compiler/rustc_const_eval/src/transform/promote_consts.rs

   1 //! A pass that promotes borrows of constant rvalues.
   2 //!
   3 //! The rvalues considered constant are trees of temps,
   4 //! each with exactly one initialization, and holding
   5 //! a constant value with no interior mutability.
   6 //! They are placed into a new MIR constant body in
   7 //! `promoted` and the borrow rvalue is replaced with
   8 //! a `Literal::Promoted` using the index into `promoted`
   9 //! of that constant MIR.
  10 //!
  11 //! This pass assumes that every use is dominated by an
  12 //! initialization and can otherwise silence errors, if
  13 //! move analysis runs after promotion on broken MIR.
  14
  15 use rustc_hir as hir;
  16 use rustc_middle::mir::traversal::ReversePostorder;
  17 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  18 use rustc_middle::mir::*;
  19 use rustc_middle::ty::cast::CastTy;
  20 use rustc_middle::ty::subst::InternalSubsts;
  21 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
  22 use rustc_span::Span;
  23
  24 use rustc_index::vec::{Idx, IndexVec};
  25
  26 use std::cell::Cell;
  27 use std::{cmp, iter, mem};
  28
  29 use crate::transform::check_consts::{qualifs, ConstCx};
  30 use crate::transform::MirPass;
  31
  32 /// A `MirPass` for promotion.
  33 ///
  34 /// Promotion is the extraction of promotable temps into separate MIR bodies so they can have
  35 /// `'static` lifetime.
  36 ///
  37 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  38 /// newly created `Constant`.
  39 #[derive(Default)]
  40 pub struct PromoteTemps<'tcx> {
  41     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  42 }
  43
  44 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  45     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  46         // There's not really any point in promoting errorful MIR.
  47         //
  48         // This does not include MIR that failed const-checking, which we still try to promote.
  49         if body.return_ty().references_error() {
  50             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  51             return;
  52         }
  53
  54         if body.source.promoted.is_some() {
  55             return;
  56         }
  57
  58         let mut rpo = traversal::reverse_postorder(body);
  59         let ccx = ConstCx::new(tcx, body);
  60         let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  61
  62         let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates);
  63
  64         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
  65         self.promoted_fragments.set(promoted);
  66     }
  67 }
  68
  69 /// State of a temporary during collection and promotion.
  70 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  71 pub enum TempState {
  72     /// No references to this temp.
  73     Undefined,
  74     /// One direct assignment and any number of direct uses.
  75     /// A borrow of this temp is promotable if the assigned
  76     /// value is qualified as constant.
  77     Defined { location: Location, uses: usize },
  78     /// Any other combination of assignments/uses.
  79     Unpromotable,
  80     /// This temp was part of an rvalue which got extracted
  81     /// during promotion and needs cleanup.
  82     PromotedOut,
  83 }
  84
  85 impl TempState {
  86     pub fn is_promotable(&self) -> bool {
  87         debug!("is_promotable: self={:?}", self);
  88         matches!(self, TempState::Defined { .. })
  89     }
  90 }
  91
  92 /// A "root candidate" for promotion, which will become the
  93 /// returned value in a promoted MIR, unless it's a subset
  94 /// of a larger candidate.
  95 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  96 pub enum Candidate {
  97     /// Borrow of a constant temporary, candidate for lifetime extension.
  98     Ref(Location),
  99 }
 100
 101 impl Candidate {
 102     fn source_info(&self, body: &Body<'_>) -> SourceInfo {
 103         match self {
 104             Candidate::Ref(location) => *body.source_info(*location),
 105         }
 106     }
 107 }
 108
 109 struct Collector<'a, 'tcx> {
 110     ccx: &'a ConstCx<'a, 'tcx>,
 111     temps: IndexVec<Local, TempState>,
 112     candidates: Vec<Candidate>,
 113 }
 114
 115 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 116     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 117         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 118         // We're only interested in temporaries and the return place
 119         match self.ccx.body.local_kind(index) {
 120             LocalKind::Temp | LocalKind::ReturnPointer => {}
 121             LocalKind::Arg | LocalKind::Var => return,
 122         }
 123
 124         // Ignore drops, if the temp gets promoted,
 125         // then it's constant and thus drop is noop.
