src/librustc_mir/transform/check_consts/validation.rs

   1 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
   2
   3 use rustc::middle::lang_items;
   4 use rustc::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
   5 use rustc::mir::*;
   6 use rustc::traits::{self, TraitEngine};
   7 use rustc::ty::cast::CastTy;
   8 use rustc::ty::{self, TyCtxt};
   9 use rustc_error_codes::*;
  10 use rustc_hir::{def_id::DefId, HirId};
  11 use rustc_index::bit_set::BitSet;
  12 use rustc_span::symbol::sym;
  13 use rustc_span::Span;
  14
  15 use std::borrow::Cow;
  16 use std::ops::Deref;
  17
  18 use self::old_dataflow::IndirectlyMutableLocals;
  19 use super::ops::{self, NonConstOp};
  20 use super::qualifs::{self, HasMutInterior, NeedsDrop};
  21 use super::resolver::FlowSensitiveAnalysis;
  22 use super::{is_lang_panic_fn, ConstKind, Item, Qualif};
  23 use crate::dataflow::{self as old_dataflow, generic as dataflow};
  24
  25 pub type IndirectlyMutableResults<'mir, 'tcx> =
  26     old_dataflow::DataflowResultsCursor<'mir, 'tcx, IndirectlyMutableLocals<'mir, 'tcx>>;
  27
  28 struct QualifCursor<'a, 'mir, 'tcx, Q: Qualif> {
  29     cursor: dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'a, 'mir, 'tcx, Q>>,
  30     in_any_value_of_ty: BitSet<Local>,
  31 }
  32
  33 impl<Q: Qualif> QualifCursor<'a, 'mir, 'tcx, Q> {
  34     pub fn new(q: Q, item: &'a Item<'mir, 'tcx>, dead_unwinds: &BitSet<BasicBlock>) -> Self {
  35         let analysis = FlowSensitiveAnalysis::new(q, item);
  36         let results =
  37             dataflow::Engine::new(item.tcx, &item.body, item.def_id, dead_unwinds, analysis)
  38                 .iterate_to_fixpoint();
  39         let cursor = dataflow::ResultsCursor::new(*item.body, results);
  40
  41         let mut in_any_value_of_ty = BitSet::new_empty(item.body.local_decls.len());
  42         for (local, decl) in item.body.local_decls.iter_enumerated() {
  43             if Q::in_any_value_of_ty(item, decl.ty) {
  44                 in_any_value_of_ty.insert(local);
  45             }
  46         }
  47
  48         QualifCursor { cursor, in_any_value_of_ty }
  49     }
  50 }
  51
  52 pub struct Qualifs<'a, 'mir, 'tcx> {
  53     has_mut_interior: QualifCursor<'a, 'mir, 'tcx, HasMutInterior>,
  54     needs_drop: QualifCursor<'a, 'mir, 'tcx, NeedsDrop>,
  55     indirectly_mutable: IndirectlyMutableResults<'mir, 'tcx>,
  56 }
  57
  58 impl Qualifs<'a, 'mir, 'tcx> {
  59     fn indirectly_mutable(&mut self, local: Local, location: Location) -> bool {
  60         self.indirectly_mutable.seek(location);
  61         self.indirectly_mutable.get().contains(local)
  62     }
  63
  64     /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
  65     ///
  66     /// Only updates the cursor if absolutely necessary
  67     fn needs_drop_lazy_seek(&mut self, local: Local, location: Location) -> bool {
  68         if !self.needs_drop.in_any_value_of_ty.contains(local) {
  69             return false;
  70         }
  71
  72         self.needs_drop.cursor.seek_before(location);
  73         self.needs_drop.cursor.get().contains(local) || self.indirectly_mutable(local, location)
  74     }
  75
  76     /// Returns `true` if `local` is `HasMutInterior` at the given `Location`.
  77     ///
  78     /// Only updates the cursor if absolutely necessary.
