compiler/rustc_mir/src/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::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         matches!(self, TempState::Defined { .. } )
  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, candidate for lifetime extension.
 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| tcx.sess.check_name(a, 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     /// Determines if this code could be executed at runtime and thus is subject to codegen.
 301     /// That means even unused constants need to be evaluated.
 302     ///
 303     /// `const_kind` should not be used in this file other than through this method!
 304     fn maybe_runtime(&self) -> bool {
 305         match self.const_kind {
 306             None | Some(hir::ConstContext::ConstFn) => true,
 307             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const) => false,
 308         }
 309     }
 310
 311     fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
 312         match candidate {
 313             Candidate::Ref(loc) => {
 314                 assert!(!self.explicit);
 315
 316                 let statement = &self.body[loc.block].statements[loc.statement_index];
 317                 match &statement.kind {
 318                     StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
 319                         match kind {
 320                             BorrowKind::Shared | BorrowKind::Mut { .. } => {}
 321
 322                             // FIXME(eddyb) these aren't promoted here but *could*
 323                             // be promoted as part of a larger value because
 324                             // `validate_rvalue`  doesn't check them, need to
 325                             // figure out what is the intended behavior.
 326                             BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
 327                         }
 328
 329                         // We can only promote interior borrows of promotable temps (non-temps
 330                         // don't get promoted anyway).
 331                         self.validate_local(place.local)?;
 332
 333                         if place.projection.contains(&ProjectionElem::Deref) {
 334                             return Err(Unpromotable);
 335                         }
 336
 337                         let mut has_mut_interior =
 338                             self.qualif_local::<qualifs::HasMutInterior>(place.local);
 339                         // HACK(eddyb) this should compute the same thing as
 340                         // `<HasMutInterior as Qualif>::in_projection` from
 341                         // `check_consts::qualifs` but without recursion.
 342                         if has_mut_interior {
 343                             // This allows borrowing fields which don't have
 344                             // `HasMutInterior`, from a type that does, e.g.:
 345                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 346                             let mut place_projection = &place.projection[..];
 347                             // FIXME(eddyb) use a forward loop instead of a reverse one.
 348                             while let &[ref proj_base @ .., elem] = place_projection {
 349                                 // FIXME(eddyb) this is probably excessive, with
 350                                 // the exception of `union` member accesses.
 351                                 let ty =
 352                                     Place::ty_from(place.local, proj_base, self.body, self.tcx)
 353                                         .projection_ty(self.tcx, elem)
 354                                         .ty;
 355                                 if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) {
 356                                     has_mut_interior = false;
 357                                     break;
 358                                 }
 359
 360                                 place_projection = proj_base;
 361                             }
 362                         }
 363
 364                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
 365                         if has_mut_interior {
 366                             return Err(Unpromotable);
 367                         }
 368                         if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
 369                             return Err(Unpromotable);
 370                         }
 371
 372                         if let BorrowKind::Mut { .. } = kind {
 373                             let ty = place.ty(self.body, self.tcx).ty;
 374
 375                             // In theory, any zero-sized value could be borrowed
 376                             // mutably without consequences. However, only &mut []
 377                             // is allowed right now.
 378                             if let ty::Array(_, len) = ty.kind() {
 379                                 match len.try_eval_usize(self.tcx, self.param_env) {
 380                                     Some(0) => {}
 381                                     _ => return Err(Unpromotable),
 382                                 }
 383                             } else {
 384                                 return Err(Unpromotable);
 385                             }
 386                         }
 387
 388                         Ok(())
 389                     }
 390                     _ => bug!(),
 391                 }
 392             }
 393             Candidate::Repeat(loc) => {
 394                 assert!(!self.explicit);
 395
 396                 let statement = &self.body[loc.block].statements[loc.statement_index];
 397                 match &statement.kind {
 398                     StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => {
 399                         if !self.tcx.features().const_in_array_repeat_expressions {
 400                             return Err(Unpromotable);
 401                         }
 402
 403                         self.validate_operand(operand)
 404                     }
 405                     _ => bug!(),
 406                 }
 407             }
 408             Candidate::Argument { bb, index } => {
 409                 assert!(self.explicit);
 410
 411                 let terminator = self.body[bb].terminator();
 412                 match &terminator.kind {
 413                     TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]),
 414                     _ => bug!(),
 415                 }
 416             }
 417             Candidate::InlineAsm { bb, index } => {
 418                 assert!(self.explicit);
 419
 420                 let terminator = self.body[bb].terminator();
 421                 match &terminator.kind {
 422                     TerminatorKind::InlineAsm { operands, .. } => match &operands[index] {
 423                         InlineAsmOperand::Const { value } => self.validate_operand(value),
 424                         _ => bug!(),
 425                     },
 426                     _ => bug!(),
 427                 }
 428             }
 429         }
 430     }
 431
 432     // FIXME(eddyb) maybe cache this?
