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 irrelevent.
 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(box Static {
 500                     kind: StaticKind::Promoted { .. },
 501                     ..
 502                 }),
 503                 projection: [],
 504             } => bug!("qualifying already promoted MIR"),
 505             PlaceRef {
 506                 base: PlaceBase::Static(box Static {
 507                     kind: StaticKind::Static,
 508                     def_id,
 509                     ..
 510                 }),
 511                 projection: [],
 512             } => {
 513                 // Only allow statics (not consts) to refer to other statics.
 514                 // FIXME(eddyb) does this matter at all for promotion?
 515                 let is_static = self.const_kind.map_or(false, |k| k.is_static());
 516                 if !is_static {
 517                     return Err(Unpromotable);
 518                 }
 519
 520                 let is_thread_local = self.tcx.has_attr(*def_id, sym::thread_local);
 521                 if is_thread_local {
 522                     return Err(Unpromotable);
 523                 }
 524
 525                 Ok(())
 526             }
 527             PlaceRef {
 528                 base: _,
 529                 projection: [proj_base @ .., elem],
 530             } => {
 531                 match *elem {
 532                     ProjectionElem::Deref |
 533                     ProjectionElem::Downcast(..) => return Err(Unpromotable),
 534
 535                     ProjectionElem::ConstantIndex {..} |
 536                     ProjectionElem::Subslice {..} => {}
 537
 538                     ProjectionElem::Index(local) => {
 539                         self.validate_local(local)?;
 540                     }
 541
 542                     ProjectionElem::Field(..) => {
 543                         if self.const_kind.is_none() {
 544                             let base_ty =
 545                                 Place::ty_from(place.base, proj_base, self.body, self.tcx).ty;
 546                             if let Some(def) = base_ty.ty_adt_def() {
 547                                 // No promotion of union field accesses.
 548                                 if def.is_union() {
 549                                     return Err(Unpromotable);
 550                                 }
 551                             }
 552                         }
 553                     }
 554                 }
 555
 556                 self.validate_place(PlaceRef {
 557                     base: place.base,
 558                     projection: proj_base,
 559                 })
 560             }
 561         }
 562     }
 563
 564     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 565         match operand {
 566             Operand::Copy(place) |
 567             Operand::Move(place) => self.validate_place(place.as_ref()),
 568
 569             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 570             // `validate_rvalue` upon access.
 571             Operand::Constant(_) => Ok(()),
 572         }
 573     }
 574
 575     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 576         match *rvalue {
 577             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 578                 let operand_ty = operand.ty(self.body, self.tcx);
 579                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 580                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 581                 match (cast_in, cast_out) {
 582                     (CastTy::Ptr(_), CastTy::Int(_)) |
 583                     (CastTy::FnPtr, CastTy::Int(_)) => {
 584                         // in normal functions, mark such casts as not promotable
 585                         return Err(Unpromotable);
 586                     }
 587                     _ => {}
 588                 }
 589             }
 590
 591             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 592                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
 593                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 594                             op == BinOp::Le || op == BinOp::Lt ||
 595                             op == BinOp::Ge || op == BinOp::Gt ||
 596                             op == BinOp::Offset);
 597
 598                     // raw pointer operations are not allowed inside promoteds
 599                     return Err(Unpromotable);
 600                 }
 601             }
 602
 603             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 604
 605             _ => {}
 606         }
 607
 608         match rvalue {
 609             Rvalue::NullaryOp(..) => Ok(()),
 610
 611             Rvalue::Discriminant(place) |
 612             Rvalue::Len(place) => self.validate_place(place.as_ref()),
 613
 614             Rvalue::Use(operand) |
 615             Rvalue::Repeat(operand, _) |
 616             Rvalue::UnaryOp(_, operand) |
 617             Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 618
 619             Rvalue::BinaryOp(_, lhs, rhs) |
 620             Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 621                 self.validate_operand(lhs)?;
 622                 self.validate_operand(rhs)
 623             }
 624
 625             Rvalue::Ref(_, kind, place) => {
 626                 if let BorrowKind::Mut { .. } = kind {
 627                     let ty = place.ty(self.body, self.tcx).ty;
 628
 629                     // In theory, any zero-sized value could be borrowed
 630                     // mutably without consequences. However, only &mut []
 631                     // is allowed right now, and only in functions.
 632                     if self.const_kind == Some(ConstKind::StaticMut) {
 633                         // Inside a `static mut`, &mut [...] is also allowed.
