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::{cmp, 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 `Constant`.
  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                         };
 314                         self.validate_local(base)?;
 315
 316                         if place.projection.contains(&ProjectionElem::Deref) {
 317                             return Err(Unpromotable);
 318                         }
 319
 320                         let mut has_mut_interior =
 321                             self.qualif_local::<qualifs::HasMutInterior>(base);
 322                         // HACK(eddyb) this should compute the same thing as
 323                         // `<HasMutInterior as Qualif>::in_projection` from
 324                         // `check_consts::qualifs` but without recursion.
 325                         if has_mut_interior {
 326                             // This allows borrowing fields which don't have
 327                             // `HasMutInterior`, from a type that does, e.g.:
 328                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 329                             let mut place_projection = &place.projection[..];
 330                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 331                             while let [proj_base @ .., elem] = place_projection {
 332                                 // FIXME(eddyb) this is probably excessive, with
 333                                 // the exception of `union` member accesses.
 334                                 let ty =
 335                                     Place::ty_from(&place.base, proj_base, *self.body, self.tcx)
 336                                         .projection_ty(self.tcx, elem)
 337                                         .ty;
 338                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 339                                     has_mut_interior = false;
 340                                     break;
 341                                 }
 342
 343                                 place_projection = proj_base;
 344                             }
 345                         }
 346
 347                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 348                         if has_mut_interior {
 349                             return Err(Unpromotable);
 350                         }
 351                         if self.qualif_local::<qualifs::NeedsDrop>(base) {
 352                             return Err(Unpromotable);
 353                         }
 354
 355                         if let BorrowKind::Mut { .. } = kind {
 356                             let ty = place.ty(*self.body, self.tcx).ty;
 357
 358                             // In theory, any zero-sized value could be borrowed
 359                             // mutably without consequences. However, only &mut []
 360                             // is allowed right now, and only in functions.
 361                             if self.const_kind == Some(ConstKind::StaticMut) {
 362                                 // Inside a `static mut`, &mut [...] is also allowed.
 363                                 match ty.kind {
 364                                     ty::Array(..) | ty::Slice(_) => {}
 365                                     _ => return Err(Unpromotable),
 366                                 }
 367                             } else if let ty::Array(_, len) = ty.kind {
 368                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 369                                 // seems unnecessary, given that this is merely a ZST.
 370                                 match len.try_eval_usize(self.tcx, self.param_env) {
 371                                     Some(0) if self.const_kind.is_none() => {}
 372                                     _ => return Err(Unpromotable),
 373                                 }
 374                             } else {
 375                                 return Err(Unpromotable);
 376                             }
 377                         }
 378
 379                         Ok(())
 380                     }
 381                     _ => bug!(),
 382                 }
 383             }
 384             Candidate::Repeat(loc) => {
 385                 assert!(!self.explicit);
 386
 387                 let statement = &self.body[loc.block].statements[loc.statement_index];
 388                 match &statement.kind {
 389                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 390                         if !self.tcx.features().const_in_array_repeat_expressions {
 391                             return Err(Unpromotable);
 392                         }
 393
 394                         self.validate_operand(operand)
 395                     }
 396                     _ => bug!(),
 397                 }
 398             }
 399             Candidate::Argument { bb, index } => {
 400                 assert!(self.explicit);
 401
 402                 let terminator = self.body[bb].terminator();
 403                 match &terminator.kind {
 404                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 405                     _ => bug!(),
 406                 }
 407             }
 408         }
 409     }
 410
 411     // FIXME(eddyb) maybe cache this?
