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                         self.validate_local(place.local)?;
 312
 313                         if place.projection.contains(&ProjectionElem::Deref) {
 314                             return Err(Unpromotable);
 315                         }
 316
 317                         let mut has_mut_interior =
 318                             self.qualif_local::<qualifs::HasMutInterior>(place.local);
 319                         // HACK(eddyb) this should compute the same thing as
 320                         // `<HasMutInterior as Qualif>::in_projection` from
 321                         // `check_consts::qualifs` but without recursion.
 322                         if has_mut_interior {
 323                             // This allows borrowing fields which don't have
 324                             // `HasMutInterior`, from a type that does, e.g.:
 325                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 326                             let mut place_projection = &place.projection[..];
 327                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 328                             while let [proj_base @ .., elem] = place_projection {
 329                                 // FIXME(eddyb) this is probably excessive, with
 330                                 // the exception of `union` member accesses.
 331                                 let ty =
 332                                     Place::ty_from(&place.local, proj_base, *self.body, self.tcx)
 333                                         .projection_ty(self.tcx, elem)
 334                                         .ty;
 335                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 336                                     has_mut_interior = false;
 337                                     break;
 338                                 }
 339
 340                                 place_projection = proj_base;
 341                             }
 342                         }
 343
 344                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 345                         if has_mut_interior {
 346                             return Err(Unpromotable);
 347                         }
 348                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 349                             return Err(Unpromotable);
 350                         }
 351
 352                         if let BorrowKind::Mut { .. } = kind {
 353                             let ty = place.ty(*self.body, self.tcx).ty;
 354
 355                             // In theory, any zero-sized value could be borrowed
 356                             // mutably without consequences. However, only &mut []
 357                             // is allowed right now, and only in functions.
 358                             if self.const_kind == Some(ConstKind::StaticMut) {
 359                                 // Inside a `static mut`, &mut [...] is also allowed.
 360                                 match ty.kind {
 361                                     ty::Array(..) | ty::Slice(_) => {}
 362                                     _ => return Err(Unpromotable),
 363                                 }
 364                             } else if let ty::Array(_, len) = ty.kind {
 365                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 366                                 // seems unnecessary, given that this is merely a ZST.
 367                                 match len.try_eval_usize(self.tcx, self.param_env) {
 368                                     Some(0) if self.const_kind.is_none() => {}
 369                                     _ => return Err(Unpromotable),
 370                                 }
 371                             } else {
 372                                 return Err(Unpromotable);
 373                             }
 374                         }
 375
 376                         Ok(())
 377                     }
 378                     _ => bug!(),
 379                 }
 380             }
 381             Candidate::Repeat(loc) => {
 382                 assert!(!self.explicit);
 383
 384                 let statement = &self.body[loc.block].statements[loc.statement_index];
 385                 match &statement.kind {
 386                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 387                         if !self.tcx.features().const_in_array_repeat_expressions {
 388                             return Err(Unpromotable);
 389                         }
 390
 391                         self.validate_operand(operand)
 392                     }
 393                     _ => bug!(),
 394                 }
 395             }
 396             Candidate::Argument { bb, index } => {
 397                 assert!(self.explicit);
 398
 399                 let terminator = self.body[bb].terminator();
 400                 match &terminator.kind {
 401                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 402                     _ => bug!(),
 403                 }
 404             }
 405         }
 406     }
 407
 408     // FIXME(eddyb) maybe cache this?
 409     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 410         let per_local = &|l| self.qualif_local::<Q>(l);
 411
 412         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 413             let num_stmts = self.body[loc.block].statements.len();
 414
 415             if loc.statement_index < num_stmts {
 416                 let statement = &self.body[loc.block].statements[loc.statement_index];
 417                 match &statement.kind {
 418                     StatementKind::Assign(box (_, rhs)) => Q::in_rvalue(&self.item, per_local, rhs),
 419                     _ => {
 420                         span_bug!(
 421                             statement.source_info.span,
 422                             "{:?} is not an assignment",
 423                             statement
 424                         );
 425                     }
 426                 }
 427             } else {
 428                 let terminator = self.body[loc.block].terminator();
 429                 match &terminator.kind {
 430                     TerminatorKind::Call { func, args, .. } => {
 431                         let return_ty = self.body.local_decls[local].ty;
 432                         Q::in_call(&self.item, per_local, func, args, return_ty)
 433                     }
 434                     kind => {
 435                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 436                     }
 437                 }
 438             }
 439         } else {
 440             let span = self.body.local_decls[local].source_info.span;
 441             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 442         }
 443     }
 444
 445     // FIXME(eddyb) maybe cache this?