 634                         match ty.kind {
 635                             ty::Array(..) | ty::Slice(_) => {}
 636                             _ => return Err(Unpromotable),
 637                         }
 638                     } else if let ty::Array(_, len) = ty.kind {
 639                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 640                         // const context which seems unnecessary given that this is merely a ZST.
 641                         match len.try_eval_usize(self.tcx, self.param_env) {
 642                             Some(0) if self.const_kind.is_none() => {},
 643                             _ => return Err(Unpromotable),
 644                         }
 645                     } else {
 646                         return Err(Unpromotable);
 647                     }
 648                 }
 649
 650                 // Special-case reborrows to be more like a copy of the reference.
 651                 let mut place = place.as_ref();
 652                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 653                     let base_ty =
 654                         Place::ty_from(&place.base, proj_base, self.body, self.tcx).ty;
 655                     if let ty::Ref(..) = base_ty.kind {
 656                         place = PlaceRef {
 657                             base: &place.base,
 658                             projection: proj_base,
 659                         };
 660                     }
 661                 }
 662
 663                 self.validate_place(place)?;
 664
 665                 // HACK(eddyb) this should compute the same thing as
 666                 // `<HasMutInterior as Qualif>::in_projection` from
 667                 // `check_consts::qualifs` but without recursion.
 668                 let mut has_mut_interior = match place.base {
 669                     PlaceBase::Local(local) => {
 670                         self.qualif_local::<qualifs::HasMutInterior>(*local)
 671                     }
 672                     PlaceBase::Static(_) => false,
 673                 };
 674                 if has_mut_interior {
 675                     let mut place_projection = place.projection;
 676                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 677                     while let [proj_base @ .., elem] = place_projection {
 678                         // FIXME(eddyb) this is probably excessive, with
 679                         // the exception of `union` member accesses.
 680                         let ty = Place::ty_from(place.base, proj_base, self.body, self.tcx)
 681                             .projection_ty(self.tcx, elem)
 682                             .ty;
 683                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 684                             has_mut_interior = false;
 685                             break;
 686                         }
 687
 688                         place_projection = proj_base;
 689                     }
 690                 }
 691                 if has_mut_interior {
 692                     return Err(Unpromotable);
 693                 }
 694
 695                 Ok(())
 696             }
 697
 698             Rvalue::Aggregate(_, ref operands) => {
 699                 for o in operands {
 700                     self.validate_operand(o)?;
 701                 }
 702
 703                 Ok(())
 704             }
 705         }
 706     }
 707
 708     fn validate_call(
 709         &self,
 710         callee: &Operand<'tcx>,
 711         args: &[Operand<'tcx>],
 712     ) -> Result<(), Unpromotable> {
 713         let fn_ty = callee.ty(self.body, self.tcx);
 714
 715         if !self.explicit && self.const_kind.is_none() {
 716             if let ty::FnDef(def_id, _) = fn_ty.kind {
 717                 // Never promote runtime `const fn` calls of
 718                 // functions without `#[rustc_promotable]`.
 719                 if !self.tcx.is_promotable_const_fn(def_id) {
 720                     return Err(Unpromotable);
 721                 }
 722             }
 723         }
 724
 725         let is_const_fn = match fn_ty.kind {
 726             ty::FnDef(def_id, _) => {
 727                 self.tcx.is_const_fn(def_id) ||
 728                 self.tcx.is_unstable_const_fn(def_id).is_some() ||
 729                 is_lang_panic_fn(self.tcx, self.def_id)
 730             }
 731             _ => false,
 732         };
 733         if !is_const_fn {
 734             return Err(Unpromotable);
 735         }
 736
 737         self.validate_operand(callee)?;
 738         for arg in args {
 739             self.validate_operand(arg)?;
 740         }
 741
 742         Ok(())
 743     }
 744 }
 745
 746 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 747 pub fn validate_candidates(
 748     tcx: TyCtxt<'tcx>,
 749     body: &Body<'tcx>,
 750     def_id: DefId,
 751     temps: &IndexVec<Local, TempState>,
 752     candidates: &[Candidate],
 753 ) -> Vec<Candidate> {
 754     let mut validator = Validator {
 755         item: Item::new(tcx, def_id, body),
 756         temps,
 757         explicit: false,
 758     };
 759
 760     candidates.iter().copied().filter(|&candidate| {
 761         validator.explicit = candidate.forces_explicit_promotion();
 762
 763         // FIXME(eddyb) also emit the errors for shuffle indices
 764         // and `#[rustc_args_required_const]` arguments here.