  79     fn has_mut_interior_lazy_seek(&mut self, local: Local, location: Location) -> bool {
  80         if !self.has_mut_interior.in_any_value_of_ty.contains(local) {
  81             return false;
  82         }
  83
  84         self.has_mut_interior.cursor.seek_before(location);
  85         self.has_mut_interior.cursor.get().contains(local)
  86             || self.indirectly_mutable(local, location)
  87     }
  88
  89     /// Returns `true` if `local` is `HasMutInterior`, but requires the `has_mut_interior` and
  90     /// `indirectly_mutable` cursors to be updated beforehand.
  91     fn has_mut_interior_eager_seek(&self, local: Local) -> bool {
  92         if !self.has_mut_interior.in_any_value_of_ty.contains(local) {
  93             return false;
  94         }
  95
  96         self.has_mut_interior.cursor.get().contains(local)
  97             || self.indirectly_mutable.get().contains(local)
  98     }
  99
 100     fn in_return_place(&mut self, item: &Item<'_, 'tcx>) -> ConstQualifs {
 101         // Find the `Return` terminator if one exists.
 102         //
 103         // If no `Return` terminator exists, this MIR is divergent. Just return the conservative
 104         // qualifs for the return type.
 105         let return_block = item
 106             .body
 107             .basic_blocks()
 108             .iter_enumerated()
 109             .find(|(_, block)| match block.terminator().kind {
 110                 TerminatorKind::Return => true,
 111                 _ => false,
 112             })
 113             .map(|(bb, _)| bb);
 114
 115         let return_block = match return_block {
 116             None => return qualifs::in_any_value_of_ty(item, item.body.return_ty()),
 117             Some(bb) => bb,
 118         };
 119
 120         let return_loc = item.body.terminator_loc(return_block);
 121
 122         ConstQualifs {
 123             needs_drop: self.needs_drop_lazy_seek(RETURN_PLACE, return_loc),
 124             has_mut_interior: self.has_mut_interior_lazy_seek(RETURN_PLACE, return_loc),
 125         }
 126     }
 127 }
 128
 129 pub struct Validator<'a, 'mir, 'tcx> {
 130     item: &'a Item<'mir, 'tcx>,
 131     qualifs: Qualifs<'a, 'mir, 'tcx>,
 132
 133     /// The span of the current statement.
 134     span: Span,
 135 }
 136
 137 impl Deref for Validator<'_, 'mir, 'tcx> {
 138     type Target = Item<'mir, 'tcx>;
 139
 140     fn deref(&self) -> &Self::Target {
 141         &self.item
 142     }
 143 }
 144
 145 impl Validator<'a, 'mir, 'tcx> {
 146     pub fn new(item: &'a Item<'mir, 'tcx>) -> Self {
 147         let dead_unwinds = BitSet::new_empty(item.body.basic_blocks().len());
 148
 149         let needs_drop = QualifCursor::new(NeedsDrop, item, &dead_unwinds);
 150
 151         let has_mut_interior = QualifCursor::new(HasMutInterior, item, &dead_unwinds);
 152
 153         let indirectly_mutable = old_dataflow::do_dataflow(
 154             item.tcx,
 155             &*item.body,
 156             item.def_id,
 157             &item.tcx.get_attrs(item.def_id),
 158             &dead_unwinds,
 159             old_dataflow::IndirectlyMutableLocals::new(item.tcx, *item.body, item.param_env),
 160             |_, local| old_dataflow::DebugFormatted::new(&local),
 161         );
 162
 163         let indirectly_mutable =
 164             old_dataflow::DataflowResultsCursor::new(indirectly_mutable, *item.body);
 165
 166         let qualifs = Qualifs { needs_drop, has_mut_interior, indirectly_mutable };
 167
 168         Validator { span: item.body.span, item, qualifs }
 169     }
 170
 171     pub fn check_body(&mut self) {
 172         let Item { tcx, body, def_id, const_kind, .. } = *self.item;
 173
 174         let use_min_const_fn_checks = (const_kind == Some(ConstKind::ConstFn)
 175             && tcx.is_min_const_fn(def_id))
 176             && !tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you;
 177
 178         if use_min_const_fn_checks {
 179             // Enforce `min_const_fn` for stable `const fn`s.