 126         // Non-uses are also irrelevant.
 127         if context.is_drop() || !context.is_use() {
 128             debug!(
 129                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 130                 context.is_drop(),
 131                 context.is_use(),
 132             );
 133             return;
 134         }
 135
 136         let temp = &mut self.temps[index];
 137         debug!("visit_local: temp={:?}", temp);
 138         if *temp == TempState::Undefined {
 139             match context {
 140                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 141                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 142                     *temp = TempState::Defined { location, uses: 0 };
 143                     return;
 144                 }
 145                 _ => { /* mark as unpromotable below */ }
 146             }
 147         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 148             // We always allow borrows, even mutable ones, as we need
 149             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 150             let allowed_use = match context {
 151                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 152                 | PlaceContext::NonMutatingUse(_) => true,
 153                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 154             };
 155             debug!("visit_local: allowed_use={:?}", allowed_use);
 156             if allowed_use {
 157                 *uses += 1;
 158                 return;
 159             }
 160             /* mark as unpromotable below */
 161         }
 162         *temp = TempState::Unpromotable;
 163     }
 164
 165     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 166         self.super_rvalue(rvalue, location);
 167
 168         match *rvalue {
 169             Rvalue::Ref(..) => {
 170                 self.candidates.push(Candidate::Ref(location));
 171             }
 172             _ => {}
 173         }
 174     }
 175 }
 176
 177 pub fn collect_temps_and_candidates(
 178     ccx: &ConstCx<'mir, 'tcx>,
 179     rpo: &mut ReversePostorder<'_, 'tcx>,
 180 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 181     let mut collector = Collector {
 182         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
 183         candidates: vec![],
 184         ccx,
 185     };
 186     for (bb, data) in rpo {
 187         collector.visit_basic_block_data(bb, data);
 188     }
 189     (collector.temps, collector.candidates)
 190 }
 191
 192 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 193 ///
 194 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 195 struct Validator<'a, 'tcx> {
 196     ccx: &'a ConstCx<'a, 'tcx>,
 197     temps: &'a IndexVec<Local, TempState>,
 198 }
 199
 200 impl std::ops::Deref for Validator<'a, 'tcx> {
 201     type Target = ConstCx<'a, 'tcx>;
 202
 203     fn deref(&self) -> &Self::Target {
 204         &self.ccx
 205     }
 206 }
 207
 208 struct Unpromotable;
 209
 210 impl<'tcx> Validator<'_, 'tcx> {
 211     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 212         match candidate {
 213             Candidate::Ref(loc) => {
 214                 let statement = &self.body[loc.block].statements[loc.statement_index];
 215                 match &statement.kind {
 216                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 217                         // We can only promote interior borrows of promotable temps (non-temps
 218                         // don't get promoted anyway).
 219                         self.validate_local(place.local)?;
 220
 221                         // The reference operation itself must be promotable.
 222                         // (Needs to come after `validate_local` to avoid ICEs.)
 223                         self.validate_ref(*kind, place)?;
 224
 225                         // We do not check all the projections (they do not get promoted anyway),
 226                         // but we do stay away from promoting anything involving a dereference.
 227                         if place.projection.contains(&ProjectionElem::Deref) {
 228                             return Err(Unpromotable);
 229                         }
 230
 231                         // We cannot promote things that need dropping, since the promoted value
 232                         // would not get dropped.
 233                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 234                             return Err(Unpromotable);
 235                         }
 236
 237                         Ok(())
 238                     }
 239                     _ => bug!(),
 240                 }
 241             }
 242         }
 243     }
 244
 245     // FIXME(eddyb) maybe cache this?
 246     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 247         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 248             let num_stmts = self.body[loc.block].statements.len();
 249
 250             if loc.statement_index < num_stmts {
 251                 let statement = &self.body[loc.block].statements[loc.statement_index];
 252                 match &statement.kind {
 253                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 254                         &self.ccx,
 255                         &mut |l| self.qualif_local::<Q>(l),
 256                         rhs,
 257                     ),
 258                     _ => {
 259                         span_bug!(
 260                             statement.source_info.span,
 261                             "{:?} is not an assignment",
 262                             statement
 263                         );
 264                     }
 265                 }
 266             } else {
 267                 let terminator = self.body[loc.block].terminator();
 268                 match &terminator.kind {
 269                     TerminatorKind::Call { .. } => {
 270                         let return_ty = self.body.local_decls[local].ty;
 271                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 272                     }
 273                     kind => {
 274                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 275                     }
 276                 }
 277             }
 278         } else {
 279             let span = self.body.local_decls[local].source_info.span;
 280             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 281         }
 282     }
 283
 284     // FIXME(eddyb) maybe cache this?