 433     fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool {
 434         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 435             let num_stmts = self.body[loc.block].statements.len();
 436
 437             if loc.statement_index < num_stmts {
 438                 let statement = &self.body[loc.block].statements[loc.statement_index];
 439                 match &statement.kind {
 440                     StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>(
 441                         &self.ccx,
 442                         &mut |l| self.qualif_local::<Q>(l),
 443                         rhs,
 444                     ),
 445                     _ => {
 446                         span_bug!(
 447                             statement.source_info.span,
 448                             "{:?} is not an assignment",
 449                             statement
 450                         );
 451                     }
 452                 }
 453             } else {
 454                 let terminator = self.body[loc.block].terminator();
 455                 match &terminator.kind {
 456                     TerminatorKind::Call { .. } => {
 457                         let return_ty = self.body.local_decls[local].ty;
 458                         Q::in_any_value_of_ty(&self.ccx, return_ty)
 459                     }
 460                     kind => {
 461                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 462                     }
 463                 }
 464             }
 465         } else {
 466             let span = self.body.local_decls[local].source_info.span;
 467             span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local);
 468         }
 469     }
 470
 471     // FIXME(eddyb) maybe cache this?
 472     fn validate_local(&self, local: Local) -> Result<(), Unpromotable> {
 473         if let TempState::Defined { location: loc, .. } = self.temps[local] {
 474             let num_stmts = self.body[loc.block].statements.len();
 475
 476             if loc.statement_index < num_stmts {
 477                 let statement = &self.body[loc.block].statements[loc.statement_index];
 478                 match &statement.kind {
 479                     StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs),
 480                     _ => {
 481                         span_bug!(
 482                             statement.source_info.span,
 483                             "{:?} is not an assignment",
 484                             statement
 485                         );
 486                     }
 487                 }
 488             } else {
 489                 let terminator = self.body[loc.block].terminator();
 490                 match &terminator.kind {
 491                     TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
 492                     TerminatorKind::Yield { .. } => Err(Unpromotable),
 493                     kind => {
 494                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 495                     }
 496                 }
 497             }
 498         } else {
 499             Err(Unpromotable)
 500         }
 501     }
 502
 503     fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> {
 504         match place {
 505             PlaceRef { local, projection: [] } => self.validate_local(local),
 506             PlaceRef { local, projection: [proj_base @ .., elem] } => {
 507                 // Validate topmost projection, then recurse.
 508                 match *elem {
 509                     ProjectionElem::Deref => {
 510                         let mut promotable = false;
 511                         // This is a special treatment for cases like *&STATIC where STATIC is a
 512                         // global static variable.
 513                         // This pattern is generated only when global static variables are directly
 514                         // accessed and is qualified for promotion safely.
 515                         if let TempState::Defined { location, .. } = self.temps[local] {
 516                             let def_stmt =
 517                                 self.body[location.block].statements.get(location.statement_index);
 518                             if let Some(Statement {
 519                                 kind:
 520                                     StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(c)))),
 521                                 ..
 522                             }) = def_stmt
 523                             {
 524                                 if let Some(did) = c.check_static_ptr(self.tcx) {
 525                                     // Evaluating a promoted may not read statics except if it got
 526                                     // promoted from a static (this is a CTFE check). So we
 527                                     // can only promote static accesses inside statics.
 528                                     if let Some(hir::ConstContext::Static(..)) = self.const_kind {
 529                                         // The `is_empty` predicate is introduced to exclude the case
 530                                         // where the projection operations are [ .field, * ].
