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