 446     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 447         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 448             let num_stmts = self.body[loc.block].statements.len();
 449
 450             if loc.statement_index < num_stmts {
 451                 let statement = &self.body[loc.block].statements[loc.statement_index];
 452                 match &statement.kind {
 453                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 454                     _ => {
 455                         span_bug!(
 456                             statement.source_info.span,
 457                             "{:?} is not an assignment",
 458                             statement
 459                         );
 460                     }
 461                 }
 462             } else {
 463                 let terminator = self.body[loc.block].terminator();
 464                 match &terminator.kind {
 465                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 466                     kind => {
 467                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 468                     }
 469                 }
 470             }
 471         } else {
 472             Err(Unpromotable)
 473         }
 474     }
 475
 476     fn validate_place(&self, place: PlaceRef<'_, 'tcx>) -> Result<(), Unpromotable> {
 477         match place {
 478             PlaceRef { local, projection: [] } => self.validate_local(*local),
 479             PlaceRef { local: _, projection: [proj_base @ .., elem] } => {
 480                 match *elem {
 481                     ProjectionElem::Deref | ProjectionElem::Downcast(..) => {
 482                         return Err(Unpromotable);
 483                     }
 484
 485                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 486
 487                     ProjectionElem::Index(local) => {
 488                         self.validate_local(local)?;
 489                     }
 490
 491                     ProjectionElem::Field(..) => {
 492                         if self.const_kind.is_none() {
 493                             let base_ty =
 494                                 Place::ty_from(place.local, proj_base, *self.body, self.tcx).ty;
 495                             if let Some(def) = base_ty.ty_adt_def() {
 496                                 // No promotion of union field accesses.
 497                                 if def.is_union() {
 498                                     return Err(Unpromotable);
 499                                 }
 500                             }
 501                         }
 502                     }
 503                 }
 504
 505                 self.validate_place(PlaceRef { local: place.local, projection: proj_base })
 506             }
 507         }
 508     }
 509
 510     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 511         match operand {
 512             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 513
 514             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 515             // `validate_rvalue` upon access.
 516             Operand::Constant(c) => {
 517                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 518                     // Only allow statics (not consts) to refer to other statics.
 519                     // FIXME(eddyb) does this matter at all for promotion?
 520                     let is_static = self.const_kind.map_or(false, |k| k.is_static());
 521                     if !is_static {
 522                         return Err(Unpromotable);
 523                     }
 524
 525                     let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 526                     if is_thread_local {
 527                         return Err(Unpromotable);
 528                     }
 529                 }
 530
 531                 Ok(())
 532             }
 533         }
 534     }
 535
 536     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 537         match *rvalue {
 538             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 539                 let operand_ty = operand.ty(*self.body, self.tcx);
 540                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 541                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 542                 match (cast_in, cast_out) {
 543                     (CastTy::Ptr(_), CastTy::Int(_)) | (CastTy::FnPtr, CastTy::Int(_)) => {
 544                         // in normal functions, mark such casts as not promotable
 545                         return Err(Unpromotable);
 546                     }
 547                     _ => {}
 548                 }
 549             }
 550
 551             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 552                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(*self.body, self.tcx).kind {
 553                     assert!(
 554                         op == BinOp::Eq
 555                             || op == BinOp::Ne
 556                             || op == BinOp::Le
 557                             || op == BinOp::Lt
 558                             || op == BinOp::Ge
 559                             || op == BinOp::Gt
 560                             || op == BinOp::Offset
 561                     );
 562
 563                     // raw pointer operations are not allowed inside promoteds
 564                     return Err(Unpromotable);
 565                 }
 566             }
 567
 568             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 569
 570             _ => {}
 571         }
 572
 573         match rvalue {
 574             Rvalue::NullaryOp(..) => Ok(()),
 575
 576             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 577
 578             Rvalue::Use(operand)
 579             | Rvalue::Repeat(operand, _)
 580             | Rvalue::UnaryOp(_, operand)
 581             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 582
 583             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 584                 self.validate_operand(lhs)?;
 585                 self.validate_operand(rhs)
 586             }
 587
 588             Rvalue::AddressOf(_, place) => {
 589                 // Raw reborrows can come from reference to pointer coercions,
 590                 // so are allowed.
