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_ast::ast::LitKind;
  16 use rustc_hir::def_id::DefId;
  17 use rustc_middle::mir::traversal::ReversePostorder;
  18 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  19 use rustc_middle::mir::*;
  20 use rustc_middle::ty::cast::CastTy;
  21 use rustc_middle::ty::subst::InternalSubsts;
  22 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
  23 use rustc_span::symbol::sym;
  24 use rustc_span::{Span, DUMMY_SP};
  25
  26 use rustc_index::vec::{Idx, IndexVec};
  27 use rustc_target::spec::abi::Abi;
  28
  29 use std::cell::Cell;
  30 use std::{cmp, iter, mem};
  31
  32 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
  33 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstKind, Item};
  34 use crate::transform::{MirPass, MirSource};
  35
  36 /// A `MirPass` for promotion.
  37 ///
  38 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
  39 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
  40 ///
  41 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  42 /// newly created `Constant`.
  43 #[derive(Default)]
  44 pub struct PromoteTemps<'tcx> {
  45     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  46 }
  47
  48 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  49     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
  50         // There's not really any point in promoting errorful MIR.
  51         //
  52         // This does not include MIR that failed const-checking, which we still try to promote.
  53         if body.return_ty().references_error() {
  54             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  55             return;
  56         }
  57
  58         if src.promoted.is_some() {
  59             return;
  60         }
  61
  62         let def_id = src.def_id();
  63
  64         let mut rpo = traversal::reverse_postorder(body);
  65         let (temps, all_candidates) = collect_temps_and_candidates(tcx, body, &mut rpo);
  66
  67         let promotable_candidates = validate_candidates(tcx, 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                         self.validate_local(place.local)?;
 311
 312                         if place.projection.contains(&ProjectionElem::Deref) {
 313                             return Err(Unpromotable);
 314                         }
 315
 316                         let mut has_mut_interior =
 317                             self.qualif_local::<qualifs::HasMutInterior>(place.local);
 318                         // HACK(eddyb) this should compute the same thing as
 319                         // `<HasMutInterior as Qualif>::in_projection` from
 320                         // `check_consts::qualifs` but without recursion.
 321                         if has_mut_interior {
 322                             // This allows borrowing fields which don't have
 323                             // `HasMutInterior`, from a type that does, e.g.:
 324                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 325                             let mut place_projection = &place.projection[..];
 326                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 327                             while let [proj_base @ .., elem] = place_projection {
 328                                 // FIXME(eddyb) this is probably excessive, with
 329                                 // the exception of `union` member accesses.
 330                                 let ty =
 331                                     Place::ty_from(place.local, proj_base, self.body, self.tcx)
 332                                         .projection_ty(self.tcx, elem)
 333                                         .ty;
 334                                 if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 335                                     has_mut_interior = false;
 336                                     break;
 337                                 }
 338
 339                                 place_projection = proj_base;
 340                             }
 341                         }
 342
 343                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 344                         if has_mut_interior {
 345                             return Err(Unpromotable);
 346                         }
 347                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 348                             return Err(Unpromotable);
 349                         }
 350
 351                         if let BorrowKind::Mut { .. } = kind {
 352                             let ty = place.ty(self.body, self.tcx).ty;
 353
 354                             // In theory, any zero-sized value could be borrowed
 355                             // mutably without consequences. However, only &mut []
 356                             // is allowed right now, and only in functions.
 357                             if self.const_kind == Some(ConstKind::StaticMut) {
 358                                 // Inside a `static mut`, &mut [...] is also allowed.
 359                                 match ty.kind {
 360                                     ty::Array(..) | ty::Slice(_) => {}
 361                                     _ => return Err(Unpromotable),
 362                                 }
 363                             } else if let ty::Array(_, len) = ty.kind {
 364                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 365                                 // seems unnecessary, given that this is merely a ZST.
