src/librustc_mir/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::def_id::DefId;
  16 use rustc::mir::*;
  17 use rustc::mir::visit::{PlaceContext, MutatingUseContext, MutVisitor, Visitor};
  18 use rustc::mir::traversal::ReversePostorder;
  19 use rustc::ty::{self, List, TyCtxt, TypeFoldable};
  20 use rustc::ty::subst::InternalSubsts;
  21 use rustc::ty::cast::CastTy;
  22 use syntax::ast::LitKind;
  23 use syntax::symbol::sym;
  24 use syntax_pos::{Span, DUMMY_SP};
  25
  26 use rustc_index::vec::{IndexVec, Idx};
  27 use rustc_target::spec::abi::Abi;
  28
  29 use std::cell::Cell;
  30 use std::{iter, mem, usize};
  31
  32 use crate::transform::{MirPass, MirSource};
  33 use crate::transform::check_consts::{qualifs, Item, ConstKind, is_lang_panic_fn};
  34
  35 /// A `MirPass` for promotion.
  36 ///
  37 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
  38 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
  39 ///
  40 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  41 /// newly created `StaticKind::Promoted`.
  42 #[derive(Default)]
  43 pub struct PromoteTemps<'tcx> {
  44     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  45 }
  46
  47 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  48     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'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 src.promoted.is_some() {
  58             return;
  59         }
  60
  61         let def_id = src.def_id();
  62
  63         let mut rpo = traversal::reverse_postorder(body);
  64         let (temps, all_candidates) = collect_temps_and_candidates(tcx, body, &mut rpo);
  65
  66         let promotable_candidates = validate_candidates(tcx, body, def_id, &temps, &all_candidates);
  67
  68         let promoted = promote_candidates(def_id, body, tcx, temps, promotable_candidates);
  69         self.promoted_fragments.set(promoted);
  70     }
  71 }
  72
  73 /// State of a temporary during collection and promotion.
  74 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  75 pub enum TempState {
  76     /// No references to this temp.
  77     Undefined,
  78     /// One direct assignment and any number of direct uses.
  79     /// A borrow of this temp is promotable if the assigned
  80     /// value is qualified as constant.
  81     Defined {
  82         location: Location,
  83         uses: usize
  84     },
  85     /// Any other combination of assignments/uses.
  86     Unpromotable,
  87     /// This temp was part of an rvalue which got extracted
  88     /// during promotion and needs cleanup.
  89     PromotedOut
  90 }
  91
  92 impl TempState {
  93     pub fn is_promotable(&self) -> bool {
  94         debug!("is_promotable: self={:?}", self);
  95         if let TempState::Defined { .. } = *self {
  96             true
  97         } else {
  98             false
  99         }
 100     }
 101 }
 102
 103 /// A "root candidate" for promotion, which will become the
 104 /// returned value in a promoted MIR, unless it's a subset
 105 /// of a larger candidate.
 106 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 107 pub enum Candidate {
 108     /// Borrow of a constant temporary.
 109     Ref(Location),
 110
 111     /// Promotion of the `x` in `[x; 32]`.
 112     Repeat(Location),
 113
 114     /// Currently applied to function calls where the callee has the unstable
 115     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
 116     /// intrinsic. The intrinsic requires the arguments are indeed constant and
 117     /// the attribute currently provides the semantic requirement that arguments
 118     /// must be constant.
 119     Argument { bb: BasicBlock, index: usize },
 120 }
 121
 122 impl Candidate {
 123     /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
 124     fn forces_explicit_promotion(&self) -> bool {
 125         match self {
 126             Candidate::Ref(_) |
 127             Candidate::Repeat(_) => false,
 128             Candidate::Argument { .. } => true,
 129         }
 130     }
 131 }
 132
 133 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
 134     let attrs = tcx.get_attrs(def_id);
 135     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
 136     let mut ret = vec![];
 137     for meta in attr.meta_item_list()? {
 138         match meta.literal()?.kind {
 139             LitKind::Int(a, _) => { ret.push(a as usize); }
 140             _ => return None,
 141         }
 142     }
 143     Some(ret)
 144 }
 145
 146 struct Collector<'a, 'tcx> {
 147     tcx: TyCtxt<'tcx>,
 148     body: &'a Body<'tcx>,
 149     temps: IndexVec<Local, TempState>,
 150     candidates: Vec<Candidate>,
 151     span: Span,
 152 }
 153
 154 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 155     fn visit_local(&mut self,
 156                    &index: &Local,
 157                    context: PlaceContext,
 158                    location: Location) {
 159         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 160         // We're only interested in temporaries and the return place
 161         match self.body.local_kind(index) {
 162             | LocalKind::Temp
 163             | LocalKind::ReturnPointer
 164             => {},
 165             | LocalKind::Arg
 166             | LocalKind::Var
 167             => return,
 168         }
 169
 170         // Ignore drops, if the temp gets promoted,
 171         // then it's constant and thus drop is noop.