 531                                         // The reason is because promotion will be illegal if field
 532                                         // accesses precede the dereferencing.
 533                                         // Discussion can be found at
 534                                         // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
 535                                         // There may be opportunity for generalization, but this needs to be
 536                                         // accounted for.
 537                                         if proj_base.is_empty()
 538                                             && !self.tcx.is_thread_local_static(did)
 539                                         {
 540                                             promotable = true;
 541                                         }
 542                                     }
 543                                 }
 544                             }
 545                         }
 546                         if !promotable {
 547                             return Err(Unpromotable);
 548                         }
 549                     }
 550                     ProjectionElem::Downcast(..) => {
 551                         return Err(Unpromotable);
 552                     }
 553
 554                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
 555
 556                     ProjectionElem::Index(local) => {
 557                         self.validate_local(local)?;
 558                     }
 559
 560                     ProjectionElem::Field(..) => {
 561                         if self.maybe_runtime() {
 562                             let base_ty =
 563                                 Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 564                             if let Some(def) = base_ty.ty_adt_def() {
 565                                 // No promotion of union field accesses.
 566                                 if def.is_union() {
 567                                     return Err(Unpromotable);
 568                                 }
 569                             }
 570                         }
 571                     }
 572                 }
 573
 574                 self.validate_place(PlaceRef { local: place.local, projection: proj_base })
 575             }
 576         }
 577     }
 578
 579     fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> {
 580         match operand {
 581             Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),
 582
 583             // The qualifs for a constant (e.g. `HasMutInterior`) are checked in
 584             // `validate_rvalue` upon access.
 585             Operand::Constant(c) => {
 586                 if let Some(def_id) = c.check_static_ptr(self.tcx) {
 587                     // Only allow statics (not consts) to refer to other statics.
 588                     // FIXME(eddyb) does this matter at all for promotion?
 589                     // FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
 590                     // and in `const` this cannot occur anyway. The only concern is that we might
 591                     // promote even `let x = &STATIC` which would be useless, but this applies to
 592                     // promotion inside statics as well.
 593                     let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
 594                     if !is_static {
 595                         return Err(Unpromotable);
 596                     }
 597
 598                     let is_thread_local = self.tcx.is_thread_local_static(def_id);
 599                     if is_thread_local {
 600                         return Err(Unpromotable);
 601                     }
 602                 }
 603
 604                 Ok(())
 605             }
 606         }
 607     }
 608
 609     fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
 610         match *rvalue {
 611             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 612                 let operand_ty = operand.ty(self.body, self.tcx);
 613                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 614                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 615                 match (cast_in, cast_out) {
 616                     (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
 617                         // ptr-to-int casts are not possible in consts and thus not promotable
 618                         return Err(Unpromotable);
 619                     }
 620                     _ => {}
 621                 }
 622             }
 623
 624             Rvalue::BinaryOp(op, ref lhs, _) => {
 625                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind() {
 626                     assert!(
 627                         op == BinOp::Eq
 628                             || op == BinOp::Ne
 629                             || op == BinOp::Le
 630                             || op == BinOp::Lt
 631                             || op == BinOp::Ge
 632                             || op == BinOp::Gt
 633                             || op == BinOp::Offset
 634                     );
 635
 636                     // raw pointer operations are not allowed inside consts and thus not promotable
 637                     return Err(Unpromotable);
 638                 }
 639             }
 640
 641             Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
 642
 643             // FIXME(RalfJung): the rest is *implicitly considered promotable*... that seems dangerous.
 644             _ => {}
 645         }
 646
 647         match rvalue {
 648             Rvalue::ThreadLocalRef(_) => Err(Unpromotable),
 649
 650             Rvalue::NullaryOp(..) => Ok(()),
 651
 652             Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
 653
 654             Rvalue::Use(operand)
 655             | Rvalue::Repeat(operand, _)
 656             | Rvalue::UnaryOp(_, operand)
 657             | Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
 658
 659             Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 660                 self.validate_operand(lhs)?;
 661                 self.validate_operand(rhs)
 662             }
 663
 664             Rvalue::AddressOf(_, place) => {
 665                 // We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
 666                 // no problem, only using it is.
