compiler/rustc_const_eval/src/transform/check_consts/check.rs

   1 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
   2
   3 use rustc_errors::{Applicability, Diagnostic, ErrorReported};
   4 use rustc_hir::def_id::DefId;
   5 use rustc_hir::{self as hir, HirId, LangItem};
   6 use rustc_index::bit_set::BitSet;
   7 use rustc_infer::infer::TyCtxtInferExt;
   8 use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
   9 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
  10 use rustc_middle::mir::*;
  11 use rustc_middle::ty::cast::CastTy;
  12 use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts};
  13 use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt};
  14 use rustc_middle::ty::{Binder, TraitPredicate, TraitRef};
  15 use rustc_mir_dataflow::impls::MaybeMutBorrowedLocals;
  16 use rustc_mir_dataflow::{self, Analysis};
  17 use rustc_span::{sym, Span, Symbol};
  18 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  19 use rustc_trait_selection::traits::{self, SelectionContext, TraitEngine};
  20
  21 use std::mem;
  22 use std::ops::Deref;
  23
  24 use super::ops::{self, NonConstOp, Status};
  25 use super::qualifs::{self, CustomEq, HasMutInterior, NeedsDrop, NeedsNonConstDrop};
  26 use super::resolver::FlowSensitiveAnalysis;
  27 use super::{is_lang_panic_fn, is_lang_special_const_fn, ConstCx, Qualif};
  28 use crate::const_eval::is_unstable_const_fn;
  29
  30 // We are using `MaybeMutBorrowedLocals` as a proxy for whether an item may have been mutated
  31 // through a pointer prior to the given point. This is okay even though `MaybeMutBorrowedLocals`
  32 // kills locals upon `StorageDead` because a local will never be used after a `StorageDead`.
  33 type IndirectlyMutableResults<'mir, 'tcx> =
  34     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, MaybeMutBorrowedLocals<'mir, 'tcx>>;
  35
  36 type QualifResults<'mir, 'tcx, Q> =
  37     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
  38
  39 #[derive(Default)]
  40 pub struct Qualifs<'mir, 'tcx> {
  41     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
  42     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
  43     needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
  44     indirectly_mutable: Option<IndirectlyMutableResults<'mir, 'tcx>>,
  45 }
  46
  47 impl Qualifs<'mir, 'tcx> {
  48     pub fn indirectly_mutable(
  49         &mut self,
  50         ccx: &'mir ConstCx<'mir, 'tcx>,
  51         local: Local,
  52         location: Location,
  53     ) -> bool {
  54         let indirectly_mutable = self.indirectly_mutable.get_or_insert_with(|| {
  55             let ConstCx { tcx, body, param_env, .. } = *ccx;
  56
  57             // We can use `unsound_ignore_borrow_on_drop` here because custom drop impls are not
  58             // allowed in a const.
  59             //
  60             // FIXME(ecstaticmorse): Someday we want to allow custom drop impls. How do we do this
  61             // without breaking stable code?
  62             MaybeMutBorrowedLocals::mut_borrows_only(tcx, &body, param_env)
  63                 .unsound_ignore_borrow_on_drop()
  64                 .into_engine(tcx, &body)
  65                 .pass_name("const_qualification")
  66                 .iterate_to_fixpoint()
  67                 .into_results_cursor(&body)
  68         });
  69
  70         indirectly_mutable.seek_before_primary_effect(location);
  71         indirectly_mutable.get().contains(local)
  72     }
  73
  74     /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
  75     ///
  76     /// Only updates the cursor if absolutely necessary
  77     pub fn needs_drop(
  78         &mut self,
  79         ccx: &'mir ConstCx<'mir, 'tcx>,
  80         local: Local,
  81         location: Location,
  82     ) -> bool {
  83         let ty = ccx.body.local_decls[local].ty;
  84         if !NeedsDrop::in_any_value_of_ty(ccx, ty) {
  85             return false;
  86         }
  87
  88         let needs_drop = self.needs_drop.get_or_insert_with(|| {
  89             let ConstCx { tcx, body, .. } = *ccx;
  90
  91             FlowSensitiveAnalysis::new(NeedsDrop, ccx)
  92                 .into_engine(tcx, &body)
  93                 .iterate_to_fixpoint()
  94                 .into_results_cursor(&body)
  95         });
  96
  97         needs_drop.seek_before_primary_effect(location);
  98         needs_drop.get().contains(local) || self.indirectly_mutable(ccx, local, location)
  99     }
 100
 101     /// Returns `true` if `local` is `NeedsNonConstDrop` at the given `Location`.
 102     ///
 103     /// Only updates the cursor if absolutely necessary
 104     pub fn needs_non_const_drop(
 105         &mut self,
 106         ccx: &'mir ConstCx<'mir, 'tcx>,
 107         local: Local,
 108         location: Location,
 109     ) -> bool {
 110         let ty = ccx.body.local_decls[local].ty;
 111         if !NeedsNonConstDrop::in_any_value_of_ty(ccx, ty) {
 112             return false;
 113         }
 114
 115         let needs_non_const_drop = self.needs_non_const_drop.get_or_insert_with(|| {
 116             let ConstCx { tcx, body, .. } = *ccx;
 117
 118             FlowSensitiveAnalysis::new(NeedsNonConstDrop, ccx)
 119                 .into_engine(tcx, &body)
 120                 .iterate_to_fixpoint()
 121                 .into_results_cursor(&body)
 122         });
 123
 124         needs_non_const_drop.seek_before_primary_effect(location);
 125         needs_non_const_drop.get().contains(local) || self.indirectly_mutable(ccx, local, location)
 126     }
 127
 128     /// Returns `true` if `local` is `HasMutInterior` at the given `Location`.
 129     ///
 130     /// Only updates the cursor if absolutely necessary.