 412     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 413         let per_local = &|l| self.qualif_local::<Q>(l);
 414
 415         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 416             let num_stmts = self.body[loc.block].statements.len();
 417
 418             if loc.statement_index < num_stmts {
 419                 let statement = &self.body[loc.block].statements[loc.statement_index];
 420                 match &statement.kind {
 421                     StatementKind::Assign(box (_, rhs)) => Q::in_rvalue(&self.item, per_local, rhs),
 422                     _ => {
 423                         span_bug!(
 424                             statement.source_info.span,
 425                             "{:?} is not an assignment",
 426                             statement
 427                         );
 428                     }
 429                 }
 430             } else {
 431                 let terminator = self.body[loc.block].terminator();
 432                 match &terminator.kind {
 433                     TerminatorKind::Call { func, args, .. } => {
 434                         let return_ty = self.body.local_decls[local].ty;
 435                         Q::in_call(&self.item, per_local, func, args, return_ty)
 436                     }
 437                     kind => {
 438                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 439                     }
 440                 }
 441             }
 442         } else {
 443             let span = self.body.local_decls[local].source_info.span;
 444             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 445         }
 446     }
 447
 448     // FIXME(eddyb) maybe cache this?
 449     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 450         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 451             let num_stmts = self.body[loc.block].statements.len();
 452
 453             if loc.statement_index < num_stmts {
 454                 let statement = &self.body[loc.block].statements[loc.statement_index];
 455                 match &statement.kind {
 456                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 457                     _ => {
 458                         span_bug!(
 459                             statement.source_info.span,
 460                             "{:?} is not an assignment",
 461                             statement
 462                         );
 463                     }
 464                 }
 465             } else {
 466                 let terminator = self.body[loc.block].terminator();
 467                 match &terminator.kind {
 468                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 469                     kind => {
 470                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 471                     }
 472                 }
 473             }
 474         } else {
 475             Err(Unpromotable)
 476         }
 477     }
 478
 479     fn validate_place(&self, place: PlaceRef<'_, 'tcx>) -> Result<(), Unpromotable> {
 480         match place {
 481             PlaceRef { base: PlaceBase::Local(local), projection: [] } => {
 482                 self.validate_local(*local)
 483             }
 484             PlaceRef { base: _, projection: [proj_base @ .., elem] } => {
 485                 match *elem {
 486                     ProjectionElem::Deref | ProjectionElem::Downcast(..) => {
 487                         return Err(Unpromotable);
 488                     }
 489
 490                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 491
 492                     ProjectionElem::Index(local) => {
 493                         self.validate_local(local)?;
 494                     }
 495
 496                     ProjectionElem::Field(..) => {
 497                         if self.const_kind.is_none() {
 498                             let base_ty =
 499                                 Place::ty_from(place.base, proj_base, *self.body, self.tcx).ty;
 500                             if let Some(def) = base_ty.ty_adt_def() {
 501                                 // No promotion of union field accesses.
 502                                 if def.is_union() {
 503                                     return Err(Unpromotable);
 504                                 }
 505                             }
 506                         }
 507                     }
 508                 }
 509
 510                 self.validate_place(PlaceRef { base: place.base, projection: proj_base })
 511             }
 512         }
 513     }
 514
 515     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 516         match operand {
 517             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 518
 519             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 520             // `validate_rvalue` upon access.
 521             Operand::Constant(c) => {
 522                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 523                     // Only allow statics (not consts) to refer to other statics.
 524                     // FIXME(eddyb) does this matter at all for promotion?
 525                     let is_static = self.const_kind.map_or(false, |k| k.is_static());
 526                     if !is_static {
 527                         return Err(Unpromotable);
 528                     }
 529
 530                     let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 531                     if is_thread_local {
 532                         return Err(Unpromotable);
 533                     }
 534                 }
 535
 536                 Ok(())
 537             }
 538         }
 539     }
 540
 541     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 542         match *rvalue {
 543             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 544                 let operand_ty = operand.ty(*self.body, self.tcx);
 545                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 546                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 547                 match (cast_in, cast_out) {
 548                     (CastTy::Ptr(_), CastTy::Int(_)) | (CastTy::FnPtr, CastTy::Int(_)) => {
 549                         // in normal functions, mark such casts as not promotable
 550                         return Err(Unpromotable);
 551                     }
 552                     _ => {}
 553                 }
 554             }
 555
 556             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 557                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(*self.body, self.tcx).kind {
 558                     assert!(
 559                         op == BinOp::Eq
 560                             || op == BinOp::Ne
 561                             || op == BinOp::Le
 562                             || op == BinOp::Lt
 563                             || op == BinOp::Ge
 564                             || op == BinOp::Gt
 565                             || op == BinOp::Offset
 566                     );
 567
 568                     // raw pointer operations are not allowed inside promoteds
 569                     return Err(Unpromotable);
 570                 }
 571             }
 572
 573             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 574
 575             _ => {}
 576         }
 577
 578         match rvalue {
 579             Rvalue::NullaryOp(..) => Ok(()),
 580
 581             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 582
 583             Rvalue::Use(operand)
 584             | Rvalue::Repeat(operand, _)
 585             | Rvalue::UnaryOp(_, operand)
 586             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 587
 588             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 589                 self.validate_operand(lhs)?;
 590                 self.validate_operand(rhs)
 591             }
 592
 593             Rvalue::AddressOf(_, place) => {
 594                 // Raw reborrows can come from reference to pointer coercions,
 595                 // so are allowed.