 667                 if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
 668                     let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 669                     if let ty::Ref(..) = base_ty.kind() {
 670                         return self.validate_place(PlaceRef {
 671                             local: place.local,
 672                             projection: proj_base,
 673                         });
 674                     }
 675                 }
 676                 Err(Unpromotable)
 677             }
 678
 679             Rvalue::Ref(_, kind, place) => {
 680                 if let BorrowKind::Mut { .. } = kind {
 681                     let ty = place.ty(self.body, self.tcx).ty;
 682
 683                     // In theory, any zero-sized value could be borrowed
 684                     // mutably without consequences. However, only &mut []
 685                     // is allowed right now.
 686                     if let ty::Array(_, len) = ty.kind() {
 687                         match len.try_eval_usize(self.tcx, self.param_env) {
 688                             Some(0) => {}
 689                             _ => return Err(Unpromotable),
 690                         }
 691                     } else {
 692                         return Err(Unpromotable);
 693                     }
 694                 }
 695
 696                 // Special-case reborrows to be more like a copy of the reference.
 697                 let mut place = place.as_ref();
 698                 if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
 699                     let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
 700                     if let ty::Ref(..) = base_ty.kind() {
 701                         place = PlaceRef { local: place.local, projection: proj_base };
 702                     }
 703                 }
 704
 705                 self.validate_place(place)?;
 706
 707                 // HACK(eddyb) this should compute the same thing as
 708                 // `<HasMutInterior as Qualif>::in_projection` from
 709                 // `check_consts::qualifs` but without recursion.
 710                 let mut has_mut_interior =
 711                     self.qualif_local::<qualifs::HasMutInterior>(place.local);
 712                 if has_mut_interior {
 713                     let mut place_projection = place.projection;
 714                     // FIXME(eddyb) use a forward loop instead of a reverse one.
 715                     while let &[ref proj_base @ .., elem] = place_projection {
 716                         // FIXME(eddyb) this is probably excessive, with
 717                         // the exception of `union` member accesses.
 718                         let ty = Place::ty_from(place.local, proj_base, self.body, self.tcx)
 719                             .projection_ty(self.tcx, elem)
 720                             .ty;
 721                         if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) {
 722                             has_mut_interior = false;
 723                             break;
 724                         }
 725
 726                         place_projection = proj_base;
 727                     }
 728                 }
 729                 if has_mut_interior {
 730                     return Err(Unpromotable);
 731                 }
 732
 733                 Ok(())
 734             }
 735
 736             Rvalue::Aggregate(_, ref operands) => {
 737                 for o in operands {
 738                     self.validate_operand(o)?;
 739                 }
 740
 741                 Ok(())
 742             }
 743         }
 744     }
 745
 746     fn validate_call(
 747         &self,
 748         callee: &Operand<'tcx>,
 749         args: &[Operand<'tcx>],
 750     ) -> Result<(), Unpromotable> {
 751         let fn_ty = callee.ty(self.body, self.tcx);
 752
 753         if !self.explicit && self.maybe_runtime() {
 754             if let ty::FnDef(def_id, _) = *fn_ty.kind() {
 755                 // Never promote runtime `const fn` calls of
 756                 // functions without `#[rustc_promotable]`.
 757                 if !self.tcx.is_promotable_const_fn(def_id) {
 758                     return Err(Unpromotable);
 759                 }
 760             }
 761         }
 762
 763         let is_const_fn = match *fn_ty.kind() {
 764             ty::FnDef(def_id, _) => {
 765                 is_const_fn(self.tcx, def_id)
 766                     || is_unstable_const_fn(self.tcx, def_id).is_some()
 767                     || is_lang_panic_fn(self.tcx, self.def_id.to_def_id())
 768             }
 769             _ => false,
 770         };
 771         if !is_const_fn {
 772             return Err(Unpromotable);
 773         }
 774
 775         self.validate_operand(callee)?;
 776         for arg in args {
 777             self.validate_operand(arg)?;
 778         }
 779
 780         Ok(())
 781     }
 782 }
 783
 784 // FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`.