 591                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 592                     let base_ty = Place::ty_from(&place.local, proj_base, *self.body, self.tcx).ty;
 593                     if let ty::Ref(..) = base_ty.kind {
 594                         return self.validate_place(PlaceRef {
 595                             local: &place.local,
 596                             projection: proj_base,
 597                         });
 598                     }
 599                 }
 600                 Err(Unpromotable)
 601             }
 602
 603             Rvalue::Ref(_, kind, place) => {
 604                 if let BorrowKind::Mut { .. } = kind {
 605                     let ty = place.ty(*self.body, self.tcx).ty;
 606
 607                     // In theory, any zero-sized value could be borrowed
 608                     // mutably without consequences. However, only &mut []
 609                     // is allowed right now, and only in functions.
 610                     if self.const_kind == Some(ConstKind::StaticMut) {
 611                         // Inside a `static mut`, &mut [...] is also allowed.
 612                         match ty.kind {
 613                             ty::Array(..) | ty::Slice(_) => {}
 614                             _ => return Err(Unpromotable),
 615                         }
 616                     } else if let ty::Array(_, len) = ty.kind {
 617                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 618                         // const context which seems unnecessary given that this is merely a ZST.
 619                         match len.try_eval_usize(self.tcx, self.param_env) {
 620                             Some(0) if self.const_kind.is_none() => {}
 621                             _ => return Err(Unpromotable),
 622                         }
 623                     } else {
 624                         return Err(Unpromotable);
 625                     }
 626                 }
 627
 628                 // Special-case reborrows to be more like a copy of the reference.
 629                 let mut place = place.as_ref();
 630                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 631                     let base_ty = Place::ty_from(&place.local, proj_base, *self.body, self.tcx).ty;
 632                     if let ty::Ref(..) = base_ty.kind {
 633                         place = PlaceRef { local: &place.local, projection: proj_base };
 634                     }
 635                 }
 636
 637                 self.validate_place(place)?;
 638
 639                 // HACK(eddyb) this should compute the same thing as
 640                 // `<HasMutInterior as Qualif>::in_projection` from
 641                 // `check_consts::qualifs` but without recursion.
 642                 let mut has_mut_interior =
 643                     self.qualif_local::<qualifs::HasMutInterior>(*place.local);
 644                 if has_mut_interior {
 645                     let mut place_projection = place.projection;
 646                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 647                     while let [proj_base @ .., elem] = place_projection {
 648                         // FIXME(eddyb) this is probably excessive, with
 649                         // the exception of `union` member accesses.
 650                         let ty = Place::ty_from(place.local, proj_base, *self.body, self.tcx)
 651                             .projection_ty(self.tcx, elem)
 652                             .ty;
 653                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 654                             has_mut_interior = false;
 655                             break;
 656                         }
 657
 658                         place_projection = proj_base;
 659                     }
 660                 }
 661                 if has_mut_interior {
 662                     return Err(Unpromotable);
 663                 }
 664
 665                 Ok(())
 666             }
 667
 668             Rvalue::Aggregate(_, ref operands) => {
 669                 for o in operands {
 670                     self.validate_operand(o)?;
 671                 }
 672
 673                 Ok(())
 674             }
 675         }
 676     }
 677
 678     fn validate_call(
 679         &self,
 680         callee: &Operand<'tcx>,
 681         args: &[Operand<'tcx>],
 682     ) -> Result<(), Unpromotable> {
 683         let fn_ty = callee.ty(*self.body, self.tcx);
 684
 685         if !self.explicit && self.const_kind.is_none() {
 686             if let ty::FnDef(def_id, _) = fn_ty.kind {
 687                 // Never promote runtime `const fn` calls of
 688                 // functions without `#[rustc_promotable]`.
