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_errors::struct_span_err;
   4 use rustc_hir::lang_items;
   5 use rustc_hir::{def_id::DefId, HirId};
   6 use rustc_infer::infer::TyCtxtInferExt;
   7 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
   8 use rustc_middle::mir::*;
   9 use rustc_middle::ty::cast::CastTy;
  10 use rustc_middle::ty::{self, Instance, InstanceDef, TyCtxt};
  11 use rustc_span::Span;
  12 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  13 use rustc_trait_selection::traits::{self, TraitEngine};
  14
  15 use std::borrow::Cow;
  16 use std::ops::Deref;
  17
  18 use super::ops::{self, NonConstOp};
  19 use super::qualifs::{self, CustomEq, HasMutInterior, NeedsDrop};
  20 use super::resolver::FlowSensitiveAnalysis;
  21 use super::{is_lang_panic_fn, ConstCx, ConstKind, Qualif};
  22 use crate::const_eval::{is_const_fn, is_unstable_const_fn};
  23 use crate::dataflow::MaybeMutBorrowedLocals;
  24 use crate::dataflow::{self, Analysis};
  25
  26 // We are using `MaybeMutBorrowedLocals` as a proxy for whether an item may have been mutated
  27 // through a pointer prior to the given point. This is okay even though `MaybeMutBorrowedLocals`
  28 // kills locals upon `StorageDead` because a local will never be used after a `StorageDead`.
  29 type IndirectlyMutableResults<'mir, 'tcx> =
  30     dataflow::ResultsCursor<'mir, 'tcx, MaybeMutBorrowedLocals<'mir, 'tcx>>;
  31
  32 type QualifResults<'mir, 'tcx, Q> =
  33     dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
  34
  35 #[derive(Default)]
  36 pub struct Qualifs<'mir, 'tcx> {
  37     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
  38     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
  39     indirectly_mutable: Option<IndirectlyMutableResults<'mir, 'tcx>>,
  40 }
  41
  42 impl Qualifs<'mir, 'tcx> {
  43     fn indirectly_mutable(
  44         &mut self,
  45         ccx: &'mir ConstCx<'mir, 'tcx>,
  46         local: Local,
  47         location: Location,
  48     ) -> bool {
  49         let indirectly_mutable = self.indirectly_mutable.get_or_insert_with(|| {
  50             let ConstCx { tcx, body, def_id, param_env, .. } = *ccx;
  51
  52             // We can use `unsound_ignore_borrow_on_drop` here because custom drop impls are not
  53             // allowed in a const.
  54             //
  55             // FIXME(ecstaticmorse): Someday we want to allow custom drop impls. How do we do this
  56             // without breaking stable code?
  57             MaybeMutBorrowedLocals::mut_borrows_only(tcx, &body, param_env)
  58                 .unsound_ignore_borrow_on_drop()
  59                 .into_engine(tcx, &body, def_id)
  60                 .iterate_to_fixpoint()
  61                 .into_results_cursor(&body)
  62         });
  63
  64         indirectly_mutable.seek_before_primary_effect(location);
  65         indirectly_mutable.get().contains(local)
  66     }
  67
  68     /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
  69     ///
  70     /// Only updates the cursor if absolutely necessary
  71     fn needs_drop(
  72         &mut self,
  73         ccx: &'mir ConstCx<'mir, 'tcx>,
  74         local: Local,
  75         location: Location,
  76     ) -> bool {
  77         let ty = ccx.body.local_decls[local].ty;
  78         if !NeedsDrop::in_any_value_of_ty(ccx, ty) {
  79             return false;
  80         }
  81
  82         let needs_drop = self.needs_drop.get_or_insert_with(|| {
  83             let ConstCx { tcx, body, def_id, .. } = *ccx;
  84
  85             FlowSensitiveAnalysis::new(NeedsDrop, ccx)
  86                 .into_engine(tcx, &body, def_id)
  87                 .iterate_to_fixpoint()
  88                 .into_results_cursor(&body)
  89         });
  90
  91         needs_drop.seek_before_primary_effect(location);
  92         needs_drop.get().contains(local) || self.indirectly_mutable(ccx, local, location)
  93     }
  94
  95     /// Returns `true` if `local` is `HasMutInterior` at the given `Location`.
  96     ///
  97     /// Only updates the cursor if absolutely necessary.
