src/librustc_mir/transform/promote_consts.rs

   1 //! A pass that promotes borrows of constant rvalues.
   2 //!
   3 //! The rvalues considered constant are trees of temps,
   4 //! each with exactly one initialization, and holding
   5 //! a constant value with no interior mutability.
   6 //! They are placed into a new MIR constant body in
   7 //! `promoted` and the borrow rvalue is replaced with
   8 //! a `Literal::Promoted` using the index into `promoted`
   9 //! of that constant MIR.
  10 //!
  11 //! This pass assumes that every use is dominated by an
  12 //! initialization and can otherwise silence errors, if
  13 //! move analysis runs after promotion on broken MIR.
  14
  15 use rustc::mir::traversal::ReversePostorder;
  16 use rustc::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  17 use rustc::mir::*;
  18 use rustc::ty::cast::CastTy;
  19 use rustc::ty::subst::InternalSubsts;
  20 use rustc::ty::{self, List, TyCtxt, TypeFoldable};
  21 use rustc_hir::def_id::DefId;
  22 use rustc_span::symbol::sym;
  23 use rustc_span::{Span, DUMMY_SP};
  24 use syntax::ast::LitKind;
  25
  26 use rustc_index::vec::{Idx, IndexVec};
  27 use rustc_target::spec::abi::Abi;
  28
  29 use std::cell::Cell;
  30 use std::{iter, mem, usize};
  31
  32 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstKind, Item};
  33 use crate::transform::{MirPass, MirSource};
  34
  35 /// A `MirPass` for promotion.
  36 ///
  37 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
  38 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
  39 ///
  40 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  41 /// newly created `StaticKind::Promoted`.
  42 #[derive(Default)]
  43 pub struct PromoteTemps<'tcx> {
  44     pub promoted_fragments: Cell<IndexVec<Promoted, BodyAndCache<'tcx>>>,
  45 }
  46
  47 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  48     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  49         // There's not really any point in promoting errorful MIR.
  50         //
  51         // This does not include MIR that failed const-checking, which we still try to promote.
  52         if body.return_ty().references_error() {
  53             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  54             return;
  55         }
  56
  57         if src.promoted.is_some() {
  58             return;
  59         }
  60
  61         let def_id = src.def_id();
  62
  63         let mut rpo = traversal::reverse_postorder(body);
  64         let (temps, all_candidates) = collect_temps_and_candidates(tcx, body, &mut rpo);
  65
  66         let promotable_candidates =
  67             validate_candidates(tcx, read_only!(body), def_id, &temps, &all_candidates);
  68
  69         let promoted = promote_candidates(def_id, body, tcx, temps, promotable_candidates);
  70         self.promoted_fragments.set(promoted);
  71     }
  72 }
  73
  74 /// State of a temporary during collection and promotion.
  75 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  76 pub enum TempState {
  77     /// No references to this temp.
  78     Undefined,
  79     /// One direct assignment and any number of direct uses.
  80     /// A borrow of this temp is promotable if the assigned
  81     /// value is qualified as constant.
  82     Defined { location: Location, uses: usize },
  83     /// Any other combination of assignments/uses.
  84     Unpromotable,
  85     /// This temp was part of an rvalue which got extracted
  86     /// during promotion and needs cleanup.
  87     PromotedOut,
  88 }
  89
  90 impl TempState {
  91     pub fn is_promotable(&self) -> bool {
  92         debug!("is_promotable: self={:?}", self);
  93         if let TempState::Defined { .. } = *self { true } else { false }
  94     }
  95 }
  96
  97 /// A "root candidate" for promotion, which will become the
  98 /// returned value in a promoted MIR, unless it's a subset
  99 /// of a larger candidate.
 100 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 101 pub enum Candidate {
 102     /// Borrow of a constant temporary.
 103     Ref(Location),
 104
 105     /// Promotion of the `x` in `[x; 32]`.
 106     Repeat(Location),
 107
 108     /// Currently applied to function calls where the callee has the unstable
 109     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
 110     /// intrinsic. The intrinsic requires the arguments are indeed constant and
 111     /// the attribute currently provides the semantic requirement that arguments
 112     /// must be constant.
