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 as hir;
  17 use rustc_hir::def_id::DefId;
  18 use rustc_middle::mir::traversal::ReversePostorder;
  19 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
  20 use rustc_middle::mir::*;
  21 use rustc_middle::ty::cast::CastTy;
  22 use rustc_middle::ty::subst::InternalSubsts;
  23 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
  24 use rustc_span::symbol::sym;
  25 use rustc_span::{Span, DUMMY_SP};
  26
  27 use rustc_index::vec::{Idx, IndexVec};
  28 use rustc_target::spec::abi::Abi;
  29
  30 use std::cell::Cell;
  31 use std::{cmp, iter, mem};
  32
  33 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
  34 use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstCx};
  35 use crate::transform::{MirPass, MirSource};
  36
  37 /// A `MirPass` for promotion.
  38 ///
  39 /// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits
  40 /// errors when promotion of `#[rustc_args_required_const]` arguments fails.
  41 ///
  42 /// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
  43 /// newly created `Constant`.
  44 #[derive(Default)]
  45 pub struct PromoteTemps<'tcx> {
  46     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
  47 }
  48
  49 impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
  50     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
  51         // There's not really any point in promoting errorful MIR.
  52         //
  53         // This does not include MIR that failed const-checking, which we still try to promote.
  54         if body.return_ty().references_error() {
  55             tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
  56             return;
  57         }
  58
  59         if src.promoted.is_some() {
  60             return;
  61         }
  62
  63         let def = src.with_opt_param().expect_local();
  64
  65         let mut rpo = traversal::reverse_postorder(body);
  66         let ccx = ConstCx::new(tcx, def.did, body);
  67         let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo);
  68
  69         let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates);
  70
  71         let promoted = promote_candidates(def.to_global(), body, tcx, temps, promotable_candidates);
  72         self.promoted_fragments.set(promoted);
  73     }
  74 }
  75
  76 /// State of a temporary during collection and promotion.
  77 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  78 pub enum TempState {
  79     /// No references to this temp.
  80     Undefined,
  81     /// One direct assignment and any number of direct uses.
  82     /// A borrow of this temp is promotable if the assigned
  83     /// value is qualified as constant.
  84     Defined { location: Location, uses: usize },
  85     /// Any other combination of assignments/uses.
  86     Unpromotable,
  87     /// This temp was part of an rvalue which got extracted
  88     /// during promotion and needs cleanup.
  89     PromotedOut,
  90 }
  91
  92 impl TempState {
  93     pub fn is_promotable(&self) -> bool {
  94         debug!("is_promotable: self={:?}", self);
  95         if let TempState::Defined { .. } = *self { true } else { false }
  96     }
  97 }
  98
  99 /// A "root candidate" for promotion, which will become the
 100 /// returned value in a promoted MIR, unless it's a subset
 101 /// of a larger candidate.
 102 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 103 pub enum Candidate {
 104     /// Borrow of a constant temporary.
 105     Ref(Location),
 106
 107     /// Promotion of the `x` in `[x; 32]`.
 108     Repeat(Location),
 109
 110     /// Currently applied to function calls where the callee has the unstable
 111     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
 112     /// intrinsic. The intrinsic requires the arguments are indeed constant and
 113     /// the attribute currently provides the semantic requirement that arguments
 114     /// must be constant.
 115     Argument { bb: BasicBlock, index: usize },
 116
 117     /// `const` operand in asm!.
 118     InlineAsm { bb: BasicBlock, index: usize },
 119 }
 120
 121 impl Candidate {
 122     /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`.
 123     fn forces_explicit_promotion(&self) -> bool {
 124         match self {
 125             Candidate::Ref(_) | Candidate::Repeat(_) => false,
 126             Candidate::Argument { .. } | Candidate::InlineAsm { .. } => true,
 127         }
 128     }
 129 }
 130
 131 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> {
 132     let attrs = tcx.get_attrs(def_id);
 133     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
 134     let mut ret = vec![];
 135     for meta in attr.meta_item_list()? {
 136         match meta.literal()?.kind {
 137             LitKind::Int(a, _) => {
 138                 ret.push(a as usize);
 139             }
 140             _ => return None,
 141         }
 142     }
 143     Some(ret)
 144 }
 145
 146 struct Collector<'a, 'tcx> {
 147     ccx: &'a ConstCx<'a, 'tcx>,
 148     temps: IndexVec<Local, TempState>,
 149     candidates: Vec<Candidate>,
 150 }
 151
 152 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 153     fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) {
 154         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
 155         // We're only interested in temporaries and the return place
 156         match self.ccx.body.local_kind(index) {
 157             LocalKind::Temp | LocalKind::ReturnPointer => {}
 158             LocalKind::Arg | LocalKind::Var => return,
 159         }
 160
 161         // Ignore drops, if the temp gets promoted,
 162         // then it's constant and thus drop is noop.
 163         // Non-uses are also irrelevant.
