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::mir::traversal::ReversePostorder;
  16 use rustc::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  17 use rustc::mir::*;
  18 use rustc::ty::cast::CastTy;
  19 use rustc::ty::subst::InternalSubsts;
  20 use rustc::ty::{self, List, TyCtxt, TypeFoldable};
  21 use rustc_hir::def_id::DefId;
  22 use rustc_span::symbol::sym;
  23 use rustc_span::{Span, DUMMY_SP};
  24 use syntax::ast::LitKind;
  25
  26 use rustc_index::vec::{Idx, IndexVec};
  27 use rustc_target::spec::abi::Abi;
  28
  29 use std::cell::Cell;
  30 use std::{iter, mem, usize};
  31
  32 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
  33 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstKind, Item};
  34 use crate::transform::{MirPass, MirSource};
  35
  36 /// A `MirPass` for promotion.
  37 ///
  38 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
  39 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
  40 ///
  41 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  42 /// newly created `StaticKind::Promoted`.
  43 #[derive(Default)]
  44 pub struct PromoteTemps<'tcx> {
  45     pub promoted_fragments: Cell<IndexVec<Promoted, BodyAndCache<'tcx>>>,
  46 }
  47
  48 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  49     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  50         // There's not really any point in promoting errorful MIR.
  51         //
  52         // This does not include MIR that failed const-checking, which we still try to promote.
  53         if body.return_ty().references_error() {
  54             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  55             return;
  56         }
  57
  58         if src.promoted.is_some() {
  59             return;
  60         }
  61
  62         let def_id = src.def_id();
  63
  64         let mut rpo = traversal::reverse_postorder(body);
  65         let (temps, all_candidates) = collect_temps_and_candidates(tcx, body, &mut rpo);
  66
  67         let promotable_candidates =
  68             validate_candidates(tcx, read_only!(body), def_id, &temps, &all_candidates);
  69
  70         let promoted = promote_candidates(def_id, body, tcx, temps, promotable_candidates);
  71         self.promoted_fragments.set(promoted);
  72     }
  73 }
  74
  75 /// State of a temporary during collection and promotion.
  76 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  77 pub enum TempState {
  78     /// No references to this temp.
  79     Undefined,
  80     /// One direct assignment and any number of direct uses.
  81     /// A borrow of this temp is promotable if the assigned
  82     /// value is qualified as constant.
  83     Defined { location: Location, uses: usize },
  84     /// Any other combination of assignments/uses.
  85     Unpromotable,
  86     /// This temp was part of an rvalue which got extracted
  87     /// during promotion and needs cleanup.
  88     PromotedOut,
  89 }
  90
  91 impl TempState {
  92     pub fn is_promotable(&self) -> bool {
  93         debug!("is_promotable: self={:?}", self);
  94         if let TempState::Defined { .. } = *self { true } else { false }
  95     }
  96 }
  97
  98 /// A "root candidate" for promotion, which will become the
  99 /// returned value in a promoted MIR, unless it's a subset
 100 /// of a larger candidate.
 101 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 102 pub enum Candidate {
 103     /// Borrow of a constant temporary.
 104     Ref(Location),
 105
 106     /// Promotion of the `x` in `[x; 32]`.
 107     Repeat(Location),
 108
 109     /// Currently applied to function calls where the callee has the unstable
 110     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
 111     /// intrinsic. The intrinsic requires the arguments are indeed constant and
 112     /// the attribute currently provides the semantic requirement that arguments
 113     /// must be constant.
 114     Argument { bb: BasicBlock, index: usize },
 115 }
 116
 117 impl Candidate {
 118     /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
 119     fn forces_explicit_promotion(&self) -> bool {
 120         match self {
 121             Candidate::Ref(_) | Candidate::Repeat(_) => false,
 122             Candidate::Argument { .. } => true,
 123         }
 124     }
 125 }
 126
 127 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
 128     let attrs = tcx.get_attrs(def_id);
 129     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
 130     let mut ret = vec![];
 131     for meta in attr.meta_item_list()? {
 132         match meta.literal()?.kind {
 133             LitKind::Int(a, _) => {
 134                 ret.push(a as usize);
 135             }
 136             _ => return None,
 137         }
 138     }
 139     Some(ret)
 140 }
 141
 142 struct Collector<'a, 'tcx> {
 143     tcx: TyCtxt<'tcx>,
 144     body: &'a Body<'tcx>,
 145     temps: IndexVec<Local, TempState>,
 146     candidates: Vec<Candidate>,
 147     span: Span,
 148 }
 149
 150 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 151     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 152         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 153         // We're only interested in temporaries and the return place
 154         match self.body.local_kind(index) {
 155             LocalKind::Temp | LocalKind::ReturnPointer => {}
 156             LocalKind::Arg | LocalKind::Var => return,
 157         }
 158
 159         // Ignore drops, if the temp gets promoted,
 160         // then it's constant and thus drop is noop.