  98     fn has_mut_interior(
  99         &mut self,
 100         ccx: &'mir ConstCx<'mir, 'tcx>,
 101         local: Local,
 102         location: Location,
 103     ) -> bool {
 104         let ty = ccx.body.local_decls[local].ty;
 105         if !HasMutInterior::in_any_value_of_ty(ccx, ty) {
 106             return false;
 107         }
 108
 109         let has_mut_interior = self.has_mut_interior.get_or_insert_with(|| {
 110             let ConstCx { tcx, body, def_id, .. } = *ccx;
 111
 112             FlowSensitiveAnalysis::new(HasMutInterior, ccx)
 113                 .into_engine(tcx, &body, def_id)
 114                 .iterate_to_fixpoint()
 115                 .into_results_cursor(&body)
 116         });
 117
 118         has_mut_interior.seek_before_primary_effect(location);
 119         has_mut_interior.get().contains(local) || self.indirectly_mutable(ccx, local, location)
 120     }
 121
 122     fn in_return_place(&mut self, ccx: &'mir ConstCx<'mir, 'tcx>) -> ConstQualifs {
 123         // Find the `Return` terminator if one exists.
 124         //
 125         // If no `Return` terminator exists, this MIR is divergent. Just return the conservative
 126         // qualifs for the return type.
 127         let return_block = ccx
 128             .body
 129             .basic_blocks()
 130             .iter_enumerated()
 131             .find(|(_, block)| match block.terminator().kind {
 132                 TerminatorKind::Return => true,
 133                 _ => false,
 134             })
 135             .map(|(bb, _)| bb);
 136
 137         let return_block = match return_block {
 138             None => return qualifs::in_any_value_of_ty(ccx, ccx.body.return_ty()),
 139             Some(bb) => bb,
 140         };
 141
 142         let return_loc = ccx.body.terminator_loc(return_block);
 143
 144         let custom_eq = match ccx.const_kind() {
 145             // We don't care whether a `const fn` returns a value that is not structurally
 146             // matchable. Functions calls are opaque and always use type-based qualification, so
 147             // this value should never be used.
 148             ConstKind::ConstFn => true,
 149
 150             // If we know that all values of the return type are structurally matchable, there's no
 151             // need to run dataflow.
 152             ConstKind::Const | ConstKind::Static | ConstKind::StaticMut
 153                 if !CustomEq::in_any_value_of_ty(ccx, ccx.body.return_ty()) =>
 154             {
 155                 false
 156             }
 157
 158             ConstKind::Const | ConstKind::Static | ConstKind::StaticMut => {
 159                 let mut cursor = FlowSensitiveAnalysis::new(CustomEq, ccx)
 160                     .into_engine(ccx.tcx, &ccx.body, ccx.def_id)
 161                     .iterate_to_fixpoint()
 162                     .into_results_cursor(&ccx.body);
 163
 164                 cursor.seek_after_primary_effect(return_loc);
 165                 cursor.contains(RETURN_PLACE)
 166             }
 167         };
 168
 169         ConstQualifs {
 170             needs_drop: self.needs_drop(ccx, RETURN_PLACE, return_loc),
 171             has_mut_interior: self.has_mut_interior(ccx, RETURN_PLACE, return_loc),
 172             custom_eq,
 173         }
 174     }
 175 }
 176
 177 pub struct Validator<'mir, 'tcx> {
 178     ccx: &'mir ConstCx<'mir, 'tcx>,
 179     qualifs: Qualifs<'mir, 'tcx>,
 180
 181     /// The span of the current statement.
 182     span: Span,
 183 }
 184
 185 impl Deref for Validator<'mir, 'tcx> {
 186     type Target = ConstCx<'mir, 'tcx>;
 187
 188     fn deref(&self) -> &Self::Target {
 189         &self.ccx
 190     }
 191 }
 192
 193 impl Validator<'mir, 'tcx> {
 194     pub fn new(ccx: &'mir ConstCx<'mir, 'tcx>) -> Self {
 195         Validator { span: ccx.body.span, ccx, qualifs: Default::default() }
 196     }
 197
 198     pub fn check_body(&mut self) {
 199         let ConstCx { tcx, body, def_id, const_kind, .. } = *self.ccx;
 200
 201         let use_min_const_fn_checks = (const_kind == Some(ConstKind::ConstFn)
 202             && crate::const_eval::is_min_const_fn(tcx, def_id))
 203             && !tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you;
 204
 205         if use_min_const_fn_checks {
 206             // Enforce `min_const_fn` for stable `const fn`s.