 113     Argument { bb: BasicBlock, index: usize },
 114 }
 115
 116 impl Candidate {
 117     /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
 118     fn forces_explicit_promotion(&self) -> bool {
 119         match self {
 120             Candidate::Ref(_) | Candidate::Repeat(_) => false,
 121             Candidate::Argument { .. } => true,
 122         }
 123     }
 124 }
 125
 126 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
 127     let attrs = tcx.get_attrs(def_id);
 128     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
 129     let mut ret = vec![];
 130     for meta in attr.meta_item_list()? {
 131         match meta.literal()?.kind {
 132             LitKind::Int(a, _) => {
 133                 ret.push(a as usize);
 134             }
 135             _ => return None,
 136         }
 137     }
 138     Some(ret)
 139 }
 140
 141 struct Collector<'a, 'tcx> {
 142     tcx: TyCtxt<'tcx>,
 143     body: &'a Body<'tcx>,
 144     temps: IndexVec<Local, TempState>,
 145     candidates: Vec<Candidate>,
 146     span: Span,
 147 }
 148
 149 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 150     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 151         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 152         // We're only interested in temporaries and the return place
 153         match self.body.local_kind(index) {
 154             LocalKind::Temp | LocalKind::ReturnPointer => {}
 155             LocalKind::Arg | LocalKind::Var => return,
 156         }
 157
 158         // Ignore drops, if the temp gets promoted,
 159         // then it's constant and thus drop is noop.
 160         // Non-uses are also irrelevant.
 161         if context.is_drop() || !context.is_use() {
 162             debug!(
 163                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 164                 context.is_drop(),
 165                 context.is_use(),
 166             );
 167             return;
 168         }
 169
 170         let temp = &mut self.temps[index];
 171         debug!("visit_local: temp={:?}", temp);
 172         if *temp == TempState::Undefined {
 173             match context {
 174                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 175                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 176                     *temp = TempState::Defined { location, uses: 0 };
 177                     return;
 178                 }
 179                 _ => { /* mark as unpromotable below */ }
 180             }
 181         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 182             // We always allow borrows, even mutable ones, as we need
 183             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 184             let allowed_use = match context {
 185                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 186                 | PlaceContext::NonMutatingUse(_) => true,
 187                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 188             };
 189             debug!("visit_local: allowed_use={:?}", allowed_use);
 190             if allowed_use {
 191                 *uses += 1;
 192                 return;
 193             }
 194             /* mark as unpromotable below */
 195         }
 196         *temp = TempState::Unpromotable;
 197     }
 198
 199     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 200         self.super_rvalue(rvalue, location);
 201
 202         match *rvalue {
 203             Rvalue::Ref(..) => {
 204                 self.candidates.push(Candidate::Ref(location));
 205             }
 206             Rvalue::Repeat(..) if self.tcx.features().const_in_array_repeat_expressions => {
 207                 // FIXME(#49147) only promote the element when it isn't `Copy`
 208                 // (so that code that can copy it at runtime is unaffected).
 209                 self.candidates.push(Candidate::Repeat(location));
 210             }
 211             _ => {}
 212         }
 213     }
 214
 215     fn visit_terminator_kind(&mut self, kind: &TerminatorKind<'tcx>, location: Location) {
 216         self.super_terminator_kind(kind, location);
 217
 218         if let TerminatorKind::Call { ref func, .. } = *kind {
 219             if let ty::FnDef(def_id, _) = func.ty(self.body, self.tcx).kind {
 220                 let fn_sig = self.tcx.fn_sig(def_id);
 221                 if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
 222                     let name = self.tcx.item_name(def_id);
 223                     // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
 224                     if name.as_str().starts_with("simd_shuffle") {
 225                         self.candidates.push(Candidate::Argument { bb: location.block, index: 2 });
 226
 227                         return; // Don't double count `simd_shuffle` candidates
 228                     }
 229                 }
 230
 231                 if let Some(constant_args) = args_required_const(self.tcx, def_id) {
 232                     for index in constant_args {
 233                         self.candidates.push(Candidate::Argument { bb: location.block, index });
 234                     }
 235                 }
 236             }
 237         }
 238     }
 239
 240     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 241         self.span = source_info.span;
 242     }
 243 }
 244
 245 pub fn collect_temps_and_candidates(
 246     tcx: TyCtxt<'tcx>,
 247     body: &Body<'tcx>,
 248     rpo: &mut ReversePostorder<'_, 'tcx>,
 249 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 250     let mut collector = Collector {
 251         tcx,
 252         body,
 253         temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls),
 254         candidates: vec![],
 255         span: body.span,
 256     };
 257     for (bb, data) in rpo {
 258         collector.visit_basic_block_data(bb, data);
 259     }
 260     (collector.temps, collector.candidates)
 261 }
 262
 263 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 264 ///
 265 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 266 struct Validator<'a, 'tcx> {
 267     item: Item<'a, 'tcx>,
 268     temps: &'a IndexVec<Local, TempState>,
 269
 270     /// Explicit promotion happens e.g. for constant arguments declared via
 271     /// `rustc_args_required_const`.
