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::ReversePostorderIter;
  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::ConstsPromoted)
  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 (mut temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  64
  65         let promotable_candidates = validate_candidates(&ccx, &mut 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, valid: Result<(), ()> },
  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, valid: Err(()) };
 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 ReversePostorderIter<'_, '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 mut 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(&mut 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>(&mut 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     fn validate_local(&mut self, local: Local) -> Result<(), Unpromotable> {
 276         if let TempState::Defined { location: loc, uses, valid } = self.temps[local] {
 277             valid.or_else(|_| {
 278                 let ok = {
 279                     let block = &self.body[loc.block];
 280                     let num_stmts = block.statements.len();
 281
 282                     if loc.statement_index < num_stmts {
 283                         let statement = &block.statements[loc.statement_index];
 284                         match &statement.kind {
 285                             StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 286                             _ => {
 287                                 span_bug!(
 288                                     statement.source_info.span,
 289                                     "{:?} is not an assignment",
 290                                     statement
 291                                 );
 292                             }
 293                         }
 294                     } else {
 295                         let terminator = block.terminator();
 296                         match &terminator.kind {
 297                             TerminatorKind::Call { func, args, .. } => {
 298                                 self.validate_call(func, args)
 299                             }
 300                             TerminatorKind::Yield { .. } => Err(Unpromotable),
 301                             kind => {
 302                                 span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 303                             }
 304                         }
 305                     }
 306                 };
 307                 self.temps[local] = match ok {
 308                     Ok(()) => TempState::Defined { location: loc, uses, valid: Ok(()) },
 309                     Err(_) => TempState::Unpromotable,
 310                 };
 311                 ok
 312             })
 313         } else {
 314             Err(Unpromotable)
 315         }
 316     }
 317
 318     fn validate_place(&mut self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 319         match place.last_projection() {
 320             None => self.validate_local(place.local),
 321             Some((place_base, elem)) => {
 322                 // Validate topmost projection, then recurse.
 323                 match elem {
 324                     ProjectionElem::Deref => {
 325                         let mut promotable = false;
 326                         // We need to make sure this is a `Deref` of a local with no further projections.
 327                         // Discussion can be found at
 328                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 329                         if let Some(local) = place_base.as_local() {
 330                             // This is a special treatment for cases like *&STATIC where STATIC is a
 331                             // global static variable.
 332                             // This pattern is generated only when global static variables are directly
 333                             // accessed and is qualified for promotion safely.
 334                             if let TempState::Defined { location, .. } = self.temps[local] {
 335                                 let def_stmt = self.body[location.block]
 336                                     .statements
 337                                     .get(location.statement_index);
 338                                 if let Some(Statement {
 339                                     kind:
 340                                         StatementKind::Assign(box (
 341                                             _,
 342                                             Rvalue::Use(Operand::Constant(c)),
 343                                         )),
 344                                     ..
 345                                 }) = def_stmt
 346                                 {
 347                                     if let Some(did) = c.check_static_ptr(self.tcx) {
 348                                         // Evaluating a promoted may not read statics except if it got
 349                                         // promoted from a static (this is a CTFE check). So we
 350                                         // can only promote static accesses inside statics.
 351                                         if let Some(hir::ConstContext::Static(..)) = self.const_kind
 352                                         {
 353                                             if !self.tcx.is_thread_local_static(did) {
 354                                                 promotable = true;
 355                                             }
 356                                         }
 357                                     }
 358                                 }
 359                             }
 360                         }
 361                         if !promotable {
 362                             return Err(Unpromotable);
 363                         }
 364                     }
 365                     ProjectionElem::Downcast(..) => {
 366                         return Err(Unpromotable);
 367                     }
 368
 369                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 370
 371                     ProjectionElem::Index(local) => {
 372                         let mut promotable = false;
 373                         // Only accept if we can predict the index and are indexing an array.
 374                         let val =
 375                             if let TempState::Defined { location: loc, .. } = self.temps[local] {
 376                                 let block = &self.body[loc.block];
 377                                 if loc.statement_index < block.statements.len() {
 378                                     let statement = &block.statements[loc.statement_index];
 379                                     match &statement.kind {
 380                                         StatementKind::Assign(box (
 381                                             _,
 382                                             Rvalue::Use(Operand::Constant(c)),
 383                                         )) => c.literal.try_eval_usize(self.tcx, self.param_env),
 384                                         _ => None,
 385                                     }
 386                                 } else {
 387                                     None
 388                                 }
 389                             } else {
 390                                 None
 391                             };
 392                         if let Some(idx) = val {
 393                             // Determine the type of the thing we are indexing.
