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::error_reporting::InferCtxtExt;
  18 use rustc_trait_selection::traits::SelectionContext;
  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<'mir, 'tcx> 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         tainted_by_errors: 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)| matches!(block.terminator().kind, TerminatorKind::Return))
 135             .map(|(bb, _)| bb);
 136
 137         let Some(return_block) = return_block else {
 138             return qualifs::in_any_value_of_ty(ccx, ccx.body.return_ty(), tainted_by_errors);
 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             tainted_by_errors,
 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<'tcx>) {
 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<'tcx>>(&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, fn_span, from_hir_call, .. } => {
 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 {
 799                             caller,
 800                             callee,
 801                             substs,
 802                             span: *fn_span,
 803                             from_hir_call: *from_hir_call,
 804                         });
 805                         return;
 806                     }
 807
 808                     let trait_ref = TraitRef::from_method(tcx, trait_id, substs);
 809                     let poly_trait_pred = Binder::dummy(TraitPredicate {
 810                         trait_ref,
 811                         constness: ty::BoundConstness::ConstIfConst,
 812                         polarity: ty::ImplPolarity::Positive,
 813                     });
 814                     let obligation =
 815                         Obligation::new(ObligationCause::dummy(), param_env, poly_trait_pred);
 816
 817                     let implsrc = tcx.infer_ctxt().enter(|infcx| {
 818                         let mut selcx = SelectionContext::new(&infcx);
 819                         selcx.select(&obligation)
 820                     });
 821
 822                     match implsrc {
 823                         Ok(Some(ImplSource::Param(_, ty::BoundConstness::ConstIfConst))) => {
 824                             debug!(
 825                                 "const_trait_impl: provided {:?} via where-clause in {:?}",
 826                                 trait_ref, param_env
 827                             );
 828                             return;
 829                         }
 830                         Ok(Some(ImplSource::UserDefined(data))) => {
 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 let hir::Constness::NotConst = tcx.impl_constness(data.impl_def_id) {
 844                                 self.check_op(ops::FnCallNonConst {
 845                                     caller,
 846                                     callee,
 847                                     substs,
 848                                     span: *fn_span,
 849                                     from_hir_call: *from_hir_call,
 850                                 });
 851                                 return;
 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                                 && tcx.has_attr(caller, sym::default_method_body_is_const)
 861                                 && callee_trait == tcx.trait_of_item(caller)
 862                                 // Can only call methods when it's `<Self as TheTrait>::f`.
 863                                 && tcx.types.self_param == substs.type_at(0)
 864                             {
 865                                 nonconst_call_permission = true;
 866                             }
 867
 868                             if !nonconst_call_permission {
 869                                 let obligation = Obligation::new(
 870                                     ObligationCause::dummy_with_span(*fn_span),
 871                                     param_env,
 872                                     tcx.mk_predicate(
 873                                         poly_trait_pred.map_bound(ty::PredicateKind::Trait),
 874                                     ),
 875                                 );
 876
 877                                 // improve diagnostics by showing what failed. Our requirements are stricter this time
 878                                 // as we are going to error again anyways.
 879                                 tcx.infer_ctxt().enter(|infcx| {
 880                                     if let Err(e) = implsrc {
 881                                         infcx.report_selection_error(
 882                                             obligation.clone(),
 883                                             &obligation,
 884                                             &e,
 885                                             false,
 886                                         );
 887                                     }
 888                                 });
 889
 890                                 self.check_op(ops::FnCallNonConst {
 891                                     caller,
 892                                     callee,
 893                                     substs,
 894                                     span: *fn_span,
 895                                     from_hir_call: *from_hir_call,
 896                                 });
 897                                 return;
 898                             }
 899                         }
 900                         _ => {}
 901                     }
 902
 903                     // Resolve a trait method call to its concrete implementation, which may be in a
 904                     // `const` trait impl.
 905                     let instance = Instance::resolve(tcx, param_env, callee, substs);
 906                     debug!("Resolving ({:?}) -> {:?}", callee, instance);
 907                     if let Ok(Some(func)) = instance {
 908                         if let InstanceDef::Item(def) = func.def {
 909                             callee = def.did;
 910                         }
 911                     }
 912                 }
 913
 914                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
 915
 916                 // `begin_panic` and `panic_display` are generic functions that accept
 917                 // types other than str. Check to enforce that only str can be used in
 918                 // const-eval.