 272     /// Implicit promotion has almost the same rules, except that disallows `const fn`
 273     /// except for those marked `#[rustc_promotable]`. This is to avoid changing
 274     /// a legitimate run-time operation into a failing compile-time operation
 275     /// e.g. due to addresses being compared inside the function.
 276     explicit: bool,
 277 }
 278
 279 impl std::ops::Deref for Validator<'a, 'tcx> {
 280     type Target = Item<'a, 'tcx>;
 281
 282     fn deref(&self) -> &Self::Target {
 283         &self.item
 284     }
 285 }
 286
 287 struct Unpromotable;
 288
 289 impl<'tcx> Validator<'_, 'tcx> {
 290     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 291         match candidate {
 292             Candidate::Ref(loc) => {
 293                 assert!(!self.explicit);
 294
 295                 let statement = &self.body[loc.block].statements[loc.statement_index];
 296                 match &statement.kind {
 297                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 298                         match kind {
 299                             BorrowKind::Shared | BorrowKind::Mut { .. } => {}
 300
 301                             // FIXME(eddyb) these aren't promoted here but *could*
 302                             // be promoted as part of a larger value because
 303                             // `validate_rvalue`  doesn't check them, need to
 304                             // figure out what is the intended behavior.
 305                             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 306                         }
 307
 308                         // We can only promote interior borrows of promotable temps (non-temps
 309                         // don't get promoted anyway).
 310                         let base = match place.base {
 311                             PlaceBase::Local(local) => local,
 312                             _ => return Err(Unpromotable),
 313                         };
 314                         self.validate_local(base)?;
 315
 316                         if place.projection.contains(&ProjectionElem::Deref) {
 317                             return Err(Unpromotable);
 318                         }
 319
 320                         let mut has_mut_interior =
 321                             self.qualif_local::<qualifs::HasMutInterior>(base);
 322                         // HACK(eddyb) this should compute the same thing as
 323                         // `<HasMutInterior as Qualif>::in_projection` from
 324                         // `check_consts::qualifs` but without recursion.
 325                         if has_mut_interior {
 326                             // This allows borrowing fields which don't have
 327                             // `HasMutInterior`, from a type that does, e.g.:
 328                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 329                             let mut place_projection = &place.projection[..];
 330                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 331                             while let [proj_base @ .., elem] = place_projection {
 332                                 // FIXME(eddyb) this is probably excessive, with
 333                                 // the exception of `union` member accesses.
 334                                 let ty =
 335                                     Place::ty_from(&place.base, proj_base, *self.body, self.tcx)
 336                                         .projection_ty(self.tcx, elem)
 337                                         .ty;
 338                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 339                                     has_mut_interior = false;
 340                                     break;
 341                                 }
 342
 343                                 place_projection = proj_base;
 344                             }
 345                         }
 346
 347                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 348                         if has_mut_interior {
 349                             return Err(Unpromotable);
 350                         }
 351                         if self.qualif_local::<qualifs::NeedsDrop>(base) {
 352                             return Err(Unpromotable);
 353                         }
 354
 355                         if let BorrowKind::Mut { .. } = kind {
 356                             let ty = place.ty(*self.body, self.tcx).ty;
 357
 358                             // In theory, any zero-sized value could be borrowed
 359                             // mutably without consequences. However, only &mut []
 360                             // is allowed right now, and only in functions.
 361                             if self.const_kind == Some(ConstKind::StaticMut) {
 362                                 // Inside a `static mut`, &mut [...] is also allowed.
 363                                 match ty.kind {
 364                                     ty::Array(..) | ty::Slice(_) => {}
 365                                     _ => return Err(Unpromotable),
 366                                 }
 367                             } else if let ty::Array(_, len) = ty.kind {
 368                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 369                                 // seems unnecessary, given that this is merely a ZST.
 370                                 match len.try_eval_usize(self.tcx, self.param_env) {
 371                                     Some(0) if self.const_kind.is_none() => {}
 372                                     _ => return Err(Unpromotable),
 373                                 }
 374                             } else {
 375                                 return Err(Unpromotable);
 376                             }
 377                         }
 378
 379                         Ok(())
 380                     }
 381                     _ => bug!(),
 382                 }
 383             }
 384             Candidate::Repeat(loc) => {
 385                 assert!(!self.explicit);
 386
 387                 let statement = &self.body[loc.block].statements[loc.statement_index];
 388                 match &statement.kind {
 389                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 390                         if !self.tcx.features().const_in_array_repeat_expressions {
 391                             return Err(Unpromotable);
 392                         }
 393
 394                         self.validate_operand(operand)
 395                     }
 396                     _ => bug!(),
 397                 }
 398             }
 399             Candidate::Argument { bb, index } => {
 400                 assert!(self.explicit);
 401
 402                 let terminator = self.body[bb].terminator();
 403                 match &terminator.kind {
 404                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 405                     _ => bug!(),
 406                 }
 407             }
 408         }
 409     }
 410
 411     // FIXME(eddyb) maybe cache this?