 180             use crate::transform::qualify_min_const_fn::is_min_const_fn;
 181             if let Err((span, err)) = is_min_const_fn(tcx, def_id, &body) {
 182                 error_min_const_fn_violation(tcx, span, err);
 183                 return;
 184             }
 185         }
 186
 187         check_short_circuiting_in_const_local(self.item);
 188
 189         if body.is_cfg_cyclic() {
 190             // We can't provide a good span for the error here, but this should be caught by the
 191             // HIR const-checker anyways.
 192             self.check_op_spanned(ops::Loop, body.span);
 193         }
 194
 195         self.visit_body(body);
 196
 197         // Ensure that the end result is `Sync` in a non-thread local `static`.
 198         let should_check_for_sync =
 199             const_kind == Some(ConstKind::Static) && !tcx.has_attr(def_id, sym::thread_local);
 200
 201         if should_check_for_sync {
 202             let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
 203             check_return_ty_is_sync(tcx, &body, hir_id);
 204         }
 205     }
 206
 207     pub fn qualifs_in_return_place(&mut self) -> ConstQualifs {
 208         self.qualifs.in_return_place(self.item)
 209     }
 210
 211     /// Emits an error at the given `span` if an expression cannot be evaluated in the current
 212     /// context.
 213     pub fn check_op_spanned<O>(&mut self, op: O, span: Span)
 214     where
 215         O: NonConstOp,
 216     {
 217         trace!("check_op: op={:?}", op);
 218
 219         if op.is_allowed_in_item(self) {
 220             return;
 221         }
 222
 223         // If an operation is supported in miri (and is not already controlled by a feature gate) it
 224         // can be turned on with `-Zunleash-the-miri-inside-of-you`.
 225         let is_unleashable = O::IS_SUPPORTED_IN_MIRI && O::feature_gate(self.tcx).is_none();
 226
 227         if is_unleashable && self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
 228             self.tcx.sess.span_warn(span, "skipping const checks");
 229             return;
 230         }
 231
 232         op.emit_error(self, span);
 233     }
 234
 235     /// Emits an error if an expression cannot be evaluated in the current context.
 236     pub fn check_op(&mut self, op: impl NonConstOp) {
 237         let span = self.span;
 238         self.check_op_spanned(op, span)
 239     }
 240
 241     fn check_static(&mut self, def_id: DefId, span: Span) {
 242         let is_thread_local = self.tcx.has_attr(def_id, sym::thread_local);
 243         if is_thread_local {
 244             self.check_op_spanned(ops::ThreadLocalAccess, span)
 245         } else {
 246             self.check_op_spanned(ops::StaticAccess, span)
 247         }
 248     }
 249
 250     fn check_immutable_borrow_like(&mut self, location: Location, place: &Place<'tcx>) {
 251         // FIXME: Change the `in_*` methods to take a `FnMut` so we don't have to manually
 252         // seek the cursors beforehand.
 253         self.qualifs.has_mut_interior.cursor.seek_before(location);
 254         self.qualifs.indirectly_mutable.seek(location);
 255
 256         let borrowed_place_has_mut_interior = HasMutInterior::in_place(
 257             &self.item,
 258             &|local| self.qualifs.has_mut_interior_eager_seek(local),
 259             place.as_ref(),
 260         );
 261
 262         if borrowed_place_has_mut_interior {
 263             self.check_op(ops::CellBorrow);
 264         }
 265     }
 266 }
 267
 268 impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
 269     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
 270         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
 271
 272         // Just as the old checker did, we skip const-checking basic blocks on the unwind path.
 273         // These blocks often drop locals that would otherwise be returned from the function.
 274         //
 275         // FIXME: This shouldn't be unsound since a panic at compile time will cause a compiler
 276         // error anyway, but maybe we should do more here?
