src/librustc_mir/transform/promote_consts.rs

   1 //! A pass that promotes borrows of constant rvalues.
   2 //!
   3 //! The rvalues considered constant are trees of temps,
   4 //! each with exactly one initialization, and holding
   5 //! a constant value with no interior mutability.
   6 //! They are placed into a new MIR constant body in
   7 //! `promoted` and the borrow rvalue is replaced with
   8 //! a `Literal::Promoted` using the index into `promoted`
   9 //! of that constant MIR.
  10 //!
  11 //! This pass assumes that every use is dominated by an
  12 //! initialization and can otherwise silence errors, if
  13 //! move analysis runs after promotion on broken MIR.
  14
  15 use rustc::hir::def_id::DefId;
  16 use rustc::mir::*;
  17 use rustc::mir::interpret::ConstValue;
  18 use rustc::mir::visit::{PlaceContext, MutatingUseContext, MutVisitor, Visitor};
  19 use rustc::mir::traversal::ReversePostorder;
  20 use rustc::ty::{self, List, TyCtxt};
  21 use rustc::ty::subst::InternalSubsts;
  22 use rustc::ty::cast::CastTy;
  23 use syntax::ast::LitKind;
  24 use syntax::symbol::sym;
  25 use syntax_pos::{Span, DUMMY_SP};
  26
  27 use rustc_index::vec::{IndexVec, Idx};
  28 use rustc_target::spec::abi::Abi;
  29
  30 use std::{iter, mem, usize};
  31
  32 use crate::transform::check_consts::{qualifs, Item, ConstKind, is_lang_panic_fn};
  33
  34 /// State of a temporary during collection and promotion.
  35 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  36 pub enum TempState {
  37     /// No references to this temp.
  38     Undefined,
  39     /// One direct assignment and any number of direct uses.
  40     /// A borrow of this temp is promotable if the assigned
  41     /// value is qualified as constant.
  42     Defined {
  43         location: Location,
  44         uses: usize
  45     },
  46     /// Any other combination of assignments/uses.
  47     Unpromotable,
  48     /// This temp was part of an rvalue which got extracted
  49     /// during promotion and needs cleanup.
  50     PromotedOut
  51 }
  52
  53 impl TempState {
  54     pub fn is_promotable(&self) -> bool {
  55         debug!("is_promotable: self={:?}", self);
  56         if let TempState::Defined { .. } = *self {
  57             true
  58         } else {
  59             false
  60         }
  61     }
  62 }
  63
  64 /// A "root candidate" for promotion, which will become the
  65 /// returned value in a promoted MIR, unless it's a subset
  66 /// of a larger candidate.
  67 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  68 pub enum Candidate {
  69     /// Borrow of a constant temporary.
  70     Ref(Location),
  71
  72     /// Promotion of the `x` in `[x; 32]`.
  73     Repeat(Location),
  74
  75     /// Currently applied to function calls where the callee has the unstable
  76     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
  77     /// intrinsic. The intrinsic requires the arguments are indeed constant and
  78     /// the attribute currently provides the semantic requirement that arguments
  79     /// must be constant.
  80     Argument { bb: BasicBlock, index: usize },
  81 }
  82
  83 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
  84     let attrs = tcx.get_attrs(def_id);
  85     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
  86     let mut ret = vec![];
  87     for meta in attr.meta_item_list()? {
  88         match meta.literal()?.kind {
  89             LitKind::Int(a, _) => { ret.push(a as usize); }
  90             _ => return None,
  91         }
  92     }
  93     Some(ret)
  94 }
  95
  96 struct Collector<'a, 'tcx> {
  97     tcx: TyCtxt<'tcx>,
  98     body: &'a Body<'tcx>,
  99     temps: IndexVec<Local, TempState>,
 100     candidates: Vec<Candidate>,
 101     span: Span,
 102 }
 103
 104 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 105     fn visit_local(&mut self,
 106                    &index: &Local,
 107                    context: PlaceContext,
 108                    location: Location) {
 109         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 110         // We're only interested in temporaries and the return place
 111         match self.body.local_kind(index) {
 112             | LocalKind::Temp
 113             | LocalKind::ReturnPointer
 114             => {},
 115             | LocalKind::Arg
 116             | LocalKind::Var
 117             => return,
 118         }
 119
 120         // Ignore drops, if the temp gets promoted,
 121         // then it's constant and thus drop is noop.
 122         // Non-uses are also irrelevent.