 765
 766         let is_promotable = validator.validate_candidate(candidate).is_ok();
 767         match candidate {
 768             Candidate::Argument { bb, index } if !is_promotable => {
 769                 let span = body[bb].terminator().source_info.span;
 770                 let msg = format!("argument {} is required to be a constant", index + 1);
 771                 tcx.sess.span_err(span, &msg);
 772             }
 773             _ => ()
 774         }
 775
 776         is_promotable
 777     }).collect()
 778 }
 779
 780 struct Promoter<'a, 'tcx> {
 781     tcx: TyCtxt<'tcx>,
 782     source: &'a mut Body<'tcx>,
 783     promoted: Body<'tcx>,
 784     temps: &'a mut IndexVec<Local, TempState>,
 785
 786     /// If true, all nested temps are also kept in the
 787     /// source MIR, not moved to the promoted MIR.
 788     keep_original: bool,
 789 }
 790
 791 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 792     fn new_block(&mut self) -> BasicBlock {
 793         let span = self.promoted.span;
 794         self.promoted.basic_blocks_mut().push(BasicBlockData {
 795             statements: vec![],
 796             terminator: Some(Terminator {
 797                 source_info: SourceInfo {
 798                     span,
 799                     scope: OUTERMOST_SOURCE_SCOPE
 800                 },
 801                 kind: TerminatorKind::Return
 802             }),
 803             is_cleanup: false
 804         })
 805     }
 806
 807     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 808         let last = self.promoted.basic_blocks().last().unwrap();
 809         let data = &mut self.promoted[last];
 810         data.statements.push(Statement {
 811             source_info: SourceInfo {
 812                 span,
 813                 scope: OUTERMOST_SOURCE_SCOPE
 814             },
 815             kind: StatementKind::Assign(box(Place::from(dest), rvalue))
 816         });
 817     }
 818
 819     fn is_temp_kind(&self, local: Local) -> bool {
 820         self.source.local_kind(local) == LocalKind::Temp
 821     }
 822
 823     /// Copies the initialization of this temp to the
 824     /// promoted MIR, recursing through temps.
 825     fn promote_temp(&mut self, temp: Local) -> Local {
 826         let old_keep_original = self.keep_original;
 827         let loc = match self.temps[temp] {
 828             TempState::Defined { location, uses } if uses > 0 => {
 829                 if uses > 1 {
 830                     self.keep_original = true;
 831                 }
 832                 location
 833             }
 834             state =>  {
 835                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}",
 836                           temp, state);
 837             }
 838         };
 839         if !self.keep_original {
 840             self.temps[temp] = TempState::PromotedOut;
 841         }
 842
 843         let num_stmts = self.source[loc.block].statements.len();
 844         let new_temp = self.promoted.local_decls.push(
 845             LocalDecl::new_temp(self.source.local_decls[temp].ty,
 846                                 self.source.local_decls[temp].source_info.span));
 847
 848         debug!("promote({:?} @ {:?}/{:?}, {:?})",
 849                temp, loc, num_stmts, self.keep_original);
 850
 851         // First, take the Rvalue or Call out of the source MIR,
 852         // or duplicate it, depending on keep_original.