 277         if block.is_cleanup {
 278             return;
 279         }
 280
 281         self.super_basic_block_data(bb, block);
 282     }
 283
 284     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 285         trace!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);
 286
 287         // Special-case reborrows to be more like a copy of a reference.
 288         match *rvalue {
 289             Rvalue::Ref(_, kind, ref place) => {
 290                 if let Some(reborrowed_proj) = place_as_reborrow(self.tcx, *self.body, place) {
 291                     let ctx = match kind {
 292                         BorrowKind::Shared => {
 293                             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
 294                         }
 295                         BorrowKind::Shallow => {
 296                             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
 297                         }
 298                         BorrowKind::Unique => {
 299                             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
 300                         }
 301                         BorrowKind::Mut { .. } => {
 302                             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 303                         }
 304                     };
 305                     self.visit_place_base(&place.base, ctx, location);
 306                     self.visit_projection(&place.base, reborrowed_proj, ctx, location);
 307                     return;
 308                 }
 309             }
 310             Rvalue::AddressOf(mutbl, ref place) => {
 311                 if let Some(reborrowed_proj) = place_as_reborrow(self.tcx, *self.body, place) {
 312                     let ctx = match mutbl {
 313                         Mutability::Not => {
 314                             PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
 315                         }
 316                         Mutability::Mut => PlaceContext::MutatingUse(MutatingUseContext::AddressOf),
 317                     };
 318                     self.visit_place_base(&place.base, ctx, location);
 319                     self.visit_projection(&place.base, reborrowed_proj, ctx, location);
 320                     return;
 321                 }
 322             }
 323             _ => {}
 324         }
 325
 326         self.super_rvalue(rvalue, location);
 327
 328         match *rvalue {
 329             Rvalue::Use(_)
 330             | Rvalue::Repeat(..)
 331             | Rvalue::UnaryOp(UnOp::Neg, _)
 332             | Rvalue::UnaryOp(UnOp::Not, _)
 333             | Rvalue::NullaryOp(NullOp::SizeOf, _)
 334             | Rvalue::CheckedBinaryOp(..)
 335             | Rvalue::Cast(CastKind::Pointer(_), ..)
 336             | Rvalue::Discriminant(..)
 337             | Rvalue::Len(_)
 338             | Rvalue::Aggregate(..) => {}
 339
 340             Rvalue::Ref(_, kind @ BorrowKind::Mut { .. }, ref place)
 341             | Rvalue::Ref(_, kind @ BorrowKind::Unique, ref place) => {
 342                 let ty = place.ty(*self.body, self.tcx).ty;
 343                 let is_allowed = match ty.kind {
 344                     // Inside a `static mut`, `&mut [...]` is allowed.
 345                     ty::Array(..) | ty::Slice(_) if self.const_kind() == ConstKind::StaticMut => {
 346                         true
 347                     }
 348
 349                     // FIXME(ecstaticmorse): We could allow `&mut []` inside a const context given
 350                     // that this is merely a ZST and it is already eligible for promotion.
 351                     // This may require an RFC?
 352                     /*
 353                     ty::Array(_, len) if len.try_eval_usize(cx.tcx, cx.param_env) == Some(0)
 354                         => true,
 355                     */
 356                     _ => false,
 357                 };
 358
 359                 if !is_allowed {
 360                     if let BorrowKind::Mut { .. } = kind {
 361                         self.check_op(ops::MutBorrow);
 362                     } else {
 363                         self.check_op(ops::CellBorrow);
 364                     }
 365                 }
 366             }
 367
 368             Rvalue::AddressOf(Mutability::Mut, _) => self.check_op(ops::MutAddressOf),
 369
 370             // At the moment, `PlaceBase::Static` is only used for promoted MIR.
