {
 598                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 599                     if let ty::Ref(..) = base_ty.kind() {
 600                         return self.validate_place(place_base);
 601                     }
 602                 }
 603                 return Err(Unpromotable);
 604             }
 605
 606             Rvalue::Ref(_, kind, place) => {
 607                 // Special-case reborrows to be more like a copy of the reference.
 608                 let mut place_simplified = place.as_ref();
 609                 if let Some((place_base, ProjectionElem::Deref)) =
 610                     place_simplified.last_projection()
 611                 {
 612                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 613                     if let ty::Ref(..) = base_ty.kind() {
 614                         place_simplified = place_base;
 615                     }
 616                 }
 617
 618                 self.validate_place(place_simplified)?;
 619
 620                 // Check that the reference is fine (using the original place!).
 621                 // (Needs to come after `validate_place` to avoid ICEs.)
 622                 self.validate_ref(*kind, place)?;
 623             }
 624
 625             Rvalue::Aggregate(_, operands) => {
 626                 for o in operands {
 627                     self.validate_operand(o)?;
 628                 }
 629             }
 630         }
 631
 632         Ok(())
 633     }
 634
 635     fn validate_call(
 636         &self,
 637         callee: &Operand<'tcx>,
 638         args: &[Operand<'tcx>],
 639     ) -> Result<(), Unpromotable> {
 640         let fn_ty = callee.ty(self.body, self.tcx);
 641
 642         // Inside const/static items, we promote all (eligible) function calls.
 643         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 644         let promote_all_const_fn = matches!(
 645             self.const_kind,
 646             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 647         );
 648         if !promote_all_const_fn {
 649             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 650                 // Never promote runtime `const fn` calls of
 651                 // functions without `#[rustc_promotable]`.
 652                 if !self.tcx.is_promotable_const_fn(def_id) {
 653                     return Err(Unpromotable);
 654                 }
 655             }
 656         }
 657
 658         let is_const_fn = match *fn_ty.kind() {
 659             ty::FnDef(def_id, _) => {
 660                 is_const_fn(self.tcx, def_id)
 661                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 662                     || is_lang_panic_fn(self.tcx, def_id)
 663             }
 664             _ => false,
 665         };
 666         if !is_const_fn {
 667             return Err(Unpromotable);
 668         }
 669
 670         self.validate_operand(callee)?;
 671         for arg in args {
 672             self.validate_operand(arg)?;
 673         }
 674
 675         Ok(())
 676     }
 677 }
 678
 679 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 680 pub fn validate_candidates(
 681     ccx: &ConstCx<'_, '_>,
 682     temps: &IndexVec<Local, TempState>,
 683     candidates: &[Candidate],
 684 ) -> Vec<Candidate> {
 685     let validator = Validator { ccx, temps };
 686
 687     candidates
 688         .iter()
 689         .copied()
 690         .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
 691         .collect()
 692 }
 693
 694 struct Promoter<'a, 'tcx> {
 695     tcx: TyCtxt<'tcx>,
 696     source: &'a mut Body<'tcx>,
 697     promoted: Body<'tcx>,
 698     temps: &'a mut IndexVec<Local, TempState>,
 699     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 700
 701     /// If true, all nested temps are also kept in the
 702     /// source MIR, not moved to the promoted MIR.
 703     keep_original: bool,
 704 }
 705
 706 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 707     fn new_block(&mut self) -> BasicBlock {
 708         let span = self.promoted.span;
 709         self.promoted.basic_blocks_mut().push(BasicBlockData {
 710             statements: vec![],
 711             terminator: Some(Terminator {
 712                 source_info: SourceInfo::outermost(span),
 713                 kind: TerminatorKind::Return,
 714             }),
 715             is_cleanup: false,
 716         })
 717     }
 718
 719     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 720         let last = self.promoted.basic_blocks().last().unwrap();
 721         let data = &mut self.promoted[last];
 722         data.statements.push(Statement {
 723             source_info: SourceInfo::outermost(span),
 724             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 725         });
 726     }
 727
 728     fn is_temp_kind(&self, local: Local) -> bool {
 729         self.source.local_kind(local) == LocalKind::Temp
 730     }
 731
 732     /// Copies the initialization of this temp to the
 733     /// promoted MIR, recursing through temps.
 734     fn promote_temp(&mut self, temp: Local) -> Local {
 735         let old_keep_original = self.keep_original;
 736         let loc = match self.temps[temp] {
 737             TempState::Defined { location, uses } if uses > 0 => {
 738                 if uses > 1 {
 739                     self.keep_original = true;
 740                 }
 741                 location
 742             }
 743             state => {
 744                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 745             }
 746         };
 747         if !self.keep_original {
 748             self.temps[temp] = TempState::PromotedOut;
 749         }
 750
 751         let num_stmts = self.source[loc.block].statements.len();
 752         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 753             self.source.local_decls[temp].ty,
 754             self.source.local_decls[temp].source_info.span,
 755         ));
 756
 757         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 758
 759         // First, take the Rvalue or Call out of the source MIR,
 760         // or duplicate it, depending on keep_original.