 131     pub fn has_mut_interior(
 132         &mut self,
 133         ccx: &'mir ConstCx<'mir, 'tcx>,
 134         local: Local,
 135         location: Location,
 136     ) -> bool {
 137         let ty = ccx.body.local_decls[local].ty;
 138         if !HasMutInterior::in_any_value_of_ty(ccx, ty) {
 139             return false;
 140         }
 141
 142         let has_mut_interior = self.has_mut_interior.get_or_insert_with(|| {
 143             let ConstCx { tcx, body, .. } = *ccx;
 144
 145             FlowSensitiveAnalysis::new(HasMutInterior, ccx)
 146                 .into_engine(tcx, &body)
 147                 .iterate_to_fixpoint()
 148                 .into_results_cursor(&body)
 149         });
 150
 151         has_mut_interior.seek_before_primary_effect(location);
 152         has_mut_interior.get().contains(local) || self.indirectly_mutable(ccx, local, location)
 153     }
 154
 155     fn in_return_place(
 156         &mut self,
 157         ccx: &'mir ConstCx<'mir, 'tcx>,
 158         error_occured: Option<ErrorReported>,
 159     ) -> ConstQualifs {
 160         // Find the `Return` terminator if one exists.
 161         //
 162         // If no `Return` terminator exists, this MIR is divergent. Just return the conservative
 163         // qualifs for the return type.
 164         let return_block = ccx
 165             .body
 166             .basic_blocks()
 167             .iter_enumerated()
 168             .find(|(_, block)| match block.terminator().kind {
 169                 TerminatorKind::Return => true,
 170                 _ => false,
 171             })
 172             .map(|(bb, _)| bb);
 173
 174         let return_block = match return_block {
 175             None => return qualifs::in_any_value_of_ty(ccx, ccx.body.return_ty(), error_occured),
 176             Some(bb) => bb,
 177         };
 178
 179         let return_loc = ccx.body.terminator_loc(return_block);
 180
 181         let custom_eq = match ccx.const_kind() {
 182             // We don't care whether a `const fn` returns a value that is not structurally
 183             // matchable. Functions calls are opaque and always use type-based qualification, so
 184             // this value should never be used.
 185             hir::ConstContext::ConstFn => true,
 186
 187             // If we know that all values of the return type are structurally matchable, there's no
 188             // need to run dataflow.
 189             _ if !CustomEq::in_any_value_of_ty(ccx, ccx.body.return_ty()) => false,
 190
 191             hir::ConstContext::Const | hir::ConstContext::Static(_) => {
 192                 let mut cursor = FlowSensitiveAnalysis::new(CustomEq, ccx)
 193                     .into_engine(ccx.tcx, &ccx.body)
 194                     .iterate_to_fixpoint()
 195                     .into_results_cursor(&ccx.body);
 196
 197                 cursor.seek_after_primary_effect(return_loc);
 198                 cursor.contains(RETURN_PLACE)
 199             }
 200         };
 201
 202         ConstQualifs {
 203             needs_drop: self.needs_drop(ccx, RETURN_PLACE, return_loc),
 204             needs_non_const_drop: self.needs_non_const_drop(ccx, RETURN_PLACE, return_loc),
 205             has_mut_interior: self.has_mut_interior(ccx, RETURN_PLACE, return_loc),
 206             custom_eq,
 207             error_occured,
 208         }
 209     }
 210 }
 211
 212 pub struct Checker<'mir, 'tcx> {
 213     ccx: &'mir ConstCx<'mir, 'tcx>,
 214     qualifs: Qualifs<'mir, 'tcx>,
 215
 216     /// The span of the current statement.
 217     span: Span,
 218
 219     /// A set that stores for each local whether it has a `StorageDead` for it somewhere.
 220     local_has_storage_dead: Option<BitSet<Local>>,
 221
 222     error_emitted: Option<ErrorReported>,
 223     secondary_errors: Vec<Diagnostic>,
 224 }
 225
 226 impl Deref for Checker<'mir, 'tcx> {
 227     type Target = ConstCx<'mir, 'tcx>;
 228
 229     fn deref(&self) -> &Self::Target {
 230         &self.ccx
 231     }
 232 }
 233
 234 impl Checker<'mir, 'tcx> {
 235     pub fn new(ccx: &'mir ConstCx<'mir, 'tcx>) -> Self {
 236         Checker {
 237             span: ccx.body.span,
 238             ccx,
 239             qualifs: Default::default(),
 240             local_has_storage_dead: None,
 241             error_emitted: None,
 242             secondary_errors: Vec::new(),
 243         }
 244     }
 245
 246     pub fn check_body(&mut self) {
 247         let ConstCx { tcx, body, .. } = *self.ccx;
 248         let def_id = self.ccx.def_id();
 249
 250         // `async` functions cannot be `const fn`. This is checked during AST lowering, so there's
 251         // no need to emit duplicate errors here.
 252         if is_async_fn(self.ccx) || body.generator.is_some() {
 253             tcx.sess.delay_span_bug(body.span, "`async` functions cannot be `const fn`");
 254             return;
 255         }
 256
 257         // The local type and predicate checks are not free and only relevant for `const fn`s.
 258         if self.const_kind() == hir::ConstContext::ConstFn {
 259             // Prevent const trait methods from being annotated as `stable`.
 260             // FIXME: Do this as part of stability checking.
 261             if self.is_const_stable_const_fn() {
 262                 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 263                 if crate::const_eval::is_parent_const_impl_raw(tcx, hir_id) {
 264                     self.ccx
 265                         .tcx
 266                         .sess
 267                         .struct_span_err(self.span, "trait methods cannot be stable const fn")
 268                         .emit();
 269                 }
 270             }
 271
 272             self.check_item_predicates();
 273
 274             for (idx, local) in body.local_decls.iter_enumerated() {
 275                 // Handle the return place below.