 123         if context.is_drop() || !context.is_use() {
 124             debug!(
 125                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 126                 context.is_drop(), context.is_use(),
 127             );
 128             return;
 129         }
 130
 131         let temp = &mut self.temps[index];
 132         debug!("visit_local: temp={:?}", temp);
 133         if *temp == TempState::Undefined {
 134             match context {
 135                 PlaceContext::MutatingUse(MutatingUseContext::Store) |
 136                 PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 137                     *temp = TempState::Defined {
 138                         location,
 139                         uses: 0
 140                     };
 141                     return;
 142                 }
 143                 _ => { /* mark as unpromotable below */ }
 144             }
 145         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 146             // We always allow borrows, even mutable ones, as we need
 147             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 148             let allowed_use = context.is_borrow() || context.is_nonmutating_use();
 149             debug!("visit_local: allowed_use={:?}", allowed_use);
 150             if allowed_use {
 151                 *uses += 1;
 152                 return;
 153             }
 154             /* mark as unpromotable below */
 155         }
 156         *temp = TempState::Unpromotable;
 157     }
 158
 159     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 160         self.super_rvalue(rvalue, location);
 161
 162         match *rvalue {
 163             Rvalue::Ref(..) => {
 164                 self.candidates.push(Candidate::Ref(location));
 165             }
 166             Rvalue::Repeat(..) if self.tcx.features().const_in_array_repeat_expressions => {
 167                 // FIXME(#49147) only promote the element when it isn't `Copy`
 168                 // (so that code that can copy it at runtime is unaffected).
 169                 self.candidates.push(Candidate::Repeat(location));
 170             }
 171             _ => {}
 172         }
 173     }
 174
 175     fn visit_terminator_kind(&mut self,
 176                              kind: &TerminatorKind<'tcx>,
 177                              location: Location) {
 178         self.super_terminator_kind(kind, location);
 179
 180         if let TerminatorKind::Call { ref func, .. } = *kind {
 181             if let ty::FnDef(def_id, _) = func.ty(self.body, self.tcx).kind {
 182                 let fn_sig = self.tcx.fn_sig(def_id);
 183                 if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
 184                     let name = self.tcx.item_name(def_id);
 185                     // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
 186                     if name.as_str().starts_with("simd_shuffle") {
 187                         self.candidates.push(Candidate::Argument {
 188                             bb: location.block,
 189                             index: 2,
 190                         });
 191                     }
 192                 }
 193
 194                 if let Some(constant_args) = args_required_const(self.tcx, def_id) {
 195                     for index in constant_args {
 196                         self.candidates.push(Candidate::Argument { bb: location.block, index });
 197                     }
 198                 }
 199             }
 200         }
 201     }
 202
 203     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 204         self.span = source_info.span;
 205     }
 206 }
 207
 208 pub fn collect_temps_and_candidates(
 209     tcx: TyCtxt<'tcx>,
 210     body: &Body<'tcx>,
 211     rpo: &mut ReversePostorder<'_, 'tcx>,
 212 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 213     let mut collector = Collector {
 214         tcx,
 215         body,
 216         temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls),
 217         candidates: vec![],
 218         span: body.span,
 219     };
 220     for (bb, data) in rpo {
 221         collector.visit_basic_block_data(bb, data);
 222     }
 223     (collector.temps, collector.candidates)
 224 }
 225
 226 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 227 ///
 228 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 229 struct Validator<'a, 'tcx> {
 230     item: Item<'a, 'tcx>,
 231     temps: &'a IndexVec<Local, TempState>,
 232
 233     /// Explicit promotion happens e.g. for constant arguments declared via
 234     /// `rustc_args_required_const`.
 235     /// Implicit promotion has almost the same rules, except that disallows `const fn`
 236     /// except for those marked `#[rustc_promotable]`. This is to avoid changing
 237     /// a legitimate run-time operation into a failing compile-time operation
 238     /// e.g. due to addresses being compared inside the function.
 239     explicit: bool,
 240 }
 241
 242 impl std::ops::Deref for Validator<'a, 'tcx> {
 243     type Target = Item<'a, 'tcx>;
 244
 245     fn deref(&self) -> &Self::Target {
 246         &self.item
 247     }
 248 }
 249
 250 struct Unpromotable;
 251
 252 impl<'tcx> Validator<'_, 'tcx> {
 253     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 254         match candidate {
 255             Candidate::Ref(loc) => {
 256                 assert!(!self.explicit);
 257
 258                 let statement = &self.body[loc.block].statements[loc.statement_index];
 259                 match &statement.kind {
 260                     StatementKind::Assign(box(_, Rvalue::Ref(_, kind, place))) => {
 261                         match kind {
 262                             BorrowKind::Shared | BorrowKind::Mut { .. } => {}
 263
 264                             // FIXME(eddyb) these aren't promoted here but *could*
 265                             // be promoted as part of a larger value because
 266                             // `validate_rvalue`  doesn't check them, need to
 267                             // figure out what is the intended behavior.