 207             use crate::transform::qualify_min_const_fn::is_min_const_fn;
 208             if let Err((span, err)) = is_min_const_fn(tcx, def_id, &body) {
 209                 error_min_const_fn_violation(tcx, span, err);
 210                 return;
 211             }
 212         }
 213
 214         check_short_circuiting_in_const_local(self.ccx);
 215
 216         if body.is_cfg_cyclic() {
 217             // We can't provide a good span for the error here, but this should be caught by the
 218             // HIR const-checker anyways.
 219             self.check_op_spanned(ops::Loop, body.span);
 220         }
 221
 222         self.visit_body(&body);
 223
 224         // Ensure that the end result is `Sync` in a non-thread local `static`.
 225         let should_check_for_sync =
 226             const_kind == Some(ConstKind::Static) && !tcx.is_thread_local_static(def_id);
 227
 228         if should_check_for_sync {
 229             let hir_id = tcx.hir().as_local_hir_id(def_id.expect_local());
 230             check_return_ty_is_sync(tcx, &body, hir_id);
 231         }
 232     }
 233
 234     pub fn qualifs_in_return_place(&mut self) -> ConstQualifs {
 235         self.qualifs.in_return_place(self.ccx)
 236     }
 237
 238     /// Emits an error at the given `span` if an expression cannot be evaluated in the current
 239     /// context.
 240     pub fn check_op_spanned<O>(&mut self, op: O, span: Span)
 241     where
 242         O: NonConstOp,
 243     {
 244         debug!("check_op: op={:?}", op);
 245
 246         if op.is_allowed_in_item(self) {
 247             return;
 248         }
 249
 250         // If an operation is supported in miri (and is not already controlled by a feature gate) it
 251         // can be turned on with `-Zunleash-the-miri-inside-of-you`.
 252         let is_unleashable = O::IS_SUPPORTED_IN_MIRI && O::feature_gate().is_none();
 253
 254         if is_unleashable && self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
 255             self.tcx.sess.span_warn(span, "skipping const checks");
 256             return;
 257         }
 258
 259         op.emit_error(self, span);
 260     }
 261
 262     /// Emits an error if an expression cannot be evaluated in the current context.
 263     pub fn check_op(&mut self, op: impl NonConstOp) {
 264         let span = self.span;
 265         self.check_op_spanned(op, span)
 266     }
 267
 268     fn check_static(&mut self, def_id: DefId, span: Span) {
 269         if self.tcx.is_thread_local_static(def_id) {
 270             self.check_op_spanned(ops::ThreadLocalAccess, span)
 271         } else {
 272             self.check_op_spanned(ops::StaticAccess, span)
 273         }
 274     }
 275 }
 276
 277 impl Visitor<'tcx> for Validator<'mir, 'tcx> {
 278     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
 279         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
 280
 281         // Just as the old checker did, we skip const-checking basic blocks on the unwind path.
 282         // These blocks often drop locals that would otherwise be returned from the function.
 283         //
 284         // FIXME: This shouldn't be unsound since a panic at compile time will cause a compiler
 285         // error anyway, but maybe we should do more here?
 286         if block.is_cleanup {
 287             return;
 288         }
 289
 290         self.super_basic_block_data(bb, block);
 291     }
 292
 293     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 294         trace!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);
 295
 296         // Special-case reborrows to be more like a copy of a reference.
 297         match *rvalue {
 298             Rvalue::Ref(_, kind, place) => {
 299                 if let Some(reborrowed_proj) = place_as_reborrow(self.tcx, self.body, place) {
 300                     let ctx = match kind {
 301                         BorrowKind::Shared => {
 302                             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
 303                         }
 304                         BorrowKind::Shallow => {
 305                             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
 306                         }
 307                         BorrowKind::Unique => {
 308                             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
 309                         }
 310                         BorrowKind::Mut { .. } => {
 311                             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 312                         }
 313                     };
 314                     self.visit_local(&place.local, ctx, location);
 315                     self.visit_projection(place.local, reborrowed_proj, ctx, location);
 316                     return;
 317                 }
 318             }
 319             Rvalue::AddressOf(mutbl, place) => {
 320                 if let Some(reborrowed_proj) = place_as_reborrow(self.tcx, self.body, place) {
 321                     let ctx = match mutbl {
 322                         Mutability::Not => {
 323                             PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
 324                         }
 325                         Mutability::Mut => PlaceContext::MutatingUse(MutatingUseContext::AddressOf),
 326                     };
 327                     self.visit_local(&place.local, ctx, location);
 328                     self.visit_projection(place.local, reborrowed_proj, ctx, location);
 329                     return;
 330                 }
 331             }
 332             _ => {}
 333         }
 334
 335         self.super_rvalue(rvalue, location);
 336
 337         match *rvalue {
 338             Rvalue::Use(_)
 339             | Rvalue::Repeat(..)