 785 pub fn validate_candidates(
 786     ccx: &ConstCx<'_, '_>,
 787     temps: &IndexVec<Local, TempState>,
 788     candidates: &[Candidate],
 789 ) -> Vec<Candidate> {
 790     let mut validator = Validator { ccx, temps, explicit: false };
 791
 792     candidates
 793         .iter()
 794         .copied()
 795         .filter(|&candidate| {
 796             validator.explicit = candidate.forces_explicit_promotion();
 797
 798             // FIXME(eddyb) also emit the errors for shuffle indices
 799             // and `#[rustc_args_required_const]` arguments here.
 800
 801             let is_promotable = validator.validate_candidate(candidate).is_ok();
 802
 803             // If we use explicit validation, we carry the risk of turning a legitimate run-time
 804             // operation into a failing compile-time operation. Make sure that does not happen
 805             // by asserting that there is no possible run-time behavior here in case promotion
 806             // fails.
 807             if validator.explicit && !is_promotable {
 808                 ccx.tcx.sess.delay_span_bug(
 809                     ccx.body.span,
 810                     "Explicit promotion requested, but failed to promote",
 811                 );
 812             }
 813
 814             match candidate {
 815                 Candidate::Argument { bb, index } | Candidate::InlineAsm { bb, index }
 816                     if !is_promotable =>
 817                 {
 818                     let span = ccx.body[bb].terminator().source_info.span;
 819                     let msg = format!("argument {} is required to be a constant", index + 1);
 820                     ccx.tcx.sess.span_err(span, &msg);
 821                 }
 822                 _ => (),
 823             }
 824
 825             is_promotable
 826         })
 827         .collect()
 828 }
 829
 830 struct Promoter<'a, 'tcx> {
 831     tcx: TyCtxt<'tcx>,
 832     source: &'a mut Body<'tcx>,
 833     promoted: Body<'tcx>,
 834     temps: &'a mut IndexVec<Local, TempState>,
 835     extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>,
 836
 837     /// If true, all nested temps are also kept in the
 838     /// source MIR, not moved to the promoted MIR.
 839     keep_original: bool,
 840 }
 841
 842 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 843     fn new_block(&mut self) -> BasicBlock {
 844         let span = self.promoted.span;
 845         self.promoted.basic_blocks_mut().push(BasicBlockData {
 846             statements: vec![],
 847             terminator: Some(Terminator {
 848                 source_info: SourceInfo::outermost(span),
 849                 kind: TerminatorKind::Return,
 850             }),
 851             is_cleanup: false,
 852         })
 853     }
 854
 855     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 856         let last = self.promoted.basic_blocks().last().unwrap();
 857         let data = &mut self.promoted[last];
 858         data.statements.push(Statement {
 859             source_info: SourceInfo::outermost(span),
 860             kind: StatementKind::Assign(box (Place::from(dest), rvalue)),
 861         });
 862     }
 863
 864     fn is_temp_kind(&self, local: Local) -> bool {
 865         self.source.local_kind(local) == LocalKind::Temp
 866     }
 867
 868     /// Copies the initialization of this temp to the
 869     /// promoted MIR, recursing through temps.
 870     fn promote_temp(&mut self, temp: Local) -> Local {
 871         let old_keep_original = self.keep_original;
 872         let loc = match self.temps[temp] {
 873             TempState::Defined { location, uses } if uses > 0 => {
 874                 if uses > 1 {
 875                     self.keep_original = true;
 876                 }
 877                 location
 878             }
 879             state => {
 880                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state);
 881             }
 882         };
 883         if !self.keep_original {
 884             self.temps[temp] = TempState::PromotedOut;
 885         }
 886
 887         let num_stmts = self.source[loc.block].statements.len();
 888         let new_temp = self.promoted.local_decls.push(LocalDecl::new(
 889             self.source.local_decls[temp].ty,
 890             self.source.local_decls[temp].source_info.span,
 891         ));
 892
 893         debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original);
 894
 895         // First, take the Rvalue or Call out of the source MIR,
 896         // or duplicate it, depending on keep_original.