 689                 if !self.tcx.is_promotable_const_fn(def_id) {
 690                     return Err(Unpromotable);
 691                 }
 692             }
 693         }
 694
 695         let is_const_fn = match fn_ty.kind {
 696             ty::FnDef(def_id, _) => {
 697                 is_const_fn(self.tcx, def_id)
 698                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 699                     || is_lang_panic_fn(self.tcx, self.def_id)
 700             }
 701             _ => false,
 702         };
 703         if !is_const_fn {
 704             return Err(Unpromotable);
 705         }
 706
 707         self.validate_operand(callee)?;
 708         for arg in args {
 709             self.validate_operand(arg)?;
 710         }
 711
 712         Ok(())
 713     }
 714 }
 715
 716 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 717 pub fn validate_candidates(
 718     tcx: TyCtxt<'tcx>,
 719     body: ReadOnlyBodyAndCache<'_, 'tcx>,
 720     def_id: DefId,
 721     temps: &IndexVec<Local, TempState>,
 722     candidates: &[Candidate],
 723 ) -> Vec<Candidate> {
 724     let mut validator = Validator { item: Item::new(tcx, def_id, body), temps, explicit: false };
 725
 726     candidates
 727         .iter()
 728         .copied()
 729         .filter(|&candidate| {
 730             validator.explicit = candidate.forces_explicit_promotion();
 731
 732             // FIXME(eddyb) also emit the errors for shuffle indices
 733             // and `#[rustc_args_required_const]` arguments here.
 734
 735             let is_promotable = validator.validate_candidate(candidate).is_ok();
 736             match candidate {
 737                 Candidate::Argument { bb, index } if !is_promotable => {
 738                     let span = body[bb].terminator().source_info.span;
 739                     let msg = format!("argument {} is required to be a constant", index + 1);
 740                     tcx.sess.span_err(span, &msg);
 741                 }
 742                 _ => (),
 743             }
 744
 745             is_promotable
 746         })
 747         .collect()
 748 }
 749
 750 struct Promoter<'a, 'tcx> {
 751     tcx: TyCtxt<'tcx>,
 752     source: &'a mut BodyAndCache<'tcx>,
 753     promoted: BodyAndCache<'tcx>,
 754     temps: &'a mut IndexVec<Local, TempState>,
 755     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 756
 757     /// If true, all nested temps are also kept in the
 758     /// source MIR, not moved to the promoted MIR.
 759     keep_original: bool,
 760 }
 761
 762 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 763     fn new_block(&mut self) -> BasicBlock {
 764         let span = self.promoted.span;
 765         self.promoted.basic_blocks_mut().push(BasicBlockData {
 766             statements: vec![],
 767             terminator: Some(Terminator {
 768                 source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 769                 kind: TerminatorKind::Return,
 770             }),
 771             is_cleanup: false,
 772         })
 773     }
 774
 775     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 776         let last = self.promoted.basic_blocks().last().unwrap();
 777         let data = &mut self.promoted[last];
 778         data.statements.push(Statement {
 779             source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 780             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 781         });
 782     }
 783
 784     fn is_temp_kind(&self, local: Local) -> bool {
 785         self.source.local_kind(local) == LocalKind::Temp
 786     }
 787
 788     /// Copies the initialization of this temp to the
 789     /// promoted MIR, recursing through temps.