 161         // Non-uses are also irrelevant.
 162         if context.is_drop() || !context.is_use() {
 163             debug!(
 164                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 165                 context.is_drop(),
 166                 context.is_use(),
 167             );
 168             return;
 169         }
 170
 171         let temp = &mut self.temps[index];
 172         debug!("visit_local: temp={:?}", temp);
 173         if *temp == TempState::Undefined {
 174             match context {
 175                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 176                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 177                     *temp = TempState::Defined { location, uses: 0 };
 178                     return;
 179                 }
 180                 _ => { /* mark as unpromotable below */ }
 181             }
 182         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 183             // We always allow borrows, even mutable ones, as we need
 184             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 185             let allowed_use = match context {
 186                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 187                 | PlaceContext::NonMutatingUse(_) => true,
 188                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 189             };
 190             debug!("visit_local: allowed_use={:?}", allowed_use);
 191             if allowed_use {
 192                 *uses += 1;
 193                 return;
 194             }
 195             /* mark as unpromotable below */
 196         }
 197         *temp = TempState::Unpromotable;
 198     }
 199
 200     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 201         self.super_rvalue(rvalue, location);
 202
 203         match *rvalue {
 204             Rvalue::Ref(..) => {
 205                 self.candidates.push(Candidate::Ref(location));
 206             }
 207             Rvalue::Repeat(..) if self.tcx.features().const_in_array_repeat_expressions => {
 208                 // FIXME(#49147) only promote the element when it isn't `Copy`
 209                 // (so that code that can copy it at runtime is unaffected).
 210                 self.candidates.push(Candidate::Repeat(location));
 211             }
 212             _ => {}
 213         }
 214     }
 215
 216     fn visit_terminator_kind(&mut self, kind: &TerminatorKind<'tcx>, location: Location) {
 217         self.super_terminator_kind(kind, location);
 218
 219         if let TerminatorKind::Call { ref func, .. } = *kind {
 220             if let ty::FnDef(def_id, _) = func.ty(self.body, self.tcx).kind {
 221                 let fn_sig = self.tcx.fn_sig(def_id);
 222                 if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
 223                     let name = self.tcx.item_name(def_id);
 224                     // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
 225                     if name.as_str().starts_with("simd_shuffle") {
 226                         self.candidates.push(Candidate::Argument { bb: location.block, index: 2 });
 227
 228                         return; // Don't double count `simd_shuffle` candidates
 229                     }
 230                 }
 231
 232                 if let Some(constant_args) = args_required_const(self.tcx, def_id) {
 233                     for index in constant_args {
 234                         self.candidates.push(Candidate::Argument { bb: location.block, index });
 235                     }
 236                 }
 237             }
 238         }
 239     }
 240
 241     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 242         self.span = source_info.span;
 243     }
 244 }
 245
 246 pub fn collect_temps_and_candidates(
 247     tcx: TyCtxt<'tcx>,
 248     body: &Body<'tcx>,
 249     rpo: &mut ReversePostorder<'_, 'tcx>,
 250 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 251     let mut collector = Collector {
 252         tcx,
 253         body,
 254         temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls),
 255         candidates: vec![],
 256         span: body.span,
 257     };
 258     for (bb, data) in rpo {
 259         collector.visit_basic_block_data(bb, data);
 260     }
 261     (collector.temps, collector.candidates)
 262 }
 263
 264 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 265 ///
 266 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 267 struct Validator<'a, 'tcx> {
 268     item: Item<'a, 'tcx>,
 269     temps: &'a IndexVec<Local, TempState>,
 270
 271     /// Explicit promotion happens e.g. for constant arguments declared via
 272     /// `rustc_args_required_const`.
 273     /// Implicit promotion has almost the same rules, except that disallows `const fn`
 274     /// except for those marked `#[rustc_promotable]`. This is to avoid changing
 275     /// a legitimate run-time operation into a failing compile-time operation
 276     /// e.g. due to addresses being compared inside the function.