 919
 920                 // const-eval of the `begin_panic` fn assumes the argument is `&str`
 921                 if Some(callee) == tcx.lang_items().begin_panic_fn() {
 922                     match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
 923                         ty::Ref(_, ty, _) if ty.is_str() => return,
 924                         _ => self.check_op(ops::PanicNonStr),
 925                     }
 926                 }
 927
 928                 // const-eval of the `panic_display` fn assumes the argument is `&&str`
 929                 if Some(callee) == tcx.lang_items().panic_display() {
 930                     match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
 931                         ty::Ref(_, ty, _) if matches!(ty.kind(), ty::Ref(_, ty, _) if ty.is_str()) =>
 932                         {
 933                             return;
 934                         }
 935                         _ => self.check_op(ops::PanicNonStr),
 936                     }
 937                 }
 938
 939                 if Some(callee) == tcx.lang_items().exchange_malloc_fn() {
 940                     self.check_op(ops::HeapAllocation);
 941                     return;
 942                 }
 943
 944                 // `async` blocks get lowered to `std::future::from_generator(/* a closure */)`.
 945                 let is_async_block = Some(callee) == tcx.lang_items().from_generator_fn();
 946                 if is_async_block {
 947                     let kind = hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block);
 948                     self.check_op(ops::Generator(kind));
 949                     return;
 950                 }
 951
 952                 let is_intrinsic = tcx.fn_sig(callee).abi() == RustIntrinsic;
 953
 954                 if !tcx.is_const_fn_raw(callee) {
 955                     if tcx.trait_of_item(callee).is_some() {
 956                         if tcx.has_attr(callee, sym::default_method_body_is_const) {
 957                             // To get to here we must have already found a const impl for the
 958                             // trait, but for it to still be non-const can be that the impl is
 959                             // using default method bodies.
 960                             nonconst_call_permission = true;
 961                         }
 962                     }
 963
 964                     if !nonconst_call_permission {
 965                         self.check_op(ops::FnCallNonConst {
 966                             caller,
 967                             callee,
 968                             substs,
 969                             span: *fn_span,
 970                             from_hir_call: *from_hir_call,
 971                         });
 972                         return;
 973                     }
 974                 }
 975
 976                 // If the `const fn` we are trying to call is not const-stable, ensure that we have
 977                 // the proper feature gate enabled.
 978                 if let Some(gate) = is_unstable_const_fn(tcx, callee) {
 979                     trace!(?gate, "calling unstable const fn");
 980                     if self.span.allows_unstable(gate) {
 981                         return;
 982                     }
 983
 984                     // Calling an unstable function *always* requires that the corresponding gate
 985                     // be enabled, even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
 986                     if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == gate) {
 987                         self.check_op(ops::FnCallUnstable(callee, Some(gate)));
 988                         return;
 989                     }
 990
 991                     // If this crate is not using stability attributes, or the caller is not claiming to be a
 992                     // stable `const fn`, that is all that is required.
 993                     if !self.ccx.is_const_stable_const_fn() {
 994                         trace!("crate not using stability attributes or caller not stably const");
 995                         return;
 996                     }
 997
 998                     // Otherwise, we are something const-stable calling a const-unstable fn.
 999
1000                     if super::rustc_allow_const_fn_unstable(tcx, caller, gate) {
1001                         trace!("rustc_allow_const_fn_unstable gate active");
1002                         return;
1003                     }
1004
1005                     self.check_op(ops::FnCallUnstable(callee, Some(gate)));
1006                     return;
1007                 }
1008
1009                 // FIXME(ecstaticmorse); For compatibility, we consider `unstable` callees that
1010                 // have no `rustc_const_stable` attributes to be const-unstable as well. This
1011                 // should be fixed later.
1012                 let callee_is_unstable_unmarked = tcx.lookup_const_stability(callee).is_none()
1013                     && tcx.lookup_stability(callee).map_or(false, |s| s.level.is_unstable());
1014                 if callee_is_unstable_unmarked {
1015                     trace!("callee_is_unstable_unmarked");
1016                     // We do not use `const` modifiers for intrinsic "functions", as intrinsics are
1017                     // `extern` funtions, and these have no way to get marked `const`. So instead we
1018                     // use `rustc_const_(un)stable` attributes to mean that the intrinsic is `const`
1019                     if self.ccx.is_const_stable_const_fn() || is_intrinsic {
1020                         self.check_op(ops::FnCallUnstable(callee, None));
1021                         return;
1022                     }
1023                 }
1024                 trace!("permitting call");
1025             }
1026
1027             // Forbid all `Drop` terminators unless the place being dropped is a local with no
1028             // projections that cannot be `NeedsNonConstDrop`.