 596                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 597                     let base_ty = Place::ty_from(&place.base, proj_base, *self.body, self.tcx).ty;
 598                     if let ty::Ref(..) = base_ty.kind {
 599                         return self
 600                             .validate_place(PlaceRef { base: &place.base, projection: proj_base });
 601                     }
 602                 }
 603                 Err(Unpromotable)
 604             }
 605
 606             Rvalue::Ref(_, kind, place) => {
 607                 if let BorrowKind::Mut { .. } = kind {
 608                     let ty = place.ty(*self.body, self.tcx).ty;
 609
 610                     // In theory, any zero-sized value could be borrowed
 611                     // mutably without consequences. However, only &mut []
 612                     // is allowed right now, and only in functions.
 613                     if self.const_kind == Some(ConstKind::StaticMut) {
 614                         // Inside a `static mut`, &mut [...] is also allowed.
 615                         match ty.kind {
 616                             ty::Array(..) | ty::Slice(_) => {}
 617                             _ => return Err(Unpromotable),
 618                         }
 619                     } else if let ty::Array(_, len) = ty.kind {
 620                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 621                         // const context which seems unnecessary given that this is merely a ZST.
 622                         match len.try_eval_usize(self.tcx, self.param_env) {
 623                             Some(0) if self.const_kind.is_none() => {}
 624                             _ => return Err(Unpromotable),
 625                         }
 626                     } else {
 627                         return Err(Unpromotable);
 628                     }
 629                 }
 630
 631                 // Special-case reborrows to be more like a copy of the reference.
 632                 let mut place = place.as_ref();
 633                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 634                     let base_ty = Place::ty_from(&place.base, proj_base, *self.body, self.tcx).ty;
 635                     if let ty::Ref(..) = base_ty.kind {
 636                         place = PlaceRef { base: &place.base, projection: proj_base };
 637                     }
 638                 }
 639
 640                 self.validate_place(place)?;
 641
 642                 // HACK(eddyb) this should compute the same thing as
 643                 // `<HasMutInterior as Qualif>::in_projection` from
 644                 // `check_consts::qualifs` but without recursion.
 645                 let mut has_mut_interior = match place.base {
 646                     PlaceBase::Local(local) => self.qualif_local::<qualifs::HasMutInterior>(*local),
 647                 };
 648                 if has_mut_interior {
 649                     let mut place_projection = place.projection;
 650                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 651                     while let [proj_base @ .., elem] = place_projection {
 652                         // FIXME(eddyb) this is probably excessive, with
 653                         // the exception of `union` member accesses.