 285     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 286         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 287             let block = &self.body[loc.block];
 288             let num_stmts = block.statements.len();
 289
 290             if loc.statement_index < num_stmts {
 291                 let statement = &block.statements[loc.statement_index];
 292                 match &statement.kind {
 293                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 294                     _ => {
 295                         span_bug!(
 296                             statement.source_info.span,
 297                             "{:?} is not an assignment",
 298                             statement
 299                         );
 300                     }
 301                 }
 302             } else {
 303                 let terminator = block.terminator();
 304                 match &terminator.kind {
 305                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 306                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 307                     kind => {
 308                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 309                     }
 310                 }
 311             }
 312         } else {
 313             Err(Unpromotable)
 314         }
 315     }
 316
 317     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 318         match place.last_projection() {
 319             None => self.validate_local(place.local),
 320             Some((place_base, elem)) => {
 321                 // Validate topmost projection, then recurse.
 322                 match elem {
 323                     ProjectionElem::Deref => {
 324                         let mut promotable = false;
 325                         // We need to make sure this is a `Deref` of a local with no further projections.
 326                         // Discussion can be found at
 327                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 328                         if let Some(local) = place_base.as_local() {
 329                             // This is a special treatment for cases like *&STATIC where STATIC is a
 330                             // global static variable.
 331                             // This pattern is generated only when global static variables are directly
 332                             // accessed and is qualified for promotion safely.
 333                             if let TempState::Defined { location, .. } = self.temps[local] {
 334                                 let def_stmt = self.body[location.block]
 335                                     .statements
 336                                     .get(location.statement_index);
 337                                 if let Some(Statement {
 338                                     kind:
 339                                         StatementKind::Assign(box (
 340                                             _,
 341                                             Rvalue::Use(Operand::Constant(c)),
 342                                         )),
 343                                     ..
 344                                 }) = def_stmt
 345                                 {
 346                                     if let Some(did) = c.check_static_ptr(self.tcx) {
 347                                         // Evaluating a promoted may not read statics except if it got
 348                                         // promoted from a static (this is a CTFE check). So we
 349                                         // can only promote static accesses inside statics.
 350                                         if let Some(hir::ConstContext::Static(..)) = self.const_kind
 351                                         {
 352                                             if !self.tcx.is_thread_local_static(did) {
 353                                                 promotable = true;
 354                                             }
 355                                         }
 356                                     }
 357                                 }
 358                             }
 359                         }
 360                         if !promotable {
 361                             return Err(Unpromotable);
 362                         }
 363                     }
 364                     ProjectionElem::Downcast(..) => {
 365                         return Err(Unpromotable);
 366                     }
 367
 368                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 369
 370                     ProjectionElem::Index(local) => {
 371                         let mut promotable = false;
 372                         // Only accept if we can predict the index and are indexing an array.
 373                         let val =
 374                             if let TempState::Defined { location: loc, .. } = self.temps[local] {
 375                                 let block = &self.body[loc.block];
 376                                 if loc.statement_index < block.statements.len() {
 377                                     let statement = &block.statements[loc.statement_index];
 378                                     match &statement.kind {
 379                                         StatementKind::Assign(box (
 380                                             _,
 381                                             Rvalue::Use(Operand::Constant(c)),
 382                                         )) => c.literal.try_eval_usize(self.tcx, self.param_env),
 383                                         _ => None,
 384                                     }
 385                                 } else {
 386                                     None
 387                                 }
 388                             } else {
 389                                 None
 390                             };
 391                         if let Some(idx) = val {
 392                             // Determine the type of the thing we are indexing.
 393                             let ty = place_base.ty(self.body, self.tcx).ty;
 394                             match ty.kind() {
 395                                 ty::Array(_, len) => {
 396                                     // It's an array; determine its length.