 371             Rvalue::Ref(_, BorrowKind::Shared, ref place)
 372             | Rvalue::Ref(_, BorrowKind::Shallow, ref place)
 373             | Rvalue::AddressOf(Mutability::Not, ref place)
 374                 if matches!(place.base, PlaceBase::Static(_)) =>
 375             {
 376                 bug!("Saw a promoted during const-checking, which must run before promotion")
 377             }
 378
 379             Rvalue::Ref(_, BorrowKind::Shared, ref place)
 380             | Rvalue::Ref(_, BorrowKind::Shallow, ref place) => {
 381                 self.check_immutable_borrow_like(location, place)
 382             }
 383
 384             Rvalue::AddressOf(Mutability::Not, ref place) => {
 385                 self.check_immutable_borrow_like(location, place)
 386             }
 387
 388             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 389                 let operand_ty = operand.ty(*self.body, self.tcx);
 390                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 391                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 392
 393                 if let (CastTy::Ptr(_), CastTy::Int(_)) | (CastTy::FnPtr, CastTy::Int(_)) =
 394                     (cast_in, cast_out)
 395                 {
 396                     self.check_op(ops::RawPtrToIntCast);
 397                 }
 398             }
 399
 400             Rvalue::BinaryOp(op, ref lhs, _) => {
 401                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(*self.body, self.tcx).kind {
 402                     assert!(
 403                         op == BinOp::Eq
 404                             || op == BinOp::Ne
 405                             || op == BinOp::Le
 406                             || op == BinOp::Lt
 407                             || op == BinOp::Ge
 408                             || op == BinOp::Gt
 409                             || op == BinOp::Offset
 410                     );
 411
 412                     self.check_op(ops::RawPtrComparison);
 413                 }
 414             }
 415
 416             Rvalue::NullaryOp(NullOp::Box, _) => {
 417                 self.check_op(ops::HeapAllocation);
 418             }
 419         }
 420     }
 421
 422     fn visit_place_base(
 423         &mut self,
 424         place_base: &PlaceBase<'tcx>,
 425         context: PlaceContext,
 426         location: Location,
 427     ) {
 428         trace!(
 429             "visit_place_base: place_base={:?} context={:?} location={:?}",
 430             place_base,
 431             context,
 432             location,
 433         );
 434         self.super_place_base(place_base, context, location);
 435
 436         match place_base {
 437             PlaceBase::Local(_) => {}
 438             PlaceBase::Static(_) => {
 439                 bug!("Promotion must be run after const validation");
 440             }
 441         }
 442     }
 443
 444     fn visit_operand(&mut self, op: &Operand<'tcx>, location: Location) {
 445         self.super_operand(op, location);
 446         if let Operand::Constant(c) = op {
 447             if let Some(def_id) = c.check_static_ptr(self.tcx) {
 448                 self.check_static(def_id, self.span);
 449             }
 450         }
 451     }
 452     fn visit_projection_elem(
 453         &mut self,
 454         place_base: &PlaceBase<'tcx>,
 455         proj_base: &[PlaceElem<'tcx>],
 456         elem: &PlaceElem<'tcx>,
 457         context: PlaceContext,
 458         location: Location,
 459     ) {
 460         trace!(
 461             "visit_projection_elem: place_base={:?} proj_base={:?} elem={:?} \
 462             context={:?} location={:?}",
 463             place_base,
 464             proj_base,
 465             elem,
 466             context,
 467             location,
 468         );
 469
 470         self.super_projection_elem(place_base, proj_base, elem, context, location);
 471
 472         match elem {
 473             ProjectionElem::Deref => {
 474                 let base_ty = Place::ty_from(place_base, proj_base, *self.body, self.tcx).ty;
 475                 if let ty::RawPtr(_) = base_ty.kind {
 476                     if proj_base.is_empty() {
 477                         if let (PlaceBase::Local(local), []) = (place_base, proj_base) {
 478                             let decl = &self.body.local_decls[*local];
 479                             if let LocalInfo::StaticRef { def_id, .. } = decl.local_info {
 480                                 let span = decl.source_info.span;
 481                                 self.check_static(def_id, span);
 482                                 return;
 483                             }
 484                         }
 485                     }
 486                     self.check_op(ops::RawPtrDeref);
 487                 }
 488
 489                 if context.is_mutating_use() {
 490                     self.check_op(ops::MutDeref);
 491                 }
 492             }
 493
 494             ProjectionElem::ConstantIndex { .. }
 495             | ProjectionElem::Subslice { .. }
 496             | ProjectionElem::Field(..)