 790     fn promote_temp(&mut self, temp: Local) -> Local {
 791         let old_keep_original = self.keep_original;
 792         let loc = match self.temps[temp] {
 793             TempState::Defined { location, uses } if uses > 0 => {
 794                 if uses > 1 {
 795                     self.keep_original = true;
 796                 }
 797                 location
 798             }
 799             state => {
 800                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 801             }
 802         };
 803         if !self.keep_original {
 804             self.temps[temp] = TempState::PromotedOut;
 805         }
 806
 807         let num_stmts = self.source[loc.block].statements.len();
 808         let new_temp = self.promoted.local_decls.push(LocalDecl::new_temp(
 809             self.source.local_decls[temp].ty,
 810             self.source.local_decls[temp].source_info.span,
 811         ));
 812
 813         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 814
 815         // First, take the Rvalue or Call out of the source MIR,
 816         // or duplicate it, depending on keep_original.
 817         if loc.statement_index < num_stmts {
 818             let (mut rvalue, source_info) = {
 819                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 820                 let rhs = match statement.kind {
 821                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 822                     _ => {
 823                         span_bug!(
 824                             statement.source_info.span,
 825                             "{:?} is not an assignment",
 826                             statement
 827                         );
 828                     }
 829                 };
 830
 831                 (
 832                     if self.keep_original {
 833                         rhs.clone()
 834                     } else {
 835                         let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 836                         mem::replace(rhs, unit)
 837                     },
 838                     statement.source_info,
 839                 )
 840             };
 841
 842             self.visit_rvalue(&mut rvalue, loc);
 843             self.assign(new_temp, rvalue, source_info.span);
 844         } else {
 845             let terminator = if self.keep_original {
 846                 self.source[loc.block].terminator().clone()
 847             } else {
 848                 let terminator = self.source[loc.block].terminator_mut();
 849                 let target = match terminator.kind {
 850                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 851                     ref kind => {
 852                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 853                     }
 854                 };
 855                 Terminator {
 856                     source_info: terminator.source_info,
 857                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 858                 }
 859             };
 860
 861             match terminator.kind {
 862                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 863                     self.visit_operand(&mut func, loc);
 864                     for arg in &mut args {
 865                         self.visit_operand(arg, loc);
 866                     }
 867
 868                     let last = self.promoted.basic_blocks().last().unwrap();
 869                     let new_target = self.new_block();
 870
 871                     *self.promoted[last].terminator_mut() = Terminator {
 872                         kind: TerminatorKind::Call {
 873                             func,
 874                             args,
 875                             cleanup: None,
 876                             destination: Some((Place::from(new_temp), new_target)),
 877                             from_hir_call,
 878                         },
 879                         ..terminator
 880                     };
 881                 }
 882                 ref kind => {
 883                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 884                 }
 885             };
 886         };
 887
 888         self.keep_original = old_keep_original;
 889         new_temp
 890     }
 891
 892     fn promote_candidate(
 893         mut self,
 894         def_id: DefId,
 895         candidate: Candidate,
 896         next_promoted_id: usize,
 897     ) -> Option<BodyAndCache<'tcx>> {
 898         let mut rvalue = {
 899             let promoted = &mut self.promoted;
 900             let promoted_id = Promoted::new(next_promoted_id);
 901             let tcx = self.tcx;
 902             let mut promoted_operand = |ty, span| {
 903                 promoted.span = span;
 904                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 905
 906                 Operand::Constant(Box::new(Constant {
 907                     span,
 908                     user_ty: None,
 909                     literal: tcx.mk_const(ty::Const {
 910                         ty,
 911                         val: ty::ConstKind::Unevaluated(
 912                             def_id,
 913                             InternalSubsts::identity_for_item(tcx, def_id),
 914                             Some(promoted_id),
 915                         ),
 916                     }),
 917                 }))
 918             };
 919             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 920             match candidate {
 921                 Candidate::Ref(loc) => {
 922                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 923                     match statement.kind {
 924                         StatementKind::Assign(box (
 925                             _,
 926                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 927                         )) => {
 928                             // Use the underlying local for this (necessarily interior) borrow.