 397                                     if let Some(len) = len.try_eval_usize(self.tcx, self.param_env)
 398                                     {
 399                                         // If the index is in-bounds, go ahead.
 400                                         if idx < len {
 401                                             promotable = true;
 402                                         }
 403                                     }
 404                                 }
 405                                 _ => {}
 406                             }
 407                         }
 408                         if !promotable {
 409                             return Err(Unpromotable);
 410                         }
 411
 412                         self.validate_local(local)?;
 413                     }
 414
 415                     ProjectionElem::Field(..) => {
 416                         let base_ty = place_base.ty(self.body, self.tcx).ty;
 417                         if base_ty.is_union() {
 418                             // No promotion of union field accesses.
 419                             return Err(Unpromotable);
 420                         }
 421                     }
 422                 }
 423
 424                 self.validate_place(place_base)
 425             }
 426         }
 427     }
 428
 429     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 430         match operand {
 431             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 432
 433             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 434             // `validate_rvalue` upon access.
 435             Operand::Constant(c) => {
 436                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 437                     // Only allow statics (not consts) to refer to other statics.
 438                     // FIXME(eddyb) does this matter at all for promotion?
 439                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 440                     // and in `const` this cannot occur anyway. The only concern is that we might
 441                     // promote even `let x = &STATIC` which would be useless, but this applies to
 442                     // promotion inside statics as well.
 443                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 444                     if !is_static {
 445                         return Err(Unpromotable);
 446                     }
 447
 448                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 449                     if is_thread_local {
 450                         return Err(Unpromotable);
 451                     }
 452                 }
 453
 454                 Ok(())
 455             }
 456         }
 457     }
 458
 459     fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
 460         match kind {
 461             // Reject these borrow types just to be safe.
 462             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
 463             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 464
 465             BorrowKind::Shared => {
 466                 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
 467                 if has_mut_interior {
 468                     return Err(Unpromotable);
 469                 }
 470             }
 471
 472             BorrowKind::Mut { .. } => {
 473                 let ty = place.ty(self.body, self.tcx).ty;
 474
 475                 // In theory, any zero-sized value could be borrowed
 476                 // mutably without consequences. However, only &mut []
 477                 // is allowed right now.
 478                 if let ty::Array(_, len) = ty.kind() {
 479                     match len.try_eval_usize(self.tcx, self.param_env) {
 480                         Some(0) => {}
 481                         _ => return Err(Unpromotable),
 482                     }
 483                 } else {
 484                     return Err(Unpromotable);
 485                 }
 486             }
 487         }
 488
 489         Ok(())
 490     }
 491
 492     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 493         match rvalue {
 494             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
 495                 self.validate_operand(operand)?;
 496             }
 497
 498             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 499                 self.validate_place(place.as_ref())?
 500             }
 501
 502             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 503
 504             Rvalue::Cast(kind, operand, cast_ty) => {
 505                 if matches!(kind, CastKind::Misc) {
 506                     let operand_ty = operand.ty(self.body, self.tcx);
 507                     let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 508                     let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 509                     if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 510                         // ptr-to-int casts are not possible in consts and thus not promotable
 511                         return Err(Unpromotable);
 512                     }
 513                     // int-to-ptr casts are fine, they just use the integer value at pointer type.
 514                 }
 515
 516                 self.validate_operand(operand)?;
 517             }
 518
 519             Rvalue::NullaryOp(op, _) => match op {
 520                 NullOp::Box => return Err(Unpromotable),
 521                 NullOp::SizeOf => {}
 522                 NullOp::AlignOf => {}
 523             },
 524
 525             Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable),
 526
 527             Rvalue::UnaryOp(op, operand) => {
 528                 match op {
 529                     // These operations can never fail.
 530                     UnOp::Neg | UnOp::Not => {}
 531                 }
 532
 533                 self.validate_operand(operand)?;
 534             }
 535
 536             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 537                 let op = *op;
 538                 let lhs_ty = lhs.ty(self.body, self.tcx);
 539
 540                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 541                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 542                     assert!(matches!(
 543                         op,
 544                         BinOp::Eq
 545                             | BinOp::Ne
 546                             | BinOp::Le
 547                             | BinOp::Lt
 548                             | BinOp::Ge
 549                             | BinOp::Gt
 550                             | BinOp::Offset
 551                     ));
 552                     return Err(Unpromotable);
 553                 }
 554
 555                 match op {
 556                     BinOp::Div | BinOp::Rem => {
 557                         if lhs_ty.is_integral() {
 558                             // Integer division: the RHS must be a non-zero const.