 172         // Non-uses are also irrelevant.
 173         if context.is_drop() || !context.is_use() {
 174             debug!(
 175                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 176                 context.is_drop(), context.is_use(),
 177             );
 178             return;
 179         }
 180
 181         let temp = &mut self.temps[index];
 182         debug!("visit_local: temp={:?}", temp);
 183         if *temp == TempState::Undefined {
 184             match context {
 185                 PlaceContext::MutatingUse(MutatingUseContext::Store) |
 186                 PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 187                     *temp = TempState::Defined {
 188                         location,
 189                         uses: 0
 190                     };
 191                     return;
 192                 }
 193                 _ => { /* mark as unpromotable below */ }
 194             }
 195         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 196             // We always allow borrows, even mutable ones, as we need
 197             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 198             let allowed_use = context.is_borrow() || context.is_nonmutating_use();
 199             debug!("visit_local: allowed_use={:?}", allowed_use);
 200             if allowed_use {
 201                 *uses += 1;
 202                 return;
 203             }
 204             /* mark as unpromotable below */
 205         }
 206         *temp = TempState::Unpromotable;
 207     }
 208
 209     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 210         self.super_rvalue(rvalue, location);
 211
 212         match *rvalue {
 213             Rvalue::Ref(..) => {
 214                 self.candidates.push(Candidate::Ref(location));
 215             }
 216             Rvalue::Repeat(..) if self.tcx.features().const_in_array_repeat_expressions => {
 217                 // FIXME(#49147) only promote the element when it isn't `Copy`
 218                 // (so that code that can copy it at runtime is unaffected).
 219                 self.candidates.push(Candidate::Repeat(location));
 220             }
 221             _ => {}
 222         }
 223     }
 224
 225     fn visit_terminator_kind(&mut self,
 226                              kind: &TerminatorKind<'tcx>,
 227                              location: Location) {
 228         self.super_terminator_kind(kind, location);
 229
 230         if let TerminatorKind::Call { ref func, .. } = *kind {
 231             if let ty::FnDef(def_id, _) = func.ty(self.body, self.tcx).kind {
 232                 let fn_sig = self.tcx.fn_sig(def_id);
 233                 if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
 234                     let name = self.tcx.item_name(def_id);
 235                     // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
 236                     if name.as_str().starts_with("simd_shuffle") {
 237                         self.candidates.push(Candidate::Argument {
 238                             bb: location.block,
 239                             index: 2,
 240                         });
 241
 242                         return; // Don't double count `simd_shuffle` candidates
 243                     }
 244                 }
 245
 246                 if let Some(constant_args) = args_required_const(self.tcx, def_id) {
 247                     for index in constant_args {
 248                         self.candidates.push(Candidate::Argument { bb: location.block, index });
 249                     }
 250                 }
 251             }
 252         }
 253     }
 254
 255     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 256         self.span = source_info.span;
 257     }
 258 }
 259
 260 pub fn collect_temps_and_candidates(
 261     tcx: TyCtxt<'tcx>,
 262     body: &Body<'tcx>,
 263     rpo: &mut ReversePostorder<'_, 'tcx>,
 264 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 265     let mut collector = Collector {
 266         tcx,
 267         body,
 268         temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls),
 269         candidates: vec![],
 270         span: body.span,
 271     };
 272     for (bb, data) in rpo {
 273         collector.visit_basic_block_data(bb, data);
 274     }
 275     (collector.temps, collector.candidates)
 276 }
 277
 278 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 279 ///
 280 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 281 struct Validator<'a, 'tcx> {
 282     item: Item<'a, 'tcx>,
 283     temps: &'a IndexVec<Local, TempState>,
 284
 285     /// Explicit promotion happens e.g. for constant arguments declared via
 286     /// `rustc_args_required_const`.
 287     /// Implicit promotion has almost the same rules, except that disallows `const fn`
 288     /// except for those marked `#[rustc_promotable]`. This is to avoid changing
 289     /// a legitimate run-time operation into a failing compile-time operation
 290     /// e.g. due to addresses being compared inside the function.