 276                 if idx == RETURN_PLACE || local.internal {
 277                     continue;
 278                 }
 279
 280                 self.span = local.source_info.span;
 281                 self.check_local_or_return_ty(local.ty, idx);
 282             }
 283
 284             // impl trait is gone in MIR, so check the return type of a const fn by its signature
 285             // instead of the type of the return place.
 286             self.span = body.local_decls[RETURN_PLACE].source_info.span;
 287             let return_ty = tcx.fn_sig(def_id).output();
 288             self.check_local_or_return_ty(return_ty.skip_binder(), RETURN_PLACE);
 289         }
 290
 291         if !tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
 292             self.visit_body(&body);
 293         }
 294
 295         // Ensure that the end result is `Sync` in a non-thread local `static`.
 296         let should_check_for_sync = self.const_kind()
 297             == hir::ConstContext::Static(hir::Mutability::Not)
 298             && !tcx.is_thread_local_static(def_id.to_def_id());
 299
 300         if should_check_for_sync {
 301             let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 302             check_return_ty_is_sync(tcx, &body, hir_id);
 303         }
 304
 305         // If we got through const-checking without emitting any "primary" errors, emit any
 306         // "secondary" errors if they occurred.
 307         let secondary_errors = mem::take(&mut self.secondary_errors);
 308         if self.error_emitted.is_none() {
 309             for error in secondary_errors {
 310                 self.tcx.sess.diagnostic().emit_diagnostic(&error);
 311             }
 312         } else {
 313             assert!(self.tcx.sess.has_errors());
 314         }
 315     }
 316
 317     fn local_has_storage_dead(&mut self, local: Local) -> bool {
 318         let ccx = self.ccx;
 319         self.local_has_storage_dead
 320             .get_or_insert_with(|| {
 321                 struct StorageDeads {
 322                     locals: BitSet<Local>,
 323                 }
 324                 impl Visitor<'tcx> for StorageDeads {
 325                     fn visit_statement(&mut self, stmt: &Statement<'tcx>, _: Location) {
 326                         if let StatementKind::StorageDead(l) = stmt.kind {
 327                             self.locals.insert(l);
 328                         }
 329                     }
 330                 }
 331                 let mut v = StorageDeads { locals: BitSet::new_empty(ccx.body.local_decls.len()) };
 332                 v.visit_body(ccx.body);
 333                 v.locals
 334             })
 335             .contains(local)
 336     }
 337
 338     pub fn qualifs_in_return_place(&mut self) -> ConstQualifs {
 339         self.qualifs.in_return_place(self.ccx, self.error_emitted)
 340     }
 341
 342     /// Emits an error if an expression cannot be evaluated in the current context.
 343     pub fn check_op(&mut self, op: impl NonConstOp) {
 344         self.check_op_spanned(op, self.span);
 345     }
 346
 347     /// Emits an error at the given `span` if an expression cannot be evaluated in the current
 348     /// context.
 349     pub fn check_op_spanned<O: NonConstOp>(&mut self, op: O, span: Span) {
 350         let gate = match op.status_in_item(self.ccx) {
 351             Status::Allowed => return,
 352
 353             Status::Unstable(gate) if self.tcx.features().enabled(gate) => {
 354                 let unstable_in_stable = self.ccx.is_const_stable_const_fn()
 355                     && !super::rustc_allow_const_fn_unstable(
 356                         self.tcx,
 357                         self.def_id().to_def_id(),
 358                         gate,
 359                     );
 360                 if unstable_in_stable {
 361                     emit_unstable_in_stable_error(self.ccx, span, gate);
 362                 }
 363
 364                 return;
 365             }
 366
 367             Status::Unstable(gate) => Some(gate),
 368             Status::Forbidden => None,
 369         };
 370
 371         if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
 372             self.tcx.sess.miri_unleashed_feature(span, gate);
 373             return;
 374         }
 375
 376         let mut err = op.build_error(self.ccx, span);
 377         assert!(err.is_error());
 378
 379         match op.importance() {
 380             ops::DiagnosticImportance::Primary => {
 381                 self.error_emitted = Some(ErrorReported);
 382                 err.emit();
 383             }
 384
 385             ops::DiagnosticImportance::Secondary => err.buffer(&mut self.secondary_errors),
 386         }
 387     }
 388
 389     fn check_static(&mut self, def_id: DefId, span: Span) {
 390         if self.tcx.is_thread_local_static(def_id) {
 391             self.tcx.sess.delay_span_bug(span, "tls access is checked in `Rvalue::ThreadLocalRef");
 392         }
 393         self.check_op_spanned(ops::StaticAccess, span)
 394     }
 395
 396     fn check_local_or_return_ty(&mut self, ty: Ty<'tcx>, local: Local) {
 397         let kind = self.body.local_kind(local);
 398
 399         for ty in ty.walk(self.tcx) {
 400             let ty = match ty.unpack() {
 401                 GenericArgKind::Type(ty) => ty,
 402
 403                 // No constraints on lifetimes or constants, except potentially
 404                 // constants' types, but `walk` will get to them as well.