 394                             let ty = place_base.ty(self.body, self.tcx).ty;
 395                             match ty.kind() {
 396                                 ty::Array(_, len) => {
 397                                     // It's an array; determine its length.
 398                                     if let Some(len) = len.try_eval_usize(self.tcx, self.param_env)
 399                                     {
 400                                         // If the index is in-bounds, go ahead.
 401                                         if idx < len {
 402                                             promotable = true;
 403                                         }
 404                                     }
 405                                 }
 406                                 _ => {}
 407                             }
 408                         }
 409                         if !promotable {
 410                             return Err(Unpromotable);
 411                         }
 412
 413                         self.validate_local(local)?;
 414                     }
 415
 416                     ProjectionElem::Field(..) => {
 417                         let base_ty = place_base.ty(self.body, self.tcx).ty;
 418                         if base_ty.is_union() {
 419                             // No promotion of union field accesses.
 420                             return Err(Unpromotable);
 421                         }
 422                     }
 423                 }
 424
 425                 self.validate_place(place_base)
 426             }
 427         }
 428     }
 429
 430     fn validate_operand(&mut self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 431         match operand {
 432             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 433
 434             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 435             // `validate_rvalue` upon access.
 436             Operand::Constant(c) => {
 437                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 438                     // Only allow statics (not consts) to refer to other statics.
 439                     // FIXME(eddyb) does this matter at all for promotion?
 440                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 441                     // and in `const` this cannot occur anyway. The only concern is that we might
 442                     // promote even `let x = &STATIC` which would be useless, but this applies to
 443                     // promotion inside statics as well.
 444                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 445                     if !is_static {
 446                         return Err(Unpromotable);
 447                     }
 448
 449                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 450                     if is_thread_local {
 451                         return Err(Unpromotable);
 452                     }
 453                 }
 454
 455                 Ok(())
 456             }
 457         }
 458     }
 459
 460     fn validate_ref(&mut self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
 461         match kind {
 462             // Reject these borrow types just to be safe.
 463             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
 464             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 465
 466             BorrowKind::Shared => {
 467                 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
 468                 if has_mut_interior {
 469                     return Err(Unpromotable);
 470                 }
 471             }
 472
 473             BorrowKind::Mut { .. } => {
 474                 let ty = place.ty(self.body, self.tcx).ty;
 475
 476                 // In theory, any zero-sized value could be borrowed
 477                 // mutably without consequences. However, only &mut []
 478                 // is allowed right now.
 479                 if let ty::Array(_, len) = ty.kind() {
 480                     match len.try_eval_usize(self.tcx, self.param_env) {
 481                         Some(0) => {}
 482                         _ => return Err(Unpromotable),
 483                     }
 484                 } else {
 485                     return Err(Unpromotable);
 486                 }
 487             }
 488         }
 489
 490         Ok(())
 491     }
 492
 493     fn validate_rvalue(&mut self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 494         match rvalue {
 495             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
 496                 self.validate_operand(operand)?;
 497             }
 498
 499             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 500                 self.validate_place(place.as_ref())?
 501             }
 502
 503             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 504
 505             Rvalue::Cast(kind, operand, cast_ty) => {
 506                 if matches!(kind, CastKind::Misc) {
 507                     let operand_ty = operand.ty(self.body, self.tcx);
 508                     let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 509                     let cast_out = CastTy::from_ty(*cast_ty).expect("bad output type for cast");
 510                     if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 511                         // ptr-to-int casts are not possible in consts and thus not promotable
 512                         return Err(Unpromotable);
 513                     }
 514                     // int-to-ptr casts are fine, they just use the integer value at pointer type.
 515                 }
 516
 517                 self.validate_operand(operand)?;
 518             }
 519
 520             Rvalue::NullaryOp(op, _) => match op {
 521                 NullOp::SizeOf => {}
 522                 NullOp::AlignOf => {}
 523             },
 524
 525             Rvalue::ShallowInitBox(_, _) => return Err(Unpromotable),
 526
 527             Rvalue::UnaryOp(op, operand) => {
 528                 match op {
 529                     // These operations can never fail.