 291     explicit: bool,
 292 }
 293
 294 impl std::ops::Deref for Validator<'a, 'tcx> {
 295     type Target = Item<'a, 'tcx>;
 296
 297     fn deref(&self) -> &Self::Target {
 298         &self.item
 299     }
 300 }
 301
 302 struct Unpromotable;
 303
 304 impl<'tcx> Validator<'_, 'tcx> {
 305     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 306         match candidate {
 307             Candidate::Ref(loc) => {
 308                 assert!(!self.explicit);
 309
 310                 let statement = &self.body[loc.block].statements[loc.statement_index];
 311                 match &statement.kind {
 312                     StatementKind::Assign(box(_, Rvalue::Ref(_, kind, place))) => {
 313                         match kind {
 314                             BorrowKind::Shared | BorrowKind::Mut { .. } => {}
 315
 316                             // FIXME(eddyb) these aren't promoted here but *could*
 317                             // be promoted as part of a larger value because
 318                             // `validate_rvalue`  doesn't check them, need to
 319                             // figure out what is the intended behavior.
 320                             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 321                         }
 322
 323                         // We can only promote interior borrows of promotable temps (non-temps
 324                         // don't get promoted anyway).
 325                         let base = match place.base {
 326                             PlaceBase::Local(local) => local,
 327                             _ => return Err(Unpromotable),
 328                         };
 329                         self.validate_local(base)?;
 330
 331                         if place.projection.contains(&ProjectionElem::Deref) {
 332                             return Err(Unpromotable);
 333                         }
 334
 335                         let mut has_mut_interior =
 336                             self.qualif_local::<qualifs::HasMutInterior>(base);
 337                         // HACK(eddyb) this should compute the same thing as
 338                         // `<HasMutInterior as Qualif>::in_projection` from
 339                         // `check_consts::qualifs` but without recursion.
 340                         if has_mut_interior {
 341                             // This allows borrowing fields which don't have
 342                             // `HasMutInterior`, from a type that does, e.g.:
 343                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 344                             let mut place_projection = &place.projection[..];
 345                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 346                             while let [proj_base @ .., elem] = place_projection {
 347                                 // FIXME(eddyb) this is probably excessive, with
 348                                 // the exception of `union` member accesses.
 349                                 let ty =
 350                                     Place::ty_from(&place.base, proj_base, self.body, self.tcx)
 351                                         .projection_ty(self.tcx, elem)
 352                                         .ty;
 353                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 354                                     has_mut_interior = false;
 355                                     break;
 356                                 }
 357
 358                                 place_projection = proj_base;
 359                             }
 360                         }
 361
 362                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 363                         if has_mut_interior {
 364                             return Err(Unpromotable);
 365                         }
 366                         if self.qualif_local::<qualifs::NeedsDrop>(base) {
 367                             return Err(Unpromotable);
 368                         }
 369
 370                         if let BorrowKind::Mut { .. } = kind {
 371                             let ty = place.ty(self.body, self.tcx).ty;
 372
 373                             // In theory, any zero-sized value could be borrowed
 374                             // mutably without consequences. However, only &mut []
 375                             // is allowed right now, and only in functions.
 376                             if self.const_kind == Some(ConstKind::StaticMut) {
 377                                 // Inside a `static mut`, &mut [...] is also allowed.
 378                                 match ty.kind {
 379                                     ty::Array(..) | ty::Slice(_) => {}
 380                                     _ => return Err(Unpromotable),
 381                                 }
 382                             } else if let ty::Array(_, len) = ty.kind {
 383                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 384                                 // seems unnecessary, given that this is merely a ZST.
 385                                 match len.try_eval_usize(self.tcx, self.param_env) {
 386                                     Some(0) if self.const_kind.is_none() => {},
 387                                     _ => return Err(Unpromotable),
 388                                 }
 389                             } else {
 390                                 return Err(Unpromotable);
 391                             }
 392                         }
 393
 394                         Ok(())
 395                     }
 396                     _ => bug!()
 397                 }
 398             }
 399             Candidate::Repeat(loc) => {
 400                 assert!(!self.explicit);
 401
 402                 let statement = &self.body[loc.block].statements[loc.statement_index];
 403                 match &statement.kind {
 404                     StatementKind::Assign(box(_, Rvalue::Repeat(ref operand, _))) => {
 405                         if !self.tcx.features().const_in_array_repeat_expressions {
 406                             return Err(Unpromotable);
 407                         }
 408
 409                         self.validate_operand(operand)
 410                     }
 411                     _ => bug!()
 412                 }
 413             },
 414             Candidate::Argument { bb, index } => {
 415                 assert!(self.explicit);
 416
 417                 let terminator = self.body[bb].terminator();
 418                 match &terminator.kind {
 419                     TerminatorKind::Call { args, .. } => {
 420                         self.validate_operand(&args[index])
 421                     }
 422                     _ => bug!()
 423                 }
 424             }
 425         }
 426     }
 427
 428     // FIXME(eddyb) maybe cache this?