 164         if context.is_drop() || !context.is_use() {
 165             debug!(
 166                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 167                 context.is_drop(),
 168                 context.is_use(),
 169             );
 170             return;
 171         }
 172
 173         let temp = &mut self.temps[index];
 174         debug!("visit_local: temp={:?}", temp);
 175         if *temp == TempState::Undefined {
 176             match context {
 177                 PlaceContext::MutatingUse(MutatingUseContext::Store)
 178                 | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 179                     *temp = TempState::Defined { location, uses: 0 };
 180                     return;
 181                 }
 182                 _ => { /* mark as unpromotable below */ }
 183             }
 184         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 185             // We always allow borrows, even mutable ones, as we need
 186             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 187             let allowed_use = match context {
 188                 PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 189                 | PlaceContext::NonMutatingUse(_) => true,
 190                 PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
 191             };
 192             debug!("visit_local: allowed_use={:?}", allowed_use);
 193             if allowed_use {
 194                 *uses += 1;
 195                 return;
 196             }
 197             /* mark as unpromotable below */
 198         }
 199         *temp = TempState::Unpromotable;
 200     }
 201
 202     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 203         self.super_rvalue(rvalue, location);
 204
 205         match *rvalue {
 206             Rvalue::Ref(..) => {
 207                 self.candidates.push(Candidate::Ref(location));
 208             }
 209             Rvalue::Repeat(..) if self.ccx.tcx.features().const_in_array_repeat_expressions => {
 210                 // FIXME(#49147) only promote the element when it isn't `Copy`
 211                 // (so that code that can copy it at runtime is unaffected).
 212                 self.candidates.push(Candidate::Repeat(location));
 213             }
 214             _ => {}
 215         }
 216     }
 217
 218     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 219         self.super_terminator(terminator, location);
 220
 221         match terminator.kind {
 222             TerminatorKind::Call { ref func, .. } => {
 223                 if let ty::FnDef(def_id, _) = func.ty(self.ccx.body, self.ccx.tcx).kind {
 224                     let fn_sig = self.ccx.tcx.fn_sig(def_id);
 225                     if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() {
 226                         let name = self.ccx.tcx.item_name(def_id);
 227                         // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles.
 228                         if name.as_str().starts_with("simd_shuffle") {
 229                             self.candidates
 230                                 .push(Candidate::Argument { bb: location.block, index: 2 });
 231
 232                             return; // Don't double count `simd_shuffle` candidates
 233                         }
 234                     }
 235
 236                     if let Some(constant_args) = args_required_const(self.ccx.tcx, def_id) {
 237                         for index in constant_args {
 238                             self.candidates.push(Candidate::Argument { bb: location.block, index });
 239                         }
 240                     }
 241                 }
 242             }
 243             TerminatorKind::InlineAsm { ref operands, .. } => {
 244                 for (index, op) in operands.iter().enumerate() {
 245                     match op {
 246                         InlineAsmOperand::Const { .. } => {
 247                             self.candidates.push(Candidate::InlineAsm { bb: location.block, index })
 248                         }
 249                         _ => {}
 250                     }
 251                 }
 252             }
 253             _ => {}
 254         }
 255     }
 256 }
 257
 258 pub fn collect_temps_and_candidates(
 259     ccx: &ConstCx<'mir, 'tcx>,
 260     rpo: &mut ReversePostorder<'_, 'tcx>,
 261 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
 262     let mut collector = Collector {
 263         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
 264         candidates: vec![],
 265         ccx,
 266     };
 267     for (bb, data) in rpo {
 268         collector.visit_basic_block_data(bb, data);
 269     }
 270     (collector.temps, collector.candidates)
 271 }
 272
 273 /// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable.
 274 ///
 275 /// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion.
 276 struct Validator<'a, 'tcx> {
 277     ccx: &'a ConstCx<'a, 'tcx>,
 278     temps: &'a IndexVec<Local, TempState>,
 279
 280     /// Explicit promotion happens e.g. for constant arguments declared via
 281     /// `rustc_args_required_const`.
 282     /// Implicit promotion has almost the same rules, except that disallows `const fn`
 283     /// except for those marked `#[rustc_promotable]`. This is to avoid changing
 284     /// a legitimate run-time operation into a failing compile-time operation
 285     /// e.g. due to addresses being compared inside the function.
 286     explicit: bool,
 287 }
 288
 289 impl std::ops::Deref for Validator<'a, 'tcx> {
 290     type Target = ConstCx<'a, 'tcx>;
 291
 292     fn deref(&self) -> &Self::Target {
 293         &self.ccx
 294     }
 295 }
 296
 297 struct Unpromotable;
 298
 299 impl<'tcx> Validator<'_, 'tcx> {
 300     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 301         match candidate {
 302             Candidate::Ref(loc) => {
 303                 assert!(!self.explicit);
 304
 305                 let statement = &self.body[loc.block].statements[loc.statement_index];
 306                 match &statement.kind {
 307                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 308                         match kind {
 309                             BorrowKind::Shared | BorrowKind::Mut { .. } => {}
 310
 311                             // FIXME(eddyb) these aren't promoted here but *could*
 312                             // be promoted as part of a larger value because
 313                             // `validate_rvalue`  doesn't check them, need to
 314                             // figure out what is the intended behavior.