 366                                 match len.try_eval_usize(self.tcx, self.param_env) {
 367                                     Some(0) if self.const_kind.is_none() => {}
 368                                     _ => return Err(Unpromotable),
 369                                 }
 370                             } else {
 371                                 return Err(Unpromotable);
 372                             }
 373                         }
 374
 375                         Ok(())
 376                     }
 377                     _ => bug!(),
 378                 }
 379             }
 380             Candidate::Repeat(loc) => {
 381                 assert!(!self.explicit);
 382
 383                 let statement = &self.body[loc.block].statements[loc.statement_index];
 384                 match &statement.kind {
 385                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 386                         if !self.tcx.features().const_in_array_repeat_expressions {
 387                             return Err(Unpromotable);
 388                         }
 389
 390                         self.validate_operand(operand)
 391                     }
 392                     _ => bug!(),
 393                 }
 394             }
 395             Candidate::Argument { bb, index } => {
 396                 assert!(self.explicit);
 397
 398                 let terminator = self.body[bb].terminator();
 399                 match &terminator.kind {
 400                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 401                     _ => bug!(),
 402                 }
 403             }
 404         }
 405     }
 406
 407     // FIXME(eddyb) maybe cache this?
 408     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 409         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 410             let num_stmts = self.body[loc.block].statements.len();
 411
 412             if loc.statement_index < num_stmts {
 413                 let statement = &self.body[loc.block].statements[loc.statement_index];
 414                 match &statement.kind {
 415                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 416                         &self.item,
 417                         &mut |l| self.qualif_local::<Q>(l),
 418                         rhs,
 419                     ),
 420                     _ => {
 421                         span_bug!(
 422                             statement.source_info.span,
 423                             "{:?} is not an assignment",
 424                             statement
 425                         );
 426                     }
 427                 }
 428             } else {
 429                 let terminator = self.body[loc.block].terminator();
 430                 match &terminator.kind {
 431                     TerminatorKind::Call { .. } => {
 432                         let return_ty = self.body.local_decls[local].ty;
 433                         Q::in_any_value_of_ty(&self.item, return_ty)
 434                     }
 435                     kind => {
 436                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 437                     }
 438                 }
 439             }
 440         } else {
 441             let span = self.body.local_decls[local].source_info.span;
 442             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 443         }
 444     }
 445
 446     // FIXME(eddyb) maybe cache this?
 447     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 448         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 449             let num_stmts = self.body[loc.block].statements.len();
 450
 451             if loc.statement_index < num_stmts {
 452                 let statement = &self.body[loc.block].statements[loc.statement_index];
 453                 match &statement.kind {
 454                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 455                     _ => {
 456                         span_bug!(
 457                             statement.source_info.span,
 458                             "{:?} is not an assignment",
 459                             statement
 460                         );
 461                     }
 462                 }
 463             } else {
 464                 let terminator = self.body[loc.block].terminator();
 465                 match &terminator.kind {
 466                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 467                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 468                     kind => {
 469                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 470                     }
 471                 }
 472             }
 473         } else {
 474             Err(Unpromotable)
 475         }
 476     }
 477
 478     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 479         match place {
 480             PlaceRef { local, projection: [] } => self.validate_local(local),
 481             PlaceRef { local: _, projection: [proj_base @ .., elem] } => {
 482                 match *elem {
 483                     ProjectionElem::Deref | ProjectionElem::Downcast(..) => {
 484                         return Err(Unpromotable);
 485                     }
 486
 487                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 488
 489                     ProjectionElem::Index(local) => {
 490                         self.validate_local(local)?;
 491                     }
 492
 493                     ProjectionElem::Field(..) => {
 494                         if self.const_kind.is_none() {
 495                             let base_ty =
 496                                 Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 497                             if let Some(def) = base_ty.ty_adt_def() {
 498                                 // No promotion of union field accesses.
 499                                 if def.is_union() {
 500                                     return Err(Unpromotable);
 501                                 }
 502                             }
 503                         }
 504                     }
 505                 }
 506
 507                 self.validate_place(PlaceRef { local: place.local, projection: proj_base })
 508             }
 509         }
 510     }
 511
 512     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 513         match operand {
 514             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 515
 516             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 517             // `validate_rvalue` upon access.
 518             Operand::Constant(c) => {
 519                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 520                     // Only allow statics (not consts) to refer to other statics.
 521                     // FIXME(eddyb) does this matter at all for promotion?