 412     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 413         let per_local = &|l| self.qualif_local::<Q>(l);
 414
 415         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 416             let num_stmts = self.body[loc.block].statements.len();
 417
 418             if loc.statement_index < num_stmts {
 419                 let statement = &self.body[loc.block].statements[loc.statement_index];
 420                 match &statement.kind {
 421                     StatementKind::Assign(box (_, rhs)) => Q::in_rvalue(&self.item, per_local, rhs),
 422                     _ => {
 423                         span_bug!(
 424                             statement.source_info.span,
 425                             "{:?} is not an assignment",
 426                             statement
 427                         );
 428                     }
 429                 }
 430             } else {
 431                 let terminator = self.body[loc.block].terminator();
 432                 match &terminator.kind {
 433                     TerminatorKind::Call { func, args, .. } => {
 434                         let return_ty = self.body.local_decls[local].ty;
 435                         Q::in_call(&self.item, per_local, func, args, return_ty)
 436                     }
 437                     kind => {
 438                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 439                     }
 440                 }
 441             }
 442         } else {
 443             let span = self.body.local_decls[local].source_info.span;
 444             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 445         }
 446     }
 447
 448     // FIXME(eddyb) maybe cache this?
 449     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 450         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 451             let num_stmts = self.body[loc.block].statements.len();
 452
 453             if loc.statement_index < num_stmts {
 454                 let statement = &self.body[loc.block].statements[loc.statement_index];
 455                 match &statement.kind {
 456                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 457                     _ => {
 458                         span_bug!(
 459                             statement.source_info.span,
 460                             "{:?} is not an assignment",
 461                             statement
 462                         );
 463                     }
 464                 }
 465             } else {
 466                 let terminator = self.body[loc.block].terminator();
 467                 match &terminator.kind {
 468                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 469                     kind => {
 470                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 471                     }
 472                 }
 473             }
 474         } else {
 475             Err(Unpromotable)
 476         }
 477     }
 478
 479     fn validate_place(&self, place: PlaceRef<'_, 'tcx>) -> Result<(), Unpromotable> {
 480         match place {
 481             PlaceRef { base: PlaceBase::Local(local), projection: [] } => {
 482                 self.validate_local(*local)
 483             }
 484             PlaceRef { base: PlaceBase::Static(_), projection: [] } => {
 485                 bug!("qualifying already promoted MIR")
 486             }
 487             PlaceRef { base: _, projection: [proj_base @ .., elem] } => {
 488                 match *elem {
 489                     ProjectionElem::Deref | ProjectionElem::Downcast(..) => {
 490                         return Err(Unpromotable);
 491                     }
 492
 493                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 494
 495                     ProjectionElem::Index(local) => {
 496                         self.validate_local(local)?;
 497                     }
 498
 499                     ProjectionElem::Field(..) => {
 500                         if self.const_kind.is_none() {
 501                             let base_ty =
 502                                 Place::ty_from(place.base, proj_base, *self.body, self.tcx).ty;
 503                             if let Some(def) = base_ty.ty_adt_def() {
 504                                 // No promotion of union field accesses.
 505                                 if def.is_union() {
 506                                     return Err(Unpromotable);
 507                                 }
 508                             }
 509                         }
 510                     }
 511                 }
 512
 513                 self.validate_place(PlaceRef { base: place.base, projection: proj_base })
 514             }
 515         }
 516     }
 517
 518     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 519         match operand {
 520             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 521
 522             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 523             // `validate_rvalue` upon access.
 524             Operand::Constant(c) => {
 525                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 526                     // Only allow statics (not consts) to refer to other statics.
 527                     // FIXME(eddyb) does this matter at all for promotion?