 559                             let const_val = match rhs {
 560                                 Operand::Constant(c) => {
 561                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 562                                 }
 563                                 _ => None,
 564                             };
 565                             match const_val {
 566                                 Some(x) if x != 0 => {}        // okay
 567                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 568                             }
 569                         }
 570                     }
 571                     // The remaining operations can never fail.
 572                     BinOp::Eq
 573                     | BinOp::Ne
 574                     | BinOp::Le
 575                     | BinOp::Lt
 576                     | BinOp::Ge
 577                     | BinOp::Gt
 578                     | BinOp::Offset
 579                     | BinOp::Add
 580                     | BinOp::Sub
 581                     | BinOp::Mul
 582                     | BinOp::BitXor
 583                     | BinOp::BitAnd
 584                     | BinOp::BitOr
 585                     | BinOp::Shl
 586                     | BinOp::Shr => {}
 587                 }
 588
 589                 self.validate_operand(lhs)?;
 590                 self.validate_operand(rhs)?;
 591             }
 592
 593             Rvalue::AddressOf(_, place) => {
 594                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 595                 // no problem, only using it is.
 596                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 597                 {
 598                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 599                     if let ty::Ref(..) = base_ty.kind() {
 600                         return self.validate_place(place_base);
 601                     }
 602                 }
 603                 return Err(Unpromotable);
 604             }
 605
 606             Rvalue::Ref(_, kind, place) => {
 607                 // Special-case reborrows to be more like a copy of the reference.
 608                 let mut place_simplified = place.as_ref();
 609                 if let Some((place_base, ProjectionElem::Deref)) =
 610                     place_simplified.last_projection()
 611                 {
 612                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 613                     if let ty::Ref(..) = base_ty.kind() {
 614                         place_simplified = place_base;
 615                     }
 616                 }
 617
 618                 self.validate_place(place_simplified)?;
 619
 620                 // Check that the reference is fine (using the original place!).
 621                 // (Needs to come after `validate_place` to avoid ICEs.)
 622                 self.validate_ref(*kind, place)?;
 623             }
 624
 625             Rvalue::Aggregate(_, operands) => {
 626                 for o in operands {
 627                     self.validate_operand(o)?;
 628                 }
 629             }
 630         }
 631
 632         Ok(())
 633     }
 634
 635     fn validate_call(
 636         &self,
 637         callee: &Operand<'tcx>,
 638         args: &[Operand<'tcx>],
 639     ) -> Result<(), Unpromotable> {
 640         let fn_ty = callee.ty(self.body, self.tcx);
 641
 642         // Inside const/static items, we promote all (eligible) function calls.
 643         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 644         let promote_all_const_fn = matches!(
 645             self.const_kind,
 646             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 647         );
 648         if !promote_all_const_fn {
 649             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 650                 // Never promote runtime `const fn` calls of
 651                 // functions without `#[rustc_promotable]`.
 652                 if !self.tcx.is_promotable_const_fn(def_id) {
 653                     return Err(Unpromotable);
 654                 }
 655             }
 656         }
 657
 658         let is_const_fn = match *fn_ty.kind() {
 659             ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
 660             _ => false,
 661         };
 662         if !is_const_fn {
 663             return Err(Unpromotable);
 664         }
 665
 666         self.validate_operand(callee)?;
 667         for arg in args {
 668             self.validate_operand(arg)?;
 669         }
 670
 671         Ok(())
 672     }
 673 }
 674
 675 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 676 pub fn validate_candidates(
 677     ccx: &ConstCx<'_, '_>,
 678     temps: &IndexVec<Local, TempState>,
 679     candidates: &[Candidate],
 680 ) -> Vec<Candidate> {
 681     let validator = Validator { ccx, temps };
 682
 683     candidates
 684         .iter()
 685         .copied()
 686         .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
 687         .collect()
 688 }
 689
 690 struct Promoter<'a, 'tcx> {
 691     tcx: TyCtxt<'tcx>,
 692     source: &'a mut Body<'tcx>,
 693     promoted: Body<'tcx>,
 694     temps: &'a mut IndexVec<Local, TempState>,
 695     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 696
 697     /// If true, all nested temps are also kept in the
 698     /// source MIR, not moved to the promoted MIR.