 897         if loc.statement_index < num_stmts {
 898             let (mut rvalue, source_info) = {
 899                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 900                 let rhs = match statement.kind {
 901                     StatementKind::Assign(box (_, ref mut rhs)) => rhs,
 902                     _ => {
 903                         span_bug!(
 904                             statement.source_info.span,
 905                             "{:?} is not an assignment",
 906                             statement
 907                         );
 908                     }
 909                 };
 910
 911                 (
 912                     if self.keep_original {
 913                         rhs.clone()
 914                     } else {
 915                         let unit = Rvalue::Use(Operand::Constant(box Constant {
 916                             span: statement.source_info.span,
 917                             user_ty: None,
 918                             literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit),
 919                         }));
 920                         mem::replace(rhs, unit)
 921                     },
 922                     statement.source_info,
 923                 )
 924             };
 925
 926             self.visit_rvalue(&mut rvalue, loc);
 927             self.assign(new_temp, rvalue, source_info.span);
 928         } else {
 929             let terminator = if self.keep_original {
 930                 self.source[loc.block].terminator().clone()
 931             } else {
 932                 let terminator = self.source[loc.block].terminator_mut();
 933                 let target = match terminator.kind {
 934                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 935                     ref kind => {
 936                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 937                     }
 938                 };
 939                 Terminator {
 940                     source_info: terminator.source_info,
 941                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }),
 942                 }
 943             };
 944
 945             match terminator.kind {
 946                 TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => {
 947                     self.visit_operand(&mut func, loc);
 948                     for arg in &mut args {
 949                         self.visit_operand(arg, loc);
 950                     }
 951
 952                     let last = self.promoted.basic_blocks().last().unwrap();
 953                     let new_target = self.new_block();
 954
 955                     *self.promoted[last].terminator_mut() = Terminator {
 956                         kind: TerminatorKind::Call {
 957                             func,
 958                             args,
 959                             cleanup: None,
 960                             destination: Some((Place::from(new_temp), new_target)),
 961                             from_hir_call,
 962                             fn_span,
 963                         },
 964                         ..terminator
 965                     };
 966                 }
 967                 ref kind => {
 968                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 969                 }
 970             };
 971         };
 972
 973         self.keep_original = old_keep_original;
 974         new_temp
 975     }
 976
 977     fn promote_candidate(
 978         mut self,
 979         def: ty::WithOptConstParam<DefId>,
 980         candidate: Candidate,
 981         next_promoted_id: usize,
 982     ) -> Option<Body<'tcx>> {
 983         let mut rvalue = {
 984             let promoted = &mut self.promoted;
 985             let promoted_id = Promoted::new(next_promoted_id);
 986             let tcx = self.tcx;
 987             let mut promoted_operand = |ty, span| {
 988                 promoted.span = span;
 989                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span);
 990
 991                 Operand::Constant(Box::new(Constant {
 992                     span,
 993                     user_ty: None,
 994                     literal: tcx.mk_const(ty::Const {
 995                         ty,
 996                         val: ty::ConstKind::Unevaluated(
 997                             def,
 998                             InternalSubsts::for_item(tcx, def.did, |param, _| {
 999                                 if let ty::GenericParamDefKind::Lifetime = param.kind {
1000                                     tcx.lifetimes.re_erased.into()
1001                                 } else {
1002                                     tcx.mk_param_from_def(param)
1003                                 }
1004                             }),
1005                             Some(promoted_id),
1006                         ),
1007                     }),
1008                 }))
1009             };
1010             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
1011             match candidate {
1012                 Candidate::Ref(loc) => {
1013                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
1014                     match statement.kind {
1015                         StatementKind::Assign(box (
1016                             _,
1017                             Rvalue::Ref(ref mut region, borrow_kind, ref mut place),
1018                         )) => {
1019                             // Use the underlying local for this (necessarily interior) borrow.
1020                             let ty = local_decls.local_decls()[place.local].ty;
1021                             let span = statement.source_info.span;
1022
1023                             let ref_ty = tcx.mk_ref(
1024                                 tcx.lifetimes.re_erased,
1025                                 ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() },
1026                             );
1027
1028                             *region = tcx.lifetimes.re_erased;
1029
1030                             let mut projection = vec![PlaceElem::Deref];
1031                             projection.extend(place.projection);
1032                             place.projection = tcx.intern_place_elems(&projection);
1033
1034                             // Create a temp to hold the promoted reference.