 528                     let is_static = self.const_kind.map_or(false, |k| k.is_static());
 529                     if !is_static {
 530                         return Err(Unpromotable);
 531                     }
 532
 533                     let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 534                     if is_thread_local {
 535                         return Err(Unpromotable);
 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(_)) | (CastTy::FnPtr, CastTy::Int(_)) => {
 552                         // in normal functions, mark such casts as not promotable
 553                         return Err(Unpromotable);
 554                     }
 555                     _ => {}
 556                 }
 557             }
 558
 559             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 560                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(*self.body, self.tcx).kind {
 561                     assert!(
 562                         op == BinOp::Eq
 563                             || op == BinOp::Ne
 564                             || op == BinOp::Le
 565                             || op == BinOp::Lt
 566                             || op == BinOp::Ge
 567                             || op == BinOp::Gt
 568                             || op == BinOp::Offset
 569                     );
 570
 571                     // raw pointer operations are not allowed inside promoteds
 572                     return Err(Unpromotable);
 573                 }
 574             }
 575
 576             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 577
 578             _ => {}
 579         }
 580
 581         match rvalue {
 582             Rvalue::NullaryOp(..) => Ok(()),
 583
 584             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 585
 586             Rvalue::Use(operand)
 587             | Rvalue::Repeat(operand, _)
 588             | Rvalue::UnaryOp(_, operand)
 589             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 590
 591             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 592                 self.validate_operand(lhs)?;
 593                 self.validate_operand(rhs)
 594             }
 595
 596             Rvalue::AddressOf(_, place) => {
 597                 // Raw reborrows can come from reference to pointer coercions,
 598                 // so are allowed.
 599                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 600                     let base_ty = Place::ty_from(&place.base, proj_base, *self.body, self.tcx).ty;
 601                     if let ty::Ref(..) = base_ty.kind {
 602                         return self
 603                             .validate_place(PlaceRef { base: &place.base, projection: proj_base });
 604                     }
 605                 }
 606                 Err(Unpromotable)
 607             }
 608
 609             Rvalue::Ref(_, kind, place) => {
 610                 if let BorrowKind::Mut { .. } = kind {
 611                     let ty = place.ty(*self.body, self.tcx).ty;
 612
 613                     // In theory, any zero-sized value could be borrowed
 614                     // mutably without consequences. However, only &mut []
 615                     // is allowed right now, and only in functions.
 616                     if self.const_kind == Some(ConstKind::StaticMut) {
 617                         // Inside a `static mut`, &mut [...] is also allowed.
 618                         match ty.kind {
 619                             ty::Array(..) | ty::Slice(_) => {}
 620                             _ => return Err(Unpromotable),
 621                         }
 622                     } else if let ty::Array(_, len) = ty.kind {
 623                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 624                         // const context which seems unnecessary given that this is merely a ZST.
 625                         match len.try_eval_usize(self.tcx, self.param_env) {
 626                             Some(0) if self.const_kind.is_none() => {}
 627                             _ => return Err(Unpromotable),
 628                         }
 629                     } else {
 630                         return Err(Unpromotable);
 631                     }
 632                 }
 633
 634                 // Special-case reborrows to be more like a copy of the reference.
 635                 let mut place = place.as_ref();
 636                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 637                     let base_ty = Place::ty_from(&place.base, proj_base, *self.body, self.tcx).ty;
 638                     if let ty::Ref(..) = base_ty.kind {
 639                         place = PlaceRef { base: &place.base, projection: proj_base };
 640                     }
 641                 }
 642
 643                 self.validate_place(place)?;
 644
 645                 // HACK(eddyb) this should compute the same thing as
 646                 // `<HasMutInterior as Qualif>::in_projection` from
 647                 // `check_consts::qualifs` but without recursion.
 648                 let mut has_mut_interior = match place.base {
 649                     PlaceBase::Local(local) => self.qualif_local::<qualifs::HasMutInterior>(*local),
 650                     PlaceBase::Static(_) => false,
 651                 };
 652                 if has_mut_interior {
 653                     let mut place_projection = place.projection;
 654                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 655                     while let [proj_base @ .., elem] = place_projection {
 656                         // FIXME(eddyb) this is probably excessive, with
 657                         // the exception of `union` member accesses.
 658                         let ty = Place::ty_from(place.base, proj_base, *self.body, self.tcx)
 659                             .projection_ty(self.tcx, elem)
 660                             .ty;
 661                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 662                             has_mut_interior = false;
 663                             break;
 664                         }
 665
 666                         place_projection = proj_base;
 667                     }
 668                 }
 669                 if has_mut_interior {
 670                     return Err(Unpromotable);
 671                 }
 672
 673                 Ok(())
 674             }
 675
 676             Rvalue::Aggregate(_, ref operands) => {
 677                 for o in operands {
 678                     self.validate_operand(o)?;
 679                 }
 680
 681                 Ok(())
 682             }
 683         }
 684     }
 685
 686     fn validate_call(
 687         &self,
 688         callee: &Operand<'tcx>,
 689         args: &[Operand<'tcx>],
 690     ) -> Result<(), Unpromotable> {
 691         let fn_ty = callee.ty(*self.body, self.tcx);
 692
 693         if !self.explicit && self.const_kind.is_none() {
 694             if let ty::FnDef(def_id, _) = fn_ty.kind {
 695                 // Never promote runtime `const fn` calls of
 696                 // functions without `#[rustc_promotable]`.