 654                         let ty = Place::ty_from(place.base, proj_base, *self.body, self.tcx)
 655                             .projection_ty(self.tcx, elem)
 656                             .ty;
 657                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 658                             has_mut_interior = false;
 659                             break;
 660                         }
 661
 662                         place_projection = proj_base;
 663                     }
 664                 }
 665                 if has_mut_interior {
 666                     return Err(Unpromotable);
 667                 }
 668
 669                 Ok(())
 670             }
 671
 672             Rvalue::Aggregate(_, ref operands) => {
 673                 for o in operands {
 674                     self.validate_operand(o)?;
 675                 }
 676
 677                 Ok(())
 678             }
 679         }
 680     }
 681
 682     fn validate_call(
 683         &self,
 684         callee: &Operand<'tcx>,
 685         args: &[Operand<'tcx>],
 686     ) -> Result<(), Unpromotable> {
 687         let fn_ty = callee.ty(*self.body, self.tcx);
 688
 689         if !self.explicit && self.const_kind.is_none() {
 690             if let ty::FnDef(def_id, _) = fn_ty.kind {
 691                 // Never promote runtime `const fn` calls of
 692                 // functions without `#[rustc_promotable]`.
 693                 if !self.tcx.is_promotable_const_fn(def_id) {
 694                     return Err(Unpromotable);
 695                 }
 696             }
 697         }
 698
 699         let is_const_fn = match fn_ty.kind {
 700             ty::FnDef(def_id, _) => {
 701                 is_const_fn(self.tcx, def_id)
 702                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 703                     || is_lang_panic_fn(self.tcx, self.def_id)
 704             }
 705             _ => false,
 706         };
 707         if !is_const_fn {
 708             return Err(Unpromotable);
 709         }
 710
 711         self.validate_operand(callee)?;
 712         for arg in args {
 713             self.validate_operand(arg)?;
 714         }
 715
 716         Ok(())
 717     }
 718 }
 719
 720 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 721 pub fn validate_candidates(
 722     tcx: TyCtxt<'tcx>,
 723     body: ReadOnlyBodyAndCache<'_, 'tcx>,
 724     def_id: DefId,
 725     temps: &IndexVec<Local, TempState>,
 726     candidates: &[Candidate],
 727 ) -> Vec<Candidate> {
 728     let mut validator = Validator { item: Item::new(tcx, def_id, body), temps, explicit: false };
 729
 730     candidates
 731         .iter()
 732         .copied()
 733         .filter(|&candidate| {
 734             validator.explicit = candidate.forces_explicit_promotion();
 735
 736             // FIXME(eddyb) also emit the errors for shuffle indices
 737             // and `#[rustc_args_required_const]` arguments here.
 738
 739             let is_promotable = validator.validate_candidate(candidate).is_ok();
 740             match candidate {
 741                 Candidate::Argument { bb, index } if !is_promotable => {
 742                     let span = body[bb].terminator().source_info.span;
 743                     let msg = format!("argument {} is required to be a constant", index + 1);
 744                     tcx.sess.span_err(span, &msg);
 745                 }
 746                 _ => (),
 747             }
 748
 749             is_promotable
 750         })
 751         .collect()
 752 }
 753
 754 struct Promoter<'a, 'tcx> {
 755     tcx: TyCtxt<'tcx>,
 756     source: &'a mut BodyAndCache<'tcx>,
 757     promoted: BodyAndCache<'tcx>,
 758     temps: &'a mut IndexVec<Local, TempState>,
 759     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 760
 761     /// If true, all nested temps are also kept in the
 762     /// source MIR, not moved to the promoted MIR.
 763     keep_original: bool,
 764 }
 765
 766 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 767     fn new_block(&mut self) -> BasicBlock {
 768         let span = self.promoted.span;
 769         self.promoted.basic_blocks_mut().push(BasicBlockData {
 770             statements: vec![],
 771             terminator: Some(Terminator {
 772                 source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 773                 kind: TerminatorKind::Return,
 774             }),
 775             is_cleanup: false,
 776         })
 777     }
 778
 779     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 780         let last = self.promoted.basic_blocks().last().unwrap();
 781         let data = &mut self.promoted[last];
 782         data.statements.push(Statement {
 783             source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 784             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 785         });
 786     }
 787
 788     fn is_temp_kind(&self, local: Local) -> bool {
 789         self.source.local_kind(local) == LocalKind::Temp
 790     }
 791
 792     /// Copies the initialization of this temp to the
 793     /// promoted MIR, recursing through temps.