1035                             // This is because `*r` requires `r` to be a local,
1036                             // otherwise we would use the `promoted` directly.
1037                             let mut promoted_ref = LocalDecl::new(ref_ty, span);
1038                             promoted_ref.source_info = statement.source_info;
1039                             let promoted_ref = local_decls.push(promoted_ref);
1040                             assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref);
1041
1042                             let promoted_ref_statement = Statement {
1043                                 source_info: statement.source_info,
1044                                 kind: StatementKind::Assign(Box::new((
1045                                     Place::from(promoted_ref),
1046                                     Rvalue::Use(promoted_operand(ref_ty, span)),
1047                                 ))),
1048                             };
1049                             self.extra_statements.push((loc, promoted_ref_statement));
1050
1051                             Rvalue::Ref(
1052                                 tcx.lifetimes.re_erased,
1053                                 borrow_kind,
1054                                 Place {
1055                                     local: mem::replace(&mut place.local, promoted_ref),
1056                                     projection: List::empty(),
1057                                 },
1058                             )
1059                         }
1060                         _ => bug!(),
1061                     }
1062                 }
1063                 Candidate::Repeat(loc) => {
1064                     let statement = &mut blocks[loc.block].statements[loc.statement_index];
1065                     match statement.kind {
1066                         StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => {
1067                             let ty = operand.ty(local_decls, self.tcx);
1068                             let span = statement.source_info.span;
1069
1070                             Rvalue::Use(mem::replace(operand, promoted_operand(ty, span)))
1071                         }
1072                         _ => bug!(),
1073                     }
1074                 }
1075                 Candidate::Argument { bb, index } => {
1076                     let terminator = blocks[bb].terminator_mut();
1077                     match terminator.kind {
1078                         TerminatorKind::Call { ref mut args, .. } => {
1079                             let ty = args[index].ty(local_decls, self.tcx);
1080                             let span = terminator.source_info.span;
1081
1082                             Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span)))
1083                         }
1084                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
1085                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
1086                         // we are seeing a `Goto`. That means that the `promote_temps` method
1087                         // already promoted this call away entirely. This case occurs when calling
1088                         // a function requiring a constant argument and as that constant value
1089                         // providing a value whose computation contains another call to a function
1090                         // requiring a constant argument.
1091                         TerminatorKind::Goto { .. } => return None,
1092                         _ => bug!(),
1093                     }
1094                 }
1095                 Candidate::InlineAsm { bb, index } => {
1096                     let terminator = blocks[bb].terminator_mut();
1097                     match terminator.kind {
1098                         TerminatorKind::InlineAsm { ref mut operands, .. } => {
1099                             match &mut operands[index] {
1100                                 InlineAsmOperand::Const { ref mut value } => {
1101                                     let ty = value.ty(local_decls, self.tcx);
1102                                     let span = terminator.source_info.span;
1103
1104                                     Rvalue::Use(mem::replace(value, promoted_operand(ty, span)))
1105                                 }
1106                                 _ => bug!(),
1107                             }
1108                         }
1109
1110                         _ => bug!(),
1111                     }
1112                 }
1113             }
1114         };
1115
1116         assert_eq!(self.new_block(), START_BLOCK);
1117         self.visit_rvalue(
1118             &mut rvalue,
1119             Location { block: BasicBlock::new(0), statement_index: usize::MAX },
1120         );
1121
1122         let span = self.promoted.span;
1123         self.assign(RETURN_PLACE, rvalue, span);
1124         Some(self.promoted)
1125     }
1126 }
1127
1128 /// Replaces all temporaries with their promoted counterparts.
1129 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
1130     fn tcx(&self) -> TyCtxt<'tcx> {
1131         self.tcx
1132     }
1133
1134     fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
1135         if self.is_temp_kind(*local) {
1136             *local = self.promote_temp(*local);
1137         }
1138     }
1139 }
1140
1141 pub fn promote_candidates<'tcx>(
1142     def: ty::WithOptConstParam<DefId>,
1143     body: &mut Body<'tcx>,
1144     tcx: TyCtxt<'tcx>,
1145     mut temps: IndexVec<Local, TempState>,
1146     candidates: Vec<Candidate>,
1147 ) -> IndexVec<Promoted, Body<'tcx>> {
1148     // Visit candidates in reverse, in case they're nested.