 530                     UnOp::Neg | UnOp::Not => {}
 531                 }
 532
 533                 self.validate_operand(operand)?;
 534             }
 535
 536             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 537                 let op = *op;
 538                 let lhs_ty = lhs.ty(self.body, self.tcx);
 539
 540                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 541                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 542                     assert!(matches!(
 543                         op,
 544                         BinOp::Eq
 545                             | BinOp::Ne
 546                             | BinOp::Le
 547                             | BinOp::Lt
 548                             | BinOp::Ge
 549                             | BinOp::Gt
 550                             | BinOp::Offset
 551                     ));
 552                     return Err(Unpromotable);
 553                 }
 554
 555                 match op {
 556                     BinOp::Div | BinOp::Rem => {
 557                         if lhs_ty.is_integral() {
 558                             // Integer division: the RHS must be a non-zero const.
 559                             let const_val = match rhs {
 560                                 Operand::Constant(c) => {
 561                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 562                                 }
 563                                 _ => None,
 564                             };
 565                             match const_val {
 566                                 Some(x) if x != 0 => {}        // okay
 567                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 568                             }
 569                         }
 570                     }
 571                     // The remaining operations can never fail.
 572                     BinOp::Eq
 573                     | BinOp::Ne
 574                     | BinOp::Le
 575                     | BinOp::Lt
 576                     | BinOp::Ge
 577                     | BinOp::Gt
 578                     | BinOp::Offset
 579                     | BinOp::Add
 580                     | BinOp::Sub
 581                     | BinOp::Mul
 582                     | BinOp::BitXor
 583                     | BinOp::BitAnd
 584                     | BinOp::BitOr
 585                     | BinOp::Shl
 586                     | BinOp::Shr => {}
 587                 }
 588
 589                 self.validate_operand(lhs)?;
 590                 self.validate_operand(rhs)?;
 591             }
 592
 593             Rvalue::AddressOf(_, place) => {
 594                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 595                 // no problem, only using it is.
 596                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 597                 {
 598                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 599                     if let ty::Ref(..) = base_ty.kind() {
 600                         return self.validate_place(place_base);
 601                     }
 602                 }
 603                 return Err(Unpromotable);
 604             }
 605
 606             Rvalue::Ref(_, kind, place) => {
 607                 // Special-case reborrows to be more like a copy of the reference.
 608                 let mut place_simplified = place.as_ref();
 609                 if let Some((place_base, ProjectionElem::Deref)) =
 610                     place_simplified.last_projection()
 611                 {
 612                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 613                     if let ty::Ref(..) = base_ty.kind() {
 614                         place_simplified = place_base;
 615                     }
 616                 }
 617
 618                 self.validate_place(place_simplified)?;
 619
 620                 // Check that the reference is fine (using the original place!).
 621                 // (Needs to come after `validate_place` to avoid ICEs.)
 622                 self.validate_ref(*kind, place)?;
 623             }
 624
 625             Rvalue::Aggregate(_, operands) => {
 626                 for o in operands {
 627                     self.validate_operand(o)?;
 628                 }
 629             }
 630         }
 631
 632         Ok(())
 633     }
 634
 635     fn validate_call(
 636         &mut self,
 637         callee: &Operand<'tcx>,
 638         args: &[Operand<'tcx>],
 639     ) -> Result<(), Unpromotable> {
 640         let fn_ty = callee.ty(self.body, self.tcx);
 641
 642         // Inside const/static items, we promote all (eligible) function calls.
 643         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 644         let promote_all_const_fn = matches!(
 645             self.const_kind,
 646             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 647         );
 648         if !promote_all_const_fn {
 649             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 650                 // Never promote runtime `const fn` calls of
 651                 // functions without `#[rustc_promotable]`.
