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