 929                             let ty = local_decls.local_decls()[place.local].ty;
 930                             let span = statement.source_info.span;
 931
 932                             let ref_ty = tcx.mk_ref(
 933                                 tcx.lifetimes.re_erased,
 934                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 935                             );
 936
 937                             *region = tcx.lifetimes.re_erased;
 938
 939                             let mut projection = vec![PlaceElem::Deref];
 940                             projection.extend(place.projection);
 941                             place.projection = tcx.intern_place_elems(&projection);
 942
 943                             // Create a temp to hold the promoted reference.
 944                             // This is because `*r` requires `r` to be a local,
 945                             // otherwise we would use the `promoted` directly.
 946                             let mut promoted_ref = LocalDecl::new_temp(ref_ty, span);
 947                             promoted_ref.source_info = statement.source_info;
 948                             let promoted_ref = local_decls.push(promoted_ref);
 949                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 950
 951                             let promoted_ref_statement = Statement {
 952                                 source_info: statement.source_info,
 953                                 kind: StatementKind::Assign(Box::new((
 954                                     Place::from(promoted_ref),
 955                                     Rvalue::Use(promoted_operand(ref_ty, span)),
 956                                 ))),
 957                             };
 958                             self.extra_statements.push((loc, promoted_ref_statement));
 959
 960                             Rvalue::Ref(
 961                                 tcx.lifetimes.re_erased,
 962                                 borrow_kind,
 963                                 Place {
 964                                     local: mem::replace(&mut place.local, promoted_ref),
 965                                     projection: List::empty(),
 966                                 },
 967                             )
 968                         }
 969                         _ => bug!(),
 970                     }
 971                 }
 972                 Candidate::Repeat(loc) => {
 973                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 974                     match statement.kind {
 975                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
 976                             let ty = operand.ty(local_decls, self.tcx);
 977                             let span = statement.source_info.span;
 978
 979                             Rvalue::Use(mem::replace(operand, promoted_operand(ty, span)))
 980                         }
 981                         _ => bug!(),
 982                     }
 983                 }
 984                 Candidate::Argument { bb, index } => {
 985                     let terminator = blocks[bb].terminator_mut();
 986                     match terminator.kind {
 987                         TerminatorKind::Call { ref mut args, .. } => {
 988                             let ty = args[index].ty(local_decls, self.tcx);
 989                             let span = terminator.source_info.span;
 990
 991                             Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span)))
 992                         }
 993                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 994                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 995                         // we are seeing a `Goto`. That means that the `promote_temps` method
 996                         // already promoted this call away entirely. This case occurs when calling
 997                         // a function requiring a constant argument and as that constant value
 998                         // providing a value whose computation contains another call to a function
 999                         // requiring a constant argument.
1000                         TerminatorKind::Goto { .. } => return None,
1001                         _ => bug!(),
1002                     }
1003                 }
1004             }
1005         };
1006
1007         assert_eq!(self.new_block(), START_BLOCK);
1008         self.visit_rvalue(
1009             &mut rvalue,
1010             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
1011         );
1012
1013         let span = self.promoted.span;
1014         self.assign(RETURN_PLACE, rvalue, span);
1015         Some(self.promoted)
1016     }
1017 }
1018
1019 /// Replaces all temporaries with their promoted counterparts.
1020 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1021     fn tcx(&self) -> TyCtxt<'tcx> {
1022         self.tcx
1023     }
1024
1025     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
1026         if self.is_temp_kind(*local) {
1027             *local = self.promote_temp(*local);
1028         }
1029     }
1030
1031     fn process_projection_elem(&mut self, elem: &PlaceElem<'tcx>) -> Option<PlaceElem<'tcx>> {
1032         match elem {
1033             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1034                 Some(PlaceElem::Index(self.promote_temp(*local)))
1035             }
1036             _ => None,
1037         }
1038     }
1039 }
1040
1041 pub fn promote_candidates<'tcx>(
1042     def_id: DefId,
1043     body: &mut BodyAndCache<'tcx>,
1044     tcx: TyCtxt<'tcx>,
1045     mut temps: IndexVec<Local, TempState>,
1046     candidates: Vec<Candidate>,
1047 ) -> IndexVec<Promoted, BodyAndCache<'tcx>> {
1048     // Visit candidates in reverse, in case they're nested.
