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