 268                             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 269                         }
 270
 271                         // We can only promote interior borrows of promotable temps (non-temps
 272                         // don't get promoted anyway).
 273                         let base = match place.base {
 274                             PlaceBase::Local(local) => local,
 275                             _ => return Err(Unpromotable),
 276                         };
 277                         self.validate_local(base)?;
 278
 279                         if place.projection.contains(&ProjectionElem::Deref) {
 280                             return Err(Unpromotable);
 281                         }
 282
 283                         let mut has_mut_interior =
 284                             self.qualif_local::<qualifs::HasMutInterior>(base);
 285                         // HACK(eddyb) this should compute the same thing as
 286                         // `<HasMutInterior as Qualif>::in_projection` from
 287                         // `check_consts::qualifs` but without recursion.
 288                         if has_mut_interior {
 289                             // This allows borrowing fields which don't have
 290                             // `HasMutInterior`, from a type that does, e.g.:
 291                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 292                             let mut place_projection = &place.projection[..];
 293                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 294                             while let [proj_base @ .., elem] = place_projection {
 295                                 // FIXME(eddyb) this is probably excessive, with
 296                                 // the exception of `union` member accesses.
 297                                 let ty =
 298                                     Place::ty_from(&place.base, proj_base, self.body, self.tcx)
 299                                         .projection_ty(self.tcx, elem)
 300                                         .ty;
 301                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 302                                     has_mut_interior = false;
 303                                     break;
 304                                 }
 305
 306                                 place_projection = proj_base;
 307                             }
 308                         }
 309
 310                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 311                         if has_mut_interior {
 312                             return Err(Unpromotable);
 313                         }
 314                         if self.qualif_local::<qualifs::NeedsDrop>(base) {
 315                             return Err(Unpromotable);
 316                         }
 317
 318                         if let BorrowKind::Mut { .. } = kind {
 319                             let ty = place.ty(self.body, self.tcx).ty;
 320
 321                             // In theory, any zero-sized value could be borrowed
 322                             // mutably without consequences. However, only &mut []
 323                             // is allowed right now, and only in functions.
 324                             if self.const_kind == Some(ConstKind::StaticMut) {
 325                                 // Inside a `static mut`, &mut [...] is also allowed.
 326                                 match ty.kind {
 327                                     ty::Array(..) | ty::Slice(_) => {}
 328                                     _ => return Err(Unpromotable),
 329                                 }
 330                             } else if let ty::Array(_, len) = ty.kind {
 331                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 332                                 // seems unnecessary, given that this is merely a ZST.
 333                                 match len.try_eval_usize(self.tcx, self.param_env) {
 334                                     Some(0) if self.const_kind.is_none() => {},
 335                                     _ => return Err(Unpromotable),
 336                                 }
 337                             } else {
 338                                 return Err(Unpromotable);
 339                             }
 340                         }
 341
 342                         Ok(())
 343                     }
 344                     _ => bug!()
 345                 }
 346             }
 347             Candidate::Repeat(loc) => {
 348                 assert!(!self.explicit);
 349
 350                 let statement = &self.body[loc.block].statements[loc.statement_index];
 351                 match &statement.kind {
 352                     StatementKind::Assign(box(_, Rvalue::Repeat(ref operand, _))) => {
 353                         if !self.tcx.features().const_in_array_repeat_expressions {
 354                             return Err(Unpromotable);
 355                         }
 356
 357                         self.validate_operand(operand)
 358                     }
 359                     _ => bug!()
 360                 }
 361             },
 362             Candidate::Argument { bb, index } => {
 363                 assert!(self.explicit);
 364
 365                 let terminator = self.body[bb].terminator();
 366                 match &terminator.kind {
 367                     TerminatorKind::Call { args, .. } => {
 368                         self.validate_operand(&args[index])
 369                     }
 370                     _ => bug!()
 371                 }
 372             }
 373         }
 374     }
 375
 376     // FIXME(eddyb) maybe cache this?
 377     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 378         let per_local = &|l| self.qualif_local::<Q>(l);
 379
 380         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 381             let num_stmts = self.body[loc.block].statements.len();
 382
 383             if loc.statement_index < num_stmts {
 384                 let statement = &self.body[loc.block].statements[loc.statement_index];
 385                 match &statement.kind {
 386                     StatementKind::Assign(box(_, rhs)) => {
 387                         Q::in_rvalue(&self.item, per_local, rhs)
 388                     }
 389                     _ => {
 390                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 391                                 statement);
 392                     }
 393                 }
 394             } else {
 395                 let terminator = self.body[loc.block].terminator();
 396                 match &terminator.kind {
 397                     TerminatorKind::Call { func, args, .. } => {
 398                         let return_ty = self.body.local_decls[local].ty;
 399                         Q::in_call(&self.item, per_local, func, args, return_ty)
 400                     }
 401                     kind => {
 402                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 403                     }
 404                 }
 405             }
 406         } else {
 407             let span = self.body.local_decls[local].source_info.span;
 408             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 409         }
 410     }
 411
 412     // FIXME(eddyb) maybe cache this?