 429     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 430         let per_local = &|l| self.qualif_local::<Q>(l);
 431
 432         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 433             let num_stmts = self.body[loc.block].statements.len();
 434
 435             if loc.statement_index < num_stmts {
 436                 let statement = &self.body[loc.block].statements[loc.statement_index];
 437                 match &statement.kind {
 438                     StatementKind::Assign(box(_, rhs)) => {
 439                         Q::in_rvalue(&self.item, per_local, rhs)
 440                     }
 441                     _ => {
 442                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 443                                 statement);
 444                     }
 445                 }
 446             } else {
 447                 let terminator = self.body[loc.block].terminator();
 448                 match &terminator.kind {
 449                     TerminatorKind::Call { func, args, .. } => {
 450                         let return_ty = self.body.local_decls[local].ty;
 451                         Q::in_call(&self.item, per_local, func, args, return_ty)
 452                     }
 453                     kind => {
 454                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 455                     }
 456                 }
 457             }
 458         } else {
 459             let span = self.body.local_decls[local].source_info.span;
 460             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 461         }
 462     }
 463
 464     // FIXME(eddyb) maybe cache this?
 465     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 466         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 467             let num_stmts = self.body[loc.block].statements.len();
 468
 469             if loc.statement_index < num_stmts {
 470                 let statement = &self.body[loc.block].statements[loc.statement_index];
 471                 match &statement.kind {
 472                     StatementKind::Assign(box(_, rhs)) => self.validate_rvalue(rhs),
 473                     _ => {
 474                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 475                                 statement);
 476                     }
 477                 }
 478             } else {
 479                 let terminator = self.body[loc.block].terminator();
 480                 match &terminator.kind {
 481                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 482                     kind => {
 483                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 484                     }
 485                 }
 486             }
 487         } else {
 488             Err(Unpromotable)
 489         }
 490     }
 491
 492     fn validate_place(&self, place: PlaceRef<'_, 'tcx>) -> Result<(), Unpromotable> {
 493         match place {
 494             PlaceRef {
 495                 base: PlaceBase::Local(local),
 496                 projection: [],
 497             } => self.validate_local(*local),
 498             PlaceRef {
 499                 base: PlaceBase::Static(_),
 500                 projection: [],
 501             } => bug!("qualifying already promoted MIR"),
 502             PlaceRef {
 503                 base: _,
 504                 projection: [proj_base @ .., elem],
 505             } => {
 506                 match *elem {
 507                     ProjectionElem::Deref |
 508                     ProjectionElem::Downcast(..) => return Err(Unpromotable),
 509
 510                     ProjectionElem::ConstantIndex {..} |
 511                     ProjectionElem::Subslice {..} => {}
 512
 513                     ProjectionElem::Index(local) => {
 514                         self.validate_local(local)?;
 515                     }
 516
 517                     ProjectionElem::Field(..) => {
 518                         if self.const_kind.is_none() {
 519                             let base_ty =
 520                                 Place::ty_from(place.base, proj_base, self.body, self.tcx).ty;
 521                             if let Some(def) = base_ty.ty_adt_def() {
 522                                 // No promotion of union field accesses.
 523                                 if def.is_union() {
 524                                     return Err(Unpromotable);
 525                                 }
 526                             }
 527                         }
 528                     }
 529                 }
 530
 531                 self.validate_place(PlaceRef {
 532                     base: place.base,
 533                     projection: proj_base,
 534                 })
 535             }
 536         }
 537     }
 538
 539     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 540         match operand {
 541             Operand::Copy(place) |
 542             Operand::Move(place) => self.validate_place(place.as_ref()),
 543
 544             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 545             // `validate_rvalue` upon access.
 546             Operand::Constant(c) => {
 547                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 548                     // Only allow statics (not consts) to refer to other statics.
 549                     // FIXME(eddyb) does this matter at all for promotion?
 550                     let is_static = self.const_kind.map_or(false, |k| k.is_static());
 551                     if !is_static {
 552                         return Err(Unpromotable);
 553                     }
 554
 555                     let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 556                     if is_thread_local {
 557                         return Err(Unpromotable);
 558                     }
 559                 }
 560
 561                 Ok(())
 562             },
 563         }
 564     }
 565
 566     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 567         match *rvalue {
 568             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 569                 let operand_ty = operand.ty(self.body, self.tcx);
 570                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 571                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 572                 match (cast_in, cast_out) {
 573                     (CastTy::Ptr(_), CastTy::Int(_)) |
 574                     (CastTy::FnPtr, CastTy::Int(_)) => {
 575                         // in normal functions, mark such casts as not promotable
 576                         return Err(Unpromotable);
 577                     }
 578                     _ => {}
 579                 }
 580             }
 581
 582             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 583                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
 584                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 585                             op == BinOp::Le || op == BinOp::Lt ||
 586                             op == BinOp::Ge || op == BinOp::Gt ||
 587                             op == BinOp::Offset);
 588
 589                     // raw pointer operations are not allowed inside promoteds
 590                     return Err(Unpromotable);
 591                 }
 592             }
 593
 594             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 595
 596             _ => {}
 597         }
 598
 599         match rvalue {
 600             Rvalue::NullaryOp(..) => Ok(()),
 601
 602             Rvalue::Discriminant(place) |
 603             Rvalue::Len(place) => self.validate_place(place.as_ref()),
 604
 605             Rvalue::Use(operand) |
 606             Rvalue::Repeat(operand, _) |
 607             Rvalue::UnaryOp(_, operand) |
 608             Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 609
 610             Rvalue::BinaryOp(_, lhs, rhs) |
 611             Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 612                 self.validate_operand(lhs)?;
 613                 self.validate_operand(rhs)
 614             }
 615
 616             Rvalue::Ref(_, kind, place) => {
 617                 if let BorrowKind::Mut { .. } = kind {
 618                     let ty = place.ty(self.body, self.tcx).ty;
 619
 620                     // In theory, any zero-sized value could be borrowed
 621                     // mutably without consequences. However, only &mut []
 622                     // is allowed right now, and only in functions.
