compiler/rustc_mir/src/transform/promote_consts.rs

   1 //! A pass that promotes borrows of constant rvalues.
   2 //!
   3 //! The rvalues considered constant are trees of temps,
   4 //! each with exactly one initialization, and holding
   5 //! a constant value with no interior mutability.
   6 //! They are placed into a new MIR constant body in
   7 //! `promoted` and the borrow rvalue is replaced with
   8 //! a `Literal::Promoted` using the index into `promoted`
   9 //! of that constant MIR.
  10 //!
  11 //! This pass assumes that every use is dominated by an
  12 //! initialization and can otherwise silence errors, if
  13 //! move analysis runs after promotion on broken MIR.
  14
  15 use rustc_ast::LitKind;
  16 use rustc_hir as hir;
  17 use rustc_hir::def_id::DefId;
  18 use rustc_middle::mir::traversal::ReversePostorder;
  19 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  20 use rustc_middle::mir::*;
  21 use rustc_middle::ty::cast::CastTy;
  22 use rustc_middle::ty::subst::InternalSubsts;
  23 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
  24 use rustc_span::symbol::sym;
  25 use rustc_span::Span;
  26
  27 use rustc_index::vec::{Idx, IndexVec};
  28 use rustc_target::spec::abi::Abi;
  29
  30 use std::cell::Cell;
  31 use std::{cmp, iter, mem};
  32
  33 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
  34 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstCx};
  35 use crate::transform::MirPass;
  36
  37 /// A `MirPass` for promotion.
  38 ///
  39 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
  40 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
  41 ///
  42 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  43 /// newly created `Constant`.
  44 #[derive(Default)]
  45 pub struct PromoteTemps<'tcx> {
  46     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  47 }
  48
  49 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  50     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  51         // There's not really any point in promoting errorful MIR.
  52         //
  53         // This does not include MIR that failed const-checking, which we still try to promote.
  54         if body.return_ty().references_error() {
  55             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  56             return;
  57         }
  58
  59         if body.source.promoted.is_some() {
  60             return;
  61         }
  62
  63         let mut rpo = traversal::reverse_postorder(body);
  64         let ccx = ConstCx::new(tcx, body);
  65         let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  66
  67         let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates);
  68
  69         let promoted = promote_candidates(body, tcx, temps, promotable_candidates);
  70         self.promoted_fragments.set(promoted);
  71     }
  72 }
  73
  74 /// State of a temporary during collection and promotion.
  75 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  76 pub enum TempState {
  77     /// No references to this temp.
  78     Undefined,
  79     /// One direct assignment and any number of direct uses.
  80     /// A borrow of this temp is promotable if the assigned
  81     /// value is qualified as constant.
  82     Defined { location: Location, uses: usize },
  83     /// Any other combination of assignments/uses.
  84     Unpromotable,
  85     /// This temp was part of an rvalue which got extracted
  86     /// during promotion and needs cleanup.
  87     PromotedOut,
  88 }
  89
  90 impl TempState {
  91     pub fn is_promotable(&self) -> bool {
  92         debug!("is_promotable: self={:?}", self);
  93         matches!(self, TempState::Defined { .. })
  94     }
  95 }
  96
  97 /// A "root candidate" for promotion, which will become the
  98 /// returned value in a promoted MIR, unless it's a subset
  99 /// of a larger candidate.
 100 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 101 pub enum Candidate {
 102     /// Borrow of a constant temporary, candidate for lifetime extension.
 103     Ref(Location),
 104
 105     /// Currently applied to function calls where the callee has the unstable
 106     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
 107     /// intrinsic. The intrinsic requires the arguments are indeed constant and
 108     /// the attribute currently provides the semantic requirement that arguments
 109     /// must be constant.
 110     Argument { bb: BasicBlock, index: usize },
 111 }
 112
 113 impl Candidate {
 114     /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
 115     fn forces_explicit_promotion(&self) -> bool {
 116         match self {
 117             Candidate::Ref(_) => false,
 118             Candidate::Argument { .. } => true,
 119         }
 120     }
 121
 122     fn source_info(&self, body: &Body<'_>) -> SourceInfo {
 123         match self {
 124             Candidate::Ref(location) => *body.source_info(*location),
 125             Candidate::Argument { bb, .. } => *body.source_info(body.terminator_loc(*bb)),
 126         }
 127     }
 128 }
 129
 130 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
 131     let attrs = tcx.get_attrs(def_id);
 132     let attr = attrs.iter().find(|a| tcx.sess.check_name(a, sym::rustc_args_required_const))?;
 133     let mut ret = vec![];
 134     for meta in attr.meta_item_list()? {
 135         match meta.literal()?.kind {
 136             LitKind::Int(a, _) => {
 137                 ret.push(a as usize);
 138             }
 139             _ => bug!("invalid arg index"),
 140         }
 141     }
 142     Some(ret)
 143 }
 144
 145 struct Collector<'a, 'tcx> {
 146     ccx: &'a ConstCx<'a, 'tcx>,
 147     temps: IndexVec<Local, TempState>,
 148     candidates: Vec<Candidate>,
 149 }
 150
 151 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 152     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 153         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 154         // We're only interested in temporaries and the return place
 155         match self.ccx.body.local_kind(index) {
 156             LocalKind::Temp | LocalKind::ReturnPointer => {}
 157             LocalKind::Arg | LocalKind::Var => return,
 158         }
 159
 160         // Ignore drops, if the temp gets promoted,
 161         // then it's constant and thus drop is noop.