 413     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 414         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 415             let num_stmts = self.body[loc.block].statements.len();
 416
 417             if loc.statement_index < num_stmts {
 418                 let statement = &self.body[loc.block].statements[loc.statement_index];
 419                 match &statement.kind {
 420                     StatementKind::Assign(box(_, rhs)) => self.validate_rvalue(rhs),
 421                     _ => {
 422                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 423                                 statement);
 424                     }
 425                 }
 426             } else {
 427                 let terminator = self.body[loc.block].terminator();
 428                 match &terminator.kind {
 429                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 430                     kind => {
 431                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 432                     }
 433                 }
 434             }
 435         } else {
 436             Err(Unpromotable)
 437         }
 438     }
 439
 440     fn validate_place(&self, place: PlaceRef<'_, 'tcx>) -> Result<(), Unpromotable> {
 441         match place {
 442             PlaceRef {
 443                 base: PlaceBase::Local(local),
 444                 projection: [],
 445             } => self.validate_local(*local),
 446             PlaceRef {
 447                 base: PlaceBase::Static(box Static {
 448                     kind: StaticKind::Promoted { .. },
 449                     ..
 450                 }),
 451                 projection: [],
 452             } => bug!("qualifying already promoted MIR"),
 453             PlaceRef {
 454                 base: PlaceBase::Static(box Static {
 455                     kind: StaticKind::Static,
 456                     def_id,
 457                     ..
 458                 }),
 459                 projection: [],
 460             } => {
 461                 // Only allow statics (not consts) to refer to other statics.
 462                 // FIXME(eddyb) does this matter at all for promotion?
 463                 let is_static = self.const_kind.map_or(false, |k| k.is_static());
 464                 if !is_static {
 465                     return Err(Unpromotable);
 466                 }
 467
 468                 let is_thread_local = self.tcx.has_attr(*def_id, sym::thread_local);
 469                 if is_thread_local {
 470                     return Err(Unpromotable);
 471                 }
 472
 473                 Ok(())
 474             }
 475             PlaceRef {
 476                 base: _,
 477                 projection: [proj_base @ .., elem],
 478             } => {
 479                 match *elem {
 480                     ProjectionElem::Deref |
 481                     ProjectionElem::Downcast(..) => return Err(Unpromotable),
 482
 483                     ProjectionElem::ConstantIndex {..} |
 484                     ProjectionElem::Subslice {..} => {}
 485
 486                     ProjectionElem::Index(local) => {
 487                         self.validate_local(local)?;
 488                     }
 489
 490                     ProjectionElem::Field(..) => {
 491                         if self.const_kind.is_none() {
 492                             let base_ty =
 493                                 Place::ty_from(place.base, proj_base, self.body, self.tcx).ty;
 494                             if let Some(def) = base_ty.ty_adt_def() {
 495                                 // No promotion of union field accesses.
 496                                 if def.is_union() {
 497                                     return Err(Unpromotable);
 498                                 }
 499                             }
 500                         }
 501                     }
 502                 }
 503
 504                 self.validate_place(PlaceRef {
 505                     base: place.base,
 506                     projection: proj_base,
 507                 })
 508             }
 509         }
 510     }
 511
 512     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 513         match operand {
 514             Operand::Copy(place) |
 515             Operand::Move(place) => self.validate_place(place.as_ref()),
 516
 517             Operand::Constant(constant) => {
 518                 if let ConstValue::Unevaluated(def_id, _) = constant.literal.val {
 519                     if self.tcx.trait_of_item(def_id).is_some() {
 520                         // Don't peek inside trait associated constants.
 521                         // (see below what we do for other consts, for now)
 522                     } else {
 523                         // HACK(eddyb) ensure that errors propagate correctly.
 524                         // FIXME(eddyb) remove this once the old promotion logic
 525                         // is gone - we can always promote constants even if they
 526                         // fail to pass const-checking, as compilation would've
 527                         // errored independently and promotion can't change that.