 794     fn promote_temp(&mut self, temp: Local) -> Local {
 795         let old_keep_original = self.keep_original;
 796         let loc = match self.temps[temp] {
 797             TempState::Defined { location, uses } if uses > 0 => {
 798                 if uses > 1 {
 799                     self.keep_original = true;
 800                 }
 801                 location
 802             }
 803             state => {
 804                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 805             }
 806         };
 807         if !self.keep_original {
 808             self.temps[temp] = TempState::PromotedOut;
 809         }
 810
 811         let num_stmts = self.source[loc.block].statements.len();
 812         let new_temp = self.promoted.local_decls.push(LocalDecl::new_temp(
 813             self.source.local_decls[temp].ty,
 814             self.source.local_decls[temp].source_info.span,
 815         ));
 816
 817         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 818
 819         // First, take the Rvalue or Call out of the source MIR,
 820         // or duplicate it, depending on keep_original.
 821         if loc.statement_index < num_stmts {
 822             let (mut rvalue, source_info) = {
 823                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 824                 let rhs = match statement.kind {
 825                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 826                     _ => {
 827                         span_bug!(
 828                             statement.source_info.span,
 829                             "{:?} is not an assignment",
 830                             statement
 831                         );
 832                     }
 833                 };
 834
 835                 (
 836                     if self.keep_original {
 837                         rhs.clone()
 838                     } else {
 839                         let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 840                         mem::replace(rhs, unit)
 841                     },
 842                     statement.source_info,
 843                 )
 844             };
 845
 846             self.visit_rvalue(&mut rvalue, loc);
 847             self.assign(new_temp, rvalue, source_info.span);
 848         } else {
 849             let terminator = if self.keep_original {
 850                 self.source[loc.block].terminator().clone()
 851             } else {
 852                 let terminator = self.source[loc.block].terminator_mut();
 853                 let target = match terminator.kind {
 854                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 855                     ref kind => {
 856                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 857                     }
 858                 };
 859                 Terminator {
 860                     source_info: terminator.source_info,
 861                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 862                 }
 863             };
 864
 865             match terminator.kind {
 866                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 867                     self.visit_operand(&mut func, loc);
 868                     for arg in &mut args {
 869                         self.visit_operand(arg, loc);
 870                     }
 871
 872                     let last = self.promoted.basic_blocks().last().unwrap();
 873                     let new_target = self.new_block();
 874
 875                     *self.promoted[last].terminator_mut() = Terminator {
 876                         kind: TerminatorKind::Call {
 877                             func,
 878                             args,
 879                             cleanup: None,
 880                             destination: Some((Place::from(new_temp), new_target)),
 881                             from_hir_call,
 882                         },
 883                         ..terminator
 884                     };
 885                 }
 886                 ref kind => {
 887                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 888                 }
 889             };
 890         };
 891
 892         self.keep_original = old_keep_original;
 893         new_temp
 894     }
 895
 896     fn promote_candidate(
 897         mut self,
 898         def_id: DefId,
 899         candidate: Candidate,
 900         next_promoted_id: usize,
 901     ) -> Option<BodyAndCache<'tcx>> {
 902         let mut rvalue = {
 903             let promoted = &mut self.promoted;
 904             let promoted_id = Promoted::new(next_promoted_id);
 905             let tcx = self.tcx;
 906             let mut promoted_operand = |ty, span| {
 907                 promoted.span = span;
 908                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 909
 910                 Operand::Constant(Box::new(Constant {
 911                     span,
 912                     user_ty: None,
 913                     literal: tcx.mk_const(ty::Const {
 914                         ty,
 915                         val: ty::ConstKind::Unevaluated(
 916                             def_id,
 917                             InternalSubsts::identity_for_item(tcx, def_id),
 918                             Some(promoted_id),
 919                         ),
 920                     }),
 921                 }))
 922             };
 923             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 924             match candidate {
 925                 Candidate::Ref(loc) => {
 926                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 927                     match statement.kind {
 928                         StatementKind::Assign(box (
 929                             _,
 930                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 931                         )) => {
 932                             // Use the underlying local for this (necessarily interior) borrow.