 315                             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 316                         }
 317
 318                         // We can only promote interior borrows of promotable temps (non-temps
 319                         // don't get promoted anyway).
 320                         self.validate_local(place.local)?;
 321
 322                         if place.projection.contains(&ProjectionElem::Deref) {
 323                             return Err(Unpromotable);
 324                         }
 325
 326                         let mut has_mut_interior =
 327                             self.qualif_local::<qualifs::HasMutInterior>(place.local);
 328                         // HACK(eddyb) this should compute the same thing as
 329                         // `<HasMutInterior as Qualif>::in_projection` from
 330                         // `check_consts::qualifs` but without recursion.
 331                         if has_mut_interior {
 332                             // This allows borrowing fields which don't have
 333                             // `HasMutInterior`, from a type that does, e.g.:
 334                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 335                             let mut place_projection = &place.projection[..];
 336                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 337                             while let &[ref proj_base @ .., elem] = place_projection {
 338                                 // FIXME(eddyb) this is probably excessive, with
 339                                 // the exception of `union` member accesses.
 340                                 let ty =
 341                                     Place::ty_from(place.local, proj_base, self.body, self.tcx)
 342                                         .projection_ty(self.tcx, elem)
 343                                         .ty;
 344                                 if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) {
 345                                     has_mut_interior = false;
 346                                     break;
 347                                 }
 348
 349                                 place_projection = proj_base;
 350                             }
 351                         }
 352
 353                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 354                         if has_mut_interior {
 355                             return Err(Unpromotable);
 356                         }
 357                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 358                             return Err(Unpromotable);
 359                         }
 360
 361                         if let BorrowKind::Mut { .. } = kind {
 362                             let ty = place.ty(self.body, self.tcx).ty;
 363
 364                             // In theory, any zero-sized value could be borrowed
 365                             // mutably without consequences. However, only &mut []
 366                             // is allowed right now, and only in functions.
 367                             if self.const_kind
 368                                 == Some(hir::ConstContext::Static(hir::Mutability::Mut))
 369                             {
 370                                 // Inside a `static mut`, &mut [...] is also allowed.
 371                                 match ty.kind {
 372                                     ty::Array(..) | ty::Slice(_) => {}
 373                                     _ => return Err(Unpromotable),
 374                                 }
 375                             } else if let ty::Array(_, len) = ty.kind {
 376                                 // FIXME(eddyb) the `self.is_non_const_fn` condition
 377                                 // seems unnecessary, given that this is merely a ZST.
 378                                 match len.try_eval_usize(self.tcx, self.param_env) {
 379                                     Some(0) if self.const_kind.is_none() => {}
 380                                     _ => return Err(Unpromotable),
 381                                 }
 382                             } else {
 383                                 return Err(Unpromotable);
 384                             }
 385                         }
 386
 387                         Ok(())
 388                     }
 389                     _ => bug!(),
 390                 }
 391             }
 392             Candidate::Repeat(loc) => {
 393                 assert!(!self.explicit);
 394
 395                 let statement = &self.body[loc.block].statements[loc.statement_index];
 396                 match &statement.kind {
 397                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 398                         if !self.tcx.features().const_in_array_repeat_expressions {
 399                             return Err(Unpromotable);
 400                         }
 401
 402                         self.validate_operand(operand)
 403                     }
 404                     _ => bug!(),
 405                 }
 406             }
 407             Candidate::Argument { bb, index } => {
 408                 assert!(self.explicit);
 409
 410                 let terminator = self.body[bb].terminator();
 411                 match &terminator.kind {
 412                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 413                     _ => bug!(),
 414                 }
 415             }
 416             Candidate::InlineAsm { bb, index } => {
 417                 assert!(self.explicit);
 418
 419                 let terminator = self.body[bb].terminator();
 420                 match &terminator.kind {
 421                     TerminatorKind::InlineAsm { operands, .. } => match &operands[index] {
 422                         InlineAsmOperand::Const { value } => self.validate_operand(value),
 423                         _ => bug!(),
 424                     },
 425                     _ => bug!(),
 426                 }
 427             }
 428         }
 429     }
 430
 431     // FIXME(eddyb) maybe cache this?
 432     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 433         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 434             let num_stmts = self.body[loc.block].statements.len();
 435
 436             if loc.statement_index < num_stmts {
 437                 let statement = &self.body[loc.block].statements[loc.statement_index];
 438                 match &statement.kind {
 439                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 440                         &self.ccx,
 441                         &mut |l| self.qualif_local::<Q>(l),
 442                         rhs,
 443                     ),
 444                     _ => {
 445                         span_bug!(
 446                             statement.source_info.span,
 447                             "{:?} is not an assignment",
 448                             statement
 449                         );
 450                     }
 451                 }
 452             } else {
 453                 let terminator = self.body[loc.block].terminator();
 454                 match &terminator.kind {
 455                     TerminatorKind::Call { .. } => {
 456                         let return_ty = self.body.local_decls[local].ty;
 457                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 458                     }
 459                     kind => {
 460                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 461                     }
 462                 }
 463             }
 464         } else {
 465             let span = self.body.local_decls[local].source_info.span;
 466             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 467         }
 468     }
 469
 470     // FIXME(eddyb) maybe cache this?
