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