 522                     let is_static = self.const_kind.map_or(false, |k| k.is_static());
 523                     if !is_static {
 524                         return Err(Unpromotable);
 525                     }
 526
 527                     let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 528                     if is_thread_local {
 529                         return Err(Unpromotable);
 530                     }
 531                 }
 532
 533                 Ok(())
 534             }
 535         }
 536     }
 537
 538     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 539         match *rvalue {
 540             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 541                 let operand_ty = operand.ty(self.body, self.tcx);
 542                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 543                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 544                 match (cast_in, cast_out) {
 545                     (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
 546                         // in normal functions, mark such casts as not promotable
 547                         return Err(Unpromotable);
 548                     }
 549                     _ => {}
 550                 }
 551             }
 552
 553             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 554                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
 555                     assert!(
 556                         op == BinOp::Eq
 557                             || op == BinOp::Ne
 558                             || op == BinOp::Le
 559                             || op == BinOp::Lt
 560                             || op == BinOp::Ge
 561                             || op == BinOp::Gt
 562                             || op == BinOp::Offset
 563                     );
 564
 565                     // raw pointer operations are not allowed inside promoteds
 566                     return Err(Unpromotable);
 567                 }
 568             }
 569
 570             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 571
 572             _ => {}
 573         }
 574
 575         match rvalue {
 576             Rvalue::NullaryOp(..) => Ok(()),
 577
 578             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 579
 580             Rvalue::Use(operand)
 581             | Rvalue::Repeat(operand, _)
 582             | Rvalue::UnaryOp(_, operand)
 583             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 584
 585             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 586                 self.validate_operand(lhs)?;
 587                 self.validate_operand(rhs)
 588             }
 589
 590             Rvalue::AddressOf(_, place) => {
 591                 // Raw reborrows can come from reference to pointer coercions,
 592                 // so are allowed.
 593                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 594                     let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 595                     if let ty::Ref(..) = base_ty.kind {
 596                         return self.validate_place(PlaceRef {
 597                             local: place.local,
 598                             projection: proj_base,
 599                         });
 600                     }
 601                 }
 602                 Err(Unpromotable)
 603             }
 604
 605             Rvalue::Ref(_, kind, place) => {
 606                 if let BorrowKind::Mut { .. } = kind {
 607                     let ty = place.ty(self.body, self.tcx).ty;
 608
 609                     // In theory, any zero-sized value could be borrowed
 610                     // mutably without consequences. However, only &mut []
 611                     // is allowed right now, and only in functions.
 612                     if self.const_kind == Some(ConstKind::StaticMut) {
 613                         // Inside a `static mut`, &mut [...] is also allowed.
 614                         match ty.kind {
 615                             ty::Array(..) | ty::Slice(_) => {}
 616                             _ => return Err(Unpromotable),
 617                         }
 618                     } else if let ty::Array(_, len) = ty.kind {
 619                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 620                         // const context which seems unnecessary given that this is merely a ZST.
 621                         match len.try_eval_usize(self.tcx, self.param_env) {
 622                             Some(0) if self.const_kind.is_none() => {}
 623                             _ => return Err(Unpromotable),
 624                         }
 625                     } else {
 626                         return Err(Unpromotable);
 627                     }
 628                 }
 629
 630                 // Special-case reborrows to be more like a copy of the reference.
 631                 let mut place = place.as_ref();
 632                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 633                     let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 634                     if let ty::Ref(..) = base_ty.kind {
 635                         place = PlaceRef { local: place.local, projection: proj_base };
 636                     }
 637                 }
 638
 639                 self.validate_place(place)?;
 640
 641                 // HACK(eddyb) this should compute the same thing as
 642                 // `<HasMutInterior as Qualif>::in_projection` from
 643                 // `check_consts::qualifs` but without recursion.
 644                 let mut has_mut_interior =
 645                     self.qualif_local::<qualifs::HasMutInterior>(place.local);
 646                 if has_mut_interior {
 647                     let mut place_projection = place.projection;
 648                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 649                     while let [proj_base @ .., elem] = place_projection {
 650                         // FIXME(eddyb) this is probably excessive, with
 651                         // the exception of `union` member accesses.