 528                         let (bits, _) = self.tcx.at(constant.span).mir_const_qualif(def_id);
 529                         if bits == super::qualify_consts::QUALIF_ERROR_BIT {
 530                             self.tcx.sess.delay_span_bug(
 531                                 constant.span,
 532                                 "promote_consts: MIR had errors",
 533                             );
 534                             return Err(Unpromotable);
 535                         }
 536                     }
 537                 }
 538
 539                 Ok(())
 540             }
 541         }
 542     }
 543
 544     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 545         match *rvalue {
 546             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 547                 let operand_ty = operand.ty(self.body, self.tcx);
 548                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 549                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 550                 match (cast_in, cast_out) {
 551                     (CastTy::Ptr(_), CastTy::Int(_)) |
 552                     (CastTy::FnPtr, CastTy::Int(_)) => {
 553                         // in normal functions, mark such casts as not promotable
 554                         return Err(Unpromotable);
 555                     }
 556                     _ => {}
 557                 }
 558             }
 559
 560             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 561                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
 562                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 563                             op == BinOp::Le || op == BinOp::Lt ||
 564                             op == BinOp::Ge || op == BinOp::Gt ||
 565                             op == BinOp::Offset);
 566
 567                     // raw pointer operations are not allowed inside promoteds
 568                     return Err(Unpromotable);
 569                 }
 570             }
 571
 572             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 573
 574             _ => {}
 575         }
 576
 577         match rvalue {
 578             Rvalue::NullaryOp(..) => Ok(()),
 579
 580             Rvalue::Discriminant(place) |
 581             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) |
 589             Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 590                 self.validate_operand(lhs)?;
 591                 self.validate_operand(rhs)
 592             }
 593
 594             Rvalue::Ref(_, kind, place) => {
 595                 if let BorrowKind::Mut { .. } = kind {
 596                     let ty = place.ty(self.body, self.tcx).ty;
 597
 598                     // In theory, any zero-sized value could be borrowed
 599                     // mutably without consequences. However, only &mut []
 600                     // is allowed right now, and only in functions.
 601                     if self.const_kind == Some(ConstKind::StaticMut) {
 602                         // Inside a `static mut`, &mut [...] is also allowed.
 603                         match ty.kind {
 604                             ty::Array(..) | ty::Slice(_) => {}
 605                             _ => return Err(Unpromotable),
 606                         }
 607                     } else if let ty::Array(_, len) = ty.kind {
 608                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 609                         // const context which seems unnecessary given that this is merely a ZST.
 610                         match len.try_eval_usize(self.tcx, self.param_env) {
 611                             Some(0) if self.const_kind.is_none() => {},
 612                             _ => return Err(Unpromotable),
 613                         }
 614                     } else {
 615                         return Err(Unpromotable);
 616                     }
 617                 }
 618
 619                 // Special-case reborrows to be more like a copy of the reference.
 620                 let mut place = place.as_ref();
 621                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 622                     let base_ty =
 623                         Place::ty_from(&place.base, proj_base, self.body, self.tcx).ty;
 624                     if let ty::Ref(..) = base_ty.kind {
 625                         place = PlaceRef {
 626                             base: &place.base,
 627                             projection: proj_base,
 628                         };
 629                     }
 630                 }
 631
 632                 self.validate_place(place)?;
 633
 634                 // HACK(eddyb) this should compute the same thing as
 635                 // `<HasMutInterior as Qualif>::in_projection` from
 636                 // `check_consts::qualifs` but without recursion.
 637                 let mut has_mut_interior = match place.base {
 638                     PlaceBase::Local(local) => {
 639                         self.qualif_local::<qualifs::HasMutInterior>(*local)
 640                     }
 641                     PlaceBase::Static(_) => false,
 642                 };
 643                 if has_mut_interior {
 644                     let mut place_projection = place.projection;
 645                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 646                     while let [proj_base @ .., elem] = place_projection {
 647                         // FIXME(eddyb) this is probably excessive, with
 648                         // the exception of `union` member accesses.
 649                         let ty = Place::ty_from(place.base, proj_base, self.body, self.tcx)
 650                             .projection_ty(self.tcx, elem)
 651                             .ty;
 652                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 653                             has_mut_interior = false;
 654                             break;
 655                         }
 656
 657                         place_projection = proj_base;
 658                     }
 659                 }
 660                 if has_mut_interior {
 661                     return Err(Unpromotable);
 662                 }
 663
 664                 Ok(())
 665             }
 666
 667             Rvalue::Aggregate(_, ref operands) => {
 668                 for o in operands {
 669                     self.validate_operand(o)?;
 670                 }
 671
 672                 Ok(())
 673             }
 674         }
 675     }
 676
 677     fn validate_call(
 678         &self,
 679         callee: &Operand<'tcx>,
 680         args: &[Operand<'tcx>],
 681     ) -> Result<(), Unpromotable> {
 682         let fn_ty = callee.ty(self.body, self.tcx);
 683
 684         if !self.explicit && self.const_kind.is_none() {
 685             if let ty::FnDef(def_id, _) = fn_ty.kind {
 686                 // Never promote runtime `const fn` calls of
 687                 // functions without `#[rustc_promotable]`.