1149     debug!("promote_candidates({:?})", candidates);
1150
1151     let mut promotions = IndexVec::new();
1152
1153     let mut extra_statements = vec![];
1154     for candidate in candidates.into_iter().rev() {
1155         match candidate {
1156             Candidate::Repeat(Location { block, statement_index })
1157             | Candidate::Ref(Location { block, statement_index }) => {
1158                 if let StatementKind::Assign(box (place, _)) =
1159                     &body[block].statements[statement_index].kind
1160                 {
1161                     if let Some(local) = place.as_local() {
1162                         if temps[local] == TempState::PromotedOut {
1163                             // Already promoted.
1164                             continue;
1165                         }
1166                     }
1167                 }
1168             }
1169             Candidate::Argument { .. } | Candidate::InlineAsm { .. } => {}
1170         }
1171
1172         // Declare return place local so that `mir::Body::new` doesn't complain.
1173         let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect();
1174
1175         let mut promoted = Body::new(
1176             IndexVec::new(),
1177             // FIXME: maybe try to filter this to avoid blowing up
1178             // memory usage?
1179             body.source_scopes.clone(),
1180             initial_locals,
1181             IndexVec::new(),
1182             0,
1183             vec![],
1184             body.span,
1185             body.generator_kind,
1186         );
1187         promoted.ignore_interior_mut_in_const_validation = true;
1188
1189         let promoter = Promoter {
1190             promoted,
1191             tcx,
1192             source: body,
1193             temps: &mut temps,
1194             extra_statements: &mut extra_statements,
1195             keep_original: false,
1196         };
1197
1198         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
1199         if let Some(promoted) = promoter.promote_candidate(def, candidate, promotions.len()) {
1200             promotions.push(promoted);
1201         }
1202     }
1203
1204     // Insert each of `extra_statements` before its indicated location, which
1205     // has to be done in reverse location order, to not invalidate the rest.
1206     extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc));
1207     for (loc, statement) in extra_statements {
1208         body[loc.block].statements.insert(loc.statement_index, statement);
1209     }
1210
1211     // Eliminate assignments to, and drops of promoted temps.
1212     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
1213     for block in body.basic_blocks_mut() {
1214         block.statements.retain(|statement| match &statement.kind {
1215             StatementKind::Assign(box (place, _)) => {
1216                 if let Some(index) = place.as_local() {
1217                     !promoted(index)
1218                 } else {
1219                     true
1220                 }
1221             }
1222             StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => {
1223                 !promoted(*index)
1224             }
1225             _ => true,
1226         });
1227         let terminator = block.terminator_mut();
1228         if let TerminatorKind::Drop { place, target, .. } = &terminator.kind {
1229             if let Some(index) = place.as_local() {
1230                 if promoted(index) {
1231                     terminator.kind = TerminatorKind::Goto { target: *target };
1232                 }
1233             }
1234         }
1235     }
1236
1237     promotions
1238 }
1239
1240 /// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should
1241 /// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path.
1242 /// Feature attribute should be suggested if `operand` can be promoted and the feature is not
1243 /// enabled.
1244 crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
1245     ccx: &ConstCx<'_, 'tcx>,
1246     operand: &Operand<'tcx>,
1247 ) -> bool {
1248     let mut rpo = traversal::reverse_postorder(&ccx.body);
1249     let (temps, _) = collect_temps_and_candidates(&ccx, &mut rpo);
1250     let validator = Validator { ccx, temps: &temps, explicit: false };
1251
1252     let should_promote = validator.validate_operand(operand).is_ok();
1253     let feature_flag = validator.ccx.tcx.features().const_in_array_repeat_expressions;
1254     debug!(
1255         "should_suggest_const_in_array_repeat_expressions_flag: def_id={:?} \
1256             should_promote={:?} feature_flag={:?}",
1257         validator.ccx.def_id, should_promote, feature_flag
1258     );
1259     should_promote && !feature_flag
1260 }