 652                         let ty = Place::ty_from(place.local, proj_base, self.body, self.tcx)
 653                             .projection_ty(self.tcx, elem)
 654                             .ty;
 655                         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
 656                             has_mut_interior = false;
 657                             break;
 658                         }
 659
 660                         place_projection = proj_base;
 661                     }
 662                 }
 663                 if has_mut_interior {
 664                     return Err(Unpromotable);
 665                 }
 666
 667                 Ok(())
 668             }
 669
 670             Rvalue::Aggregate(_, ref operands) => {
 671                 for o in operands {
 672                     self.validate_operand(o)?;
 673                 }
 674
 675                 Ok(())
 676             }
 677         }
 678     }
 679
 680     fn validate_call(
 681         &self,
 682         callee: &Operand<'tcx>,
 683         args: &[Operand<'tcx>],
 684     ) -> Result<(), Unpromotable> {
 685         let fn_ty = callee.ty(self.body, self.tcx);
 686
 687         if !self.explicit && self.const_kind.is_none() {
 688             if let ty::FnDef(def_id, _) = fn_ty.kind {
 689                 // Never promote runtime `const fn` calls of
 690                 // functions without `#[rustc_promotable]`.
 691                 if !self.tcx.is_promotable_const_fn(def_id) {
 692                     return Err(Unpromotable);
 693                 }
 694             }
 695         }
 696
 697         let is_const_fn = match fn_ty.kind {
 698             ty::FnDef(def_id, _) => {
 699                 is_const_fn(self.tcx, def_id)
 700                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 701                     || is_lang_panic_fn(self.tcx, self.def_id)
 702             }
 703             _ => false,
 704         };
 705         if !is_const_fn {
 706             return Err(Unpromotable);
 707         }
 708
 709         self.validate_operand(callee)?;
 710         for arg in args {
 711             self.validate_operand(arg)?;
 712         }
 713
 714         Ok(())
 715     }
 716 }
 717
 718 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 719 pub fn validate_candidates(
 720     tcx: TyCtxt<'tcx>,
 721     body: &Body<'tcx>,
 722     def_id: DefId,
 723     temps: &IndexVec<Local, TempState>,
 724     candidates: &[Candidate],
 725 ) -> Vec<Candidate> {
 726     let mut validator = Validator { item: Item::new(tcx, def_id, body), temps, explicit: false };
 727
 728     candidates
 729         .iter()
 730         .copied()
 731         .filter(|&candidate| {
 732             validator.explicit = candidate.forces_explicit_promotion();
 733
 734             // FIXME(eddyb) also emit the errors for shuffle indices
 735             // and `#[rustc_args_required_const]` arguments here.
 736
 737             let is_promotable = validator.validate_candidate(candidate).is_ok();
 738             match candidate {
 739                 Candidate::Argument { bb, index } if !is_promotable => {
 740                     let span = body[bb].terminator().source_info.span;
 741                     let msg = format!("argument {} is required to be a constant", index + 1);
 742                     tcx.sess.span_err(span, &msg);
 743                 }
 744                 _ => (),
 745             }
 746
 747             is_promotable
 748         })
 749         .collect()
 750 }
 751
 752 struct Promoter<'a, 'tcx> {
 753     tcx: TyCtxt<'tcx>,
 754     source: &'a mut Body<'tcx>,
 755     promoted: Body<'tcx>,
 756     temps: &'a mut IndexVec<Local, TempState>,
 757     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 758
 759     /// If true, all nested temps are also kept in the
 760     /// source MIR, not moved to the promoted MIR.
 761     keep_original: bool,
 762 }
 763
 764 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 765     fn new_block(&mut self) -> BasicBlock {
 766         let span = self.promoted.span;
 767         self.promoted.basic_blocks_mut().push(BasicBlockData {
 768             statements: vec![],
 769             terminator: Some(Terminator {
 770                 source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 771                 kind: TerminatorKind::Return,
 772             }),
 773             is_cleanup: false,
 774         })
 775     }
 776
 777     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 778         let last = self.promoted.basic_blocks().last().unwrap();
 779         let data = &mut self.promoted[last];
 780         data.statements.push(Statement {
 781             source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
 782             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 783         });
 784     }
 785
 786     fn is_temp_kind(&self, local: Local) -> bool {
 787         self.source.local_kind(local) == LocalKind::Temp
 788     }
 789
 790     /// Copies the initialization of this temp to the
 791     /// promoted MIR, recursing through temps.