 162         // Non-uses are also irrelevant.
 163         if context.is_drop() || !context.is_use() {
 164             debug!(
 165                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 166                 context.is_drop(),
 167                 context.is_use(),
 168             );
 169             return;
 170         }
 171
 172         let temp = &mut self.temps[index];
 173         debug!("visit_local: temp={:?}", temp);
 174         if *temp == TempState::Undefined {
 175             match context {
 176                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 177                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 178                     *temp = TempState::Defined { location, uses: 0 };
 179                     return;
 180                 }
 181                 _ => { /* mark as unpromotable below */ }
 182             }
 183         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 184             // We always allow borrows, even mutable ones, as we need
 185             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 186             let allowed_use = match context {
 187                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 188                 | PlaceContext::NonMutatingUse(_) => true,
 189                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 190             };
 191             debug!("visit_local: allowed_use={:?}", allowed_use);
 192             if allowed_use {
 193                 *uses += 1;
 194                 return;
 195             }
 196             /* mark as unpromotable below */
 197         }
 198         *temp = TempState::Unpromotable;
 199     }
 200
 201     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 202         self.super_rvalue(rvalue, location);
 203
 204         match *rvalue {
 205             Rvalue::Ref(..) => {
 206                 self.candidates.push(Candidate::Ref(location));
 207             }
 208             _ => {}
 209         }
 210     }
 211
 212     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 213         self.super_terminator(terminator, location);
 214
 215         if let TerminatorKind::Call { ref func, .. } = terminator.kind {
 216             if let ty::FnDef(def_id, _) = *func.ty(self.ccx.body, self.ccx.tcx).kind() {
 217                 let fn_sig = self.ccx.tcx.fn_sig(def_id);
 218                 if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
 219                     let name = self.ccx.tcx.item_name(def_id);
 220                     // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
 221                     if name.as_str().starts_with("simd_shuffle") {
 222                         self.candidates.push(Candidate::Argument { bb: location.block, index: 2 });
 223
 224                         return; // Don't double count `simd_shuffle` candidates
 225                     }
 226                 }
 227
 228                 if let Some(constant_args) = args_required_const(self.ccx.tcx, def_id) {
 229                     for index in constant_args {
 230                         self.candidates.push(Candidate::Argument { bb: location.block, index });
 231                     }
 232                 }
 233             }
 234         }
 235     }
 236 }
 237
 238 pub fn collect_temps_and_candidates(
 239     ccx: &ConstCx<'mir, 'tcx>,
 240     rpo: &mut ReversePostorder<'_, 'tcx>,
 241 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 242     let mut collector = Collector {
 243         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
 244         candidates: vec![],
 245         ccx,
 246     };
 247     for (bb, data) in rpo {
 248         collector.visit_basic_block_data(bb, data);
 249     }
 250     (collector.temps, collector.candidates)
 251 }
 252
 253 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 254 ///
 255 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 256 struct Validator<'a, 'tcx> {
 257     ccx: &'a ConstCx<'a, 'tcx>,
 258     temps: &'a IndexVec<Local, TempState>,
 259
 260     /// Explicit promotion happens e.g. for constant arguments declared via
 261     /// `rustc_args_required_const`.
 262     /// Implicit promotion has almost the same rules, except that disallows `const fn`
 263     /// except for those marked `#[rustc_promotable]`. This is to avoid changing
 264     /// a legitimate run-time operation into a failing compile-time operation
 265     /// e.g. due to addresses being compared inside the function.
 266     explicit: bool,
 267 }
 268
 269 impl std::ops::Deref for Validator<'a, 'tcx> {
 270     type Target = ConstCx<'a, 'tcx>;
 271
 272     fn deref(&self) -> &Self::Target {
 273         &self.ccx
 274     }
 275 }
 276
 277 struct Unpromotable;
 278
 279 impl<'tcx> Validator<'_, 'tcx> {
 280     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 281         match candidate {
 282             Candidate::Ref(loc) => {
 283                 assert!(!self.explicit);
 284
 285                 let statement = &self.body[loc.block].statements[loc.statement_index];
 286                 match &statement.kind {
 287                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 288                         // We can only promote interior borrows of promotable temps (non-temps
 289                         // don't get promoted anyway).
 290                         self.validate_local(place.local)?;
 291
 292                         // The reference operation itself must be promotable.
 293                         // (Needs to come after `validate_local` to avoid ICEs.)