 471     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 472         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 473             let num_stmts = self.body[loc.block].statements.len();
 474
 475             if loc.statement_index < num_stmts {
 476                 let statement = &self.body[loc.block].statements[loc.statement_index];
 477                 match &statement.kind {
 478                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 479                     _ => {
 480                         span_bug!(
 481                             statement.source_info.span,
 482                             "{:?} is not an assignment",
 483                             statement
 484                         );
 485                     }
 486                 }
 487             } else {
 488                 let terminator = self.body[loc.block].terminator();
 489                 match &terminator.kind {
 490                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 491                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 492                     kind => {
 493                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 494                     }
 495                 }
 496             }
 497         } else {
 498             Err(Unpromotable)
 499         }
 500     }
 501
 502     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 503         match place {
 504             PlaceRef { local, projection: [] } => self.validate_local(local),
 505             PlaceRef { local, projection: [proj_base @ .., elem] } => {
 506                 match *elem {
 507                     ProjectionElem::Deref => {
 508                         let mut not_promotable = true;
 509                         // This is a special treatment for cases like *&STATIC where STATIC is a
 510                         // global static variable.
 511                         // This pattern is generated only when global static variables are directly
 512                         // accessed and is qualified for promotion safely.
 513                         if let TempState::Defined { location, .. } = self.temps[local] {
 514                             let def_stmt =
 515                                 self.body[location.block].statements.get(location.statement_index);
 516                             if let Some(Statement {
 517                                 kind:
 518                                     StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(c)))),
 519                                 ..
 520                             }) = def_stmt
 521                             {
 522                                 if let Some(did) = c.check_static_ptr(self.tcx) {
 523                                     if let Some(hir::ConstContext::Static(..)) = self.const_kind {
 524                                         // The `is_empty` predicate is introduced to exclude the case
 525                                         // where the projection operations are [ .field, * ].
 526                                         // The reason is because promotion will be illegal if field
 527                                         // accesses precede the dereferencing.
 528                                         // Discussion can be found at
 529                                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 530                                         // There may be opportunity for generalization, but this needs to be
 531                                         // accounted for.
 532                                         if proj_base.is_empty()
 533                                             && !self.tcx.is_thread_local_static(did)
 534                                         {
 535                                             not_promotable = false;
 536                                         }
 537                                     }
 538                                 }
 539                             }
 540                         }
 541                         if not_promotable {
 542                             return Err(Unpromotable);
 543                         }
 544                     }
 545                     ProjectionElem::Downcast(..) => {
 546                         return Err(Unpromotable);
 547                     }
 548
 549                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 550
 551                     ProjectionElem::Index(local) => {
 552                         self.validate_local(local)?;
 553                     }
 554
 555                     ProjectionElem::Field(..) => {
 556                         if self.const_kind.is_none() {
 557                             let base_ty =
 558                                 Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 559                             if let Some(def) = base_ty.ty_adt_def() {
 560                                 // No promotion of union field accesses.
 561                                 if def.is_union() {
 562                                     return Err(Unpromotable);
 563                                 }
 564                             }
 565                         }
 566                     }
 567                 }
 568
 569                 self.validate_place(PlaceRef { local: place.local, projection: proj_base })
 570             }
 571         }
 572     }
 573
 574     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 575         match operand {
 576             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 577
 578             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 579             // `validate_rvalue` upon access.
 580             Operand::Constant(c) => {
 581                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 582                     // Only allow statics (not consts) to refer to other statics.
 583                     // FIXME(eddyb) does this matter at all for promotion?
 584                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 585                     if !is_static {
 586                         return Err(Unpromotable);
 587                     }
 588
 589                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 590                     if is_thread_local {
 591                         return Err(Unpromotable);
 592                     }
 593                 }
 594
 595                 Ok(())
 596             }
 597         }
 598     }
 599
 600     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 601         match *rvalue {
 602             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
 603                 let operand_ty = operand.ty(self.body, self.tcx);
 604                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 605                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 606                 match (cast_in, cast_out) {
 607                     (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
 608                         // in normal functions, mark such casts as not promotable
 609                         return Err(Unpromotable);
 610                     }
 611                     _ => {}
 612                 }
 613             }
 614
 615             Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
 616                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
 617                     assert!(
 618                         op == BinOp::Eq
 619                             || op == BinOp::Ne
 620                             || op == BinOp::Le
 621                             || op == BinOp::Lt
 622                             || op == BinOp::Ge
 623                             || op == BinOp::Gt
 624                             || op == BinOp::Offset
 625                     );
 626
 627                     // raw pointer operations are not allowed inside promoteds
 628                     return Err(Unpromotable);
 629                 }
 630             }
 631
 632             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 633
 634             _ => {}
 635         }
 636
 637         match rvalue {
 638             Rvalue::ThreadLocalRef(_) => Err(Unpromotable),
 639
 640             Rvalue::NullaryOp(..) => Ok(()),
 641
 642             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 643
 644             Rvalue::Use(operand)
 645             | Rvalue::Repeat(operand, _)
 646             | Rvalue::UnaryOp(_, operand)
 647             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 648
 649             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 650                 self.validate_operand(lhs)?;
 651                 self.validate_operand(rhs)
 652             }
 653
 654             Rvalue::AddressOf(_, place) => {
 655                 // Raw reborrows can come from reference to pointer coercions,
 656                 // so are allowed.