 792     fn promote_temp(&mut self, temp: Local) -> Local {
 793         let old_keep_original = self.keep_original;
 794         let loc = match self.temps[temp] {
 795             TempState::Defined { location, uses } if uses > 0 => {
 796                 if uses > 1 {
 797                     self.keep_original = true;
 798                 }
 799                 location
 800             }
 801             state => {
 802                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 803             }
 804         };
 805         if !self.keep_original {
 806             self.temps[temp] = TempState::PromotedOut;
 807         }
 808
 809         let num_stmts = self.source[loc.block].statements.len();
 810         let new_temp = self.promoted.local_decls.push(LocalDecl::new_temp(
 811             self.source.local_decls[temp].ty,
 812             self.source.local_decls[temp].source_info.span,
 813         ));
 814
 815         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 816
 817         // First, take the Rvalue or Call out of the source MIR,
 818         // or duplicate it, depending on keep_original.
 819         if loc.statement_index < num_stmts {
 820             let (mut rvalue, source_info) = {
 821                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 822                 let rhs = match statement.kind {
 823                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 824                     _ => {
 825                         span_bug!(
 826                             statement.source_info.span,
 827                             "{:?} is not an assignment",
 828                             statement
 829                         );
 830                     }
 831                 };
 832
 833                 (
 834                     if self.keep_original {
 835                         rhs.clone()
 836                     } else {
 837                         let unit = Rvalue::Use(Operand::Constant(box Constant {
 838                             span: statement.source_info.span,
 839                             user_ty: None,
 840                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit),
 841                         }));
 842                         mem::replace(rhs, unit)
 843                     },
 844                     statement.source_info,
 845                 )
 846             };
 847
 848             self.visit_rvalue(&mut rvalue, loc);
 849             self.assign(new_temp, rvalue, source_info.span);
 850         } else {
 851             let terminator = if self.keep_original {
 852                 self.source[loc.block].terminator().clone()
 853             } else {
 854                 let terminator = self.source[loc.block].terminator_mut();
 855                 let target = match terminator.kind {
 856                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 857                     ref kind => {
 858                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 859                     }
 860                 };
 861                 Terminator {
 862                     source_info: terminator.source_info,
 863                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 864                 }
 865             };
 866
 867             match terminator.kind {
 868                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 869                     self.visit_operand(&mut func, loc);
 870                     for arg in &mut args {
 871                         self.visit_operand(arg, loc);
 872                     }
 873
 874                     let last = self.promoted.basic_blocks().last().unwrap();
 875                     let new_target = self.new_block();
 876
 877                     *self.promoted[last].terminator_mut() = Terminator {
 878                         kind: TerminatorKind::Call {
 879                             func,
 880                             args,
 881                             cleanup: None,
 882                             destination: Some((Place::from(new_temp), new_target)),
 883                             from_hir_call,
 884                         },
 885                         ..terminator
 886                     };
 887                 }
 888                 ref kind => {
 889                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 890                 }
 891             };
 892         };
 893
 894         self.keep_original = old_keep_original;
 895         new_temp
 896     }
 897
 898     fn promote_candidate(
 899         mut self,
 900         def_id: DefId,
 901         candidate: Candidate,
 902         next_promoted_id: usize,
 903     ) -> Option<Body<'tcx>> {
 904         let mut rvalue = {
 905             let promoted = &mut self.promoted;
 906             let promoted_id = Promoted::new(next_promoted_id);
 907             let tcx = self.tcx;
 908             let mut promoted_operand = |ty, span| {
 909                 promoted.span = span;
 910                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 911
 912                 Operand::Constant(Box::new(Constant {
 913                     span,
 914                     user_ty: None,
 915                     literal: tcx.mk_const(ty::Const {
 916                         ty,
 917                         val: ty::ConstKind::Unevaluated(
 918                             def_id,
 919                             InternalSubsts::for_item(tcx, def_id, |param, _| {
 920                                 if let ty::GenericParamDefKind::Lifetime = param.kind {
 921                                     tcx.lifetimes.re_erased.into()
 922                                 } else {
 923                                     tcx.mk_param_from_def(param)
 924                                 }
 925                             }),
 926                             Some(promoted_id),
 927                         ),
 928                     }),
 929                 }))
 930             };
 931             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 932             match candidate {
 933                 Candidate::Ref(loc) => {
 934                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
 935                     match statement.kind {
 936                         StatementKind::Assign(box (
 937                             _,
 938                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
 939                         )) => {
 940                             // Use the underlying local for this (necessarily interior) borrow.
