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