 623                     if self.const_kind == Some(ConstKind::StaticMut) {
 624                         // Inside a `static mut`, &mut [...] is also allowed.
 625                         match ty.kind {
 626                             ty::Array(..) | ty::Slice(_) => {}
 627                             _ => return Err(Unpromotable),
 628                         }
 629                     } else if let ty::Array(_, len) = ty.kind {
 630                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 631                         // const context which seems unnecessary given that this is merely a ZST.
 632                         match len.try_eval_usize(self.tcx, self.param_env) {
 633                             Some(0) if self.const_kind.is_none() => {},
 634                             _ => return Err(Unpromotable),
 635                         }
 636                     } else {
 637                         return Err(Unpromotable);
 638                     }
 639                 }
 640
 641                 // Special-case reborrows to be more like a copy of the reference.
 642                 let mut place = place.as_ref();
 643                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 644                     let base_ty =
 645                         Place::ty_from(&place.base, proj_base, self.body, self.tcx).ty;
 646                     if let ty::Ref(..) = base_ty.kind {
 647                         place = PlaceRef {
 648                             base: &place.base,
 649                             projection: proj_base,
 650                         };
 651                     }
 652                 }
 653
 654                 self.validate_place(place)?;
 655
 656                 // HACK(eddyb) this should compute the same thing as
 657                 // `<HasMutInterior as Qualif>::in_projection` from
 658                 // `check_consts::qualifs` but without recursion.
 659                 let mut has_mut_interior = match place.base {
 660                     PlaceBase::Local(local) => {
 661                         self.qualif_local::<qualifs::HasMutInterior>(*local)
 662                     }
 663                     PlaceBase::Static(_) => false,
 664                 };
 665                 if has_mut_interior {
 666                     let mut place_projection = place.projection;
 667                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 668                     while let [proj_base @ .., elem] = place_projection {
 669                         // FIXME(eddyb) this is probably excessive, with
 670                         // the exception of `union` member accesses.
 671                         let ty = Place::ty_from(place.base, proj_base, self.body, self.tcx)
 672                             .projection_ty(self.tcx, elem)
 673                             .ty;
 674                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 675                             has_mut_interior = false;
 676                             break;
 677                         }
 678
 679                         place_projection = proj_base;
 680                     }
 681                 }
 682                 if has_mut_interior {
 683                     return Err(Unpromotable);
 684                 }
 685
 686                 Ok(())
 687             }
 688
 689             Rvalue::Aggregate(_, ref operands) => {
 690                 for o in operands {
 691                     self.validate_operand(o)?;
 692                 }
 693
 694                 Ok(())
 695             }
 696         }
 697     }
 698
 699     fn validate_call(
 700         &self,
 701         callee: &Operand<'tcx>,
 702         args: &[Operand<'tcx>],
 703     ) -> Result<(), Unpromotable> {
 704         let fn_ty = callee.ty(self.body, self.tcx);
 705
 706         if !self.explicit && self.const_kind.is_none() {
 707             if let ty::FnDef(def_id, _) = fn_ty.kind {
 708                 // Never promote runtime `const fn` calls of
 709                 // functions without `#[rustc_promotable]`.