 340             | Rvalue::UnaryOp(UnOp::Neg, _)
 341             | Rvalue::UnaryOp(UnOp::Not, _)
 342             | Rvalue::NullaryOp(NullOp::SizeOf, _)
 343             | Rvalue::CheckedBinaryOp(..)
 344             | Rvalue::Cast(CastKind::Pointer(_), ..)
 345             | Rvalue::Discriminant(..)
 346             | Rvalue::Len(_)
 347             | Rvalue::Aggregate(..) => {}
 348
 349             Rvalue::Ref(_, kind @ BorrowKind::Mut { .. }, ref place)
 350             | Rvalue::Ref(_, kind @ BorrowKind::Unique, ref place) => {
 351                 let ty = place.ty(self.body, self.tcx).ty;
 352                 let is_allowed = match ty.kind {
 353                     // Inside a `static mut`, `&mut [...]` is allowed.
 354                     ty::Array(..) | ty::Slice(_) if self.const_kind() == ConstKind::StaticMut => {
 355                         true
 356                     }
 357
 358                     // FIXME(ecstaticmorse): We could allow `&mut []` inside a const context given
 359                     // that this is merely a ZST and it is already eligible for promotion.
 360                     // This may require an RFC?
 361                     /*
 362                     ty::Array(_, len) if len.try_eval_usize(cx.tcx, cx.param_env) == Some(0)
 363                         => true,
 364                     */
 365                     _ => false,
 366                 };
 367
 368                 if !is_allowed {
 369                     if let BorrowKind::Mut { .. } = kind {
 370                         self.check_op(ops::MutBorrow);
 371                     } else {
 372                         self.check_op(ops::CellBorrow);
 373                     }
 374                 }
 375             }
 376
 377             Rvalue::AddressOf(Mutability::Mut, _) => self.check_op(ops::MutAddressOf),
 378
 379             Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Shallow, ref place)
 380             | Rvalue::AddressOf(Mutability::Not, ref place) => {
 381                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
 382                     &self.ccx,
 383                     &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
 384                     place.as_ref(),
 385                 );
 386
 387                 if borrowed_place_has_mut_interior {
 388                     self.check_op(ops::CellBorrow);
 389                 }
 390             }
 391
 392             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 393                 let operand_ty = operand.ty(self.body, self.tcx);
 394                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 395                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 396
 397                 if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 398                     self.check_op(ops::RawPtrToIntCast);
 399                 }
 400             }
 401
 402             Rvalue::BinaryOp(op, ref lhs, _) => {
 403                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
 404                     assert!(
 405                         op == BinOp::Eq
 406                             || op == BinOp::Ne
 407                             || op == BinOp::Le
 408                             || op == BinOp::Lt
 409                             || op == BinOp::Ge
 410                             || op == BinOp::Gt
 411                             || op == BinOp::Offset
 412                     );
 413
 414                     self.check_op(ops::RawPtrComparison);
 415                 }
 416             }
 417
 418             Rvalue::NullaryOp(NullOp::Box, _) => {
 419                 self.check_op(ops::HeapAllocation);
 420             }
 421         }
 422     }
 423
 424     fn visit_operand(&mut self, op: &Operand<'tcx>, location: Location) {
 425         self.super_operand(op, location);
 426         if let Operand::Constant(c) = op {
 427             if let Some(def_id) = c.check_static_ptr(self.tcx) {
 428                 self.check_static(def_id, self.span);
 429             }
 430         }
 431     }
 432     fn visit_projection_elem(
 433         &mut self,
 434         place_local: Local,
 435         proj_base: &[PlaceElem<'tcx>],
 436         elem: &PlaceElem<'tcx>,
 437         context: PlaceContext,
 438         location: Location,
 439     ) {
 440         trace!(
 441             "visit_projection_elem: place_local={:?} proj_base={:?} elem={:?} \
 442             context={:?} location={:?}",
 443             place_local,
 444             proj_base,
 445             elem,
 446             context,
 447             location,
 448         );
 449
 450         self.super_projection_elem(place_local, proj_base, elem, context, location);
 451
 452         match elem {
 453             ProjectionElem::Deref => {
 454                 let base_ty = Place::ty_from(place_local, proj_base, self.body, self.tcx).ty;
 455                 if let ty::RawPtr(_) = base_ty.kind {
 456                     if proj_base.is_empty() {
 457                         if let (local, []) = (place_local, proj_base) {
 458                             let decl = &self.body.local_decls[local];
 459                             if let LocalInfo::StaticRef { def_id, .. } = decl.local_info {
 460                                 let span = decl.source_info.span;
 461                                 self.check_static(def_id, span);
 462                                 return;
 463                             }
 464                         }
 465                     }
 466                     self.check_op(ops::RawPtrDeref);
 467                 }
 468
 469                 if context.is_mutating_use() {
 470                     self.check_op(ops::MutDeref);
 471                 }
 472             }
 473
 474             ProjectionElem::ConstantIndex { .. }
 475             | ProjectionElem::Downcast(..)