 277     explicit: bool,
 278 }
 279
 280 impl std::ops::Deref for Validator<'a, 'tcx> {
 281     type Target = Item<'a, 'tcx>;
 282
 283     fn deref(&self) -> &Self::Target {
 284         &self.item
 285     }
 286 }
 287
 288 struct Unpromotable;
 289
 290 impl<'tcx> Validator<'_, 'tcx> {
 291     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 292         match candidate {
 293             Candidate::Ref(loc) => {
 294                 assert!(!self.explicit);
 295
 296                 let statement = &self.body[loc.block].statements[loc.statement_index];
 297                 match &statement.kind {
 298                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 299                         match kind {
 300                             BorrowKind::Shared | BorrowKind::Mut { .. } => {}
 301
 302                             // FIXME(eddyb) these aren't promoted here but *could*
 303                             // be promoted as part of a larger value because
 304                             // `validate_rvalue`  doesn't check them, need to
 305                             // figure out what is the intended behavior.
 306                             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 307                         }
 308
 309                         // We can only promote interior borrows of promotable temps (non-temps
 310                         // don't get promoted anyway).
 311                         let base = match place.base {
 312                             PlaceBase::Local(local) => local,
 313                             _ => return Err(Unpromotable),
 314                         };
 315                         self.validate_local(base)?;
 316
 317                         if place.projection.contains(&ProjectionElem::Deref) {
 318                             return Err(Unpromotable);
 319                         }
 320
 321                         let mut has_mut_interior =
 322                             self.qualif_local::<qualifs::HasMutInterior>(base);
 323                         // HACK(eddyb) this should compute the same thing as
 324                         // `<HasMutInterior as Qualif>::in_projection` from
 325                         // `check_consts::qualifs` but without recursion.
 326                         if has_mut_interior {
 327                             // This allows borrowing fields which don't have
 328                             // `HasMutInterior`, from a type that does, e.g.:
 329                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 330                             let mut place_projection = &place.projection[..];
 331                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 332                             while let [proj_base @ .., elem] = place_projection {
 333                                 // FIXME(eddyb) this is probably excessive, with
 334                                 // the exception of `union` member accesses.
 335                                 let ty =
 336                                     Place::ty_from(&place.base, proj_base, *self.body, self.tcx)
 337                                         .projection_ty(self.tcx, elem)
 338                                         .ty;
 339                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 340                                     has_mut_interior = false;
 341                                     break;
 342                                 }
 343
 344                                 place_projection = proj_base;
 345                             }
 346                         }
 347
 348                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 349                         if has_mut_interior {
 350                             return Err(Unpromotable);
 351                         }
 352                         if self.qualif_local::<qualifs::NeedsDrop>(base) {
 353                             return Err(Unpromotable);
 354                         }
 355
 356                         if let BorrowKind::Mut { .. } = kind {
 357                             let ty = place.ty(*self.body, self.tcx).ty;
 358
 359                             // In theory, any zero-sized value could be borrowed
 360                             // mutably without consequences. However, only &mut []
 361                             // is allowed right now, and only in functions.
 362                             if self.const_kind == Some(ConstKind::StaticMut) {
 363                                 // Inside a `static mut`, &mut [...] is also allowed.
 364                                 match ty.kind {
 365                                     ty::Array(..) | ty::Slice(_) => {}
 366                                     _ => return Err(Unpromotable),
 367                                 }
 368                             } else if let ty::Array(_, len) = ty.kind {
 369                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 370                                 // seems unnecessary, given that this is merely a ZST.
 371                                 match len.try_eval_usize(self.tcx, self.param_env) {
 372                                     Some(0) if self.const_kind.is_none() => {}
 373                                     _ => return Err(Unpromotable),
 374                                 }
 375                             } else {
 376                                 return Err(Unpromotable);
 377                             }
 378                         }
 379
 380                         Ok(())
 381                     }
 382                     _ => bug!(),
 383                 }
 384             }
 385             Candidate::Repeat(loc) => {
 386                 assert!(!self.explicit);
 387
 388                 let statement = &self.body[loc.block].statements[loc.statement_index];
 389                 match &statement.kind {
 390                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 391                         if !self.tcx.features().const_in_array_repeat_expressions {
 392                             return Err(Unpromotable);
 393                         }
 394
 395                         self.validate_operand(operand)
 396                     }
 397                     _ => bug!(),
 398                 }
 399             }
 400             Candidate::Argument { bb, index } => {
 401                 assert!(self.explicit);
 402
 403                 let terminator = self.body[bb].terminator();
 404                 match &terminator.kind {
 405                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 406                     _ => bug!(),
 407                 }
 408             }
 409         }
 410     }
 411
 412     // FIXME(eddyb) maybe cache this?