 688                 if !self.tcx.is_promotable_const_fn(def_id) {
 689                     return Err(Unpromotable);
 690                 }
 691             }
 692         }
 693
 694         let is_const_fn = match fn_ty.kind {
 695             ty::FnDef(def_id, _) => {
 696                 self.tcx.is_const_fn(def_id) ||
 697                 self.tcx.is_unstable_const_fn(def_id).is_some() ||
 698                 is_lang_panic_fn(self.tcx, self.def_id)
 699             }
 700             _ => false,
 701         };
 702         if !is_const_fn {
 703             return Err(Unpromotable);
 704         }
 705
 706         self.validate_operand(callee)?;
 707         for arg in args {
 708             self.validate_operand(arg)?;
 709         }
 710
 711         Ok(())
 712     }
 713 }
 714
 715 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 716 pub fn validate_candidates(
 717     tcx: TyCtxt<'tcx>,
 718     body: &Body<'tcx>,
 719     def_id: DefId,
 720     temps: &IndexVec<Local, TempState>,
 721     candidates: &[Candidate],
 722 ) -> Vec<Candidate> {
 723     let mut validator = Validator {
 724         item: Item::new(tcx, def_id, body),
 725         temps,
 726         explicit: false,
 727     };
 728
 729     candidates.iter().copied().filter(|&candidate| {
 730         validator.explicit = match candidate {
 731             Candidate::Ref(_) |
 732             Candidate::Repeat(_) => false,
 733             Candidate::Argument { .. } => true,
 734         };
 735
 736         // FIXME(eddyb) also emit the errors for shuffle indices
 737         // and `#[rustc_args_required_const]` arguments here.
 738
 739         validator.validate_candidate(candidate).is_ok()
 740     }).collect()
 741 }
 742
 743 struct Promoter<'a, 'tcx> {
 744     tcx: TyCtxt<'tcx>,
 745     source: &'a mut Body<'tcx>,
 746     promoted: Body<'tcx>,
 747     temps: &'a mut IndexVec<Local, TempState>,
 748
 749     /// If true, all nested temps are also kept in the
 750     /// source MIR, not moved to the promoted MIR.
 751     keep_original: bool,
 752 }
 753
 754 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 755     fn new_block(&mut self) -> BasicBlock {
 756         let span = self.promoted.span;
 757         self.promoted.basic_blocks_mut().push(BasicBlockData {
 758             statements: vec![],
 759             terminator: Some(Terminator {
 760                 source_info: SourceInfo {
 761                     span,
 762                     scope: OUTERMOST_SOURCE_SCOPE
 763                 },
 764                 kind: TerminatorKind::Return
 765             }),
 766             is_cleanup: false
 767         })
 768     }
 769
 770     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 771         let last = self.promoted.basic_blocks().last().unwrap();
 772         let data = &mut self.promoted[last];
 773         data.statements.push(Statement {
 774             source_info: SourceInfo {
 775                 span,
 776                 scope: OUTERMOST_SOURCE_SCOPE
 777             },
 778             kind: StatementKind::Assign(box(Place::from(dest), rvalue))
 779         });
 780     }
 781
 782     fn is_temp_kind(&self, local: Local) -> bool {
 783         self.source.local_kind(local) == LocalKind::Temp
 784     }
 785
 786     /// Copies the initialization of this temp to the
 787     /// promoted MIR, recursing through temps.
 788     fn promote_temp(&mut self, temp: Local) -> Local {
 789         let old_keep_original = self.keep_original;
 790         let loc = match self.temps[temp] {
 791             TempState::Defined { location, uses } if uses > 0 => {
 792                 if uses > 1 {
 793                     self.keep_original = true;
 794                 }
 795                 location
 796             }
 797             state =>  {
 798                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}",
 799                           temp, state);
 800             }
 801         };
 802         if !self.keep_original {
 803             self.temps[temp] = TempState::PromotedOut;
 804         }
 805
 806         let num_stmts = self.source[loc.block].statements.len();
 807         let new_temp = self.promoted.local_decls.push(
 808             LocalDecl::new_temp(self.source.local_decls[temp].ty,
 809                                 self.source.local_decls[temp].source_info.span));
 810
 811         debug!("promote({:?} @ {:?}/{:?}, {:?})",
 812                temp, loc, num_stmts, self.keep_original);
 813
 814         // First, take the Rvalue or Call out of the source MIR,
 815         // or duplicate it, depending on keep_original.