1049     debug!("promote_candidates({:?})", candidates);
1050
1051     let mut promotions = IndexVec::new();
1052
1053     let mut extra_statements = vec![];
1054     for candidate in candidates.into_iter().rev() {
1055         match candidate {
1056             Candidate::Repeat(Location { block, statement_index })
1057             | Candidate::Ref(Location { block, statement_index }) => {
1058                 match &body[block].statements[statement_index].kind {
1059                     StatementKind::Assign(box (place, _)) => {
1060                         if let Some(local) = place.as_local() {
1061                             if temps[local] == TempState::PromotedOut {
1062                                 // Already promoted.
1063                                 continue;
1064                             }
1065                         }
1066                     }
1067                     _ => {}
1068                 }
1069             }
1070             Candidate::Argument { .. } => {}
1071         }
1072
1073         // Declare return place local so that `mir::Body::new` doesn't complain.
1074         let initial_locals =
1075             iter::once(LocalDecl::new_return_place(tcx.types.never, body.span)).collect();
1076
1077         let mut promoted = Body::new(
1078             IndexVec::new(),
1079             // FIXME: maybe try to filter this to avoid blowing up
1080             // memory usage?
1081             body.source_scopes.clone(),
1082             initial_locals,
1083             IndexVec::new(),
1084             0,
1085             vec![],
1086             body.span,
1087             vec![],
1088             body.generator_kind,
1089         );
1090         promoted.ignore_interior_mut_in_const_validation = true;
1091
1092         let promoter = Promoter {
1093             promoted: BodyAndCache::new(promoted),
1094             tcx,
1095             source: body,
1096             temps: &mut temps,
1097             extra_statements: &mut extra_statements,
1098             keep_original: false,
1099         };
1100
1101         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1102         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1103             promotions.push(promoted);
1104         }
1105     }
1106
1107     // Insert each of `extra_statements` before its indicated location, which
1108     // has to be done in reverse location order, to not invalidate the rest.
1109     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1110     for (loc, statement) in extra_statements {
1111         body[loc.block].statements.insert(loc.statement_index, statement);
1112     }
1113
1114     // Eliminate assignments to, and drops of promoted temps.
1115     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1116     for block in body.basic_blocks_mut() {
1117         block.statements.retain(|statement| match &statement.kind {
1118             StatementKind::Assign(box (place, _)) => {
1119                 if let Some(index) = place.as_local() {
1120                     !promoted(index)
1121                 } else {
1122                     true
1123                 }
1124             }
1125             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1126                 !promoted(*index)
1127             }
1128             _ => true,
1129         });
1130         let terminator = block.terminator_mut();
1131         match &terminator.kind {
1132             TerminatorKind::Drop { location: place, target, .. } => {
1133                 if let Some(index) = place.as_local() {
1134                     if promoted(index) {
1135                         terminator.kind = TerminatorKind::Goto { target: *target };
1136                     }
1137                 }
1138             }
1139             _ => {}
1140         }
1141     }
1142
1143     promotions
1144 }
1145
1146 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1147 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1148 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1149 /// enabled.
1150 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1151     tcx: TyCtxt<'tcx>,
1152     mir_def_id: DefId,
1153     body: ReadOnlyBodyAndCache<'_, 'tcx>,
1154     operand: &Operand<'tcx>,
1155 ) -> bool {
1156     let mut rpo = traversal::reverse_postorder(&body);
1157     let (temps, _) = collect_temps_and_candidates(tcx, &body, &mut rpo);
1158     let validator =
1159         Validator { item: Item::new(tcx, mir_def_id, body), temps: &temps, explicit: false };
1160
1161     let should_promote = validator.validate_operand(operand).is_ok();
1162     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1163     debug!(
1164         "should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1165             should_promote={:?} feature_flag={:?}",
1166         mir_def_id, should_promote, feature_flag
1167     );
1168     should_promote && !feature_flag
1169 }