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

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