 652                 if !self.tcx.is_promotable_const_fn(def_id) {
 653                     return Err(Unpromotable);
 654                 }
 655             }
 656         }
 657
 658         let is_const_fn = match *fn_ty.kind() {
 659             ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
 660             _ => false,
 661         };
 662         if !is_const_fn {
 663             return Err(Unpromotable);
 664         }
 665
 666         self.validate_operand(callee)?;
 667         for arg in args {
 668             self.validate_operand(arg)?;
 669         }
 670
 671         Ok(())
 672     }
 673 }
 674
 675 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 676 pub fn validate_candidates(
 677     ccx: &ConstCx<'_, '_>,
 678     temps: &mut IndexVec<Local, TempState>,
 679     candidates: &[Candidate],
 680 ) -> Vec<Candidate> {
 681     let mut validator = Validator { ccx, temps };
 682
 683     candidates
 684         .iter()
 685         .copied()
 686         .filter(|&candidate| validator.validate_candidate(candidate).is_ok())
 687         .collect()
 688 }
 689
 690 struct Promoter<'a, 'tcx> {
 691     tcx: TyCtxt<'tcx>,
 692     source: &'a mut Body<'tcx>,
 693     promoted: Body<'tcx>,
 694     temps: &'a mut IndexVec<Local, TempState>,
 695     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 696
 697     /// If true, all nested temps are also kept in the
 698     /// source MIR, not moved to the promoted MIR.
 699     keep_original: bool,
 700 }
 701
 702 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 703     fn new_block(&mut self) -> BasicBlock {
 704         let span = self.promoted.span;
 705         self.promoted.basic_blocks_mut().push(BasicBlockData {
 706             statements: vec![],
 707             terminator: Some(Terminator {
 708                 source_info: SourceInfo::outermost(span),
 709                 kind: TerminatorKind::Return,
 710             }),
 711             is_cleanup: false,
 712         })
 713     }
 714
 715     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 716         let last = self.promoted.basic_blocks().last().unwrap();
 717         let data = &mut self.promoted[last];
 718         data.statements.push(Statement {
 719             source_info: SourceInfo::outermost(span),
 720             kind: StatementKind::Assign(Box::new((Place::from(dest), rvalue))),
 721         });
 722     }
 723
 724     fn is_temp_kind(&self, local: Local) -> bool {
 725         self.source.local_kind(local) == LocalKind::Temp
 726     }
 727
 728     /// Copies the initialization of this temp to the
 729     /// promoted MIR, recursing through temps.
 730     fn promote_temp(&mut self, temp: Local) -> Local {
 731         let old_keep_original = self.keep_original;
 732         let loc = match self.temps[temp] {
 733             TempState::Defined { location, uses, .. } if uses > 0 => {
 734                 if uses > 1 {
 735                     self.keep_original = true;
 736                 }
 737                 location
 738             }
 739             state => {
 740                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 741             }
 742         };
 743         if !self.keep_original {
 744             self.temps[temp] = TempState::PromotedOut;
 745         }
 746
 747         let num_stmts = self.source[loc.block].statements.len();
 748         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 749             self.source.local_decls[temp].ty,
 750             self.source.local_decls[temp].source_info.span,
 751         ));
 752
 753         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 754
 755         // First, take the Rvalue or Call out of the source MIR,
 756         // or duplicate it, depending on keep_original.
 757         if loc.statement_index < num_stmts {
 758             let (mut rvalue, source_info) = {
 759                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 760                 let StatementKind::Assign(box (_, ref mut rhs)) = statement.kind else {
 761                     span_bug!(
 762                         statement.source_info.span,
 763                         "{:?} is not an assignment",
 764                         statement
 765                     );
 766                 };
 767
 768                 (
 769                     if self.keep_original {
 770                         rhs.clone()
 771                     } else {
 772                         let unit = Rvalue::Use(Operand::Constant(Box::new(Constant {
 773                             span: statement.source_info.span,
 774                             user_ty: None,
 775                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 776                         })));
 777                         mem::replace(rhs, unit)
 778                     },
 779                     statement.source_info,
 780                 )
 781             };
 782
 783             self.visit_rvalue(&mut rvalue, loc);
 784             self.assign(new_temp, rvalue, source_info.span);
 785         } else {
 786             let terminator = if self.keep_original {
 787                 self.source[loc.block].terminator().clone()
 788             } else {
 789                 let terminator = self.source[loc.block].terminator_mut();
 790                 let target = match terminator.kind {
 791                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 792                     ref kind => {
 793                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 794                     }
 795                 };
 796                 Terminator {
 797                     source_info: terminator.source_info,
 798                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 799                 }
 800             };
 801
 802             match terminator.kind {
 803                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 804                     self.visit_operand(&mut func, loc);