 294                         self.validate_ref(*kind, place)?;
 295
 296                         // We do not check all the projections (they do not get promoted anyway),
 297                         // but we do stay away from promoting anything involving a dereference.
 298                         if place.projection.contains(&ProjectionElem::Deref) {
 299                             return Err(Unpromotable);
 300                         }
 301
 302                         // We cannot promote things that need dropping, since the promoted value
 303                         // would not get dropped.
 304                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 305                             return Err(Unpromotable);
 306                         }
 307
 308                         Ok(())
 309                     }
 310                     _ => bug!(),
 311                 }
 312             }
 313             Candidate::Argument { bb, index } => {
 314                 assert!(self.explicit);
 315
 316                 let terminator = self.body[bb].terminator();
 317                 match &terminator.kind {
 318                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 319                     _ => bug!(),
 320                 }
 321             }
 322         }
 323     }
 324
 325     // FIXME(eddyb) maybe cache this?
 326     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 327         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 328             let num_stmts = self.body[loc.block].statements.len();
 329
 330             if loc.statement_index < num_stmts {
 331                 let statement = &self.body[loc.block].statements[loc.statement_index];
 332                 match &statement.kind {
 333                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 334                         &self.ccx,
 335                         &mut |l| self.qualif_local::<Q>(l),
 336                         rhs,
 337                     ),
 338                     _ => {
 339                         span_bug!(
 340                             statement.source_info.span,
 341                             "{:?} is not an assignment",
 342                             statement
 343                         );
 344                     }
 345                 }
 346             } else {
 347                 let terminator = self.body[loc.block].terminator();
 348                 match &terminator.kind {
 349                     TerminatorKind::Call { .. } => {
 350                         let return_ty = self.body.local_decls[local].ty;
 351                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 352                     }
 353                     kind => {
 354                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 355                     }
 356                 }
 357             }
 358         } else {
 359             let span = self.body.local_decls[local].source_info.span;
 360             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 361         }
 362     }
 363
 364     // FIXME(eddyb) maybe cache this?
 365     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 366         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 367             let block = &self.body[loc.block];
 368             let num_stmts = block.statements.len();
 369
 370             if loc.statement_index < num_stmts {
 371                 let statement = &block.statements[loc.statement_index];
 372                 match &statement.kind {
 373                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 374                     _ => {
 375                         span_bug!(
 376                             statement.source_info.span,
 377                             "{:?} is not an assignment",
 378                             statement
 379                         );
 380                     }
 381                 }
 382             } else {
 383                 let terminator = block.terminator();
 384                 match &terminator.kind {
 385                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 386                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 387                     kind => {
 388                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 389                     }
 390                 }
 391             }
 392         } else {
 393             Err(Unpromotable)
 394         }
 395     }
 396
 397     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 398         match place.last_projection() {
 399             None => self.validate_local(place.local),
 400             Some((place_base, elem)) => {
 401                 // Validate topmost projection, then recurse.
 402                 match elem {
 403                     ProjectionElem::Deref => {
 404                         let mut promotable = false;
 405                         // We need to make sure this is a `Deref` of a local with no further projections.
 406                         // Discussion can be found at
 407                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 408                         if let Some(local) = place_base.as_local() {
 409                             // This is a special treatment for cases like *&STATIC where STATIC is a
 410                             // global static variable.
 411                             // This pattern is generated only when global static variables are directly
 412                             // accessed and is qualified for promotion safely.
 413                             if let TempState::Defined { location, .. } = self.temps[local] {
 414                                 let def_stmt = self.body[location.block]
 415                                     .statements
 416                                     .get(location.statement_index);
 417                                 if let Some(Statement {
 418                                     kind:
 419                                         StatementKind::Assign(box (
 420                                             _,
 421                                             Rvalue::Use(Operand::Constant(c)),
 422                                         )),
 423                                     ..
 424                                 }) = def_stmt
 425                                 {
 426                                     if let Some(did) = c.check_static_ptr(self.tcx) {
 427                                         // Evaluating a promoted may not read statics except if it got
 428                                         // promoted from a static (this is a CTFE check). So we
 429                                         // can only promote static accesses inside statics.
 430                                         if let Some(hir::ConstContext::Static(..)) = self.const_kind
 431                                         {
 432                                             if !self.tcx.is_thread_local_static(did) {
 433                                                 promotable = true;
 434                                             }
 435                                         }
 436                                     }
 437                                 }
 438                             }
 439                         }
 440                         if !promotable {
 441                             return Err(Unpromotable);
 442                         }
 443                     }
 444                     ProjectionElem::Downcast(..) => {
 445                         return Err(Unpromotable);
 446                     }
 447
 448                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 449
 450                     ProjectionElem::Index(local) => {
 451                         if !self.explicit {
 452                             let mut promotable = false;
 453                             // Only accept if we can predict the index and are indexing an array.