 941                             let ty = local_decls.local_decls()[place.local].ty;
 942                             let span = statement.source_info.span;
 943
 944                             let ref_ty = tcx.mk_ref(
 945                                 tcx.lifetimes.re_erased,
 946                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
 947                             );
 948
 949                             *region = tcx.lifetimes.re_erased;
 950
 951                             let mut projection = vec![PlaceElem::Deref];
 952                             projection.extend(place.projection);
 953                             place.projection = tcx.intern_place_elems(&projection);
 954
 955                             // Create a temp to hold the promoted reference.
 956                             // This is because `*r` requires `r` to be a local,
 957                             // otherwise we would use the `promoted` directly.
 958                             let mut promoted_ref = LocalDecl::new_temp(ref_ty, span);
 959                             promoted_ref.source_info = statement.source_info;
 960                             let promoted_ref = local_decls.push(promoted_ref);
 961                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
 962
 963                             let promoted_ref_statement = Statement {
 964                                 source_info: statement.source_info,
 965                                 kind: StatementKind::Assign(Box::new((
 966                                     Place::from(promoted_ref),
 967                                     Rvalue::Use(promoted_operand(ref_ty, span)),
 968                                 ))),
 969                             };
 970                             self.extra_statements.push((loc, promoted_ref_statement));
 971
 972                             Rvalue::Ref(
 973                                 tcx.lifetimes.re_erased,
 974                                 borrow_kind,
 975                                 Place {
 976                                     local: mem::replace(&mut place.local, promoted_ref),
 977                                     projection: List::empty(),
 978                                 },
 979                             )
 980                         }
 981                         _ => bug!(),
 982                     }
 983                 }
 984                 Candidate::Repeat(loc) => {
 985                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
 986                     match statement.kind {
 987                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
 988                             let ty = operand.ty(local_decls, self.tcx);
 989                             let span = statement.source_info.span;
 990
 991                             Rvalue::Use(mem::replace(operand, promoted_operand(ty, span)))
 992                         }
 993                         _ => bug!(),
 994                     }
 995                 }
 996                 Candidate::Argument { bb, index } => {
 997                     let terminator = blocks[bb].terminator_mut();
 998                     match terminator.kind {
 999                         TerminatorKind::Call { ref mut args, .. } => {
1000                             let ty = args[index].ty(local_decls, self.tcx);
1001                             let span = terminator.source_info.span;
1002
1003                             Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span)))
1004                         }
1005                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
1006                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
1007                         // we are seeing a `Goto`. That means that the `promote_temps` method
1008                         // already promoted this call away entirely. This case occurs when calling
1009                         // a function requiring a constant argument and as that constant value
1010                         // providing a value whose computation contains another call to a function
1011                         // requiring a constant argument.
1012                         TerminatorKind::Goto { .. } => return None,
1013                         _ => bug!(),
1014                     }
1015                 }
1016             }
1017         };
1018
1019         assert_eq!(self.new_block(), START_BLOCK);
1020         self.visit_rvalue(
1021             &mut rvalue,
1022             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
1023         );
1024
1025         let span = self.promoted.span;
1026         self.assign(RETURN_PLACE, rvalue, span);
1027         Some(self.promoted)
1028     }
1029 }
1030
1031 /// Replaces all temporaries with their promoted counterparts.
1032 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1033     fn tcx(&self) -> TyCtxt<'tcx> {
1034         self.tcx
1035     }
1036
1037     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
1038         if self.is_temp_kind(*local) {
1039             *local = self.promote_temp(*local);
1040         }
1041     }
1042 }
1043
1044 pub fn promote_candidates<'tcx>(
1045     def_id: DefId,
1046     body: &mut Body<'tcx>,
1047     tcx: TyCtxt<'tcx>,
1048     mut temps: IndexVec<Local, TempState>,
1049     candidates: Vec<Candidate>,
1050 ) -> IndexVec<Promoted, Body<'tcx>> {
1051     // Visit candidates in reverse, in case they're nested.