 697                 if !self.tcx.is_promotable_const_fn(def_id) {
 698                     return Err(Unpromotable);
 699                 }
 700             }
 701         }
 702
 703         let is_const_fn = match fn_ty.kind {
 704             ty::FnDef(def_id, _) => {
 705                 self.tcx.is_const_fn(def_id)
 706                     || self.tcx.is_unstable_const_fn(def_id).is_some()
 707                     || is_lang_panic_fn(self.tcx, self.def_id)
 708             }
 709             _ => false,
 710         };
 711         if !is_const_fn {
 712             return Err(Unpromotable);
 713         }
 714
 715         self.validate_operand(callee)?;
 716         for arg in args {
 717             self.validate_operand(arg)?;
 718         }
 719
 720         Ok(())
 721     }
 722 }
 723
 724 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 725 pub fn validate_candidates(
 726     tcx: TyCtxt<'tcx>,
 727     body: ReadOnlyBodyAndCache<'_, 'tcx>,
 728     def_id: DefId,
 729     temps: &IndexVec<Local, TempState>,
 730     candidates: &[Candidate],
 731 ) -> Vec<Candidate> {
 732     let mut validator = Validator { item: Item::new(tcx, def_id, body), temps, explicit: false };
 733
 734     candidates
 735         .iter()
 736         .copied()
 737         .filter(|&candidate| {
 738             validator.explicit = candidate.forces_explicit_promotion();
 739
 740             // FIXME(eddyb) also emit the errors for shuffle indices
 741             // and `#[rustc_args_required_const]` arguments here.
 742
 743             let is_promotable = validator.validate_candidate(candidate).is_ok();
 744             match candidate {
 745                 Candidate::Argument { bb, index } if !is_promotable => {
 746                     let span = body[bb].terminator().source_info.span;
 747                     let msg = format!("argument {} is required to be a constant", index + 1);
 748                     tcx.sess.span_err(span, &msg);
 749                 }
 750                 _ => (),
 751             }
 752
 753             is_promotable
 754         })
 755         .collect()
 756 }
 757
 758 struct Promoter<'a, 'tcx> {
 759     tcx: TyCtxt<'tcx>,
 760     source: &'a mut BodyAndCache<'tcx>,
 761     promoted: BodyAndCache<'tcx>,
 762     temps: &'a mut IndexVec<Local, TempState>,
 763
 764     /// If true, all nested temps are also kept in the
 765     /// source MIR, not moved to the promoted MIR.
 766     keep_original: bool,
 767 }
 768
 769 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 770     fn new_block(&mut self) -> BasicBlock {
 771         let span = self.promoted.span;
 772         self.promoted.basic_blocks_mut().push(BasicBlockData {
 773             statements: vec![],
 774             terminator: Some(Terminator {
 775                 source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 776                 kind: TerminatorKind::Return,
 777             }),
 778             is_cleanup: false,
 779         })
 780     }
 781
 782     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 783         let last = self.promoted.basic_blocks().last().unwrap();
 784         let data = &mut self.promoted[last];
 785         data.statements.push(Statement {
 786             source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 787             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 788         });
 789     }
 790
 791     fn is_temp_kind(&self, local: Local) -> bool {
 792         self.source.local_kind(local) == LocalKind::Temp
 793     }
 794
 795     /// Copies the initialization of this temp to the
 796     /// promoted MIR, recursing through temps.
 797     fn promote_temp(&mut self, temp: Local) -> Local {
 798         let old_keep_original = self.keep_original;
 799         let loc = match self.temps[temp] {
 800             TempState::Defined { location, uses } if uses > 0 => {
 801                 if uses > 1 {
 802                     self.keep_original = true;
 803                 }
 804                 location
 805             }
 806             state => {
 807                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 808             }
 809         };
 810         if !self.keep_original {
 811             self.temps[temp] = TempState::PromotedOut;
 812         }
 813
 814         let num_stmts = self.source[loc.block].statements.len();
 815         let new_temp = self.promoted.local_decls.push(LocalDecl::new_temp(
 816             self.source.local_decls[temp].ty,
 817             self.source.local_decls[temp].source_info.span,
 818         ));
 819
 820         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 821
 822         // First, take the Rvalue or Call out of the source MIR,
 823         // or duplicate it, depending on keep_original.