 816         if loc.statement_index < num_stmts {
 817             let (mut rvalue, source_info) = {
 818                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 819                 let rhs = match statement.kind {
 820                     StatementKind::Assign(box(_, ref mut rhs)) => rhs,
 821                     _ => {
 822                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 823                                   statement);
 824                     }
 825                 };
 826
 827                 (if self.keep_original {
 828                     rhs.clone()
 829                 } else {
 830                     let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 831                     mem::replace(rhs, unit)
 832                 }, statement.source_info)
 833             };
 834
 835             self.visit_rvalue(&mut rvalue, loc);
 836             self.assign(new_temp, rvalue, source_info.span);
 837         } else {
 838             let terminator = if self.keep_original {
 839                 self.source[loc.block].terminator().clone()
 840             } else {
 841                 let terminator = self.source[loc.block].terminator_mut();
 842                 let target = match terminator.kind {
 843                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 844                     ref kind => {
 845                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 846                     }
 847                 };
 848                 Terminator {
 849                     source_info: terminator.source_info,
 850                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto {
 851                         target,
 852                     })
 853                 }
 854             };
 855
 856             match terminator.kind {
 857                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 858                     self.visit_operand(&mut func, loc);
 859                     for arg in &mut args {
 860                         self.visit_operand(arg, loc);
 861                     }
 862
 863                     let last = self.promoted.basic_blocks().last().unwrap();
 864                     let new_target = self.new_block();
 865
 866                     *self.promoted[last].terminator_mut() = Terminator {
 867                         kind: TerminatorKind::Call {
 868                             func,
 869                             args,
 870                             cleanup: None,
 871                             destination: Some(
 872                                 (Place::from(new_temp), new_target)
 873                             ),
 874                             from_hir_call,
 875                         },
 876                         ..terminator
 877                     };
 878                 }
 879                 ref kind => {
 880                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 881                 }
 882             };
 883         };
 884
 885         self.keep_original = old_keep_original;
 886         new_temp
 887     }
 888
 889     fn promote_candidate(
 890         mut self,
 891         def_id: DefId,
 892         candidate: Candidate,
 893         next_promoted_id: usize,
 894     ) -> Option<Body<'tcx>> {
 895         let mut operand = {
 896             let promoted = &mut self.promoted;
 897             let promoted_id = Promoted::new(next_promoted_id);
 898             let tcx = self.tcx;
 899             let mut promoted_place = |ty, span| {
 900                 promoted.span = span;
 901                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 902                 Place {
 903                     base: PlaceBase::Static(box Static {
 904                         kind:
 905                             StaticKind::Promoted(
 906                                 promoted_id,
 907                                 InternalSubsts::identity_for_item(tcx, def_id),
 908                             ),
 909                         ty,
 910                         def_id,
 911                     }),
 912                     projection: List::empty(),
 913                 }
 914             };
 915             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 916             match candidate {
 917                 Candidate::Ref(loc) => {
 918                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 919                     match statement.kind {
 920                         StatementKind::Assign(box(_, Rvalue::Ref(_, _, ref mut place))) => {
 921                             // Use the underlying local for this (necessarily interior) borrow.
 922                             let ty = place.base.ty(local_decls).ty;
 923                             let span = statement.source_info.span;
 924
 925                             Operand::Move(Place {
 926                                 base: mem::replace(
 927                                     &mut place.base,
 928                                     promoted_place(ty, span).base,
 929                                 ),
 930                                 projection: List::empty(),
 931                             })
 932                         }
 933                         _ => bug!()
 934                     }
 935                 }
 936                 Candidate::Repeat(loc) => {
 937                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 938                     match statement.kind {
 939                         StatementKind::Assign(box(_, Rvalue::Repeat(ref mut operand, _))) => {
 940                             let ty = operand.ty(local_decls, self.tcx);
 941                             let span = statement.source_info.span;
 942                             mem::replace(
 943                                 operand,
 944                                 Operand::Copy(promoted_place(ty, span))
 945                             )
 946                         }
 947                         _ => bug!()
 948                     }
 949                 },
 950                 Candidate::Argument { bb, index } => {
 951                     let terminator = blocks[bb].terminator_mut();
 952                     match terminator.kind {
 953                         TerminatorKind::Call { ref mut args, .. } => {
 954                             let ty = args[index].ty(local_decls, self.tcx);
 955                             let span = terminator.source_info.span;
 956                             let operand = Operand::Copy(promoted_place(ty, span));
 957                             mem::replace(&mut args[index], operand)
 958                         }
 959                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 960                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 961                         // we are seeing a `Goto`. That means that the `promote_temps` method
 962                         // already promoted this call away entirely. This case occurs when calling
 963                         // a function requiring a constant argument and as that constant value
 964                         // providing a value whose computation contains another call to a function
 965                         // requiring a constant argument.