 710                 if !self.tcx.is_promotable_const_fn(def_id) {
 711                     return Err(Unpromotable);
 712                 }
 713             }
 714         }
 715
 716         let is_const_fn = match fn_ty.kind {
 717             ty::FnDef(def_id, _) => {
 718                 self.tcx.is_const_fn(def_id) ||
 719                 self.tcx.is_unstable_const_fn(def_id).is_some() ||
 720                 is_lang_panic_fn(self.tcx, self.def_id)
 721             }
 722             _ => false,
 723         };
 724         if !is_const_fn {
 725             return Err(Unpromotable);
 726         }
 727
 728         self.validate_operand(callee)?;
 729         for arg in args {
 730             self.validate_operand(arg)?;
 731         }
 732
 733         Ok(())
 734     }
 735 }
 736
 737 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 738 pub fn validate_candidates(
 739     tcx: TyCtxt<'tcx>,
 740     body: &Body<'tcx>,
 741     def_id: DefId,
 742     temps: &IndexVec<Local, TempState>,
 743     candidates: &[Candidate],
 744 ) -> Vec<Candidate> {
 745     let mut validator = Validator {
 746         item: Item::new(tcx, def_id, body),
 747         temps,
 748         explicit: false,
 749     };
 750
 751     candidates.iter().copied().filter(|&candidate| {
 752         validator.explicit = candidate.forces_explicit_promotion();
 753
 754         // FIXME(eddyb) also emit the errors for shuffle indices
 755         // and `#[rustc_args_required_const]` arguments here.
 756
 757         let is_promotable = validator.validate_candidate(candidate).is_ok();
 758         match candidate {
 759             Candidate::Argument { bb, index } if !is_promotable => {
 760                 let span = body[bb].terminator().source_info.span;
 761                 let msg = format!("argument {} is required to be a constant", index + 1);
 762                 tcx.sess.span_err(span, &msg);
 763             }
 764             _ => ()
 765         }
 766
 767         is_promotable
 768     }).collect()
 769 }
 770
 771 struct Promoter<'a, 'tcx> {
 772     tcx: TyCtxt<'tcx>,
 773     source: &'a mut Body<'tcx>,
 774     promoted: Body<'tcx>,
 775     temps: &'a mut IndexVec<Local, TempState>,
 776
 777     /// If true, all nested temps are also kept in the
 778     /// source MIR, not moved to the promoted MIR.
 779     keep_original: bool,
 780 }
 781
 782 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 783     fn new_block(&mut self) -> BasicBlock {
 784         let span = self.promoted.span;
 785         self.promoted.basic_blocks_mut().push(BasicBlockData {
 786             statements: vec![],
 787             terminator: Some(Terminator {
 788                 source_info: SourceInfo {
 789                     span,
 790                     scope: OUTERMOST_SOURCE_SCOPE
 791                 },
 792                 kind: TerminatorKind::Return
 793             }),
 794             is_cleanup: false
 795         })
 796     }
 797
 798     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 799         let last = self.promoted.basic_blocks().last().unwrap();
 800         let data = &mut self.promoted[last];
 801         data.statements.push(Statement {
 802             source_info: SourceInfo {
 803                 span,
 804                 scope: OUTERMOST_SOURCE_SCOPE
 805             },
 806             kind: StatementKind::Assign(box(Place::from(dest), rvalue))
 807         });
 808     }
 809
 810     fn is_temp_kind(&self, local: Local) -> bool {
 811         self.source.local_kind(local) == LocalKind::Temp
 812     }
 813
 814     /// Copies the initialization of this temp to the
 815     /// promoted MIR, recursing through temps.
 816     fn promote_temp(&mut self, temp: Local) -> Local {
 817         let old_keep_original = self.keep_original;
 818         let loc = match self.temps[temp] {
 819             TempState::Defined { location, uses } if uses > 0 => {
 820                 if uses > 1 {
 821                     self.keep_original = true;
 822                 }
 823                 location
 824             }
 825             state =>  {
 826                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}",
 827                           temp, state);
 828             }
 829         };
 830         if !self.keep_original {
 831             self.temps[temp] = TempState::PromotedOut;
 832         }
 833
 834         let num_stmts = self.source[loc.block].statements.len();
 835         let new_temp = self.promoted.local_decls.push(
 836             LocalDecl::new_temp(self.source.local_decls[temp].ty,
 837                                 self.source.local_decls[temp].source_info.span));
 838
 839         debug!("promote({:?} @ {:?}/{:?}, {:?})",
 840                temp, loc, num_stmts, self.keep_original);
 841
 842         // First, take the Rvalue or Call out of the source MIR,
 843         // or duplicate it, depending on keep_original.