 497             | ProjectionElem::Index(_) => {
 498                 let base_ty = Place::ty_from(place_base, proj_base, *self.body, self.tcx).ty;
 499                 match base_ty.ty_adt_def() {
 500                     Some(def) if def.is_union() => {
 501                         self.check_op(ops::UnionAccess);
 502                     }
 503
 504                     _ => {}
 505                 }
 506             }
 507
 508             ProjectionElem::Downcast(..) => {
 509                 self.check_op(ops::Downcast);
 510             }
 511         }
 512     }
 513
 514     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 515         trace!("visit_source_info: source_info={:?}", source_info);
 516         self.span = source_info.span;
 517     }
 518
 519     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 520         trace!("visit_statement: statement={:?} location={:?}", statement, location);
 521
 522         match statement.kind {
 523             StatementKind::Assign(..) | StatementKind::SetDiscriminant { .. } => {
 524                 self.super_statement(statement, location);
 525             }
 526             StatementKind::FakeRead(FakeReadCause::ForMatchedPlace, _) => {
 527                 self.check_op(ops::IfOrMatch);
 528             }
 529             // FIXME(eddyb) should these really do nothing?
 530             StatementKind::FakeRead(..)
 531             | StatementKind::StorageLive(_)
 532             | StatementKind::StorageDead(_)
 533             | StatementKind::InlineAsm { .. }
 534             | StatementKind::Retag { .. }
 535             | StatementKind::AscribeUserType(..)
 536             | StatementKind::Nop => {}
 537         }
 538     }
 539
 540     fn visit_terminator_kind(&mut self, kind: &TerminatorKind<'tcx>, location: Location) {
 541         trace!("visit_terminator_kind: kind={:?} location={:?}", kind, location);
 542         self.super_terminator_kind(kind, location);
 543
 544         match kind {
 545             TerminatorKind::Call { func, .. } => {
 546                 let fn_ty = func.ty(*self.body, self.tcx);
 547
 548                 let def_id = match fn_ty.kind {
 549                     ty::FnDef(def_id, _) => def_id,
 550
 551                     ty::FnPtr(_) => {
 552                         self.check_op(ops::FnCallIndirect);
 553                         return;
 554                     }
 555                     _ => {
 556                         self.check_op(ops::FnCallOther);
 557                         return;
 558                     }
 559                 };
 560
 561                 // At this point, we are calling a function whose `DefId` is known...
 562                 if self.tcx.is_const_fn(def_id) {
 563                     return;
 564                 }
 565
 566                 if is_lang_panic_fn(self.tcx, def_id) {
 567                     self.check_op(ops::Panic);
 568                 } else if let Some(feature) = self.tcx.is_unstable_const_fn(def_id) {
 569                     // Exempt unstable const fns inside of macros with
 570                     // `#[allow_internal_unstable]`.
 571                     if !self.span.allows_unstable(feature) {
 572                         self.check_op(ops::FnCallUnstable(def_id, feature));
 573                     }
 574                 } else {
 575                     self.check_op(ops::FnCallNonConst(def_id));
 576                 }
 577             }
 578
 579             // Forbid all `Drop` terminators unless the place being dropped is a local with no
 580             // projections that cannot be `NeedsDrop`.
 581             TerminatorKind::Drop { location: dropped_place, .. }
 582             | TerminatorKind::DropAndReplace { location: dropped_place, .. } => {
 583                 let mut err_span = self.span;
 584
 585                 // Check to see if the type of this place can ever have a drop impl. If not, this
 586                 // `Drop` terminator is frivolous.
 587                 let ty_needs_drop =
 588                     dropped_place.ty(*self.body, self.tcx).ty.needs_drop(self.tcx, self.param_env);
 589
 590                 if !ty_needs_drop {
 591                     return;
 592                 }
 593
 594                 let needs_drop = if let Some(local) = dropped_place.as_local() {
 595                     // Use the span where the local was declared as the span of the drop error.