1052     debug!("promote_candidates({:?})", candidates);
1053
1054     let mut promotions = IndexVec::new();
1055
1056     let mut extra_statements = vec![];
1057     for candidate in candidates.into_iter().rev() {
1058         match candidate {
1059             Candidate::Repeat(Location { block, statement_index })
1060             | Candidate::Ref(Location { block, statement_index }) => {
1061                 if let StatementKind::Assign(box (place, _)) =
1062                     &body[block].statements[statement_index].kind
1063                 {
1064                     if let Some(local) = place.as_local() {
1065                         if temps[local] == TempState::PromotedOut {
1066                             // Already promoted.
1067                             continue;
1068                         }
1069                     }
1070                 }
1071             }
1072             Candidate::Argument { .. } => {}
1073         }
1074
1075         // Declare return place local so that `mir::Body::new` doesn't complain.
1076         let initial_locals =
1077             iter::once(LocalDecl::new_return_place(tcx.types.never, body.span)).collect();
1078
1079         let mut promoted = Body::new(
1080             IndexVec::new(),
1081             // FIXME: maybe try to filter this to avoid blowing up
1082             // memory usage?
1083             body.source_scopes.clone(),
1084             initial_locals,
1085             IndexVec::new(),
1086             0,
1087             vec![],
1088             body.span,
1089             vec![],
1090             body.generator_kind,
1091         );
1092         promoted.ignore_interior_mut_in_const_validation = true;
1093
1094         let promoter = Promoter {
1095             promoted,
1096             tcx,
1097             source: body,
1098             temps: &mut temps,
1099             extra_statements: &mut extra_statements,
1100             keep_original: false,
1101         };
1102
1103         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1104         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
1105             promotions.push(promoted);
1106         }
1107     }
1108
1109     // Insert each of `extra_statements` before its indicated location, which
1110     // has to be done in reverse location order, to not invalidate the rest.
1111     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1112     for (loc, statement) in extra_statements {
1113         body[loc.block].statements.insert(loc.statement_index, statement);
1114     }
1115
1116     // Eliminate assignments to, and drops of promoted temps.
1117     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1118     for block in body.basic_blocks_mut() {
1119         block.statements.retain(|statement| match &statement.kind {
1120             StatementKind::Assign(box (place, _)) => {
1121                 if let Some(index) = place.as_local() {
1122                     !promoted(index)
1123                 } else {
1124                     true
1125                 }
1126             }
1127             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1128                 !promoted(*index)
1129             }
1130             _ => true,
1131         });
1132         let terminator = block.terminator_mut();
1133         if let TerminatorKind::Drop { location: place, target, .. } = &terminator.kind {
1134             if let Some(index) = place.as_local() {
1135                 if promoted(index) {
1136                     terminator.kind = TerminatorKind::Goto { target: *target };
1137                 }
1138             }
1139         }
1140     }
1141
1142     promotions
1143 }
1144
1145 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1146 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1147 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1148 /// enabled.
1149 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1150     tcx: TyCtxt<'tcx>,
1151     mir_def_id: DefId,
1152     body: &Body<'tcx>,
1153     operand: &Operand<'tcx>,
1154 ) -> bool {
1155     let mut rpo = traversal::reverse_postorder(&body);
1156     let (temps, _) = collect_temps_and_candidates(tcx, &body, &mut rpo);
1157     let validator =
1158         Validator { item: Item::new(tcx, mir_def_id, body), temps: &temps, explicit: false };
1159
1160     let should_promote = validator.validate_operand(operand).is_ok();
1161     let feature_flag = tcx.features().const_in_array_repeat_expressions;
1162     debug!(
1163         "should_suggest_const_in_array_repeat_expressions_flag: mir_def_id={:?} \
1164             should_promote={:?} feature_flag={:?}",
1165         mir_def_id, should_promote, feature_flag
1166     );
1167     should_promote && !feature_flag
1168 }