 454                             let val = if let TempState::Defined { location: loc, .. } =
 455                                 self.temps[local]
 456                             {
 457                                 let block = &self.body[loc.block];
 458                                 if loc.statement_index < block.statements.len() {
 459                                     let statement = &block.statements[loc.statement_index];
 460                                     match &statement.kind {
 461                                         StatementKind::Assign(box (
 462                                             _,
 463                                             Rvalue::Use(Operand::Constant(c)),
 464                                         )) => c.literal.try_eval_usize(self.tcx, self.param_env),
 465                                         _ => None,
 466                                     }
 467                                 } else {
 468                                     None
 469                                 }
 470                             } else {
 471                                 None
 472                             };
 473                             if let Some(idx) = val {
 474                                 // Determine the type of the thing we are indexing.
 475                                 let ty = place_base.ty(self.body, self.tcx).ty;
 476                                 match ty.kind() {
 477                                     ty::Array(_, len) => {
 478                                         // It's an array; determine its length.
 479                                         if let Some(len) =
 480                                             len.try_eval_usize(self.tcx, self.param_env)
 481                                         {
 482                                             // If the index is in-bounds, go ahead.
 483                                             if idx < len {
 484                                                 promotable = true;
 485                                             }
 486                                         }
 487                                     }
 488                                     _ => {}
 489                                 }
 490                             }
 491                             if !promotable {
 492                                 return Err(Unpromotable);
 493                             }
 494                         }
 495                         self.validate_local(local)?;
 496                     }
 497
 498                     ProjectionElem::Field(..) => {
 499                         let base_ty = place_base.ty(self.body, self.tcx).ty;
 500                         if let Some(def) = base_ty.ty_adt_def() {
 501                             // No promotion of union field accesses.
 502                             if def.is_union() {
 503                                 return Err(Unpromotable);
 504                             }
 505                         }
 506                     }
 507                 }
 508
 509                 self.validate_place(place_base)
 510             }
 511         }
 512     }
 513
 514     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 515         match operand {
 516             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 517
 518             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 519             // `validate_rvalue` upon access.
 520             Operand::Constant(c) => {
 521                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 522                     // Only allow statics (not consts) to refer to other statics.
 523                     // FIXME(eddyb) does this matter at all for promotion?
 524                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 525                     // and in `const` this cannot occur anyway. The only concern is that we might
 526                     // promote even `let x = &STATIC` which would be useless, but this applies to
 527                     // promotion inside statics as well.
 528                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 529                     if !is_static {
 530                         return Err(Unpromotable);
 531                     }
 532
 533                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 534                     if is_thread_local {
 535                         return Err(Unpromotable);
 536                     }
 537                 }
 538
 539                 Ok(())
 540             }
 541         }
 542     }
 543
 544     fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
 545         match kind {
 546             // Reject these borrow types just to be safe.
 547             // FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
 548             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 549
 550             BorrowKind::Shared => {
 551                 let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
 552                 if has_mut_interior {
 553                     return Err(Unpromotable);
 554                 }
 555             }
 556
 557             BorrowKind::Mut { .. } => {
 558                 let ty = place.ty(self.body, self.tcx).ty;
 559
 560                 // In theory, any zero-sized value could be borrowed
 561                 // mutably without consequences. However, only &mut []
 562                 // is allowed right now.
 563                 if let ty::Array(_, len) = ty.kind() {
 564                     match len.try_eval_usize(self.tcx, self.param_env) {
 565                         Some(0) => {}
 566                         _ => return Err(Unpromotable),
 567                     }
 568                 } else {
 569                     return Err(Unpromotable);
 570                 }
 571             }
 572         }
 573
 574         Ok(())
 575     }
 576
 577     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 578         match rvalue {
 579             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
 580                 self.validate_operand(operand)?;
 581             }
 582
 583             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
 584                 self.validate_place(place.as_ref())?
 585             }
 586
 587             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 588
 589             Rvalue::Cast(kind, operand, cast_ty) => {
 590                 if matches!(kind, CastKind::Misc) {
 591                     let operand_ty = operand.ty(self.body, self.tcx);
 592                     let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 593                     let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 594                     if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 595                         // ptr-to-int casts are not possible in consts and thus not promotable
 596                         return Err(Unpromotable);
 597                     }
 598                     // int-to-ptr casts are fine, they just use the integer value at pointer type.
 599                 }
 600
 601                 self.validate_operand(operand)?;
 602             }
 603
 604             Rvalue::NullaryOp(op, _) => match op {
 605                 NullOp::Box => return Err(Unpromotable),
 606                 NullOp::SizeOf => {}
 607             },
 608
 609             Rvalue::UnaryOp(op, operand) => {
 610                 match op {
 611                     // These operations can never fail.
 612                     UnOp::Neg | UnOp::Not => {}
 613                 }
 614
 615                 self.validate_operand(operand)?;
 616             }
 617
 618             Rvalue::BinaryOp(op, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(op, box (lhs, rhs)) => {
 619                 let op = *op;
 620                 let lhs_ty = lhs.ty(self.body, self.tcx);
 621
 622                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs_ty.kind() {
 623                     // Raw and fn pointer operations are not allowed inside consts and thus not promotable.