 405                 GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
 406             };
 407
 408             match *ty.kind() {
 409                 ty::Ref(_, _, hir::Mutability::Mut) => self.check_op(ops::ty::MutRef(kind)),
 410                 ty::Opaque(..) => self.check_op(ops::ty::ImplTrait),
 411                 ty::FnPtr(..) => self.check_op(ops::ty::FnPtr(kind)),
 412
 413                 ty::Dynamic(preds, _) => {
 414                     for pred in preds.iter() {
 415                         match pred.skip_binder() {
 416                             ty::ExistentialPredicate::AutoTrait(_)
 417                             | ty::ExistentialPredicate::Projection(_) => {
 418                                 self.check_op(ops::ty::DynTrait(kind))
 419                             }
 420                             ty::ExistentialPredicate::Trait(trait_ref) => {
 421                                 if Some(trait_ref.def_id) != self.tcx.lang_items().sized_trait() {
 422                                     self.check_op(ops::ty::DynTrait(kind))
 423                                 }
 424                             }
 425                         }
 426                     }
 427                 }
 428                 _ => {}
 429             }
 430         }
 431     }
 432
 433     fn check_item_predicates(&mut self) {
 434         let ConstCx { tcx, .. } = *self.ccx;
 435
 436         let mut current = self.def_id().to_def_id();
 437         loop {
 438             let predicates = tcx.predicates_of(current);
 439             for (predicate, _) in predicates.predicates {
 440                 match predicate.kind().skip_binder() {
 441                     ty::PredicateKind::RegionOutlives(_)
 442                     | ty::PredicateKind::TypeOutlives(_)
 443                     | ty::PredicateKind::WellFormed(_)
 444                     | ty::PredicateKind::Projection(_)
 445                     | ty::PredicateKind::ConstEvaluatable(..)
 446                     | ty::PredicateKind::ConstEquate(..)
 447                     | ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
 448                     ty::PredicateKind::ObjectSafe(_) => {
 449                         bug!("object safe predicate on function: {:#?}", predicate)
 450                     }
 451                     ty::PredicateKind::ClosureKind(..) => {
 452                         bug!("closure kind predicate on function: {:#?}", predicate)
 453                     }
 454                     ty::PredicateKind::Subtype(_) | ty::PredicateKind::Coerce(_) => {
 455                         bug!("subtype/coerce predicate on function: {:#?}", predicate)
 456                     }
 457                     ty::PredicateKind::Trait(pred) => {
 458                         if Some(pred.def_id()) == tcx.lang_items().sized_trait() {
 459                             continue;
 460                         }
 461                         match pred.self_ty().kind() {
 462                             ty::Param(p) => {
 463                                 let generics = tcx.generics_of(current);
 464                                 let def = generics.type_param(p, tcx);
 465                                 let span = tcx.def_span(def.def_id);
 466
 467                                 // These are part of the function signature, so treat them like
 468                                 // arguments when determining importance.
 469                                 let kind = LocalKind::Arg;
 470
 471                                 self.check_op_spanned(ops::ty::TraitBound(kind), span);
 472                             }
 473                             // other kinds of bounds are either tautologies
 474                             // or cause errors in other passes
 475                             _ => continue,
 476                         }
 477                     }
 478                 }
 479             }
 480             match predicates.parent {
 481                 Some(parent) => current = parent,
 482                 None => break,
 483             }
 484         }
 485     }
 486
 487     fn check_mut_borrow(&mut self, local: Local, kind: hir::BorrowKind) {
 488         match self.const_kind() {
 489             // In a const fn all borrows are transient or point to the places given via
 490             // references in the arguments (so we already checked them with
 491             // TransientMutBorrow/MutBorrow as appropriate).
 492             // The borrow checker guarantees that no new non-transient borrows are created.
 493             // NOTE: Once we have heap allocations during CTFE we need to figure out
 494             // how to prevent `const fn` to create long-lived allocations that point
 495             // to mutable memory.
 496             hir::ConstContext::ConstFn => self.check_op(ops::TransientMutBorrow(kind)),
 497             _ => {
 498                 // Locals with StorageDead do not live beyond the evaluation and can
 499                 // thus safely be borrowed without being able to be leaked to the final
 500                 // value of the constant.
 501                 if self.local_has_storage_dead(local) {
 502                     self.check_op(ops::TransientMutBorrow(kind));
 503                 } else {
 504                     self.check_op(ops::MutBorrow(kind));
 505                 }
 506             }
 507         }
 508     }
 509 }
 510
 511 impl Visitor<'tcx> for Checker<'mir, 'tcx> {
 512     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
 513         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
 514
 515         // We don't const-check basic blocks on the cleanup path since we never unwind during
 516         // const-eval: a panic causes an immediate compile error. In other words, cleanup blocks
 517         // are unreachable during const-eval.
 518         //
 519         // We can't be more conservative (e.g., by const-checking cleanup blocks anyways) because
 520         // locals that would never be dropped during normal execution are sometimes dropped during
 521         // unwinding, which means backwards-incompatible live-drop errors.
 522         if block.is_cleanup {
 523             return;
 524         }
 525
 526         self.super_basic_block_data(bb, block);
 527     }
 528
 529     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 530         trace!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);
 531
 532         // Special-case reborrows to be more like a copy of a reference.
 533         match *rvalue {
 534             Rvalue::Ref(_, kind, place) => {
 535                 if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
 536                     let ctx = match kind {
 537                         BorrowKind::Shared => {
 538                             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
 539                         }
 540                         BorrowKind::Shallow => {
 541                             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
 542                         }
 543                         BorrowKind::Unique => {
 544                             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
 545                         }
 546                         BorrowKind::Mut { .. } => {
 547                             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 548                         }
 549                     };
 550                     self.visit_local(&reborrowed_place_ref.local, ctx, location);
 551                     self.visit_projection(reborrowed_place_ref, ctx, location);
 552                     return;
 553                 }
 554             }
 555             Rvalue::AddressOf(mutbl, place) => {
 556                 if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
 557                     let ctx = match mutbl {
 558                         Mutability::Not => {
 559                             PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
 560                         }
 561                         Mutability::Mut => PlaceContext::MutatingUse(MutatingUseContext::AddressOf),
 562                     };
 563                     self.visit_local(&reborrowed_place_ref.local, ctx, location);
 564                     self.visit_projection(reborrowed_place_ref, ctx, location);
 565                     return;
 566                 }
 567             }
 568             _ => {}
 569         }
 570
 571         self.super_rvalue(rvalue, location);
 572
 573         match *rvalue {
 574             Rvalue::ThreadLocalRef(_) => self.check_op(ops::ThreadLocalAccess),
 575
 576             Rvalue::Use(_)
 577             | Rvalue::Repeat(..)