 699     keep_original: bool,
 700 }
 701
 702 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 703     fn new_block(&mut self) -> BasicBlock {
 704         let span = self.promoted.span;
 705         self.promoted.basic_blocks_mut().push(BasicBlockData {
 706             statements: vec![],
 707             terminator: Some(Terminator {
 708                 source_info: SourceInfo::outermost(span),
 709                 kind: TerminatorKind::Return,
 710             }),
 711             is_cleanup: false,
 712         })
 713     }
 714
 715     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 716         let last = self.promoted.basic_blocks().last().unwrap();
 717         let data = &mut self.promoted[last];
 718         data.statements.push(Statement {
 719             source_info: SourceInfo::outermost(span),
 720             kind: StatementKind::Assign(Box::new((Place::from(dest), rvalue))),
 721         });
 722     }
 723
 724     fn is_temp_kind(&self, local: Local) -> bool {
 725         self.source.local_kind(local) == LocalKind::Temp
 726     }
 727
 728     /// Copies the initialization of this temp to the
 729     /// promoted MIR, recursing through temps.
 730     fn promote_temp(&mut self, temp: Local) -> Local {
 731         let old_keep_original = self.keep_original;
 732         let loc = match self.temps[temp] {
 733             TempState::Defined { location, uses } if uses > 0 => {
 734                 if uses > 1 {
 735                     self.keep_original = true;
 736                 }
 737                 location
 738             }
 739             state => {
 740                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 741             }
 742         };
 743         if !self.keep_original {
 744             self.temps[temp] = TempState::PromotedOut;
 745         }
 746
 747         let num_stmts = self.source[loc.block].statements.len();
 748         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 749             self.source.local_decls[temp].ty,
 750             self.source.local_decls[temp].source_info.span,
 751         ));
 752
 753         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 754
 755         // First, take the Rvalue or Call out of the source MIR,
 756         // or duplicate it, depending on keep_original.
 757         if loc.statement_index < num_stmts {
 758             let (mut rvalue, source_info) = {
 759                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 760                 let rhs = match statement.kind {
 761                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 762                     _ => {
 763                         span_bug!(
 764                             statement.source_info.span,
 765                             "{:?} is not an assignment",
 766                             statement
 767                         );
 768                     }
 769                 };
 770
 771                 (
 772                     if self.keep_original {
 773                         rhs.clone()
 774                     } else {
 775                         let unit = Rvalue::Use(Operand::Constant(Box::new(Constant {
 776                             span: statement.source_info.span,
 777                             user_ty: None,
 778                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 779                         })));
 780                         mem::replace(rhs, unit)
 781                     },
 782                     statement.source_info,
 783                 )
 784             };
 785
 786             self.visit_rvalue(&mut rvalue, loc);
 787             self.assign(new_temp, rvalue, source_info.span);
 788         } else {
 789             let terminator = if self.keep_original {
 790                 self.source[loc.block].terminator().clone()
 791             } else {
 792                 let terminator = self.source[loc.block].terminator_mut();
 793                 let target = match terminator.kind {
 794                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 795                     ref kind => {
 796                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 797                     }
 798                 };
 799                 Terminator {
 800                     source_info: terminator.source_info,
 801                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 802                 }
 803             };
 804
 805             match terminator.kind {
 806                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 807                     self.visit_operand(&mut func, loc);
 808                     for arg in &mut args {
 809                         self.visit_operand(arg, loc);
 810                     }
 811
 812                     let last = self.promoted.basic_blocks().last().unwrap();
 813                     let new_target = self.new_block();
 814
 815                     *self.promoted[last].terminator_mut() = Terminator {
 816                         kind: TerminatorKind::Call {
 817                             func,
 818                             args,
 819                             cleanup: None,
 820                             destination: Some((Place::from(new_temp), new_target)),
 821                             from_hir_call,
 822                             fn_span,
 823                         },
 824                         source_info: SourceInfo::outermost(terminator.source_info.span),
 825                         ..terminator
 826                     };
 827                 }