 413     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 414         let per_local = &|l| self.qualif_local::<Q>(l);
 415
 416         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 417             let num_stmts = self.body[loc.block].statements.len();
 418
 419             if loc.statement_index < num_stmts {
 420                 let statement = &self.body[loc.block].statements[loc.statement_index];
 421                 match &statement.kind {
 422                     StatementKind::Assign(box (_, rhs)) => Q::in_rvalue(&self.item, per_local, rhs),
 423                     _ => {
 424                         span_bug!(
 425                             statement.source_info.span,
 426                             "{:?} is not an assignment",
 427                             statement
 428                         );
 429                     }
 430                 }
 431             } else {
 432                 let terminator = self.body[loc.block].terminator();
 433                 match &terminator.kind {
 434                     TerminatorKind::Call { func, args, .. } => {
 435                         let return_ty = self.body.local_decls[local].ty;
 436                         Q::in_call(&self.item, per_local, func, args, return_ty)
 437                     }
 438                     kind => {
 439                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 440                     }
 441                 }
 442             }
 443         } else {
 444             let span = self.body.local_decls[local].source_info.span;
 445             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 446         }
 447     }
 448
 449     // FIXME(eddyb) maybe cache this?
 450     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 451         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 452             let num_stmts = self.body[loc.block].statements.len();
 453
 454             if loc.statement_index < num_stmts {
 455                 let statement = &self.body[loc.block].statements[loc.statement_index];
 456                 match &statement.kind {
 457                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 458                     _ => {
 459                         span_bug!(
 460                             statement.source_info.span,
 461                             "{:?} is not an assignment",
 462                             statement
 463                         );
 464                     }
 465                 }
 466             } else {
 467                 let terminator = self.body[loc.block].terminator();
 468                 match &terminator.kind {
 469                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 470                     kind => {
 471                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 472                     }
 473                 }
 474             }
 475         } else {
 476             Err(Unpromotable)
 477         }
 478     }
 479
 480     fn validate_place(&self, place: PlaceRef<'_, 'tcx>) -> Result<(), Unpromotable> {
 481         match place {
 482             PlaceRef { base: PlaceBase::Local(local), projection: [] } => {
 483                 self.validate_local(*local)
 484             }
 485             PlaceRef { base: PlaceBase::Static(_), projection: [] } => {
 486                 bug!("qualifying already promoted MIR")
 487             }
 488             PlaceRef { base: _, projection: [proj_base @ .., elem] } => {
 489                 match *elem {
 490                     ProjectionElem::Deref | ProjectionElem::Downcast(..) => {
 491                         return Err(Unpromotable);
 492                     }
 493
 494                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 495
 496                     ProjectionElem::Index(local) => {
 497                         self.validate_local(local)?;
 498                     }
 499
 500                     ProjectionElem::Field(..) => {
 501                         if self.const_kind.is_none() {
 502                             let base_ty =
 503                                 Place::ty_from(place.base, proj_base, *self.body, self.tcx).ty;
 504                             if let Some(def) = base_ty.ty_adt_def() {
 505                                 // No promotion of union field accesses.
 506                                 if def.is_union() {
 507                                     return Err(Unpromotable);
 508                                 }
 509                             }
 510                         }
 511                     }
 512                 }
 513
 514                 self.validate_place(PlaceRef { base: place.base, projection: proj_base })
 515             }
 516         }
 517     }
 518
 519     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 520         match operand {
 521             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 522
 523             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 524             // `validate_rvalue` upon access.
 525             Operand::Constant(c) => {
 526                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 527                     // Only allow statics (not consts) to refer to other statics.
 528                     // FIXME(eddyb) does this matter at all for promotion?
 529                     let is_static = self.const_kind.map_or(false, |k| k.is_static());
 530                     if !is_static {
 531                         return Err(Unpromotable);
 532                     }
 533
 534                     let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 535                     if is_thread_local {
 536                         return Err(Unpromotable);
 537                     }
 538                 }
 539
 540                 Ok(())
 541             }
 542         }
 543     }
 544
 545     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 546         match *rvalue {
 547             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 548                 let operand_ty = operand.ty(*self.body, self.tcx);
 549                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 550                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 551                 match (cast_in, cast_out) {
 552                     (CastTy::Ptr(_), CastTy::Int(_)) | (CastTy::FnPtr, CastTy::Int(_)) => {
 553                         // in normal functions, mark such casts as not promotable
 554                         return Err(Unpromotable);
 555                     }
 556                     _ => {}
 557                 }
 558             }
 559
 560             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 561                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(*self.body, self.tcx).kind {
 562                     assert!(
 563                         op == BinOp::Eq
 564                             || op == BinOp::Ne
 565                             || op == BinOp::Le
 566                             || op == BinOp::Lt
 567                             || op == BinOp::Ge
 568                             || op == BinOp::Gt
 569                             || op == BinOp::Offset
 570                     );
 571
 572                     // raw pointer operations are not allowed inside promoteds
 573                     return Err(Unpromotable);
 574                 }
 575             }
 576
 577             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 578
 579             _ => {}
 580         }
 581
 582         match rvalue {
 583             Rvalue::NullaryOp(..) => Ok(()),
 584
 585             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 586
 587             Rvalue::Use(operand)
 588             | Rvalue::Repeat(operand, _)
 589             | Rvalue::UnaryOp(_, operand)
 590             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 591
 592             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 593                 self.validate_operand(lhs)?;
 594                 self.validate_operand(rhs)
 595             }
 596
 597             Rvalue::AddressOf(_, place) => {
 598                 // Raw reborrows can come from reference to pointer coercions,
 599                 // so are allowed.