 824         if loc.statement_index < num_stmts {
 825             let (mut rvalue, source_info) = {
 826                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 827                 let rhs = match statement.kind {
 828                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 829                     _ => {
 830                         span_bug!(
 831                             statement.source_info.span,
 832                             "{:?} is not an assignment",
 833                             statement
 834                         );
 835                     }
 836                 };
 837
 838                 (
 839                     if self.keep_original {
 840                         rhs.clone()
 841                     } else {
 842                         let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 843                         mem::replace(rhs, unit)
 844                     },
 845                     statement.source_info,
 846                 )
 847             };
 848
 849             self.visit_rvalue(&mut rvalue, loc);
 850             self.assign(new_temp, rvalue, source_info.span);
 851         } else {
 852             let terminator = if self.keep_original {
 853                 self.source[loc.block].terminator().clone()
 854             } else {
 855                 let terminator = self.source[loc.block].terminator_mut();
 856                 let target = match terminator.kind {
 857                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 858                     ref kind => {
 859                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 860                     }
 861                 };
 862                 Terminator {
 863                     source_info: terminator.source_info,
 864                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 865                 }
 866             };
 867
 868             match terminator.kind {
 869                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 870                     self.visit_operand(&mut func, loc);
 871                     for arg in &mut args {
 872                         self.visit_operand(arg, loc);
 873                     }
 874
 875                     let last = self.promoted.basic_blocks().last().unwrap();
 876                     let new_target = self.new_block();
 877
 878                     *self.promoted[last].terminator_mut() = Terminator {
 879                         kind: TerminatorKind::Call {
 880                             func,
 881                             args,
 882                             cleanup: None,
 883                             destination: Some((Place::from(new_temp), new_target)),
 884                             from_hir_call,
 885                         },
 886                         ..terminator
 887                     };
 888                 }
 889                 ref kind => {
 890                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 891                 }
 892             };
 893         };
 894
 895         self.keep_original = old_keep_original;
 896         new_temp
 897     }
 898
 899     fn promote_candidate(
 900         mut self,
 901         def_id: DefId,
 902         candidate: Candidate,
 903         next_promoted_id: usize,
 904     ) -> Option<BodyAndCache<'tcx>> {
 905         let mut operand = {
 906             let promoted = &mut self.promoted;
 907             let promoted_id = Promoted::new(next_promoted_id);
 908             let tcx = self.tcx;
 909             let mut promoted_place = |ty, span| {
 910                 promoted.span = span;
 911                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 912                 Place {
 913                     base: PlaceBase::Static(box Static {
 914                         kind: StaticKind::Promoted(
 915                             promoted_id,
 916                             InternalSubsts::identity_for_item(tcx, def_id),
 917                         ),
 918                         ty,
 919                         def_id,
 920                     }),
 921                     projection: List::empty(),
 922                 }
 923             };
 924             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 925             match candidate {
 926                 Candidate::Ref(loc) => {
 927                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 928                     match statement.kind {
 929                         StatementKind::Assign(box (_, Rvalue::Ref(_, _, ref mut place))) => {
 930                             // Use the underlying local for this (necessarily interior) borrow.
 931                             let ty = place.base.ty(local_decls).ty;
 932                             let span = statement.source_info.span;
 933
 934                             Operand::Move(Place {
 935                                 base: mem::replace(&mut place.base, promoted_place(ty, span).base),
 936                                 projection: List::empty(),
 937                             })
 938                         }
 939                         _ => bug!(),
 940                     }
 941                 }
 942                 Candidate::Repeat(loc) => {
 943                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 944                     match statement.kind {
 945                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
 946                             let ty = operand.ty(local_decls, self.tcx);
 947                             let span = statement.source_info.span;
 948                             mem::replace(operand, Operand::Copy(promoted_place(ty, span)))
 949                         }
 950                         _ => bug!(),
 951                     }
 952                 }
 953                 Candidate::Argument { bb, index } => {
 954                     let terminator = blocks[bb].terminator_mut();
 955                     match terminator.kind {
 956                         TerminatorKind::Call { ref mut args, .. } => {
 957                             let ty = args[index].ty(local_decls, self.tcx);
 958                             let span = terminator.source_info.span;
 959                             let operand = Operand::Copy(promoted_place(ty, span));
 960                             mem::replace(&mut args[index], operand)
 961                         }
 962                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 963                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 964                         // we are seeing a `Goto`. That means that the `promote_temps` method
 965                         // already promoted this call away entirely. This case occurs when calling
 966                         // a function requiring a constant argument and as that constant value
 967                         // providing a value whose computation contains another call to a function
 968                         // requiring a constant argument.
 969                         TerminatorKind::Goto { .. } => return None,
 970                         _ => bug!(),
 971                     }
 972                 }
 973             }
 974         };
 975
 976         assert_eq!(self.new_block(), START_BLOCK);
 977         self.visit_operand(
 978             &mut operand,
 979             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
 980         );
 981
 982         let span = self.promoted.span;
 983         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
 984         Some(self.promoted)
 985     }
 986 }
 987
 988 /// Replaces all temporaries with their promoted counterparts.