 624                     assert!(matches!(
 625                         op,
 626                         BinOp::Eq
 627                             | BinOp::Ne
 628                             | BinOp::Le
 629                             | BinOp::Lt
 630                             | BinOp::Ge
 631                             | BinOp::Gt
 632                             | BinOp::Offset
 633                     ));
 634                     return Err(Unpromotable);
 635                 }
 636
 637                 match op {
 638                     BinOp::Div | BinOp::Rem => {
 639                         if !self.explicit && lhs_ty.is_integral() {
 640                             // Integer division: the RHS must be a non-zero const.
 641                             let const_val = match rhs {
 642                                 Operand::Constant(c) => {
 643                                     c.literal.try_eval_bits(self.tcx, self.param_env, lhs_ty)
 644                                 }
 645                                 _ => None,
 646                             };
 647                             match const_val {
 648                                 Some(x) if x != 0 => {}        // okay
 649                                 _ => return Err(Unpromotable), // value not known or 0 -- not okay
 650                             }
 651                         }
 652                     }
 653                     // The remaining operations can never fail.
 654                     BinOp::Eq
 655                     | BinOp::Ne
 656                     | BinOp::Le
 657                     | BinOp::Lt
 658                     | BinOp::Ge
 659                     | BinOp::Gt
 660                     | BinOp::Offset
 661                     | BinOp::Add
 662                     | BinOp::Sub
 663                     | BinOp::Mul
 664                     | BinOp::BitXor
 665                     | BinOp::BitAnd
 666                     | BinOp::BitOr
 667                     | BinOp::Shl
 668                     | BinOp::Shr => {}
 669                 }
 670
 671                 self.validate_operand(lhs)?;
 672                 self.validate_operand(rhs)?;
 673             }
 674
 675             Rvalue::AddressOf(_, place) => {
 676                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 677                 // no problem, only using it is.
 678                 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
 679                 {
 680                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 681                     if let ty::Ref(..) = base_ty.kind() {
 682                         return self.validate_place(place_base);
 683                     }
 684                 }
 685                 return Err(Unpromotable);
 686             }
 687
 688             Rvalue::Ref(_, kind, place) => {
 689                 // Special-case reborrows to be more like a copy of the reference.
 690                 let mut place_simplified = place.as_ref();
 691                 if let Some((place_base, ProjectionElem::Deref)) =
 692                     place_simplified.last_projection()
 693                 {
 694                     let base_ty = place_base.ty(self.body, self.tcx).ty;
 695                     if let ty::Ref(..) = base_ty.kind() {
 696                         place_simplified = place_base;
 697                     }
 698                 }
 699
 700                 self.validate_place(place_simplified)?;
 701
 702                 // Check that the reference is fine (using the original place!).
 703                 // (Needs to come after `validate_place` to avoid ICEs.)
 704                 self.validate_ref(*kind, place)?;
 705             }
 706
 707             Rvalue::Aggregate(_, operands) => {
 708                 for o in operands {
 709                     self.validate_operand(o)?;
 710                 }
 711             }
 712         }
 713
 714         Ok(())
 715     }
 716
 717     fn validate_call(
 718         &self,
 719         callee: &Operand<'tcx>,
 720         args: &[Operand<'tcx>],
 721     ) -> Result<(), Unpromotable> {
 722         let fn_ty = callee.ty(self.body, self.tcx);
 723
 724         // When doing explicit promotion and inside const/static items, we promote all (eligible) function calls.
 725         // Everywhere else, we require `#[rustc_promotable]` on the callee.
 726         let promote_all_const_fn = self.explicit
 727             || matches!(
 728                 self.const_kind,
 729                 Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
 730             );
 731         if !promote_all_const_fn {
 732             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 733                 // Never promote runtime `const fn` calls of
 734                 // functions without `#[rustc_promotable]`.
 735                 if !self.tcx.is_promotable_const_fn(def_id) {
 736                     return Err(Unpromotable);
 737                 }
 738             }
 739         }
 740
 741         let is_const_fn = match *fn_ty.kind() {
 742             ty::FnDef(def_id, _) => {
 743                 is_const_fn(self.tcx, def_id)
 744                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 745                     || is_lang_panic_fn(self.tcx, def_id)
 746             }
 747             _ => false,
 748         };
 749         if !is_const_fn {
 750             return Err(Unpromotable);
 751         }
 752
 753         self.validate_operand(callee)?;
 754         for arg in args {
 755             self.validate_operand(arg)?;
 756         }
 757
 758         Ok(())
 759     }
 760 }
 761
 762 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 763 pub fn validate_candidates(
 764     ccx: &ConstCx<'_, '_>,
 765     temps: &IndexVec<Local, TempState>,
 766     candidates: &[Candidate],
 767 ) -> Vec<Candidate> {
 768     let mut validator = Validator { ccx, temps, explicit: false };
 769
 770     candidates
 771         .iter()
 772         .copied()
 773         .filter(|&candidate| {
 774             validator.explicit = candidate.forces_explicit_promotion();
 775
 776             // FIXME(eddyb) also emit the errors for shuffle indices
 777             // and `#[rustc_args_required_const]` arguments here.