 600                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 601                     let base_ty = Place::ty_from(&place.base, proj_base, *self.body, self.tcx).ty;
 602                     if let ty::Ref(..) = base_ty.kind {
 603                         return self
 604                             .validate_place(PlaceRef { base: &place.base, projection: proj_base });
 605                     }
 606                 }
 607                 Err(Unpromotable)
 608             }
 609
 610             Rvalue::Ref(_, kind, place) => {
 611                 if let BorrowKind::Mut { .. } = kind {
 612                     let ty = place.ty(*self.body, self.tcx).ty;
 613
 614                     // In theory, any zero-sized value could be borrowed
 615                     // mutably without consequences. However, only &mut []
 616                     // is allowed right now, and only in functions.
 617                     if self.const_kind == Some(ConstKind::StaticMut) {
 618                         // Inside a `static mut`, &mut [...] is also allowed.
 619                         match ty.kind {
 620                             ty::Array(..) | ty::Slice(_) => {}
 621                             _ => return Err(Unpromotable),
 622                         }
 623                     } else if let ty::Array(_, len) = ty.kind {
 624                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 625                         // const context which seems unnecessary given that this is merely a ZST.
 626                         match len.try_eval_usize(self.tcx, self.param_env) {
 627                             Some(0) if self.const_kind.is_none() => {}
 628                             _ => return Err(Unpromotable),
 629                         }
 630                     } else {
 631                         return Err(Unpromotable);
 632                     }
 633                 }
 634
 635                 // Special-case reborrows to be more like a copy of the reference.
 636                 let mut place = place.as_ref();
 637                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 638                     let base_ty = Place::ty_from(&place.base, proj_base, *self.body, self.tcx).ty;
 639                     if let ty::Ref(..) = base_ty.kind {
 640                         place = PlaceRef { base: &place.base, projection: proj_base };
 641                     }
 642                 }
 643
 644                 self.validate_place(place)?;
 645
 646                 // HACK(eddyb) this should compute the same thing as
 647                 // `<HasMutInterior as Qualif>::in_projection` from
 648                 // `check_consts::qualifs` but without recursion.
 649                 let mut has_mut_interior = match place.base {
 650                     PlaceBase::Local(local) => self.qualif_local::<qualifs::HasMutInterior>(*local),
 651                     PlaceBase::Static(_) => false,
 652                 };
 653                 if has_mut_interior {
 654                     let mut place_projection = place.projection;
 655                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 656                     while let [proj_base @ .., elem] = place_projection {
 657                         // FIXME(eddyb) this is probably excessive, with
 658                         // the exception of `union` member accesses.
 659                         let ty = Place::ty_from(place.base, proj_base, *self.body, self.tcx)
 660                             .projection_ty(self.tcx, elem)
 661                             .ty;
 662                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 663                             has_mut_interior = false;
 664                             break;
 665                         }
 666
 667                         place_projection = proj_base;
 668                     }
 669                 }
 670                 if has_mut_interior {
 671                     return Err(Unpromotable);
 672                 }
 673
 674                 Ok(())
 675             }
 676
 677             Rvalue::Aggregate(_, ref operands) => {
 678                 for o in operands {
 679                     self.validate_operand(o)?;
 680                 }
 681
 682                 Ok(())
 683             }
 684         }
 685     }
 686
 687     fn validate_call(
 688         &self,
 689         callee: &Operand<'tcx>,
 690         args: &[Operand<'tcx>],
 691     ) -> Result<(), Unpromotable> {
 692         let fn_ty = callee.ty(*self.body, self.tcx);
 693
 694         if !self.explicit && self.const_kind.is_none() {
 695             if let ty::FnDef(def_id, _) = fn_ty.kind {
 696                 // Never promote runtime `const fn` calls of
 697                 // functions without `#[rustc_promotable]`.