 853         if loc.statement_index < num_stmts {
 854             let (mut rvalue, source_info) = {
 855                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 856                 let rhs = match statement.kind {
 857                     StatementKind::Assign(box(_, ref mut rhs)) => rhs,
 858                     _ => {
 859                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 860                                   statement);
 861                     }
 862                 };
 863
 864                 (if self.keep_original {
 865                     rhs.clone()
 866                 } else {
 867                     let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 868                     mem::replace(rhs, unit)
 869                 }, statement.source_info)
 870             };
 871
 872             self.visit_rvalue(&mut rvalue, loc);
 873             self.assign(new_temp, rvalue, source_info.span);
 874         } else {
 875             let terminator = if self.keep_original {
 876                 self.source[loc.block].terminator().clone()
 877             } else {
 878                 let terminator = self.source[loc.block].terminator_mut();
 879                 let target = match terminator.kind {
 880                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 881                     ref kind => {
 882                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 883                     }
 884                 };
 885                 Terminator {
 886                     source_info: terminator.source_info,
 887                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto {
 888                         target,
 889                     })
 890                 }
 891             };
 892
 893             match terminator.kind {
 894                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 895                     self.visit_operand(&mut func, loc);
 896                     for arg in &mut args {
 897                         self.visit_operand(arg, loc);
 898                     }
 899
 900                     let last = self.promoted.basic_blocks().last().unwrap();
 901                     let new_target = self.new_block();
 902
 903                     *self.promoted[last].terminator_mut() = Terminator {
 904                         kind: TerminatorKind::Call {
 905                             func,
 906                             args,
 907                             cleanup: None,
 908                             destination: Some(
 909                                 (Place::from(new_temp), new_target)
 910                             ),
 911                             from_hir_call,
 912                         },
 913                         ..terminator
 914                     };
 915                 }
 916                 ref kind => {
 917                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 918                 }
 919             };
 920         };
 921
 922         self.keep_original = old_keep_original;
 923         new_temp
 924     }
 925
 926     fn promote_candidate(
 927         mut self,
 928         def_id: DefId,
 929         candidate: Candidate,
 930         next_promoted_id: usize,
 931     ) -> Option<Body<'tcx>> {
 932         let mut operand = {
 933             let promoted = &mut self.promoted;
 934             let promoted_id = Promoted::new(next_promoted_id);
 935             let tcx = self.tcx;
 936             let mut promoted_place = |ty, span| {
 937                 promoted.span = span;
 938                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 939                 Place {
 940                     base: PlaceBase::Static(box Static {
 941                         kind:
 942                             StaticKind::Promoted(
 943                                 promoted_id,
 944                                 InternalSubsts::identity_for_item(tcx, def_id),
 945                             ),
 946                         ty,
 947                         def_id,
 948                     }),
 949                     projection: List::empty(),
 950                 }
 951             };
 952             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 953             match candidate {
 954                 Candidate::Ref(loc) => {
 955                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 956                     match statement.kind {
 957                         StatementKind::Assign(box(_, Rvalue::Ref(_, _, ref mut place))) => {
 958                             // Use the underlying local for this (necessarily interior) borrow.
 959                             let ty = place.base.ty(local_decls).ty;
 960                             let span = statement.source_info.span;
 961
 962                             Operand::Move(Place {
 963                                 base: mem::replace(
 964                                     &mut place.base,
 965                                     promoted_place(ty, span).base,
 966                                 ),
 967                                 projection: List::empty(),
 968                             })
 969                         }
 970                         _ => bug!()
 971                     }
 972                 }
 973                 Candidate::Repeat(loc) => {
 974                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 975                     match statement.kind {
 976                         StatementKind::Assign(box(_, Rvalue::Repeat(ref mut operand, _))) => {
 977                             let ty = operand.ty(local_decls, self.tcx);
 978                             let span = statement.source_info.span;
 979                             mem::replace(
 980                                 operand,
 981                                 Operand::Copy(promoted_place(ty, span))
 982                             )
 983                         }
 984                         _ => bug!()
 985                     }
 986                 },
 987                 Candidate::Argument { bb, index } => {
 988                     let terminator = blocks[bb].terminator_mut();
 989                     match terminator.kind {
 990                         TerminatorKind::Call { ref mut args, .. } => {
 991                             let ty = args[index].ty(local_decls, self.tcx);
 992                             let span = terminator.source_info.span;
 993                             let operand = Operand::Copy(promoted_place(ty, span));
 994                             mem::replace(&mut args[index], operand)
 995                         }
 996                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 997                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 998                         // we are seeing a `Goto`. That means that the `promote_temps` method
 999                         // already promoted this call away entirely. This case occurs when calling
1000                         // a function requiring a constant argument and as that constant value
1001                         // providing a value whose computation contains another call to a function
1002                         // requiring a constant argument.
1003                         TerminatorKind::Goto { .. } => return None,
1004                         _ => bug!()
1005                     }
1006                 }
1007             }
1008         };
1009
1010         assert_eq!(self.new_block(), START_BLOCK);
1011         self.visit_operand(&mut operand, Location {
1012             block: BasicBlock::new(0),
1013             statement_index: usize::MAX
1014         });
1015
1016         let span = self.promoted.span;
1017         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
1018         Some(self.promoted)
1019     }
1020 }
1021
1022 /// Replaces all temporaries with their promoted counterparts.