 778
 779             let is_promotable = validator.validate_candidate(candidate).is_ok();
 780
 781             // If we use explicit validation, we carry the risk of turning a legitimate run-time
 782             // operation into a failing compile-time operation. Make sure that does not happen
 783             // by asserting that there is no possible run-time behavior here in case promotion
 784             // fails.
 785             if validator.explicit && !is_promotable {
 786                 ccx.tcx.sess.delay_span_bug(
 787                     ccx.body.span,
 788                     "Explicit promotion requested, but failed to promote",
 789                 );
 790             }
 791
 792             match candidate {
 793                 Candidate::Argument { bb, index } if !is_promotable => {
 794                     let span = ccx.body[bb].terminator().source_info.span;
 795                     let msg = format!("argument {} is required to be a constant", index + 1);
 796                     ccx.tcx.sess.span_err(span, &msg);
 797                 }
 798                 _ => (),
 799             }
 800
 801             is_promotable
 802         })
 803         .collect()
 804 }
 805
 806 struct Promoter<'a, 'tcx> {
 807     tcx: TyCtxt<'tcx>,
 808     source: &'a mut Body<'tcx>,
 809     promoted: Body<'tcx>,
 810     temps: &'a mut IndexVec<Local, TempState>,
 811     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 812
 813     /// If true, all nested temps are also kept in the
 814     /// source MIR, not moved to the promoted MIR.
 815     keep_original: bool,
 816 }
 817
 818 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 819     fn new_block(&mut self) -> BasicBlock {
 820         let span = self.promoted.span;
 821         self.promoted.basic_blocks_mut().push(BasicBlockData {
 822             statements: vec![],
 823             terminator: Some(Terminator {
 824                 source_info: SourceInfo::outermost(span),
 825                 kind: TerminatorKind::Return,
 826             }),
 827             is_cleanup: false,
 828         })
 829     }
 830
 831     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 832         let last = self.promoted.basic_blocks().last().unwrap();
 833         let data = &mut self.promoted[last];
 834         data.statements.push(Statement {
 835             source_info: SourceInfo::outermost(span),
 836             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 837         });
 838     }
 839
 840     fn is_temp_kind(&self, local: Local) -> bool {
 841         self.source.local_kind(local) == LocalKind::Temp
 842     }
 843
 844     /// Copies the initialization of this temp to the
 845     /// promoted MIR, recursing through temps.
 846     fn promote_temp(&mut self, temp: Local) -> Local {
 847         let old_keep_original = self.keep_original;
 848         let loc = match self.temps[temp] {
 849             TempState::Defined { location, uses } if uses > 0 => {
 850                 if uses > 1 {
 851                     self.keep_original = true;
 852                 }
 853                 location
 854             }
 855             state => {
 856                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 857             }
 858         };
 859         if !self.keep_original {
 860             self.temps[temp] = TempState::PromotedOut;
 861         }
 862
 863         let num_stmts = self.source[loc.block].statements.len();
 864         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 865             self.source.local_decls[temp].ty,
 866             self.source.local_decls[temp].source_info.span,
 867         ));
 868
 869         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 870
 871         // First, take the Rvalue or Call out of the source MIR,
 872         // or duplicate it, depending on keep_original.