1029             TerminatorKind::Drop { place: dropped_place, .. }
1030             | TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
1031                 // If we are checking live drops after drop-elaboration, don't emit duplicate
1032                 // errors here.
1033                 if super::post_drop_elaboration::checking_enabled(self.ccx) {
1034                     return;
1035                 }
1036
1037                 let mut err_span = self.span;
1038                 let ty_of_dropped_place = dropped_place.ty(self.body, self.tcx).ty;
1039
1040                 let ty_needs_non_const_drop =
1041                     qualifs::NeedsNonConstDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place);
1042
1043                 debug!(?ty_of_dropped_place, ?ty_needs_non_const_drop);
1044
1045                 if !ty_needs_non_const_drop {
1046                     return;
1047                 }
1048
1049                 let needs_non_const_drop = if let Some(local) = dropped_place.as_local() {
1050                     // Use the span where the local was declared as the span of the drop error.
1051                     err_span = self.body.local_decls[local].source_info.span;
1052                     self.qualifs.needs_non_const_drop(self.ccx, local, location)
1053                 } else {
1054                     true
1055                 };
1056
1057                 if needs_non_const_drop {
1058                     self.check_op_spanned(
1059                         ops::LiveDrop { dropped_at: Some(terminator.source_info.span) },
1060                         err_span,
1061                     );
1062                 }
1063             }
1064
1065             TerminatorKind::InlineAsm { .. } => self.check_op(ops::InlineAsm),
1066
1067             TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
1068                 self.check_op(ops::Generator(hir::GeneratorKind::Gen))
1069             }
1070
1071             TerminatorKind::Abort => {
1072                 // Cleanup blocks are skipped for const checking (see `visit_basic_block_data`).
1073                 span_bug!(self.span, "`Abort` terminator outside of cleanup block")
1074             }
1075
1076             TerminatorKind::Assert { .. }
1077             | TerminatorKind::FalseEdge { .. }
1078             | TerminatorKind::FalseUnwind { .. }
1079             | TerminatorKind::Goto { .. }
1080             | TerminatorKind::Resume
1081             | TerminatorKind::Return
1082             | TerminatorKind::SwitchInt { .. }
1083             | TerminatorKind::Unreachable => {}
1084         }
1085     }
1086 }
1087
1088 fn place_as_reborrow<'tcx>(
1089     tcx: TyCtxt<'tcx>,
1090     body: &Body<'tcx>,
1091     place: Place<'tcx>,
1092 ) -> Option<PlaceRef<'tcx>> {
1093     match place.as_ref().last_projection() {
1094         Some((place_base, ProjectionElem::Deref)) => {
1095             // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
1096             // that points to the allocation for the static. Don't treat these as reborrows.
1097             if body.local_decls[place_base.local].is_ref_to_static() {
1098                 None
1099             } else {
1100                 // Ensure the type being derefed is a reference and not a raw pointer.
1101                 // This is sufficient to prevent an access to a `static mut` from being marked as a
1102                 // reborrow, even if the check above were to disappear.
1103                 let inner_ty = place_base.ty(body, tcx).ty;
1104
1105                 if let ty::Ref(..) = inner_ty.kind() {
1106                     return Some(place_base);
1107                 } else {
1108                     return None;
1109                 }
1110             }
1111         }
1112         _ => None,
1113     }
1114 }
1115
1116 fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
1117     ty.is_bool() || ty.is_integral() || ty.is_char()
1118 }
1119
1120 fn is_async_fn(ccx: &ConstCx<'_, '_>) -> bool {
1121     ccx.fn_sig().map_or(false, |sig| sig.header.asyncness == hir::IsAsync::Async)
1122 }
1123
1124 fn emit_unstable_in_stable_error(ccx: &ConstCx<'_, '_>, span: Span, gate: Symbol) {
1125     let attr_span = ccx.fn_sig().map_or(ccx.body.span, |sig| sig.span.shrink_to_lo());
1126
1127     ccx.tcx
1128         .sess
1129         .struct_span_err(
1130             span,
1131             &format!("const-stable function cannot use `#[feature({})]`", gate.as_str()),
1132         )
1133         .span_suggestion(
1134             attr_span,
1135             "if it is not part of the public API, make this function unstably const",
1136             concat!(r#"#[rustc_const_unstable(feature = "...", issue = "...")]"#, '\n').to_owned(),
1137             Applicability::HasPlaceholders,
1138         )
1139         .span_suggestion(
1140             attr_span,
1141             "otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks",
1142             format!("#[rustc_allow_const_fn_unstable({})]\n", gate),
1143             Applicability::MaybeIncorrect,
1144         )
1145         .emit();
1146 }