 828                 ref kind => {
 829                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 830                 }
 831             };
 832         };
 833
 834         self.keep_original = old_keep_original;
 835         new_temp
 836     }
 837
 838     fn promote_candidate(
 839         mut self,
 840         candidate: Candidate,
 841         next_promoted_id: usize,
 842     ) -> Option<Body<'tcx>> {
 843         let def = self.source.source.with_opt_param();
 844         let mut rvalue = {
 845             let promoted = &mut self.promoted;
 846             let promoted_id = Promoted::new(next_promoted_id);
 847             let tcx = self.tcx;
 848             let mut promoted_operand = |ty, span| {
 849                 promoted.span = span;
 850                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 851
 852                 Operand::Constant(Box::new(Constant {
 853                     span,
 854                     user_ty: None,
 855                     literal: tcx
 856                         .mk_const(ty::Const {
 857                             ty,
 858                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 859                                 def,
 860                                 substs_: Some(InternalSubsts::for_item(
 861                                     tcx,
 862                                     def.did,
 863                                     |param, _| {
 864                                         if let ty::GenericParamDefKind::Lifetime = param.kind {
 865                                             tcx.lifetimes.re_erased.into()
 866                                         } else {
 867                                             tcx.mk_param_from_def(param)
 868                                         }
 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             tcx,
 994             body.source, // `promoted` gets filled in below
 995             IndexVec::new(),
 996             IndexVec::from_elem_n(scope, 1),
 997             initial_locals,
 998             IndexVec::new(),
 999             0,
1000             vec![],
1001             body.span,
1002             body.generator_kind(),
1003         );
1004
1005         let promoter = Promoter {
1006             promoted,
1007             tcx,
1008             source: body,
1009             temps: &mut temps,
1010             extra_statements: &mut extra_statements,
1011             keep_original: false,
1012         };
1013
1014         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1015         if let Some(mut promoted) = promoter.promote_candidate(candidate, promotions.len()) {
1016             promoted.source.promoted = Some(promotions.next_index());
1017             promotions.push(promoted);
1018         }
1019     }
1020
1021     // Insert each of `extra_statements` before its indicated location, which
1022     // has to be done in reverse location order, to not invalidate the rest.
1023     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1024     for (loc, statement) in extra_statements {
1025         body[loc.block].statements.insert(loc.statement_index, statement);
1026     }
1027
1028     // Eliminate assignments to, and drops of promoted temps.
1029     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1030     for block in body.basic_blocks_mut() {
1031         block.statements.retain(|statement| match &statement.kind {
1032             StatementKind::Assign(box (place, _)) => {
1033                 if let Some(index) = place.as_local() {
1034                     !promoted(index)
1035                 } else {
1036                     true
1037                 }
1038             }
1039             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1040                 !promoted(*index)
1041             }
1042             _ => true,
1043         });
1044         let terminator = block.terminator_mut();
1045         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1046             if let Some(index) = place.as_local() {
1047                 if promoted(index) {
1048                     terminator.kind = TerminatorKind::Goto { target: *target };
1049                 }
1050             }
1051         }
1052     }
1053
1054     promotions
1055 }
1056
1057 /// This function returns `true` if the function being called in the array
1058 /// repeat expression is a `const` function.
1059 pub fn is_const_fn_in_array_repeat_expression<'tcx>(
1060     ccx: &ConstCx<'_, 'tcx>,
1061     place: &Place<'tcx>,
1062     body: &Body<'tcx>,
1063 ) -> bool {
1064     match place.as_local() {
1065         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1066         Some(local) if body.local_decls[local].is_user_variable() => return false,
1067         None => return false,
1068         _ => {}
1069     }
1070
1071     for block in body.basic_blocks() {
1072         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1073             &block.terminator
1074         {
1075             if let Operand::Constant(box Constant { literal, .. }) = func {
1076                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1077                     if let Some((destination_place, _)) = destination {
1078                         if destination_place == place {
1079                             if ccx.tcx.is_const_fn(def_id) {
1080                                 return true;
1081                             }
1082                         }
1083                     }
1084                 }
1085             }
1086         }
1087     }
1088
1089     false
1090 }