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