 698                 if !self.tcx.is_promotable_const_fn(def_id) {
 699                     return Err(Unpromotable);
 700                 }
 701             }
 702         }
 703
 704         let is_const_fn = match fn_ty.kind {
 705             ty::FnDef(def_id, _) => {
 706                 is_const_fn(self.tcx, def_id)
 707                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 708                     || is_lang_panic_fn(self.tcx, self.def_id)
 709             }
 710             _ => false,
 711         };
 712         if !is_const_fn {
 713             return Err(Unpromotable);
 714         }
 715
 716         self.validate_operand(callee)?;
 717         for arg in args {
 718             self.validate_operand(arg)?;
 719         }
 720
 721         Ok(())
 722     }
 723 }
 724
 725 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 726 pub fn validate_candidates(
 727     tcx: TyCtxt<'tcx>,
 728     body: ReadOnlyBodyAndCache<'_, 'tcx>,
 729     def_id: DefId,
 730     temps: &IndexVec<Local, TempState>,
 731     candidates: &[Candidate],
 732 ) -> Vec<Candidate> {
 733     let mut validator = Validator { item: Item::new(tcx, def_id, body), temps, explicit: false };
 734
 735     candidates
 736         .iter()
 737         .copied()
 738         .filter(|&candidate| {
 739             validator.explicit = candidate.forces_explicit_promotion();
 740
 741             // FIXME(eddyb) also emit the errors for shuffle indices
 742             // and `#[rustc_args_required_const]` arguments here.
 743
 744             let is_promotable = validator.validate_candidate(candidate).is_ok();
 745             match candidate {
 746                 Candidate::Argument { bb, index } if !is_promotable => {
 747                     let span = body[bb].terminator().source_info.span;
 748                     let msg = format!("argument {} is required to be a constant", index + 1);
 749                     tcx.sess.span_err(span, &msg);
 750                 }
 751                 _ => (),
 752             }
 753
 754             is_promotable
 755         })
 756         .collect()
 757 }
 758
 759 struct Promoter<'a, 'tcx> {
 760     tcx: TyCtxt<'tcx>,
 761     source: &'a mut BodyAndCache<'tcx>,
 762     promoted: BodyAndCache<'tcx>,
 763     temps: &'a mut IndexVec<Local, TempState>,
 764
 765     /// If true, all nested temps are also kept in the
 766     /// source MIR, not moved to the promoted MIR.
 767     keep_original: bool,
 768 }
 769
 770 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 771     fn new_block(&mut self) -> BasicBlock {
 772         let span = self.promoted.span;
 773         self.promoted.basic_blocks_mut().push(BasicBlockData {
 774             statements: vec![],
 775             terminator: Some(Terminator {
 776                 source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 777                 kind: TerminatorKind::Return,
 778             }),
 779             is_cleanup: false,
 780         })
 781     }
 782
 783     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 784         let last = self.promoted.basic_blocks().last().unwrap();
 785         let data = &mut self.promoted[last];
 786         data.statements.push(Statement {
 787             source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 788             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 789         });
 790     }
 791
 792     fn is_temp_kind(&self, local: Local) -> bool {
 793         self.source.local_kind(local) == LocalKind::Temp
 794     }
 795
 796     /// Copies the initialization of this temp to the
 797     /// promoted MIR, recursing through temps.
 798     fn promote_temp(&mut self, temp: Local) -> Local {
 799         let old_keep_original = self.keep_original;
 800         let loc = match self.temps[temp] {
 801             TempState::Defined { location, uses } if uses > 0 => {
 802                 if uses > 1 {
 803                     self.keep_original = true;
 804                 }
 805                 location
 806             }
 807             state => {
 808                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 809             }
 810         };
 811         if !self.keep_original {
 812             self.temps[temp] = TempState::PromotedOut;
 813         }
 814
 815         let num_stmts = self.source[loc.block].statements.len();
 816         let new_temp = self.promoted.local_decls.push(LocalDecl::new_temp(
 817             self.source.local_decls[temp].ty,
 818             self.source.local_decls[temp].source_info.span,
 819         ));
 820
 821         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 822
 823         // First, take the Rvalue or Call out of the source MIR,
 824         // or duplicate it, depending on keep_original.