 966                         TerminatorKind::Goto { .. } => return None,
 967                         _ => bug!()
 968                     }
 969                 }
 970             }
 971         };
 972
 973         assert_eq!(self.new_block(), START_BLOCK);
 974         self.visit_operand(&mut operand, Location {
 975             block: BasicBlock::new(0),
 976             statement_index: usize::MAX
 977         });
 978
 979         let span = self.promoted.span;
 980         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
 981         Some(self.promoted)
 982     }
 983 }
 984
 985 /// Replaces all temporaries with their promoted counterparts.
 986 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 987     fn tcx(&self) -> TyCtxt<'tcx> {
 988         self.tcx
 989     }
 990
 991     fn visit_local(&mut self,
 992                    local: &mut Local,
 993                    _: PlaceContext,
 994                    _: Location) {
 995         if self.is_temp_kind(*local) {
 996             *local = self.promote_temp(*local);
 997         }
 998     }
 999
1000     fn process_projection_elem(
1001         &mut self,
1002         elem: &PlaceElem<'tcx>,
1003     ) -> Option<PlaceElem<'tcx>> {
1004         match elem {
1005             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1006                 Some(PlaceElem::Index(self.promote_temp(*local)))
1007             }
1008             _ => None,
1009         }
1010     }
1011 }
1012
1013 pub fn promote_candidates<'tcx>(
1014     def_id: DefId,
1015     body: &mut Body<'tcx>,
1016     tcx: TyCtxt<'tcx>,
1017     mut temps: IndexVec<Local, TempState>,
1018     candidates: Vec<Candidate>,
1019 ) -> IndexVec<Promoted, Body<'tcx>> {
1020     // Visit candidates in reverse, in case they're nested.
1021     debug!("promote_candidates({:?})", candidates);
1022
1023     let mut promotions = IndexVec::new();
1024
1025     for candidate in candidates.into_iter().rev() {
1026         match candidate {
1027             Candidate::Repeat(Location { block, statement_index }) |
1028             Candidate::Ref(Location { block, statement_index }) => {
1029                 match &body[block].statements[statement_index].kind {
1030                     StatementKind::Assign(box(place, _)) => {
1031                         if let Some(local) = place.as_local() {
1032                             if temps[local] == TempState::PromotedOut {
1033                                 // Already promoted.
1034                                 continue;
1035                             }
1036                         }
1037                     }
1038                     _ => {}
1039                 }
1040             }
1041             Candidate::Argument { .. } => {}
1042         }
1043
1044
1045         // Declare return place local so that `mir::Body::new` doesn't complain.
1046         let initial_locals = iter::once(
1047             LocalDecl::new_return_place(tcx.types.never, body.span)
1048         ).collect();
1049
1050         let promoter = Promoter {
1051             promoted: Body::new(
1052                 IndexVec::new(),
1053                 // FIXME: maybe try to filter this to avoid blowing up
1054                 // memory usage?
1055                 body.source_scopes.clone(),
1056                 body.source_scope_local_data.clone(),
1057                 None,
1058                 initial_locals,
1059                 IndexVec::new(),
1060                 0,
1061                 vec![],
1062                 body.span,
1063                 vec![],
1064             ),
1065             tcx,
1066             source: body,
1067             temps: &mut temps,
1068             keep_original: false
1069         };
1070
1071         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1072         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1073             promotions.push(promoted);
1074         }
1075     }
1076
1077     // Eliminate assignments to, and drops of promoted temps.
1078     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1079     for block in body.basic_blocks_mut() {
1080         block.statements.retain(|statement| {
1081             match &statement.kind {
1082                 StatementKind::Assign(box(place, _)) => {
1083                     if let Some(index) = place.as_local() {
1084                         !promoted(index)
1085                     } else {
1086                         true
1087                     }
1088                 }
1089                 StatementKind::StorageLive(index) |
1090                 StatementKind::StorageDead(index) => {
1091                     !promoted(*index)
1092                 }
1093                 _ => true
1094             }
1095         });
1096         let terminator = block.terminator_mut();
1097         match &terminator.kind {
1098             TerminatorKind::Drop { location: place, target, .. } => {
1099                 if let Some(index) = place.as_local() {
1100                     if promoted(index) {
1101                         terminator.kind = TerminatorKind::Goto {
1102                             target: *target,
1103                         };
1104                     }
1105                 }
1106             }
1107             _ => {}
1108         }
1109     }
1110
1111     promotions
1112 }