 761         if loc.statement_index < num_stmts {
 762             let (mut rvalue, source_info) = {
 763                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 764                 let rhs = match statement.kind {
 765                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 766                     _ => {
 767                         span_bug!(
 768                             statement.source_info.span,
 769                             "{:?} is not an assignment",
 770                             statement
 771                         );
 772                     }
 773                 };
 774
 775                 (
 776                     if self.keep_original {
 777                         rhs.clone()
 778                     } else {
 779                         let unit = Rvalue::Use(Operand::Constant(box Constant {
 780                             span: statement.source_info.span,
 781                             user_ty: None,
 782                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 783                         }));
 784                         mem::replace(rhs, unit)
 785                     },
 786                     statement.source_info,
 787                 )
 788             };
 789
 790             self.visit_rvalue(&mut rvalue, loc);
 791             self.assign(new_temp, rvalue, source_info.span);
 792         } else {
 793             let terminator = if self.keep_original {
 794                 self.source[loc.block].terminator().clone()
 795             } else {
 796                 let terminator = self.source[loc.block].terminator_mut();
 797                 let target = match terminator.kind {
 798                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 799                     ref kind => {
 800                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 801                     }
 802                 };
 803                 Terminator {
 804                     source_info: terminator.source_info,
 805                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 806                 }
 807             };
 808
 809             match terminator.kind {
 810                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 811                     self.visit_operand(&mut func, loc);
 812                     for arg in &mut args {
 813                         self.visit_operand(arg, loc);
 814                     }
 815
 816                     let last = self.promoted.basic_blocks().last().unwrap();
 817                     let new_target = self.new_block();
 818
 819                     *self.promoted[last].terminator_mut() = Terminator {
 820                         kind: TerminatorKind::Call {
 821                             func,
 822                             args,
 823                             cleanup: None,
 824                             destination: Some((Place::from(new_temp), new_target)),
 825                             from_hir_call,
 826                             fn_span,
 827                         },
 828                         source_info: SourceInfo::outermost(terminator.source_info.span),
 829                         ..terminator
 830                     };
 831                 }
 832                 ref kind => {
 833                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 834                 }
 835             };
 836         };
 837
 838         self.keep_original = old_keep_original;
 839         new_temp
 840     }
 841
 842     fn promote_candidate(
 843         mut self,
 844         candidate: Candidate,
 845         next_promoted_id: usize,
 846     ) -> Option<Body<'tcx>> {
 847         let def = self.source.source.with_opt_param();
 848         let mut rvalue = {
 849             let promoted = &mut self.promoted;
 850             let promoted_id = Promoted::new(next_promoted_id);
 851             let tcx = self.tcx;
 852             let mut promoted_operand = |ty, span| {
 853                 promoted.span = span;
 854                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 855
 856                 Operand::Constant(Box::new(Constant {
 857                     span,
 858                     user_ty: None,
 859                     literal: tcx
 860                         .mk_const(ty::Const {
 861                             ty,
 862                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 863                                 def,
 864                                 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
 865                                     if let ty::GenericParamDefKind::Lifetime = param.kind {
 866                                         tcx.lifetimes.re_erased.into()
 867                                     } else {
 868                                         tcx.mk_param_from_def(param)
 869                                     }
 870                                 }),
 871                                 promoted: Some(promoted_id),
 872                             }),
 873                         })
 874                         .into(),
 875                 }))
 876             };
 877             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 878             match candidate {
 879                 Candidate::Ref(loc) => {
 880                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
 881                     match statement.kind {
 882                         StatementKind::Assign(box (
 883                             _,
 884                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 885                         )) => {
 886                             // Use the underlying local for this (necessarily interior) borrow.
 887                             let ty = local_decls.local_decls()[place.local].ty;
 888                             let span = statement.source_info.span;
 889
 890                             let ref_ty = tcx.mk_ref(
 891                                 tcx.lifetimes.re_erased,
 892                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 893                             );
 894
 895                             *region = tcx.lifetimes.re_erased;
 896
 897                             let mut projection = vec![PlaceElem::Deref];
 898                             projection.extend(place.projection);
 899                             place.projection = tcx.intern_place_elems(&projection);
 900
 901                             // Create a temp to hold the promoted reference.
 902                             // This is because `*r` requires `r` to be a local,
 903                             // otherwise we would use the `promoted` directly.
 904                             let mut promoted_ref = LocalDecl::new(ref_ty, span);
 905                             promoted_ref.source_info = statement.source_info;
 906                             let promoted_ref = local_decls.push(promoted_ref);
 907                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 908
 909                             let promoted_ref_statement = Statement {
 910                                 source_info: statement.source_info,
 911                                 kind: StatementKind::Assign(Box::new((
 912                                     Place::from(promoted_ref),
 913                                     Rvalue::Use(promoted_operand(ref_ty, span)),
 914                                 ))),
 915                             };
 916                             self.extra_statements.push((loc, promoted_ref_statement));
 917
 918                             Rvalue::Ref(
 919                                 tcx.lifetimes.re_erased,
 920                                 borrow_kind,
 921                                 Place {
 922                                     local: mem::replace(&mut place.local, promoted_ref),
 923                                     projection: List::empty(),
 924                                 },
 925                             )
 926                         }
 927                         _ => bug!(),
 928                     }
 929                 }
 930             }
 931         };
 932
 933         assert_eq!(self.new_block(), START_BLOCK);
 934         self.visit_rvalue(
 935             &mut rvalue,
 936             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 937         );
 938
 939         let span = self.promoted.span;
 940         self.assign(RETURN_PLACE, rvalue, span);
 941         Some(self.promoted)
 942     }
 943 }
 944
 945 /// Replaces all temporaries with their promoted counterparts.