 989 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 990     fn tcx(&self) -> TyCtxt<'tcx> {
 991         self.tcx
 992     }
 993
 994     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
 995         if self.is_temp_kind(*local) {
 996             *local = self.promote_temp(*local);
 997         }
 998     }
 999
1000     fn process_projection_elem(&mut self, elem: &PlaceElem<'tcx>) -> Option<PlaceElem<'tcx>> {
1001         match elem {
1002             PlaceElem::Index(local) if self.is_temp_kind(*local) => {
1003                 Some(PlaceElem::Index(self.promote_temp(*local)))
1004             }
1005             _ => None,
1006         }
1007     }
1008 }
1009
1010 pub fn promote_candidates<'tcx>(
1011     def_id: DefId,
1012     body: &mut BodyAndCache<'tcx>,
1013     tcx: TyCtxt<'tcx>,
1014     mut temps: IndexVec<Local, TempState>,
1015     candidates: Vec<Candidate>,
1016 ) -> IndexVec<Promoted, BodyAndCache<'tcx>> {
1017     // Visit candidates in reverse, in case they're nested.
1018     debug!("promote_candidates({:?})", candidates);
1019
1020     let mut promotions = IndexVec::new();
1021
1022     for candidate in candidates.into_iter().rev() {
1023         match candidate {
1024             Candidate::Repeat(Location { block, statement_index })
1025             | Candidate::Ref(Location { block, statement_index }) => {
1026                 match &body[block].statements[statement_index].kind {
1027                     StatementKind::Assign(box (place, _)) => {
1028                         if let Some(local) = place.as_local() {
1029                             if temps[local] == TempState::PromotedOut {
1030                                 // Already promoted.
1031                                 continue;
1032                             }
1033                         }
1034                     }
1035                     _ => {}
1036                 }
1037             }
1038             Candidate::Argument { .. } => {}
1039         }
1040
1041         // Declare return place local so that `mir::Body::new` doesn't complain.
1042         let initial_locals =
1043             iter::once(LocalDecl::new_return_place(tcx.types.never, body.span)).collect();
1044
1045         let promoter = Promoter {
1046             promoted: BodyAndCache::new(Body::new(
1047                 IndexVec::new(),
1048                 // FIXME: maybe try to filter this to avoid blowing up
1049                 // memory usage?
1050                 body.source_scopes.clone(),
1051                 initial_locals,
1052                 IndexVec::new(),
1053                 0,
1054                 vec![],
1055                 body.span,
1056                 vec![],
1057                 body.generator_kind,
1058             )),
1059             tcx,
1060             source: body,
1061             temps: &mut temps,
1062             keep_original: false,
1063         };
1064
1065         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1066         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1067             promotions.push(promoted);
1068         }
1069     }
1070
1071     // Eliminate assignments to, and drops of promoted temps.
1072     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1073     for block in body.basic_blocks_mut() {
1074         block.statements.retain(|statement| match &statement.kind {
1075             StatementKind::Assign(box (place, _)) => {
1076                 if let Some(index) = place.as_local() {
1077                     !promoted(index)
1078                 } else {
1079                     true
1080                 }
1081             }
1082             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1083                 !promoted(*index)
1084             }
1085             _ => true,
1086         });
1087         let terminator = block.terminator_mut();
1088         match &terminator.kind {
1089             TerminatorKind::Drop { location: place, target, .. } => {
1090                 if let Some(index) = place.as_local() {
1091                     if promoted(index) {
1092                         terminator.kind = TerminatorKind::Goto { target: *target };
1093                     }
1094                 }
1095             }
1096             _ => {}
1097         }
1098     }
1099
1100     promotions
1101 }
1102
1103 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1104 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1105 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1106 /// enabled.
1107 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1108     tcx: TyCtxt<'tcx>,
1109     mir_def_id: DefId,
1110     body: ReadOnlyBodyAndCache<'_, 'tcx>,
1111     operand: &Operand<'tcx>,
1112 ) -> bool {
1113     let mut rpo = traversal::reverse_postorder(&body);
1114     let (temps, _) = collect_temps_and_candidates(tcx, &body, &mut rpo);
1115     let validator =
1116         Validator { item: Item::new(tcx, mir_def_id, body), temps: &temps, explicit: false };
1117
1118     let should_promote = validator.validate_operand(operand).is_ok();
1119     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1120     debug!(
1121         "should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1122             should_promote={:?} feature_flag={:?}",
1123         mir_def_id, should_promote, feature_flag
1124     );
1125     should_promote && !feature_flag
1126 }