 844         if loc.statement_index < num_stmts {
 845             let (mut rvalue, source_info) = {
 846                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 847                 let rhs = match statement.kind {
 848                     StatementKind::Assign(box(_, ref mut rhs)) => rhs,
 849                     _ => {
 850                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 851                                   statement);
 852                     }
 853                 };
 854
 855                 (if self.keep_original {
 856                     rhs.clone()
 857                 } else {
 858                     let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 859                     mem::replace(rhs, unit)
 860                 }, statement.source_info)
 861             };
 862
 863             self.visit_rvalue(&mut rvalue, loc);
 864             self.assign(new_temp, rvalue, source_info.span);
 865         } else {
 866             let terminator = if self.keep_original {
 867                 self.source[loc.block].terminator().clone()
 868             } else {
 869                 let terminator = self.source[loc.block].terminator_mut();
 870                 let target = match terminator.kind {
 871                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 872                     ref kind => {
 873                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 874                     }
 875                 };
 876                 Terminator {
 877                     source_info: terminator.source_info,
 878                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto {
 879                         target,
 880                     })
 881                 }
 882             };
 883
 884             match terminator.kind {
 885                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 886                     self.visit_operand(&mut func, loc);
 887                     for arg in &mut args {
 888                         self.visit_operand(arg, loc);
 889                     }
 890
 891                     let last = self.promoted.basic_blocks().last().unwrap();
 892                     let new_target = self.new_block();
 893
 894                     *self.promoted[last].terminator_mut() = Terminator {
 895                         kind: TerminatorKind::Call {
 896                             func,
 897                             args,
 898                             cleanup: None,
 899                             destination: Some(
 900                                 (Place::from(new_temp), new_target)
 901                             ),
 902                             from_hir_call,
 903                         },
 904                         ..terminator
 905                     };
 906                 }
 907                 ref kind => {
 908                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 909                 }
 910             };
 911         };
 912
 913         self.keep_original = old_keep_original;
 914         new_temp
 915     }
 916
 917     fn promote_candidate(
 918         mut self,
 919         def_id: DefId,
 920         candidate: Candidate,
 921         next_promoted_id: usize,
 922     ) -> Option<Body<'tcx>> {
 923         let mut operand = {
 924             let promoted = &mut self.promoted;
 925             let promoted_id = Promoted::new(next_promoted_id);
 926             let tcx = self.tcx;
 927             let mut promoted_place = |ty, span| {
 928                 promoted.span = span;
 929                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 930                 Place {
 931                     base: PlaceBase::Static(box Static {
 932                         kind:
 933                             StaticKind::Promoted(
 934                                 promoted_id,
 935                                 InternalSubsts::identity_for_item(tcx, def_id),
 936                             ),
 937                         ty,
 938                         def_id,
 939                     }),
 940                     projection: List::empty(),
 941                 }
 942             };
 943             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 944             match candidate {
 945                 Candidate::Ref(loc) => {
 946                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 947                     match statement.kind {
 948                         StatementKind::Assign(box(_, Rvalue::Ref(_, _, ref mut place))) => {
 949                             // Use the underlying local for this (necessarily interior) borrow.
 950                             let ty = place.base.ty(local_decls).ty;
 951                             let span = statement.source_info.span;
 952
 953                             Operand::Move(Place {
 954                                 base: mem::replace(
 955                                     &mut place.base,
 956                                     promoted_place(ty, span).base,
 957                                 ),
 958                                 projection: List::empty(),
 959                             })
 960                         }
 961                         _ => bug!()
 962                     }
 963                 }
 964                 Candidate::Repeat(loc) => {
 965                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 966                     match statement.kind {
 967                         StatementKind::Assign(box(_, Rvalue::Repeat(ref mut operand, _))) => {
 968                             let ty = operand.ty(local_decls, self.tcx);
 969                             let span = statement.source_info.span;
 970                             mem::replace(
 971                                 operand,
 972                                 Operand::Copy(promoted_place(ty, span))
 973                             )
 974                         }
 975                         _ => bug!()
 976                     }
 977                 },
 978                 Candidate::Argument { bb, index } => {
 979                     let terminator = blocks[bb].terminator_mut();
 980                     match terminator.kind {
 981                         TerminatorKind::Call { ref mut args, .. } => {
 982                             let ty = args[index].ty(local_decls, self.tcx);
 983                             let span = terminator.source_info.span;
 984                             let operand = Operand::Copy(promoted_place(ty, span));
 985                             mem::replace(&mut args[index], operand)
 986                         }
 987                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 988                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 989                         // we are seeing a `Goto`. That means that the `promote_temps` method
 990                         // already promoted this call away entirely. This case occurs when calling
 991                         // a function requiring a constant argument and as that constant value
 992                         // providing a value whose computation contains another call to a function
 993                         // requiring a constant argument.
 994                         TerminatorKind::Goto { .. } => return None,
 995                         _ => bug!()
 996                     }
 997                 }
 998             }
 999         };
1000
1001         assert_eq!(self.new_block(), START_BLOCK);
1002         self.visit_operand(&mut operand, Location {
1003             block: BasicBlock::new(0),
1004             statement_index: usize::MAX
1005         });
1006
1007         let span = self.promoted.span;
1008         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
1009         Some(self.promoted)
1010     }
1011 }
1012
1013 /// Replaces all temporaries with their promoted counterparts.