 873         if loc.statement_index < num_stmts {
 874             let (mut rvalue, source_info) = {
 875                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 876                 let rhs = match statement.kind {
 877                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 878                     _ => {
 879                         span_bug!(
 880                             statement.source_info.span,
 881                             "{:?} is not an assignment",
 882                             statement
 883                         );
 884                     }
 885                 };
 886
 887                 (
 888                     if self.keep_original {
 889                         rhs.clone()
 890                     } else {
 891                         let unit = Rvalue::Use(Operand::Constant(box Constant {
 892                             span: statement.source_info.span,
 893                             user_ty: None,
 894                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit).into(),
 895                         }));
 896                         mem::replace(rhs, unit)
 897                     },
 898                     statement.source_info,
 899                 )
 900             };
 901
 902             self.visit_rvalue(&mut rvalue, loc);
 903             self.assign(new_temp, rvalue, source_info.span);
 904         } else {
 905             let terminator = if self.keep_original {
 906                 self.source[loc.block].terminator().clone()
 907             } else {
 908                 let terminator = self.source[loc.block].terminator_mut();
 909                 let target = match terminator.kind {
 910                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 911                     ref kind => {
 912                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 913                     }
 914                 };
 915                 Terminator {
 916                     source_info: terminator.source_info,
 917                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 918                 }
 919             };
 920
 921             match terminator.kind {
 922                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 923                     self.visit_operand(&mut func, loc);
 924                     for arg in &mut args {
 925                         self.visit_operand(arg, loc);
 926                     }
 927
 928                     let last = self.promoted.basic_blocks().last().unwrap();
 929                     let new_target = self.new_block();
 930
 931                     *self.promoted[last].terminator_mut() = Terminator {
 932                         kind: TerminatorKind::Call {
 933                             func,
 934                             args,
 935                             cleanup: None,
 936                             destination: Some((Place::from(new_temp), new_target)),
 937                             from_hir_call,
 938                             fn_span,
 939                         },
 940                         source_info: SourceInfo::outermost(terminator.source_info.span),
 941                         ..terminator
 942                     };
 943                 }
 944                 ref kind => {
 945                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 946                 }
 947             };
 948         };
 949
 950         self.keep_original = old_keep_original;
 951         new_temp
 952     }
 953
 954     fn promote_candidate(
 955         mut self,
 956         candidate: Candidate,
 957         next_promoted_id: usize,
 958     ) -> Option<Body<'tcx>> {
 959         let def = self.source.source.with_opt_param();
 960         let mut rvalue = {
 961             let promoted = &mut self.promoted;
 962             let promoted_id = Promoted::new(next_promoted_id);
 963             let tcx = self.tcx;
 964             let mut promoted_operand = |ty, span| {
 965                 promoted.span = span;
 966                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 967
 968                 Operand::Constant(Box::new(Constant {
 969                     span,
 970                     user_ty: None,
 971                     literal: tcx
 972                         .mk_const(ty::Const {
 973                             ty,
 974                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
 975                                 def,
 976                                 substs: InternalSubsts::for_item(tcx, def.did, |param, _| {
 977                                     if let ty::GenericParamDefKind::Lifetime = param.kind {
 978                                         tcx.lifetimes.re_erased.into()
 979                                     } else {
 980                                         tcx.mk_param_from_def(param)
 981                                     }
 982                                 }),
 983                                 promoted: Some(promoted_id),
 984                             }),
 985                         })
 986                         .into(),
 987                 }))
 988             };
 989             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 990             match candidate {
 991                 Candidate::Ref(loc) => {
 992                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
 993                     match statement.kind {
 994                         StatementKind::Assign(box (
 995                             _,
 996                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 997                         )) => {
 998                             // Use the underlying local for this (necessarily interior) borrow.
 999                             let ty = local_decls.local_decls()[place.local].ty;
1000                             let span = statement.source_info.span;
1001
1002                             let ref_ty = tcx.mk_ref(
1003                                 tcx.lifetimes.re_erased,
1004                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
1005                             );
1006
1007                             *region = tcx.lifetimes.re_erased;
1008
1009                             let mut projection = vec![PlaceElem::Deref];
1010                             projection.extend(place.projection);
1011                             place.projection = tcx.intern_place_elems(&projection);
1012
1013                             // Create a temp to hold the promoted reference.
1014                             // This is because `*r` requires `r` to be a local,
1015                             // otherwise we would use the `promoted` directly.
1016                             let mut promoted_ref = LocalDecl::new(ref_ty, span);
1017                             promoted_ref.source_info = statement.source_info;
1018                             let promoted_ref = local_decls.push(promoted_ref);
1019                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
1020
1021                             let promoted_ref_statement = Statement {
1022                                 source_info: statement.source_info,
1023                                 kind: StatementKind::Assign(Box::new((
1024                                     Place::from(promoted_ref),
1025                                     Rvalue::Use(promoted_operand(ref_ty, span)),
1026                                 ))),
1027                             };
1028                             self.extra_statements.push((loc, promoted_ref_statement));
1029
1030                             Rvalue::Ref(
1031                                 tcx.lifetimes.re_erased,
1032                                 borrow_kind,
1033                                 Place {
1034                                     local: mem::replace(&mut place.local, promoted_ref),
1035                                     projection: List::empty(),
1036                                 },
1037                             )
1038                         }
1039                         _ => bug!(),
1040                     }
1041                 }
1042                 Candidate::Argument { bb, index } => {
1043                     let terminator = blocks[bb].terminator_mut();
1044                     match terminator.kind {
1045                         TerminatorKind::Call { ref mut args, .. } => {
1046                             let ty = args[index].ty(local_decls, self.tcx);
1047                             let span = terminator.source_info.span;
1048
1049                             Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span)))
1050                         }
1051                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
1052                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
1053                         // we are seeing a `Goto`. That means that the `promote_temps` method
1054                         // already promoted this call away entirely. This case occurs when calling
1055                         // a function requiring a constant argument and as that constant value
1056                         // providing a value whose computation contains another call to a function
1057                         // requiring a constant argument.