 825         if loc.statement_index < num_stmts {
 826             let (mut rvalue, source_info) = {
 827                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 828                 let rhs = match statement.kind {
 829                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 830                     _ => {
 831                         span_bug!(
 832                             statement.source_info.span,
 833                             "{:?} is not an assignment",
 834                             statement
 835                         );
 836                     }
 837                 };
 838
 839                 (
 840                     if self.keep_original {
 841                         rhs.clone()
 842                     } else {
 843                         let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 844                         mem::replace(rhs, unit)
 845                     },
 846                     statement.source_info,
 847                 )
 848             };
 849
 850             self.visit_rvalue(&mut rvalue, loc);
 851             self.assign(new_temp, rvalue, source_info.span);
 852         } else {
 853             let terminator = if self.keep_original {
 854                 self.source[loc.block].terminator().clone()
 855             } else {
 856                 let terminator = self.source[loc.block].terminator_mut();
 857                 let target = match terminator.kind {
 858                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 859                     ref kind => {
 860                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 861                     }
 862                 };
 863                 Terminator {
 864                     source_info: terminator.source_info,
 865                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 866                 }
 867             };
 868
 869             match terminator.kind {
 870                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 871                     self.visit_operand(&mut func, loc);
 872                     for arg in &mut args {
 873                         self.visit_operand(arg, loc);
 874                     }
 875
 876                     let last = self.promoted.basic_blocks().last().unwrap();
 877                     let new_target = self.new_block();
 878
 879                     *self.promoted[last].terminator_mut() = Terminator {
 880                         kind: TerminatorKind::Call {
 881                             func,
 882                             args,
 883                             cleanup: None,
 884                             destination: Some((Place::from(new_temp), new_target)),
 885                             from_hir_call,
 886                         },
 887                         ..terminator
 888                     };
 889                 }
 890                 ref kind => {
 891                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 892                 }
 893             };
 894         };
 895
 896         self.keep_original = old_keep_original;
 897         new_temp
 898     }
 899
 900     fn promote_candidate(
 901         mut self,
 902         def_id: DefId,
 903         candidate: Candidate,
 904         next_promoted_id: usize,
 905     ) -> Option<BodyAndCache<'tcx>> {
 906         let mut operand = {
 907             let promoted = &mut self.promoted;
 908             let promoted_id = Promoted::new(next_promoted_id);
 909             let tcx = self.tcx;
 910             let mut promoted_place = |ty, span| {
 911                 promoted.span = span;
 912                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 913                 Place {
 914                     base: PlaceBase::Static(box Static {
 915                         kind: StaticKind::Promoted(
 916                             promoted_id,
 917                             InternalSubsts::identity_for_item(tcx, def_id),
 918                         ),
 919                         ty,
 920                         def_id,
 921                     }),
 922                     projection: List::empty(),
 923                 }
 924             };
 925             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 926             match candidate {
 927                 Candidate::Ref(loc) => {
 928                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 929                     match statement.kind {
 930                         StatementKind::Assign(box (_, Rvalue::Ref(_, _, ref mut place))) => {
 931                             // Use the underlying local for this (necessarily interior) borrow.
 932                             let ty = place.base.ty(local_decls).ty;
 933                             let span = statement.source_info.span;
 934
 935                             Operand::Move(Place {
 936                                 base: mem::replace(&mut place.base, promoted_place(ty, span).base),
 937                                 projection: List::empty(),
 938                             })
 939                         }
 940                         _ => bug!(),
 941                     }
 942                 }
 943                 Candidate::Repeat(loc) => {
 944                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 945                     match statement.kind {
 946                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
 947                             let ty = operand.ty(local_decls, self.tcx);
 948                             let span = statement.source_info.span;
 949                             mem::replace(operand, Operand::Copy(promoted_place(ty, span)))
 950                         }
 951                         _ => bug!(),
 952                     }
 953                 }
 954                 Candidate::Argument { bb, index } => {
 955                     let terminator = blocks[bb].terminator_mut();
 956                     match terminator.kind {
 957                         TerminatorKind::Call { ref mut args, .. } => {
 958                             let ty = args[index].ty(local_decls, self.tcx);
 959                             let span = terminator.source_info.span;
 960                             let operand = Operand::Copy(promoted_place(ty, span));
 961                             mem::replace(&mut args[index], operand)
 962                         }
 963                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 964                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 965                         // we are seeing a `Goto`. That means that the `promote_temps` method
 966                         // already promoted this call away entirely. This case occurs when calling
 967                         // a function requiring a constant argument and as that constant value
 968                         // providing a value whose computation contains another call to a function
 969                         // requiring a constant argument.
 970                         TerminatorKind::Goto { .. } => return None,
 971                         _ => bug!(),
 972                     }
 973                 }
 974             }
 975         };
 976
 977         assert_eq!(self.new_block(), START_BLOCK);
 978         self.visit_operand(
 979             &mut operand,
 980             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 981         );
 982
 983         let span = self.promoted.span;
 984         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
 985         Some(self.promoted)
 986     }
 987 }
 988
 989 /// Replaces all temporaries with their promoted counterparts.