 946 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 947     fn tcx(&self) -> TyCtxt<'tcx> {
 948         self.tcx
 949     }
 950
 951     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 952         if self.is_temp_kind(*local) {
 953             *local = self.promote_temp(*local);
 954         }
 955     }
 956 }
 957
 958 pub fn promote_candidates<'tcx>(
 959     body: &mut Body<'tcx>,
 960     tcx: TyCtxt<'tcx>,
 961     mut temps: IndexVec<Local, TempState>,
 962     candidates: Vec<Candidate>,
 963 ) -> IndexVec<Promoted, Body<'tcx>> {
 964     // Visit candidates in reverse, in case they're nested.
 965     debug!("promote_candidates({:?})", candidates);
 966
 967     let mut promotions = IndexVec::new();
 968
 969     let mut extra_statements = vec![];
 970     for candidate in candidates.into_iter().rev() {
 971         match candidate {
 972             Candidate::Ref(Location { block, statement_index }) => {
 973                 if let StatementKind::Assign(box (place, _)) =
 974                     &body[block].statements[statement_index].kind
 975                 {
 976                     if let Some(local) = place.as_local() {
 977                         if temps[local] == TempState::PromotedOut {
 978                             // Already promoted.
 979                             continue;
 980                         }
 981                     }
 982                 }
 983             }
 984         }
 985
 986         // Declare return place local so that `mir::Body::new` doesn't complain.
 987         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 988
 989         let mut scope = body.source_scopes[candidate.source_info(body).scope].clone();
 990         scope.parent_scope = None;
 991
 992         let promoted = Body::new(
 993             body.source, // `promoted` gets filled in below
 994             IndexVec::new(),
 995             IndexVec::from_elem_n(scope, 1),
 996             initial_locals,
 997             IndexVec::new(),
 998             0,
 999             vec![],
1000             body.span,
1001             body.generator_kind(),
1002         );
1003
1004         let promoter = Promoter {
1005             promoted,
1006             tcx,
1007             source: body,
1008             temps: &mut temps,
1009             extra_statements: &mut extra_statements,
1010             keep_original: false,
1011         };
1012
1013         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1014         if let Some(mut promoted) = promoter.promote_candidate(candidate, promotions.len()) {
1015             promoted.source.promoted = Some(promotions.next_index());
1016             promotions.push(promoted);
1017         }
1018     }
1019
1020     // Insert each of `extra_statements` before its indicated location, which
1021     // has to be done in reverse location order, to not invalidate the rest.
1022     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1023     for (loc, statement) in extra_statements {
1024         body[loc.block].statements.insert(loc.statement_index, statement);
1025     }
1026
1027     // Eliminate assignments to, and drops of promoted temps.
1028     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1029     for block in body.basic_blocks_mut() {
1030         block.statements.retain(|statement| match &statement.kind {
1031             StatementKind::Assign(box (place, _)) => {
1032                 if let Some(index) = place.as_local() {
1033                     !promoted(index)
1034                 } else {
1035                     true
1036                 }
1037             }
1038             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1039                 !promoted(*index)
1040             }
1041             _ => true,
1042         });
1043         let terminator = block.terminator_mut();
1044         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1045             if let Some(index) = place.as_local() {
1046                 if promoted(index) {
1047                     terminator.kind = TerminatorKind::Goto { target: *target };
1048                 }
1049             }
1050         }
1051     }
1052
1053     promotions
1054 }
1055
1056 /// This function returns `true` if the function being called in the array
1057 /// repeat expression is a `const` function.
1058 crate fn is_const_fn_in_array_repeat_expression<'tcx>(
1059     ccx: &ConstCx<'_, 'tcx>,
1060     place: &Place<'tcx>,
1061     body: &Body<'tcx>,
1062 ) -> bool {
1063     match place.as_local() {
1064         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1065         Some(local) if body.local_decls[local].is_user_variable() => return false,
1066         None => return false,
1067         _ => {}
1068     }
1069
1070     for block in body.basic_blocks() {
1071         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1072             &block.terminator
1073         {
1074             if let Operand::Constant(box Constant { literal, .. }) = func {
1075                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1076                     if let Some((destination_place, _)) = destination {
1077                         if destination_place == place {
1078                             if is_const_fn(ccx.tcx, def_id) {
1079                                 return true;
1080                             }
1081                         }
1082                     }
1083                 }
1084             }
1085         }
1086     }
1087
1088     false
1089 }