1014 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1015     fn tcx(&self) -> TyCtxt<'tcx> {
1016         self.tcx
1017     }
1018
1019     fn visit_local(&mut self,
1020                    local: &mut Local,
1021                    _: PlaceContext,
1022                    _: Location) {
1023         if self.is_temp_kind(*local) {
1024             *local = self.promote_temp(*local);
1025         }
1026     }
1027
1028     fn process_projection_elem(
1029         &mut self,
1030         elem: &PlaceElem<'tcx>,
1031     ) -> Option<PlaceElem<'tcx>> {
1032         match elem {
1033             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1034                 Some(PlaceElem::Index(self.promote_temp(*local)))
1035             }
1036             _ => None,
1037         }
1038     }
1039 }
1040
1041 pub fn promote_candidates<'tcx>(
1042     def_id: DefId,
1043     body: &mut Body<'tcx>,
1044     tcx: TyCtxt<'tcx>,
1045     mut temps: IndexVec<Local, TempState>,
1046     candidates: Vec<Candidate>,
1047 ) -> IndexVec<Promoted, Body<'tcx>> {
1048     // Visit candidates in reverse, in case they're nested.
1049     debug!("promote_candidates({:?})", candidates);
1050
1051     let mut promotions = IndexVec::new();
1052
1053     for candidate in candidates.into_iter().rev() {
1054         match candidate {
1055             Candidate::Repeat(Location { block, statement_index }) |
1056             Candidate::Ref(Location { block, statement_index }) => {
1057                 match &body[block].statements[statement_index].kind {
1058                     StatementKind::Assign(box(place, _)) => {
1059                         if let Some(local) = place.as_local() {
1060                             if temps[local] == TempState::PromotedOut {
1061                                 // Already promoted.
1062                                 continue;
1063                             }
1064                         }
1065                     }
1066                     _ => {}
1067                 }
1068             }
1069             Candidate::Argument { .. } => {}
1070         }
1071
1072
1073         // Declare return place local so that `mir::Body::new` doesn't complain.
1074         let initial_locals = iter::once(
1075             LocalDecl::new_return_place(tcx.types.never, body.span)
1076         ).collect();
1077
1078         let promoter = Promoter {
1079             promoted: Body::new(
1080                 IndexVec::new(),
1081                 // FIXME: maybe try to filter this to avoid blowing up
1082                 // memory usage?
1083                 body.source_scopes.clone(),
1084                 body.source_scope_local_data.clone(),
1085                 initial_locals,
1086                 IndexVec::new(),
1087                 0,
1088                 vec![],
1089                 body.span,
1090                 vec![],
1091                 body.generator_kind,
1092             ),
1093             tcx,
1094             source: body,
1095             temps: &mut temps,
1096             keep_original: false
1097         };
1098
1099         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1100         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1101             promotions.push(promoted);
1102         }
1103     }
1104
1105     // Eliminate assignments to, and drops of promoted temps.
1106     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1107     for block in body.basic_blocks_mut() {
1108         block.statements.retain(|statement| {
1109             match &statement.kind {
1110                 StatementKind::Assign(box(place, _)) => {
1111                     if let Some(index) = place.as_local() {
1112                         !promoted(index)
1113                     } else {
1114                         true
1115                     }
1116                 }
1117                 StatementKind::StorageLive(index) |
1118                 StatementKind::StorageDead(index) => {
1119                     !promoted(*index)
1120                 }
1121                 _ => true
1122             }
1123         });
1124         let terminator = block.terminator_mut();
1125         match &terminator.kind {
1126             TerminatorKind::Drop { location: place, target, .. } => {
1127                 if let Some(index) = place.as_local() {
1128                     if promoted(index) {
1129                         terminator.kind = TerminatorKind::Goto {
1130                             target: *target,
1131                         };
1132                     }
1133                 }
1134             }
1135             _ => {}
1136         }
1137     }
1138
1139     promotions
1140 }
1141
1142 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1143 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1144 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1145 /// enabled.
1146 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1147     tcx: TyCtxt<'tcx>,
1148     mir_def_id: DefId,
1149     body: &Body<'tcx>,
1150     operand: &Operand<'tcx>,
1151 ) -> bool {
1152     let mut rpo = traversal::reverse_postorder(body);
1153     let (temps, _) = collect_temps_and_candidates(tcx, body, &mut rpo);
1154     let validator = Validator {
1155         item: Item::new(tcx, mir_def_id, body),
1156         temps: &temps,
1157         explicit: false,
1158     };
1159
1160     let should_promote = validator.validate_operand(operand).is_ok();
1161     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1162     debug!("should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1163             should_promote={:?} feature_flag={:?}", mir_def_id, should_promote, feature_flag);
1164     should_promote && !feature_flag
1165 }