 596                     err_span = self.body.local_decls[local].source_info.span;
 597                     self.qualifs.needs_drop_lazy_seek(local, location)
 598                 } else {
 599                     true
 600                 };
 601
 602                 if needs_drop {
 603                     self.check_op_spanned(ops::LiveDrop, err_span);
 604                 }
 605             }
 606
 607             _ => {}
 608         }
 609     }
 610 }
 611
 612 fn error_min_const_fn_violation(tcx: TyCtxt<'_>, span: Span, msg: Cow<'_, str>) {
 613     struct_span_err!(tcx.sess, span, E0723, "{}", msg)
 614         .note("for more information, see issue https://github.com/rust-lang/rust/issues/57563")
 615         .help("add `#![feature(const_fn)]` to the crate attributes to enable")
 616         .emit();
 617 }
 618
 619 fn check_short_circuiting_in_const_local(item: &Item<'_, 'tcx>) {
 620     let body = item.body;
 621
 622     if body.control_flow_destroyed.is_empty() {
 623         return;
 624     }
 625
 626     let mut locals = body.vars_iter();
 627     if let Some(local) = locals.next() {
 628         let span = body.local_decls[local].source_info.span;
 629         let mut error = item.tcx.sess.struct_span_err(
 630             span,
 631             &format!(
 632                 "new features like let bindings are not permitted in {}s \
 633                 which also use short circuiting operators",
 634                 item.const_kind(),
 635             ),
 636         );
 637         for (span, kind) in body.control_flow_destroyed.iter() {
 638             error.span_note(
 639                 *span,
 640                 &format!(
 641                     "use of {} here does not actually short circuit due to \
 642                 the const evaluator presently not being able to do control flow. \
 643                 See https://github.com/rust-lang/rust/issues/49146 for more \
 644                 information.",
 645                     kind
 646                 ),
 647             );
 648         }
 649         for local in locals {
 650             let span = body.local_decls[local].source_info.span;
 651             error.span_note(span, "more locals defined here");
 652         }
 653         error.emit();
 654     }
 655 }
 656
 657 fn check_return_ty_is_sync(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, hir_id: HirId) {
 658     let ty = body.return_ty();
 659     tcx.infer_ctxt().enter(|infcx| {
 660         let cause = traits::ObligationCause::new(body.span, hir_id, traits::SharedStatic);
 661         let mut fulfillment_cx = traits::FulfillmentContext::new();
 662         let sync_def_id = tcx.require_lang_item(lang_items::SyncTraitLangItem, Some(body.span));
 663         fulfillment_cx.register_bound(&infcx, ty::ParamEnv::empty(), ty, sync_def_id, cause);
 664         if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
 665             infcx.report_fulfillment_errors(&err, None, false);
 666         }
 667     });
 668 }
 669
 670 fn place_as_reborrow(
 671     tcx: TyCtxt<'tcx>,
 672     body: &Body<'tcx>,
 673     place: &'a Place<'tcx>,
 674 ) -> Option<&'a [PlaceElem<'tcx>]> {
 675     place.projection.split_last().and_then(|(outermost, inner)| {
 676         if outermost != &ProjectionElem::Deref {
 677             return None;
 678         }
 679
 680         // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
 681         // that points to the allocation for the static. Don't treat these as reborrows.
 682         if let PlaceBase::Local(local) = place.base {
 683             if body.local_decls[local].is_ref_to_static() {
 684                 return None;
 685             }
 686         }
 687
 688         // Ensure the type being derefed is a reference and not a raw pointer.
 689         //
 690         // This is sufficient to prevent an access to a `static mut` from being marked as a
 691         // reborrow, even if the check above were to disappear.
 692         let inner_ty = Place::ty_from(&place.base, inner, body, tcx).ty;
 693         match inner_ty.kind {
 694             ty::Ref(..) => Some(inner),
 695             _ => None,
 696         }
 697     })
 698 }