 933                             let ty = place.base.ty(local_decls).ty;
 934                             let span = statement.source_info.span;
 935
 936                             let ref_ty = tcx.mk_ref(
 937                                 tcx.lifetimes.re_static,
 938                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 939                             );
 940
 941                             *region = tcx.lifetimes.re_static;
 942
 943                             let mut projection = vec![PlaceElem::Deref];
 944                             projection.extend(place.projection);
 945                             place.projection = tcx.intern_place_elems(&projection);
 946
 947                             // Create a temp to hold the promoted reference.
 948                             // This is because `*r` requires `r` to be a local,
 949                             // otherwise we would use the `promoted` directly.
 950                             let mut promoted_ref = LocalDecl::new_temp(ref_ty, span);
 951                             promoted_ref.source_info = statement.source_info;
 952                             let promoted_ref = local_decls.push(promoted_ref);
 953                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 954
 955                             let promoted_ref_statement = Statement {
 956                                 source_info: statement.source_info,
 957                                 kind: StatementKind::Assign(Box::new((
 958                                     Place::from(promoted_ref),
 959                                     Rvalue::Use(promoted_operand(ref_ty, span)),
 960                                 ))),
 961                             };
 962                             self.extra_statements.push((loc, promoted_ref_statement));
 963
 964                             Rvalue::Ref(
 965                                 tcx.lifetimes.re_static,
 966                                 borrow_kind,
 967                                 Place {
 968                                     base: mem::replace(
 969                                         &mut place.base,
 970                                         PlaceBase::Local(promoted_ref),
 971                                     ),
 972                                     projection: List::empty(),
 973                                 },
 974                             )
 975                         }
 976                         _ => bug!(),
 977                     }
 978                 }
 979                 Candidate::Repeat(loc) => {
 980                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 981                     match statement.kind {
 982                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
 983                             let ty = operand.ty(local_decls, self.tcx);
 984                             let span = statement.source_info.span;
 985
 986                             Rvalue::Use(mem::replace(operand, promoted_operand(ty, span)))
 987                         }
 988                         _ => bug!(),
 989                     }
 990                 }
 991                 Candidate::Argument { bb, index } => {
 992                     let terminator = blocks[bb].terminator_mut();
 993                     match terminator.kind {
 994                         TerminatorKind::Call { ref mut args, .. } => {
 995                             let ty = args[index].ty(local_decls, self.tcx);
 996                             let span = terminator.source_info.span;
 997
 998                             Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span)))
 999                         }
1000                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
1001                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
1002                         // we are seeing a `Goto`. That means that the `promote_temps` method
1003                         // already promoted this call away entirely. This case occurs when calling
1004                         // a function requiring a constant argument and as that constant value
1005                         // providing a value whose computation contains another call to a function
1006                         // requiring a constant argument.
1007                         TerminatorKind::Goto { .. } => return None,
1008                         _ => bug!(),
1009                     }
1010                 }
1011             }
1012         };
1013
1014         assert_eq!(self.new_block(), START_BLOCK);
1015         self.visit_rvalue(
1016             &mut rvalue,
1017             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
1018         );
1019
1020         let span = self.promoted.span;
1021         self.assign(RETURN_PLACE, rvalue, span);
1022         Some(self.promoted)
1023     }
1024 }
1025
1026 /// Replaces all temporaries with their promoted counterparts.
1027 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1028     fn tcx(&self) -> TyCtxt<'tcx> {
1029         self.tcx
1030     }
1031
1032     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
1033         if self.is_temp_kind(*local) {
1034             *local = self.promote_temp(*local);
1035         }
1036     }
1037
1038     fn process_projection_elem(&mut self, elem: &PlaceElem<'tcx>) -> Option<PlaceElem<'tcx>> {
1039         match elem {
1040             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1041                 Some(PlaceElem::Index(self.promote_temp(*local)))
1042             }
1043             _ => None,
1044         }
1045     }
1046 }
1047
1048 pub fn promote_candidates<'tcx>(
1049     def_id: DefId,
1050     body: &mut BodyAndCache<'tcx>,
1051     tcx: TyCtxt<'tcx>,
1052     mut temps: IndexVec<Local, TempState>,
1053     candidates: Vec<Candidate>,
1054 ) -> IndexVec<Promoted, BodyAndCache<'tcx>> {
1055     // Visit candidates in reverse, in case they're nested.