 805                     for arg in &mut args {
 806                         self.visit_operand(arg, loc);
 807                     }
 808
 809                     let last = self.promoted.basic_blocks().last().unwrap();
 810                     let new_target = self.new_block();
 811
 812                     *self.promoted[last].terminator_mut() = Terminator {
 813                         kind: TerminatorKind::Call {
 814                             func,
 815                             args,
 816                             cleanup: None,
 817                             destination: Some((Place::from(new_temp), new_target)),
 818                             from_hir_call,
 819                             fn_span,
 820                         },
 821                         source_info: SourceInfo::outermost(terminator.source_info.span),
 822                         ..terminator
 823                     };
 824                 }
 825                 ref kind => {
 826                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 827                 }
 828             };
 829         };
 830
 831         self.keep_original = old_keep_original;
 832         new_temp
 833     }
 834
 835     fn promote_candidate(mut self, candidate: Candidate, next_promoted_id: usize) -> Body<'tcx> {
 836         let def = self.source.source.with_opt_param();
 837         let mut rvalue = {
 838             let promoted = &mut self.promoted;
 839             let promoted_id = Promoted::new(next_promoted_id);
 840             let tcx = self.tcx;
 841             let mut promoted_operand = |ty, span| {
 842                 promoted.span = span;
 843                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 844
 845                 Operand::Constant(Box::new(Constant {
 846                     span,
 847                     user_ty: None,
 848                     literal: tcx
 849                         .mk_const(ty::ConstS {
 850                             ty,
 851                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 852                                 def,
 853                                 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
 854                                     if let ty::GenericParamDefKind::Lifetime = param.kind {
 855                                         tcx.lifetimes.re_erased.into()
 856                                     } else {
 857                                         tcx.mk_param_from_def(param)
 858                                     }
 859                                 }),
 860                                 promoted: Some(promoted_id),
 861                             }),
 862                         })
 863                         .into(),
 864                 }))
 865             };
 866             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 867             let loc = candidate.location;
 868             let statement = &mut blocks[loc.block].statements[loc.statement_index];
 869             match statement.kind {
 870                 StatementKind::Assign(box (
 871                     _,
 872                     Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 873                 )) => {
 874                     // Use the underlying local for this (necessarily interior) borrow.
 875                     let ty = local_decls.local_decls()[place.local].ty;
 876                     let span = statement.source_info.span;
 877
 878                     let ref_ty = tcx.mk_ref(
 879                         tcx.lifetimes.re_erased,
 880                         ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 881                     );
 882
 883                     *region = tcx.lifetimes.re_erased;
 884
 885                     let mut projection = vec![PlaceElem::Deref];
 886                     projection.extend(place.projection);
 887                     place.projection = tcx.intern_place_elems(&projection);
 888
 889                     // Create a temp to hold the promoted reference.
 890                     // This is because `*r` requires `r` to be a local,
 891                     // otherwise we would use the `promoted` directly.
 892                     let mut promoted_ref = LocalDecl::new(ref_ty, span);
 893                     promoted_ref.source_info = statement.source_info;
 894                     let promoted_ref = local_decls.push(promoted_ref);
 895                     assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 896
 897                     let promoted_ref_statement = Statement {
 898                         source_info: statement.source_info,
 899                         kind: StatementKind::Assign(Box::new((
 900                             Place::from(promoted_ref),
 901                             Rvalue::Use(promoted_operand(ref_ty, span)),
 902                         ))),
 903                     };
 904                     self.extra_statements.push((loc, promoted_ref_statement));
 905
 906                     Rvalue::Ref(
 907                         tcx.lifetimes.re_erased,
 908                         borrow_kind,
 909                         Place {
 910                             local: mem::replace(&mut place.local, promoted_ref),
 911                             projection: List::empty(),
 912                         },
 913                     )
 914                 }
 915                 _ => bug!(),
 916             }
 917         };
 918
 919         assert_eq!(self.new_block(), START_BLOCK);
 920         self.visit_rvalue(
 921             &mut rvalue,
 922             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 923         );
 924
 925         let span = self.promoted.span;
 926         self.assign(RETURN_PLACE, rvalue, span);
 927         self.promoted
 928     }
 929 }
 930
 931 /// Replaces all temporaries with their promoted counterparts.