 657                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 658                     let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 659                     if let ty::Ref(..) = base_ty.kind {
 660                         return self.validate_place(PlaceRef {
 661                             local: place.local,
 662                             projection: proj_base,
 663                         });
 664                     }
 665                 }
 666                 Err(Unpromotable)
 667             }
 668
 669             Rvalue::Ref(_, kind, place) => {
 670                 if let BorrowKind::Mut { .. } = kind {
 671                     let ty = place.ty(self.body, self.tcx).ty;
 672
 673                     // In theory, any zero-sized value could be borrowed
 674                     // mutably without consequences. However, only &mut []
 675                     // is allowed right now, and only in functions.
 676                     if self.const_kind == Some(hir::ConstContext::Static(hir::Mutability::Mut)) {
 677                         // Inside a `static mut`, &mut [...] is also allowed.
 678                         match ty.kind {
 679                             ty::Array(..) | ty::Slice(_) => {}
 680                             _ => return Err(Unpromotable),
 681                         }
 682                     } else if let ty::Array(_, len) = ty.kind {
 683                         // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
 684                         // const context which seems unnecessary given that this is merely a ZST.
 685                         match len.try_eval_usize(self.tcx, self.param_env) {
 686                             Some(0) if self.const_kind.is_none() => {}
 687                             _ => return Err(Unpromotable),
 688                         }
 689                     } else {
 690                         return Err(Unpromotable);
 691                     }
 692                 }
 693
 694                 // Special-case reborrows to be more like a copy of the reference.
 695                 let mut place = place.as_ref();
 696                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 697                     let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 698                     if let ty::Ref(..) = base_ty.kind {
 699                         place = PlaceRef { local: place.local, projection: proj_base };
 700                     }
 701                 }
 702
 703                 self.validate_place(place)?;
 704
 705                 // HACK(eddyb) this should compute the same thing as
 706                 // `<HasMutInterior as Qualif>::in_projection` from
 707                 // `check_consts::qualifs` but without recursion.
 708                 let mut has_mut_interior =
 709                     self.qualif_local::<qualifs::HasMutInterior>(place.local);
 710                 if has_mut_interior {
 711                     let mut place_projection = place.projection;
 712                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 713                     while let &[ref proj_base @ .., elem] = place_projection {
 714                         // FIXME(eddyb) this is probably excessive, with
 715                         // the exception of `union` member accesses.
 716                         let ty = Place::ty_from(place.local, proj_base, self.body, self.tcx)
 717                             .projection_ty(self.tcx, elem)
 718                             .ty;
 719                         if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) {
 720                             has_mut_interior = false;
 721                             break;
 722                         }
 723
 724                         place_projection = proj_base;
 725                     }
 726                 }
 727                 if has_mut_interior {
 728                     return Err(Unpromotable);
 729                 }
 730
 731                 Ok(())
 732             }
 733
 734             Rvalue::Aggregate(_, ref operands) => {
 735                 for o in operands {
 736                     self.validate_operand(o)?;
 737                 }
 738
 739                 Ok(())
 740             }
 741         }
 742     }
 743
 744     fn validate_call(
 745         &self,
 746         callee: &Operand<'tcx>,
 747         args: &[Operand<'tcx>],
 748     ) -> Result<(), Unpromotable> {
 749         let fn_ty = callee.ty(self.body, self.tcx);
 750
 751         if !self.explicit && self.const_kind.is_none() {
 752             if let ty::FnDef(def_id, _) = fn_ty.kind {
 753                 // Never promote runtime `const fn` calls of
 754                 // functions without `#[rustc_promotable]`.
 755                 if !self.tcx.is_promotable_const_fn(def_id) {
 756                     return Err(Unpromotable);
 757                 }
 758             }
 759         }
 760
 761         let is_const_fn = match fn_ty.kind {
 762             ty::FnDef(def_id, _) => {
 763                 is_const_fn(self.tcx, def_id)
 764                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 765                     || is_lang_panic_fn(self.tcx, self.def_id.to_def_id())
 766             }
 767             _ => false,
 768         };
 769         if !is_const_fn {
 770             return Err(Unpromotable);
 771         }
 772
 773         self.validate_operand(callee)?;
 774         for arg in args {
 775             self.validate_operand(arg)?;
 776         }
 777
 778         Ok(())
 779     }
 780 }
 781
 782 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 783 pub fn validate_candidates(
 784     ccx: &ConstCx<'_, '_>,
 785     temps: &IndexVec<Local, TempState>,
 786     candidates: &[Candidate],
 787 ) -> Vec<Candidate> {
 788     let mut validator = Validator { ccx, temps, explicit: false };
 789
 790     candidates
 791         .iter()
 792         .copied()
 793         .filter(|&candidate| {
 794             validator.explicit = candidate.forces_explicit_promotion();
 795
 796             // FIXME(eddyb) also emit the errors for shuffle indices
 797             // and `#[rustc_args_required_const]` arguments here.