 578             | Rvalue::Discriminant(..)
 579             | Rvalue::Len(_)
 580             | Rvalue::Aggregate(..) => {}
 581
 582             Rvalue::Ref(_, kind @ BorrowKind::Mut { .. }, ref place)
 583             | Rvalue::Ref(_, kind @ BorrowKind::Unique, ref place) => {
 584                 let ty = place.ty(self.body, self.tcx).ty;
 585                 let is_allowed = match ty.kind() {
 586                     // Inside a `static mut`, `&mut [...]` is allowed.
 587                     ty::Array(..) | ty::Slice(_)
 588                         if self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut) =>
 589                     {
 590                         true
 591                     }
 592
 593                     // FIXME(ecstaticmorse): We could allow `&mut []` inside a const context given
 594                     // that this is merely a ZST and it is already eligible for promotion.
 595                     // This may require an RFC?
 596                     /*
 597                     ty::Array(_, len) if len.try_eval_usize(cx.tcx, cx.param_env) == Some(0)
 598                         => true,
 599                     */
 600                     _ => false,
 601                 };
 602
 603                 if !is_allowed {
 604                     if let BorrowKind::Mut { .. } = kind {
 605                         self.check_mut_borrow(place.local, hir::BorrowKind::Ref)
 606                     } else {
 607                         self.check_op(ops::CellBorrow);
 608                     }
 609                 }
 610             }
 611
 612             Rvalue::AddressOf(Mutability::Mut, ref place) => {
 613                 self.check_mut_borrow(place.local, hir::BorrowKind::Raw)
 614             }
 615
 616             Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Shallow, ref place)
 617             | Rvalue::AddressOf(Mutability::Not, ref place) => {
 618                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
 619                     &self.ccx,
 620                     &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
 621                     place.as_ref(),
 622                 );
 623
 624                 if borrowed_place_has_mut_interior {
 625                     match self.const_kind() {
 626                         // In a const fn all borrows are transient or point to the places given via
 627                         // references in the arguments (so we already checked them with
 628                         // TransientCellBorrow/CellBorrow as appropriate).
 629                         // The borrow checker guarantees that no new non-transient borrows are created.
 630                         // NOTE: Once we have heap allocations during CTFE we need to figure out
 631                         // how to prevent `const fn` to create long-lived allocations that point
 632                         // to (interior) mutable memory.
 633                         hir::ConstContext::ConstFn => self.check_op(ops::TransientCellBorrow),
 634                         _ => {
 635                             // Locals with StorageDead are definitely not part of the final constant value, and
 636                             // it is thus inherently safe to permit such locals to have their
 637                             // address taken as we can't end up with a reference to them in the
 638                             // final value.
 639                             // Note: This is only sound if every local that has a `StorageDead` has a
 640                             // `StorageDead` in every control flow path leading to a `return` terminator.
 641                             if self.local_has_storage_dead(place.local) {
 642                                 self.check_op(ops::TransientCellBorrow);
 643                             } else {
 644                                 self.check_op(ops::CellBorrow);
 645                             }
 646                         }
 647                     }
 648                 }
 649             }
 650
 651             Rvalue::Cast(
 652                 CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
 653                 _,
 654                 _,
 655             ) => {}
 656
 657             Rvalue::Cast(
 658                 CastKind::Pointer(
 659                     PointerCast::UnsafeFnPointer
 660                     | PointerCast::ClosureFnPointer(_)
 661                     | PointerCast::ReifyFnPointer,
 662                 ),
 663                 _,
 664                 _,
 665             ) => self.check_op(ops::FnPtrCast),
 666
 667             Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, _) => {
 668                 // Nothing to check here (`check_local_or_return_ty` ensures no trait objects occur
 669                 // in the type of any local, which also excludes casts).
 670             }
 671
 672             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 673                 let operand_ty = operand.ty(self.body, self.tcx);
 674                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 675                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 676
 677                 if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 678                     self.check_op(ops::RawPtrToIntCast);
 679                 }
 680             }
 681
 682             Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) => {}
 683             Rvalue::NullaryOp(NullOp::Box, _) => self.check_op(ops::HeapAllocation),
 684             Rvalue::ShallowInitBox(_, _) => {}
 685
 686             Rvalue::UnaryOp(_, ref operand) => {
 687                 let ty = operand.ty(self.body, self.tcx);
 688                 if is_int_bool_or_char(ty) {
 689                     // Int, bool, and char operations are fine.
 690                 } else if ty.is_floating_point() {
 691                     self.check_op(ops::FloatingPointOp);
 692                 } else {
 693                     span_bug!(self.span, "non-primitive type in `Rvalue::UnaryOp`: {:?}", ty);
 694                 }
 695             }
 696
 697             Rvalue::BinaryOp(op, box (ref lhs, ref rhs))
 698             | Rvalue::CheckedBinaryOp(op, box (ref lhs, ref rhs)) => {
 699                 let lhs_ty = lhs.ty(self.body, self.tcx);
 700                 let rhs_ty = rhs.ty(self.body, self.tcx);
 701
 702                 if is_int_bool_or_char(lhs_ty) && is_int_bool_or_char(rhs_ty) {
 703                     // Int, bool, and char operations are fine.