 932 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 933     fn tcx(&self) -> TyCtxt<'tcx> {
 934         self.tcx
 935     }
 936
 937     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 938         if self.is_temp_kind(*local) {
 939             *local = self.promote_temp(*local);
 940         }
 941     }
 942 }
 943
 944 pub fn promote_candidates<'tcx>(
 945     body: &mut Body<'tcx>,
 946     tcx: TyCtxt<'tcx>,
 947     mut temps: IndexVec<Local, TempState>,
 948     candidates: Vec<Candidate>,
 949 ) -> IndexVec<Promoted, Body<'tcx>> {
 950     // Visit candidates in reverse, in case they're nested.
 951     debug!("promote_candidates({:?})", candidates);
 952
 953     let mut promotions = IndexVec::new();
 954
 955     let mut extra_statements = vec![];
 956     for candidate in candidates.into_iter().rev() {
 957         let Location { block, statement_index } = candidate.location;
 958         if let StatementKind::Assign(box (place, _)) = &body[block].statements[statement_index].kind
 959         {
 960             if let Some(local) = place.as_local() {
 961                 if temps[local] == TempState::PromotedOut {
 962                     // Already promoted.
 963                     continue;
 964                 }
 965             }
 966         }
 967
 968         // Declare return place local so that `mir::Body::new` doesn't complain.
 969         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
 970
 971         let mut scope = body.source_scopes[body.source_info(candidate.location).scope].clone();
 972         scope.parent_scope = None;
 973
 974         let promoted = Body::new(
 975             body.source, // `promoted` gets filled in below
 976             IndexVec::new(),
 977             IndexVec::from_elem_n(scope, 1),
 978             initial_locals,
 979             IndexVec::new(),
 980             0,
 981             vec![],
 982             body.span,
 983             body.generator_kind(),
 984             body.tainted_by_errors,
 985         );
 986
 987         let promoter = Promoter {
 988             promoted,
 989             tcx,
 990             source: body,
 991             temps: &mut temps,
 992             extra_statements: &mut extra_statements,
 993             keep_original: false,
 994         };
 995
 996         let mut promoted = promoter.promote_candidate(candidate, promotions.len());
 997         promoted.source.promoted = Some(promotions.next_index());
 998         promotions.push(promoted);
 999     }
1000
1001     // Insert each of `extra_statements` before its indicated location, which
1002     // has to be done in reverse location order, to not invalidate the rest.
1003     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1004     for (loc, statement) in extra_statements {
1005         body[loc.block].statements.insert(loc.statement_index, statement);
1006     }
1007
1008     // Eliminate assignments to, and drops of promoted temps.
1009     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1010     for block in body.basic_blocks_mut() {
1011         block.statements.retain(|statement| match &statement.kind {
1012             StatementKind::Assign(box (place, _)) => {
1013                 if let Some(index) = place.as_local() {
1014                     !promoted(index)
1015                 } else {
1016                     true
1017                 }
1018             }
1019             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1020                 !promoted(*index)
1021             }
1022             _ => true,
1023         });
1024         let terminator = block.terminator_mut();
1025         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1026             if let Some(index) = place.as_local() {
1027                 if promoted(index) {
1028                     terminator.kind = TerminatorKind::Goto { target: *target };
1029                 }
1030             }
1031         }
1032     }
1033
1034     promotions
1035 }
1036
1037 /// This function returns `true` if the function being called in the array
1038 /// repeat expression is a `const` function.
1039 pub fn is_const_fn_in_array_repeat_expression<'tcx>(
1040     ccx: &ConstCx<'_, 'tcx>,
1041     place: &Place<'tcx>,
1042     body: &Body<'tcx>,
1043 ) -> bool {
1044     match place.as_local() {
1045         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1046         Some(local) if body.local_decls[local].is_user_variable() => return false,
1047         None => return false,
1048         _ => {}
1049     }
1050
1051     for block in body.basic_blocks() {
1052         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1053             &block.terminator
1054         {
1055             if let Operand::Constant(box Constant { literal, .. }) = func {
1056                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1057                     if let Some((destination_place, _)) = destination {
1058                         if destination_place == place {
1059                             if ccx.tcx.is_const_fn(def_id) {
1060                                 return true;
1061                             }
1062                         }
1063                     }
1064                 }
1065             }
1066         }
1067     }
1068
1069     false
1070 }