 990 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 991     fn tcx(&self) -> TyCtxt<'tcx> {
 992         self.tcx
 993     }
 994
 995     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 996         if self.is_temp_kind(*local) {
 997             *local = self.promote_temp(*local);
 998         }
 999     }
1000
1001     fn process_projection_elem(&mut self, elem: &PlaceElem<'tcx>) -> Option<PlaceElem<'tcx>> {
1002         match elem {
1003             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1004                 Some(PlaceElem::Index(self.promote_temp(*local)))
1005             }
1006             _ => None,
1007         }
1008     }
1009 }
1010
1011 pub fn promote_candidates<'tcx>(
1012     def_id: DefId,
1013     body: &mut BodyAndCache<'tcx>,
1014     tcx: TyCtxt<'tcx>,
1015     mut temps: IndexVec<Local, TempState>,
1016     candidates: Vec<Candidate>,
1017 ) -> IndexVec<Promoted, BodyAndCache<'tcx>> {
1018     // Visit candidates in reverse, in case they're nested.
1019     debug!("promote_candidates({:?})", candidates);
1020
1021     let mut promotions = IndexVec::new();
1022
1023     for candidate in candidates.into_iter().rev() {
1024         match candidate {
1025             Candidate::Repeat(Location { block, statement_index })
1026             | Candidate::Ref(Location { block, statement_index }) => {
1027                 match &body[block].statements[statement_index].kind {
1028                     StatementKind::Assign(box (place, _)) => {
1029                         if let Some(local) = place.as_local() {
1030                             if temps[local] == TempState::PromotedOut {
1031                                 // Already promoted.
1032                                 continue;
1033                             }
1034                         }
1035                     }
1036                     _ => {}
1037                 }
1038             }
1039             Candidate::Argument { .. } => {}
1040         }
1041
1042         // Declare return place local so that `mir::Body::new` doesn't complain.
1043         let initial_locals =
1044             iter::once(LocalDecl::new_return_place(tcx.types.never, body.span)).collect();
1045
1046         let promoter = Promoter {
1047             promoted: BodyAndCache::new(Body::new(
1048                 IndexVec::new(),
1049                 // FIXME: maybe try to filter this to avoid blowing up
1050                 // memory usage?
1051                 body.source_scopes.clone(),
1052                 initial_locals,
1053                 IndexVec::new(),
1054                 0,
1055                 vec![],
1056                 body.span,
1057                 vec![],
1058                 body.generator_kind,
1059             )),
1060             tcx,
1061             source: body,
1062             temps: &mut temps,
1063             keep_original: false,
1064         };
1065
1066         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1067         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1068             promotions.push(promoted);
1069         }
1070     }
1071
1072     // Eliminate assignments to, and drops of promoted temps.
1073     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1074     for block in body.basic_blocks_mut() {
1075         block.statements.retain(|statement| match &statement.kind {
1076             StatementKind::Assign(box (place, _)) => {
1077                 if let Some(index) = place.as_local() {
1078                     !promoted(index)
1079                 } else {
1080                     true
1081                 }
1082             }
1083             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1084                 !promoted(*index)
1085             }
1086             _ => true,
1087         });
1088         let terminator = block.terminator_mut();
1089         match &terminator.kind {
1090             TerminatorKind::Drop { location: place, target, .. } => {
1091                 if let Some(index) = place.as_local() {
1092                     if promoted(index) {
1093                         terminator.kind = TerminatorKind::Goto { target: *target };
1094                     }
1095                 }
1096             }
1097             _ => {}
1098         }
1099     }
1100
1101     promotions
1102 }
1103
1104 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1105 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1106 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1107 /// enabled.
1108 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1109     tcx: TyCtxt<'tcx>,
1110     mir_def_id: DefId,
1111     body: ReadOnlyBodyAndCache<'_, 'tcx>,
1112     operand: &Operand<'tcx>,
1113 ) -> bool {
1114     let mut rpo = traversal::reverse_postorder(&body);
1115     let (temps, _) = collect_temps_and_candidates(tcx, &body, &mut rpo);
1116     let validator =
1117         Validator { item: Item::new(tcx, mir_def_id, body), temps: &temps, explicit: false };
1118
1119     let should_promote = validator.validate_operand(operand).is_ok();
1120     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1121     debug!(
1122         "should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1123             should_promote={:?} feature_flag={:?}",
1124         mir_def_id, should_promote, feature_flag
1125     );
1126     should_promote && !feature_flag
1127 }