 704                 } else if lhs_ty.is_fn_ptr() || lhs_ty.is_unsafe_ptr() {
 705                     assert_eq!(lhs_ty, rhs_ty);
 706                     assert!(
 707                         op == BinOp::Eq
 708                             || op == BinOp::Ne
 709                             || op == BinOp::Le
 710                             || op == BinOp::Lt
 711                             || op == BinOp::Ge
 712                             || op == BinOp::Gt
 713                             || op == BinOp::Offset
 714                     );
 715
 716                     self.check_op(ops::RawPtrComparison);
 717                 } else if lhs_ty.is_floating_point() || rhs_ty.is_floating_point() {
 718                     self.check_op(ops::FloatingPointOp);
 719                 } else {
 720                     span_bug!(
 721                         self.span,
 722                         "non-primitive type in `Rvalue::BinaryOp`: {:?} ⚬ {:?}",
 723                         lhs_ty,
 724                         rhs_ty
 725                     );
 726                 }
 727             }
 728         }
 729     }
 730
 731     fn visit_operand(&mut self, op: &Operand<'tcx>, location: Location) {
 732         self.super_operand(op, location);
 733         if let Operand::Constant(c) = op {
 734             if let Some(def_id) = c.check_static_ptr(self.tcx) {
 735                 self.check_static(def_id, self.span);
 736             }
 737         }
 738     }
 739     fn visit_projection_elem(
 740         &mut self,
 741         place_local: Local,
 742         proj_base: &[PlaceElem<'tcx>],
 743         elem: PlaceElem<'tcx>,
 744         context: PlaceContext,
 745         location: Location,
 746     ) {
 747         trace!(
 748             "visit_projection_elem: place_local={:?} proj_base={:?} elem={:?} \
 749             context={:?} location={:?}",
 750             place_local,
 751             proj_base,
 752             elem,
 753             context,
 754             location,
 755         );
 756
 757         self.super_projection_elem(place_local, proj_base, elem, context, location);
 758
 759         match elem {
 760             ProjectionElem::Deref => {
 761                 let base_ty = Place::ty_from(place_local, proj_base, self.body, self.tcx).ty;
 762                 if let ty::RawPtr(_) = base_ty.kind() {
 763                     if proj_base.is_empty() {
 764                         let decl = &self.body.local_decls[place_local];
 765                         if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
 766                             let span = decl.source_info.span;
 767                             self.check_static(def_id, span);
 768                             return;
 769                         }
 770                     }
 771                     self.check_op(ops::RawPtrDeref);
 772                 }
 773
 774                 if context.is_mutating_use() {
 775                     self.check_op(ops::MutDeref);
 776                 }
 777             }
 778
 779             ProjectionElem::ConstantIndex { .. }
 780             | ProjectionElem::Downcast(..)
 781             | ProjectionElem::Subslice { .. }
 782             | ProjectionElem::Field(..)
 783             | ProjectionElem::Index(_) => {}
 784         }
 785     }
 786
 787     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 788         trace!("visit_source_info: source_info={:?}", source_info);
 789         self.span = source_info.span;
 790     }
 791
 792     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 793         trace!("visit_statement: statement={:?} location={:?}", statement, location);
 794
 795         self.super_statement(statement, location);
 796
 797         match statement.kind {
 798             StatementKind::LlvmInlineAsm { .. } => {
 799                 self.check_op(ops::InlineAsm);
 800             }
 801
 802             StatementKind::Assign(..)
 803             | StatementKind::SetDiscriminant { .. }
 804             | StatementKind::FakeRead(..)
 805             | StatementKind::StorageLive(_)
 806             | StatementKind::StorageDead(_)
 807             | StatementKind::Retag { .. }
 808             | StatementKind::AscribeUserType(..)
 809             | StatementKind::Coverage(..)
 810             | StatementKind::CopyNonOverlapping(..)
 811             | StatementKind::Nop => {}
 812         }
 813     }
 814
 815     #[instrument(level = "debug", skip(self))]
 816     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 817         use rustc_target::spec::abi::Abi::RustIntrinsic;
 818
 819         self.super_terminator(terminator, location);
 820
 821         match &terminator.kind {
 822             TerminatorKind::Call { func, args, .. } => {
 823                 let ConstCx { tcx, body, param_env, .. } = *self.ccx;
 824                 let caller = self.def_id().to_def_id();
 825
 826                 let fn_ty = func.ty(body, tcx);
 827
 828                 let (mut callee, mut substs) = match *fn_ty.kind() {
 829                     ty::FnDef(def_id, substs) => (def_id, substs),
 830
 831                     ty::FnPtr(_) => {
 832                         self.check_op(ops::FnCallIndirect);
 833                         return;
 834                     }
 835                     _ => {
 836                         span_bug!(terminator.source_info.span, "invalid callee of type {:?}", fn_ty)
 837                     }
 838                 };
 839
 840                 let mut nonconst_call_permission = false;
 841
 842                 // Attempting to call a trait method?
 843                 if let Some(trait_id) = tcx.trait_of_item(callee) {
 844                     trace!("attempting to call a trait method");
 845                     if !self.tcx.features().const_trait_impl {
 846                         self.check_op(ops::FnCallNonConst);
 847                         return;
 848                     }
 849
 850                     let trait_ref = TraitRef::from_method(tcx, trait_id, substs);
 851                     let obligation = Obligation::new(
 852                         ObligationCause::dummy(),
 853                         param_env,
 854                         Binder::dummy(TraitPredicate {
 855                             trait_ref,
 856                             constness: ty::BoundConstness::ConstIfConst,
 857                         }),
 858                     );
 859
 860                     let implsrc = tcx.infer_ctxt().enter(|infcx| {
 861                         let mut selcx =
 862                             SelectionContext::with_constness(&infcx, hir::Constness::Const);
 863                         selcx.select(&obligation)
 864                     });
 865
 866                     match implsrc {
 867                         Ok(Some(ImplSource::Param(_, ty::BoundConstness::ConstIfConst))) => {
 868                             debug!(
 869                                 "const_trait_impl: provided {:?} via where-clause in {:?}",
 870                                 trait_ref, param_env
 871                             );
 872                             return;
 873                         }
 874                         Ok(Some(ImplSource::UserDefined(data))) => {
 875                             let callee_name = tcx.item_name(callee);
 876                             if let Some(&did) = tcx
 877                                 .associated_item_def_ids(data.impl_def_id)
 878                                 .iter()
 879                                 .find(|did| tcx.item_name(**did) == callee_name)
 880                             {
 881                                 // using internal substs is ok here, since this is only
 882                                 // used for the `resolve` call below
 883                                 substs = InternalSubsts::identity_for_item(tcx, did);
 884                                 callee = did;
 885                             }
 886                         }
 887                         _ if !tcx.is_const_fn_raw(callee) => {
 888                             // At this point, it is only legal when the caller is marked with
 889                             // #[default_method_body_is_const], and the callee is in the same
 890                             // trait.