 798
 799             let is_promotable = validator.validate_candidate(candidate).is_ok();
 800
 801             // If we use explicit validation, we carry the risk of turning a legitimate run-time
 802             // operation into a failing compile-time operation. Make sure that does not happen
 803             // by asserting that there is no possible run-time behavior here in case promotion
 804             // fails.
 805             if validator.explicit && !is_promotable {
 806                 ccx.tcx.sess.delay_span_bug(
 807                     ccx.body.span,
 808                     "Explicit promotion requested, but failed to promote",
 809                 );
 810             }
 811
 812             match candidate {
 813                 Candidate::Argument { bb, index } | Candidate::InlineAsm { bb, index }
 814                     if !is_promotable =>
 815                 {
 816                     let span = ccx.body[bb].terminator().source_info.span;
 817                     let msg = format!("argument {} is required to be a constant", index + 1);
 818                     ccx.tcx.sess.span_err(span, &msg);
 819                 }
 820                 _ => (),
 821             }
 822
 823             is_promotable
 824         })
 825         .collect()
 826 }
 827
 828 struct Promoter<'a, 'tcx> {
 829     tcx: TyCtxt<'tcx>,
 830     source: &'a mut Body<'tcx>,
 831     promoted: Body<'tcx>,
 832     temps: &'a mut IndexVec<Local, TempState>,
 833     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 834
 835     /// If true, all nested temps are also kept in the
 836     /// source MIR, not moved to the promoted MIR.
 837     keep_original: bool,
 838 }
 839
 840 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 841     fn new_block(&mut self) -> BasicBlock {
 842         let span = self.promoted.span;
 843         self.promoted.basic_blocks_mut().push(BasicBlockData {
 844             statements: vec![],
 845             terminator: Some(Terminator {
 846                 source_info: SourceInfo::outermost(span),
 847                 kind: TerminatorKind::Return,
 848             }),
 849             is_cleanup: false,
 850         })
 851     }
 852
 853     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 854         let last = self.promoted.basic_blocks().last().unwrap();
 855         let data = &mut self.promoted[last];
 856         data.statements.push(Statement {
 857             source_info: SourceInfo::outermost(span),
 858             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 859         });
 860     }
 861
 862     fn is_temp_kind(&self, local: Local) -> bool {
 863         self.source.local_kind(local) == LocalKind::Temp
 864     }
 865
 866     /// Copies the initialization of this temp to the
 867     /// promoted MIR, recursing through temps.
 868     fn promote_temp(&mut self, temp: Local) -> Local {
 869         let old_keep_original = self.keep_original;
 870         let loc = match self.temps[temp] {
 871             TempState::Defined { location, uses } if uses > 0 => {
 872                 if uses > 1 {
 873                     self.keep_original = true;
 874                 }
 875                 location
 876             }
 877             state => {
 878                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 879             }
 880         };
 881         if !self.keep_original {
 882             self.temps[temp] = TempState::PromotedOut;
 883         }
 884
 885         let num_stmts = self.source[loc.block].statements.len();
 886         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 887             self.source.local_decls[temp].ty,
 888             self.source.local_decls[temp].source_info.span,
 889         ));
 890
 891         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 892
 893         // First, take the Rvalue or Call out of the source MIR,
 894         // or duplicate it, depending on keep_original.
 895         if loc.statement_index < num_stmts {
 896             let (mut rvalue, source_info) = {
 897                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 898                 let rhs = match statement.kind {
 899                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 900                     _ => {
 901                         span_bug!(
 902                             statement.source_info.span,
 903                             "{:?} is not an assignment",
 904                             statement
 905                         );
 906                     }
 907                 };
 908
 909                 (
 910                     if self.keep_original {
 911                         rhs.clone()
 912                     } else {
 913                         let unit = Rvalue::Use(Operand::Constant(box Constant {
 914                             span: statement.source_info.span,
 915                             user_ty: None,
 916                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit),
 917                         }));
 918                         mem::replace(rhs, unit)
 919                     },
 920                     statement.source_info,
 921                 )
 922             };
 923
 924             self.visit_rvalue(&mut rvalue, loc);
 925             self.assign(new_temp, rvalue, source_info.span);
 926         } else {
 927             let terminator = if self.keep_original {
 928                 self.source[loc.block].terminator().clone()
 929             } else {
 930                 let terminator = self.source[loc.block].terminator_mut();
 931                 let target = match terminator.kind {
 932                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 933                     ref kind => {
 934                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 935                     }
 936                 };
 937                 Terminator {
 938                     source_info: terminator.source_info,
 939                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 940                 }
 941             };
 942
 943             match terminator.kind {
 944                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 945                     self.visit_operand(&mut func, loc);
 946                     for arg in &mut args {
 947                         self.visit_operand(arg, loc);
 948                     }
 949
 950                     let last = self.promoted.basic_blocks().last().unwrap();
 951                     let new_target = self.new_block();
 952
 953                     *self.promoted[last].terminator_mut() = Terminator {
 954                         kind: TerminatorKind::Call {
 955                             func,
 956                             args,
 957                             cleanup: None,