1058                         TerminatorKind::Goto { .. } => return None,
1059                         _ => bug!(),
1060                     }
1061                 }
1062             }
1063         };
1064
1065         assert_eq!(self.new_block(), START_BLOCK);
1066         self.visit_rvalue(
1067             &mut rvalue,
1068             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
1069         );
1070
1071         let span = self.promoted.span;
1072         self.assign(RETURN_PLACE, rvalue, span);
1073         Some(self.promoted)
1074     }
1075 }
1076
1077 /// Replaces all temporaries with their promoted counterparts.
1078 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1079     fn tcx(&self) -> TyCtxt<'tcx> {
1080         self.tcx
1081     }
1082
1083     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
1084         if self.is_temp_kind(*local) {
1085             *local = self.promote_temp(*local);
1086         }
1087     }
1088 }
1089
1090 pub fn promote_candidates<'tcx>(
1091     body: &mut Body<'tcx>,
1092     tcx: TyCtxt<'tcx>,
1093     mut temps: IndexVec<Local, TempState>,
1094     candidates: Vec<Candidate>,
1095 ) -> IndexVec<Promoted, Body<'tcx>> {
1096     // Visit candidates in reverse, in case they're nested.
1097     debug!("promote_candidates({:?})", candidates);
1098
1099     let mut promotions = IndexVec::new();
1100
1101     let mut extra_statements = vec![];
1102     for candidate in candidates.into_iter().rev() {
1103         match candidate {
1104             Candidate::Ref(Location { block, statement_index }) => {
1105                 if let StatementKind::Assign(box (place, _)) =
1106                     &body[block].statements[statement_index].kind
1107                 {
1108                     if let Some(local) = place.as_local() {
1109                         if temps[local] == TempState::PromotedOut {
1110                             // Already promoted.
1111                             continue;
1112                         }
1113                     }
1114                 }
1115             }
1116             Candidate::Argument { .. } => {}
1117         }
1118
1119         // Declare return place local so that `mir::Body::new` doesn't complain.
1120         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
1121
1122         let mut scope = body.source_scopes[candidate.source_info(body).scope].clone();
1123         scope.parent_scope = None;
1124
1125         let promoted = Body::new(
1126             body.source, // `promoted` gets filled in below
1127             IndexVec::new(),
1128             IndexVec::from_elem_n(scope, 1),
1129             initial_locals,
1130             IndexVec::new(),
1131             0,
1132             vec![],
1133             body.span,
1134             body.generator_kind(),
1135         );
1136
1137         let promoter = Promoter {
1138             promoted,
1139             tcx,
1140             source: body,
1141             temps: &mut temps,
1142             extra_statements: &mut extra_statements,
1143             keep_original: false,
1144         };
1145
1146         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1147         if let Some(mut promoted) = promoter.promote_candidate(candidate, promotions.len()) {
1148             promoted.source.promoted = Some(promotions.next_index());
1149             promotions.push(promoted);
1150         }
1151     }
1152
1153     // Insert each of `extra_statements` before its indicated location, which
1154     // has to be done in reverse location order, to not invalidate the rest.
1155     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1156     for (loc, statement) in extra_statements {
1157         body[loc.block].statements.insert(loc.statement_index, statement);
1158     }
1159
1160     // Eliminate assignments to, and drops of promoted temps.
1161     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1162     for block in body.basic_blocks_mut() {
1163         block.statements.retain(|statement| match &statement.kind {
1164             StatementKind::Assign(box (place, _)) => {
1165                 if let Some(index) = place.as_local() {
1166                     !promoted(index)
1167                 } else {
1168                     true
1169                 }
1170             }
1171             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1172                 !promoted(*index)
1173             }
1174             _ => true,
1175         });
1176         let terminator = block.terminator_mut();
1177         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1178             if let Some(index) = place.as_local() {
1179                 if promoted(index) {
1180                     terminator.kind = TerminatorKind::Goto { target: *target };
1181                 }
1182             }
1183         }
1184     }
1185
1186     promotions
1187 }
1188
1189 /// This function returns `true` if the function being called in the array
1190 /// repeat expression is a `const` function.
1191 crate fn is_const_fn_in_array_repeat_expression<'tcx>(
1192     ccx: &ConstCx<'_, 'tcx>,
1193     place: &Place<'tcx>,
1194     body: &Body<'tcx>,
1195 ) -> bool {
1196     match place.as_local() {
1197         // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
1198         Some(local) if body.local_decls[local].is_user_variable() => return false,
1199         None => return false,
1200         _ => {}
1201     }
1202
1203     for block in body.basic_blocks() {
1204         if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
1205             &block.terminator
1206         {
1207             if let Operand::Constant(box Constant { literal, .. }) = func {
1208                 if let ty::FnDef(def_id, _) = *literal.ty().kind() {
1209                     if let Some((destination_place, _)) = destination {
1210                         if destination_place == place {
1211                             if is_const_fn(ccx.tcx, def_id) {
1212                                 return true;
1213                             }
1214                         }
1215                     }
1216                 }
1217             }
1218         }
1219     }
1220
1221     false
1222 }