 891                             let callee_trait = tcx.trait_of_item(callee);
 892                             if callee_trait.is_some() {
 893                                 if tcx.has_attr(caller, sym::default_method_body_is_const) {
 894                                     if tcx.trait_of_item(caller) == callee_trait {
 895                                         nonconst_call_permission = true;
 896                                     }
 897                                 }
 898                             }
 899
 900                             if !nonconst_call_permission {
 901                                 self.check_op(ops::FnCallNonConst);
 902                                 return;
 903                             }
 904                         }
 905                         _ => {}
 906                     }
 907
 908                     // Resolve a trait method call to its concrete implementation, which may be in a
 909                     // `const` trait impl.
 910                     let instance = Instance::resolve(tcx, param_env, callee, substs);
 911                     debug!("Resolving ({:?}) -> {:?}", callee, instance);
 912                     if let Ok(Some(func)) = instance {
 913                         if let InstanceDef::Item(def) = func.def {
 914                             callee = def.did;
 915                         }
 916                     }
 917                 }
 918
 919                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
 920                 if is_lang_special_const_fn(tcx, callee) {
 921                     // `begin_panic` and `panic_display` are generic functions that accept
 922                     // types other than str. Check to enforce that only str can be used in
 923                     // const-eval.
 924
 925                     // const-eval of the `begin_panic` fn assumes the argument is `&str`
 926                     if Some(callee) == tcx.lang_items().begin_panic_fn() {
 927                         match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
 928                             ty::Ref(_, ty, _) if ty.is_str() => (),
 929                             _ => self.check_op(ops::PanicNonStr),
 930                         }
 931                     }
 932
 933                     // const-eval of the `panic_display` fn assumes the argument is `&&str`
 934                     if Some(callee) == tcx.lang_items().panic_display() {
 935                         match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
 936                             ty::Ref(_, ty, _) if matches!(ty.kind(), ty::Ref(_, ty, _) if ty.is_str()) =>
 937                                 {}
 938                             _ => self.check_op(ops::PanicNonStr),
 939                         }
 940                     }
 941
 942                     if is_lang_panic_fn(tcx, callee) {
 943                         // run stability check on non-panic special const fns.
 944                         return;
 945                     }
 946                 }
 947
 948                 if Some(callee) == tcx.lang_items().exchange_malloc_fn() {
 949                     self.check_op(ops::HeapAllocation);
 950                     return;
 951                 }
 952
 953                 // `async` blocks get lowered to `std::future::from_generator(/* a closure */)`.
 954                 let is_async_block = Some(callee) == tcx.lang_items().from_generator_fn();
 955                 if is_async_block {
 956                     let kind = hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block);
 957                     self.check_op(ops::Generator(kind));
 958                     return;
 959                 }
 960
 961                 let is_intrinsic = tcx.fn_sig(callee).abi() == RustIntrinsic;
 962
 963                 if !tcx.is_const_fn_raw(callee) {
 964                     if tcx.trait_of_item(callee).is_some() {
 965                         if tcx.has_attr(callee, sym::default_method_body_is_const) {
 966                             // To get to here we must have already found a const impl for the
 967                             // trait, but for it to still be non-const can be that the impl is
 968                             // using default method bodies.
 969                             nonconst_call_permission = true;
 970                         }
 971                     }
 972
 973                     if !nonconst_call_permission {
 974                         self.check_op(ops::FnCallNonConst);
 975                         return;
 976                     }
 977                 }
 978
 979                 // If the `const fn` we are trying to call is not const-stable, ensure that we have
 980                 // the proper feature gate enabled.
 981                 if let Some(gate) = is_unstable_const_fn(tcx, callee) {
 982                     trace!(?gate, "calling unstable const fn");
 983                     if self.span.allows_unstable(gate) {
 984                         return;
 985                     }
 986
 987                     // Calling an unstable function *always* requires that the corresponding gate
 988                     // be enabled, even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
 989                     if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == gate) {
 990                         self.check_op(ops::FnCallUnstable(callee, Some(gate)));
 991                         return;
 992                     }
 993
 994                     // If this crate is not using stability attributes, or the caller is not claiming to be a
 995                     // stable `const fn`, that is all that is required.
 996                     if !self.ccx.is_const_stable_const_fn() {
 997                         trace!("crate not using stability attributes or caller not stably const");
 998                         return;
 999                     }
1000
1001                     // Otherwise, we are something const-stable calling a const-unstable fn.
1002
1003                     if super::rustc_allow_const_fn_unstable(tcx, caller, gate) {
1004                         trace!("rustc_allow_const_fn_unstable gate active");
1005                         return;
1006                     }
1007
1008                     self.check_op(ops::FnCallUnstable(callee, Some(gate)));
1009                     return;
1010                 }
1011
1012                 // FIXME(ecstaticmorse); For compatibility, we consider `unstable` callees that
1013                 // have no `rustc_const_stable` attributes to be const-unstable as well. This
1014                 // should be fixed later.