 958                             destination: Some((Place::from(new_temp), new_target)),
 959                             from_hir_call,
 960                             fn_span,
 961                         },
 962                         ..terminator
 963                     };
 964                 }
 965                 ref kind => {
 966                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 967                 }
 968             };
 969         };
 970
 971         self.keep_original = old_keep_original;
 972         new_temp
 973     }
 974
 975     fn promote_candidate(
 976         mut self,
 977         def: ty::WithOptConstParam<DefId>,
 978         candidate: Candidate,
 979         next_promoted_id: usize,
 980     ) -> Option<Body<'tcx>> {
 981         let mut rvalue = {
 982             let promoted = &mut self.promoted;
 983             let promoted_id = Promoted::new(next_promoted_id);
 984             let tcx = self.tcx;
 985             let mut promoted_operand = |ty, span| {
 986                 promoted.span = span;
 987                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 988
 989                 Operand::Constant(Box::new(Constant {
 990                     span,
 991                     user_ty: None,
 992                     literal: tcx.mk_const(ty::Const {
 993                         ty,
 994                         val: ty::ConstKind::Unevaluated(
 995                             def,
 996                             InternalSubsts::for_item(tcx, def.did, |param, _| {
 997                                 if let ty::GenericParamDefKind::Lifetime = param.kind {
 998                                     tcx.lifetimes.re_erased.into()
 999                                 } else {
1000                                     tcx.mk_param_from_def(param)
1001                                 }
1002                             }),
1003                             Some(promoted_id),
1004                         ),
1005                     }),
1006                 }))
1007             };
1008             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
1009             match candidate {
1010                 Candidate::Ref(loc) => {
1011                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
1012                     match statement.kind {
1013                         StatementKind::Assign(box (
1014                             _,
1015                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
1016                         )) => {
1017                             // Use the underlying local for this (necessarily interior) borrow.
1018                             let ty = local_decls.local_decls()[place.local].ty;
1019                             let span = statement.source_info.span;
1020
1021                             let ref_ty = tcx.mk_ref(
1022                                 tcx.lifetimes.re_erased,
1023                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
1024                             );
1025
1026                             *region = tcx.lifetimes.re_erased;
1027
1028                             let mut projection = vec![PlaceElem::Deref];
1029                             projection.extend(place.projection);
1030                             place.projection = tcx.intern_place_elems(&projection);
1031
1032                             // Create a temp to hold the promoted reference.
1033                             // This is because `*r` requires `r` to be a local,
1034                             // otherwise we would use the `promoted` directly.
1035                             let mut promoted_ref = LocalDecl::new(ref_ty, span);
1036                             promoted_ref.source_info = statement.source_info;
1037                             let promoted_ref = local_decls.push(promoted_ref);
1038                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
1039
1040                             let promoted_ref_statement = Statement {
1041                                 source_info: statement.source_info,
1042                                 kind: StatementKind::Assign(Box::new((
1043                                     Place::from(promoted_ref),
1044                                     Rvalue::Use(promoted_operand(ref_ty, span)),
1045                                 ))),
1046                             };
1047                             self.extra_statements.push((loc, promoted_ref_statement));
1048
1049                             Rvalue::Ref(
1050                                 tcx.lifetimes.re_erased,
1051                                 borrow_kind,
1052                                 Place {
1053                                     local: mem::replace(&mut place.local, promoted_ref),
1054                                     projection: List::empty(),
1055                                 },
1056                             )
1057                         }
1058                         _ => bug!(),
1059                     }
1060                 }
1061                 Candidate::Repeat(loc) => {
1062                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
1063                     match statement.kind {
1064                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
1065                             let ty = operand.ty(local_decls, self.tcx);
1066                             let span = statement.source_info.span;
1067
1068                             Rvalue::Use(mem::replace(operand, promoted_operand(ty, span)))
1069                         }
1070                         _ => bug!(),
1071                     }
1072                 }
1073                 Candidate::Argument { bb, index } => {
1074                     let terminator = blocks[bb].terminator_mut();
1075                     match terminator.kind {
1076                         TerminatorKind::Call { ref mut args, .. } => {
1077                             let ty = args[index].ty(local_decls, self.tcx);
1078                             let span = terminator.source_info.span;
1079
1080                             Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span)))
1081                         }
1082                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
1083                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
1084                         // we are seeing a `Goto`. That means that the `promote_temps` method
1085                         // already promoted this call away entirely. This case occurs when calling
1086                         // a function requiring a constant argument and as that constant value
1087                         // providing a value whose computation contains another call to a function
1088                         // requiring a constant argument.