1056     debug!("promote_candidates({:?})", candidates);
1057
1058     let mut promotions = IndexVec::new();
1059
1060     let mut extra_statements = vec![];
1061     for candidate in candidates.into_iter().rev() {
1062         match candidate {
1063             Candidate::Repeat(Location { block, statement_index })
1064             | Candidate::Ref(Location { block, statement_index }) => {
1065                 match &body[block].statements[statement_index].kind {
1066                     StatementKind::Assign(box (place, _)) => {
1067                         if let Some(local) = place.as_local() {
1068                             if temps[local] == TempState::PromotedOut {
1069                                 // Already promoted.
1070                                 continue;
1071                             }
1072                         }
1073                     }
1074                     _ => {}
1075                 }
1076             }
1077             Candidate::Argument { .. } => {}
1078         }
1079
1080         // Declare return place local so that `mir::Body::new` doesn't complain.
1081         let initial_locals =
1082             iter::once(LocalDecl::new_return_place(tcx.types.never, body.span)).collect();
1083
1084         let mut promoted = Body::new(
1085             IndexVec::new(),
1086             // FIXME: maybe try to filter this to avoid blowing up
1087             // memory usage?
1088             body.source_scopes.clone(),
1089             initial_locals,
1090             IndexVec::new(),
1091             0,
1092             vec![],
1093             body.span,
1094             vec![],
1095             body.generator_kind,
1096         );
1097         promoted.ignore_interior_mut_in_const_validation = true;
1098
1099         let promoter = Promoter {
1100             promoted: BodyAndCache::new(promoted),
1101             tcx,
1102             source: body,
1103             temps: &mut temps,
1104             extra_statements: &mut extra_statements,
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     // Insert each of `extra_statements` before its indicated location, which
1115     // has to be done in reverse location order, to not invalidate the rest.
1116     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1117     for (loc, statement) in extra_statements {
1118         body[loc.block].statements.insert(loc.statement_index, statement);
1119     }
1120
1121     // Eliminate assignments to, and drops of promoted temps.
1122     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1123     for block in body.basic_blocks_mut() {
1124         block.statements.retain(|statement| match &statement.kind {
1125             StatementKind::Assign(box (place, _)) => {
1126                 if let Some(index) = place.as_local() {
1127                     !promoted(index)
1128                 } else {
1129                     true
1130                 }
1131             }
1132             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1133                 !promoted(*index)
1134             }
1135             _ => true,
1136         });
1137         let terminator = block.terminator_mut();
1138         match &terminator.kind {
1139             TerminatorKind::Drop { location: place, target, .. } => {
1140                 if let Some(index) = place.as_local() {
1141                     if promoted(index) {
1142                         terminator.kind = TerminatorKind::Goto { target: *target };
1143                     }
1144                 }
1145             }
1146             _ => {}
1147         }
1148     }
1149
1150     promotions
1151 }
1152
1153 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1154 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1155 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1156 /// enabled.
1157 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1158     tcx: TyCtxt<'tcx>,
1159     mir_def_id: DefId,
1160     body: ReadOnlyBodyAndCache<'_, 'tcx>,
1161     operand: &Operand<'tcx>,
1162 ) -> bool {
1163     let mut rpo = traversal::reverse_postorder(&body);
1164     let (temps, _) = collect_temps_and_candidates(tcx, &body, &mut rpo);
1165     let validator =
1166         Validator { item: Item::new(tcx, mir_def_id, body), temps: &temps, explicit: false };
1167
1168     let should_promote = validator.validate_operand(operand).is_ok();
1169     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1170     debug!(
1171         "should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1172             should_promote={:?} feature_flag={:?}",
1173         mir_def_id, should_promote, feature_flag
1174     );
1175     should_promote && !feature_flag
1176 }