1015                 let callee_is_unstable_unmarked = tcx.lookup_const_stability(callee).is_none()
1016                     && tcx.lookup_stability(callee).map_or(false, |s| s.level.is_unstable());
1017                 if callee_is_unstable_unmarked {
1018                     trace!("callee_is_unstable_unmarked");
1019                     // We do not use `const` modifiers for intrinsic "functions", as intrinsics are
1020                     // `extern` funtions, and these have no way to get marked `const`. So instead we
1021                     // use `rustc_const_(un)stable` attributes to mean that the intrinsic is `const`
1022                     if self.ccx.is_const_stable_const_fn() || is_intrinsic {
1023                         self.check_op(ops::FnCallUnstable(callee, None));
1024                         return;
1025                     }
1026                 }
1027                 trace!("permitting call");
1028             }
1029
1030             // Forbid all `Drop` terminators unless the place being dropped is a local with no
1031             // projections that cannot be `NeedsNonConstDrop`.
1032             TerminatorKind::Drop { place: dropped_place, .. }
1033             | TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
1034                 // If we are checking live drops after drop-elaboration, don't emit duplicate
1035                 // errors here.
1036                 if super::post_drop_elaboration::checking_enabled(self.ccx) {
1037                     return;
1038                 }
1039
1040                 let mut err_span = self.span;
1041
1042                 let ty_needs_non_const_drop = qualifs::NeedsNonConstDrop::in_any_value_of_ty(
1043                     self.ccx,
1044                     dropped_place.ty(self.body, self.tcx).ty,
1045                 );
1046
1047                 if !ty_needs_non_const_drop {
1048                     return;
1049                 }
1050
1051                 let needs_non_const_drop = if let Some(local) = dropped_place.as_local() {
1052                     // Use the span where the local was declared as the span of the drop error.
1053                     err_span = self.body.local_decls[local].source_info.span;
1054                     self.qualifs.needs_non_const_drop(self.ccx, local, location)
1055                 } else {
1056                     true
1057                 };
1058
1059                 if needs_non_const_drop {
1060                     self.check_op_spanned(
1061                         ops::LiveDrop { dropped_at: Some(terminator.source_info.span) },
1062                         err_span,
1063                     );
1064                 }
1065             }
1066
1067             TerminatorKind::InlineAsm { .. } => self.check_op(ops::InlineAsm),
1068
1069             TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
1070                 self.check_op(ops::Generator(hir::GeneratorKind::Gen))
1071             }
1072
1073             TerminatorKind::Abort => {
1074                 // Cleanup blocks are skipped for const checking (see `visit_basic_block_data`).
1075                 span_bug!(self.span, "`Abort` terminator outside of cleanup block")
1076             }
1077
1078             TerminatorKind::Assert { .. }
1079             | TerminatorKind::FalseEdge { .. }
1080             | TerminatorKind::FalseUnwind { .. }
1081             | TerminatorKind::Goto { .. }
1082             | TerminatorKind::Resume
1083             | TerminatorKind::Return
1084             | TerminatorKind::SwitchInt { .. }
1085             | TerminatorKind::Unreachable => {}
1086         }
1087     }
1088 }
1089
1090 fn check_return_ty_is_sync(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, hir_id: HirId) {
1091     let ty = body.return_ty();
1092     tcx.infer_ctxt().enter(|infcx| {
1093         let cause = traits::ObligationCause::new(body.span, hir_id, traits::SharedStatic);
1094         let mut fulfillment_cx = traits::FulfillmentContext::new();
1095         let sync_def_id = tcx.require_lang_item(LangItem::Sync, Some(body.span));
1096         fulfillment_cx.register_bound(&infcx, ty::ParamEnv::empty(), ty, sync_def_id, cause);
1097         if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
1098             infcx.report_fulfillment_errors(&err, None, false);
1099         }
1100     });
1101 }
1102
1103 fn place_as_reborrow(
1104     tcx: TyCtxt<'tcx>,
1105     body: &Body<'tcx>,
1106     place: Place<'tcx>,
1107 ) -> Option<PlaceRef<'tcx>> {
1108     match place.as_ref().last_projection() {
1109         Some((place_base, ProjectionElem::Deref)) => {
1110             // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
1111             // that points to the allocation for the static. Don't treat these as reborrows.
1112             if body.local_decls[place_base.local].is_ref_to_static() {
1113                 None
1114             } else {
1115                 // Ensure the type being derefed is a reference and not a raw pointer.
1116                 // This is sufficient to prevent an access to a `static mut` from being marked as a
1117                 // reborrow, even if the check above were to disappear.
1118                 let inner_ty = place_base.ty(body, tcx).ty;
1119
1120                 if let ty::Ref(..) = inner_ty.kind() {
1121                     return Some(place_base);
1122                 } else {
1123                     return None;
1124                 }
1125             }
1126         }
1127         _ => None,
1128     }
1129 }
1130
1131 fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
1132     ty.is_bool() || ty.is_integral() || ty.is_char()
1133 }
1134
1135 fn is_async_fn(ccx: &ConstCx<'_, '_>) -> bool {
1136     ccx.fn_sig().map_or(false, |sig| sig.header.asyncness == hir::IsAsync::Async)
1137 }
1138
1139 fn emit_unstable_in_stable_error(ccx: &ConstCx<'_, '_>, span: Span, gate: Symbol) {
1140     let attr_span = ccx.fn_sig().map_or(ccx.body.span, |sig| sig.span.shrink_to_lo());
1141
1142     ccx.tcx
1143         .sess
1144         .struct_span_err(
1145             span,
1146             &format!("const-stable function cannot use `#[feature({})]`", gate.as_str()),
1147         )
1148         .span_suggestion(
1149             attr_span,
1150             "if it is not part of the public API, make this function unstably const",
1151             concat!(r#"#[rustc_const_unstable(feature = "...", issue = "...")]"#, '\n').to_owned(),
1152             Applicability::HasPlaceholders,
1153         )
1154         .span_suggestion(
1155             attr_span,
1156             "otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks",
1157             format!("#[rustc_allow_const_fn_unstable({})]\n", gate),
1158             Applicability::MaybeIncorrect,
1159         )
1160         .emit();
1161 }