1089                         TerminatorKind::Goto { .. } => return None,
1090                         _ => bug!(),
1091                     }
1092                 }
1093                 Candidate::InlineAsm { bb, index } => {
1094                     let terminator = blocks[bb].terminator_mut();
1095                     match terminator.kind {
1096                         TerminatorKind::InlineAsm { ref mut operands, .. } => {
1097                             match &mut operands[index] {
1098                                 InlineAsmOperand::Const { ref mut value } => {
1099                                     let ty = value.ty(local_decls, self.tcx);
1100                                     let span = terminator.source_info.span;
1101
1102                                     Rvalue::Use(mem::replace(value, promoted_operand(ty, span)))
1103                                 }
1104                                 _ => bug!(),
1105                             }
1106                         }
1107
1108                         _ => bug!(),
1109                     }
1110                 }
1111             }
1112         };
1113
1114         assert_eq!(self.new_block(), START_BLOCK);
1115         self.visit_rvalue(
1116             &mut rvalue,
1117             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
1118         );
1119
1120         let span = self.promoted.span;
1121         self.assign(RETURN_PLACE, rvalue, span);
1122         Some(self.promoted)
1123     }
1124 }
1125
1126 /// Replaces all temporaries with their promoted counterparts.
1127 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1128     fn tcx(&self) -> TyCtxt<'tcx> {
1129         self.tcx
1130     }
1131
1132     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
1133         if self.is_temp_kind(*local) {
1134             *local = self.promote_temp(*local);
1135         }
1136     }
1137 }
1138
1139 pub fn promote_candidates<'tcx>(
1140     def: ty::WithOptConstParam<DefId>,
1141     body: &mut Body<'tcx>,
1142     tcx: TyCtxt<'tcx>,
1143     mut temps: IndexVec<Local, TempState>,
1144     candidates: Vec<Candidate>,
1145 ) -> IndexVec<Promoted, Body<'tcx>> {
1146     // Visit candidates in reverse, in case they're nested.
1147     debug!("promote_candidates({:?})", candidates);
1148
1149     let mut promotions = IndexVec::new();
1150
1151     let mut extra_statements = vec![];
1152     for candidate in candidates.into_iter().rev() {
1153         match candidate {
1154             Candidate::Repeat(Location { block, statement_index })
1155             | Candidate::Ref(Location { block, statement_index }) => {
1156                 if let StatementKind::Assign(box (place, _)) =
1157                     &body[block].statements[statement_index].kind
1158                 {
1159                     if let Some(local) = place.as_local() {
1160                         if temps[local] == TempState::PromotedOut {
1161                             // Already promoted.
1162                             continue;
1163                         }
1164                     }
1165                 }
1166             }
1167             Candidate::Argument { .. } | Candidate::InlineAsm { .. } => {}
1168         }
1169
1170         // Declare return place local so that `mir::Body::new` doesn't complain.
1171         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
1172
1173         let mut promoted = Body::new(
1174             IndexVec::new(),
1175             // FIXME: maybe try to filter this to avoid blowing up
1176             // memory usage?
1177             body.source_scopes.clone(),
1178             initial_locals,
1179             IndexVec::new(),
1180             0,
1181             vec![],
1182             body.span,
1183             body.generator_kind,
1184         );
1185         promoted.ignore_interior_mut_in_const_validation = true;
1186
1187         let promoter = Promoter {
1188             promoted,
1189             tcx,
1190             source: body,
1191             temps: &mut temps,
1192             extra_statements: &mut extra_statements,
1193             keep_original: false,
1194         };
1195
1196         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1197         if let Some(promoted) = promoter.promote_candidate(def, candidate, promotions.len()) {
1198             promotions.push(promoted);
1199         }
1200     }
1201
1202     // Insert each of `extra_statements` before its indicated location, which
1203     // has to be done in reverse location order, to not invalidate the rest.
1204     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1205     for (loc, statement) in extra_statements {
1206         body[loc.block].statements.insert(loc.statement_index, statement);
1207     }
1208
1209     // Eliminate assignments to, and drops of promoted temps.
1210     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1211     for block in body.basic_blocks_mut() {
1212         block.statements.retain(|statement| match &statement.kind {
1213             StatementKind::Assign(box (place, _)) => {
1214                 if let Some(index) = place.as_local() {
1215                     !promoted(index)
1216                 } else {
1217                     true
1218                 }
1219             }
1220             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1221                 !promoted(*index)
1222             }
1223             _ => true,
1224         });
1225         let terminator = block.terminator_mut();
1226         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1227             if let Some(index) = place.as_local() {
1228                 if promoted(index) {
1229                     terminator.kind = TerminatorKind::Goto { target: *target };
1230                 }
1231             }
1232         }
1233     }
1234
1235     promotions
1236 }
1237
1238 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1239 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1240 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1241 /// enabled.
1242 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1243     ccx: &ConstCx<'_, 'tcx>,
1244     operand: &Operand<'tcx>,
1245 ) -> bool {
1246     let mut rpo = traversal::reverse_postorder(&ccx.body);
1247     let (temps, _) = collect_temps_and_candidates(&ccx, &mut rpo);
1248     let validator = Validator { ccx, temps: &temps, explicit: false };
1249
1250     let should_promote = validator.validate_operand(operand).is_ok();
1251     let feature_flag = validator.ccx.tcx.features().const_in_array_repeat_expressions;
1252     debug!(
1253         "should_suggest_const_in_array_repeat_expressions_flag: def_id={:?} \
1254             should_promote={:?} feature_flag={:?}",
1255         validator.ccx.def_id, should_promote, feature_flag
1256     );
1257     should_promote && !feature_flag
1258 }