1023 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1024     fn tcx(&self) -> TyCtxt<'tcx> {
1025         self.tcx
1026     }
1027
1028     fn visit_local(&mut self,
1029                    local: &mut Local,
1030                    _: PlaceContext,
1031                    _: Location) {
1032         if self.is_temp_kind(*local) {
1033             *local = self.promote_temp(*local);
1034         }
1035     }
1036
1037     fn process_projection_elem(
1038         &mut self,
1039         elem: &PlaceElem<'tcx>,
1040     ) -> Option<PlaceElem<'tcx>> {
1041         match elem {
1042             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1043                 Some(PlaceElem::Index(self.promote_temp(*local)))
1044             }
1045             _ => None,
1046         }
1047     }
1048 }
1049
1050 pub fn promote_candidates<'tcx>(
1051     def_id: DefId,
1052     body: &mut Body<'tcx>,
1053     tcx: TyCtxt<'tcx>,
1054     mut temps: IndexVec<Local, TempState>,
1055     candidates: Vec<Candidate>,
1056 ) -> IndexVec<Promoted, Body<'tcx>> {
1057     // Visit candidates in reverse, in case they're nested.
1058     debug!("promote_candidates({:?})", candidates);
1059
1060     let mut promotions = IndexVec::new();
1061
1062     for candidate in candidates.into_iter().rev() {
1063         match candidate {
1064             Candidate::Repeat(Location { block, statement_index }) |
1065             Candidate::Ref(Location { block, statement_index }) => {
1066                 match &body[block].statements[statement_index].kind {
1067                     StatementKind::Assign(box(place, _)) => {
1068                         if let Some(local) = place.as_local() {
1069                             if temps[local] == TempState::PromotedOut {
1070                                 // Already promoted.
1071                                 continue;
1072                             }
1073                         }
1074                     }
1075                     _ => {}
1076                 }
1077             }
1078             Candidate::Argument { .. } => {}
1079         }
1080
1081
1082         // Declare return place local so that `mir::Body::new` doesn't complain.
1083         let initial_locals = iter::once(
1084             LocalDecl::new_return_place(tcx.types.never, body.span)
1085         ).collect();
1086
1087         let promoter = Promoter {
1088             promoted: Body::new(
1089                 IndexVec::new(),
1090                 // FIXME: maybe try to filter this to avoid blowing up
1091                 // memory usage?
1092                 body.source_scopes.clone(),
1093                 body.source_scope_local_data.clone(),
1094                 None,
1095                 initial_locals,
1096                 IndexVec::new(),
1097                 0,
1098                 vec![],
1099                 body.span,
1100                 vec![],
1101             ),
1102             tcx,
1103             source: body,
1104             temps: &mut temps,
1105             keep_original: false
1106         };
1107
1108         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1109         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1110             promotions.push(promoted);
1111         }
1112     }
1113
1114     // Eliminate assignments to, and drops of promoted temps.
1115     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1116     for block in body.basic_blocks_mut() {
1117         block.statements.retain(|statement| {
1118             match &statement.kind {
1119                 StatementKind::Assign(box(place, _)) => {
1120                     if let Some(index) = place.as_local() {
1121                         !promoted(index)
1122                     } else {
1123                         true
1124                     }
1125                 }
1126                 StatementKind::StorageLive(index) |
1127                 StatementKind::StorageDead(index) => {
1128                     !promoted(*index)
1129                 }
1130                 _ => true
1131             }
1132         });
1133         let terminator = block.terminator_mut();
1134         match &terminator.kind {
1135             TerminatorKind::Drop { location: place, target, .. } => {
1136                 if let Some(index) = place.as_local() {
1137                     if promoted(index) {
1138                         terminator.kind = TerminatorKind::Goto {
1139                             target: *target,
1140                         };
1141                     }
1142                 }
1143             }
1144             _ => {}
1145         }
1146     }
1147
1148     promotions
1149 }
1150
1151 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1152 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1153 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1154 /// enabled.
1155 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1156     tcx: TyCtxt<'tcx>,
1157     mir_def_id: DefId,
1158     body: &Body<'tcx>,
1159     operand: &Operand<'tcx>,
1160 ) -> bool {
1161     let mut rpo = traversal::reverse_postorder(body);
1162     let (temps, _) = collect_temps_and_candidates(tcx, body, &mut rpo);
1163     let validator = Validator {
1164         item: Item::new(tcx, mir_def_id, body),
1165         temps: &temps,
1166         explicit: false,
1167     };
1168
1169     let should_promote = validator.validate_operand(operand).is_ok();
1170     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1171     debug!("should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1172             should_promote={:?} feature_flag={:?}", mir_def_id, should_promote, feature_flag);
1173     should_promote && !feature_flag
1174 }