 476             | ProjectionElem::Subslice { .. }
 477             | ProjectionElem::Field(..)
 478             | ProjectionElem::Index(_) => {
 479                 let base_ty = Place::ty_from(place_local, proj_base, self.body, self.tcx).ty;
 480                 match base_ty.ty_adt_def() {
 481                     Some(def) if def.is_union() => {
 482                         self.check_op(ops::UnionAccess);
 483                     }
 484
 485                     _ => {}
 486                 }
 487             }
 488         }
 489     }
 490
 491     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 492         trace!("visit_source_info: source_info={:?}", source_info);
 493         self.span = source_info.span;
 494     }
 495
 496     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 497         trace!("visit_statement: statement={:?} location={:?}", statement, location);
 498
 499         match statement.kind {
 500             StatementKind::Assign(..) | StatementKind::SetDiscriminant { .. } => {
 501                 self.super_statement(statement, location);
 502             }
 503
 504             StatementKind::FakeRead(
 505                 FakeReadCause::ForMatchedPlace
 506                 | FakeReadCause::ForMatchGuard
 507                 | FakeReadCause::ForGuardBinding,
 508                 _,
 509             ) => {
 510                 self.super_statement(statement, location);
 511                 self.check_op(ops::IfOrMatch);
 512             }
 513             StatementKind::LlvmInlineAsm { .. } => {
 514                 self.super_statement(statement, location);
 515                 self.check_op(ops::InlineAsm);
 516             }
 517
 518             StatementKind::FakeRead(FakeReadCause::ForLet | FakeReadCause::ForIndex, _)
 519             | StatementKind::StorageLive(_)
 520             | StatementKind::StorageDead(_)
 521             | StatementKind::Retag { .. }
 522             | StatementKind::AscribeUserType(..)
 523             | StatementKind::Nop => {}
 524         }
 525     }
 526
 527     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 528         trace!("visit_terminator: terminator={:?} location={:?}", terminator, location);
 529         self.super_terminator(terminator, location);
 530
 531         match &terminator.kind {
 532             TerminatorKind::Call { func, .. } => {
 533                 let fn_ty = func.ty(self.body, self.tcx);
 534
 535                 let (def_id, substs) = match fn_ty.kind {
 536                     ty::FnDef(def_id, substs) => (def_id, substs),
 537
 538                     ty::FnPtr(_) => {
 539                         self.check_op(ops::FnCallIndirect);
 540                         return;
 541                     }
 542                     _ => {
 543                         span_bug!(terminator.source_info.span, "invalid callee of type {:?}", fn_ty)
 544                     }
 545                 };
 546
 547                 // At this point, we are calling a function whose `DefId` is known...
 548                 if is_const_fn(self.tcx, def_id) {
 549                     return;
 550                 }
 551
 552                 // See if this is a trait method for a concrete type whose impl of that trait is
 553                 // `const`.
 554                 if self.tcx.features().const_trait_impl {
 555                     let instance = Instance::resolve(self.tcx, self.param_env, def_id, substs);
 556                     debug!("Resolving ({:?}) -> {:?}", def_id, instance);
 557                     if let Ok(Some(func)) = instance {
 558                         if let InstanceDef::Item(def_id) = func.def {
 559                             if is_const_fn(self.tcx, def_id) {
 560                                 return;
 561                             }
 562                         }
 563                     }
 564                 }
 565
 566                 if is_lang_panic_fn(self.tcx, def_id) {
 567                     self.check_op(ops::Panic);
 568                 } else if let Some(feature) = is_unstable_const_fn(self.tcx, 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(self.ccx, 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             // FIXME: Some of these are only caught by `min_const_fn`, but should error here
 608             // instead.
 609             TerminatorKind::Abort
 610             | TerminatorKind::Assert { .. }
 611             | TerminatorKind::FalseEdges { .. }
 612             | TerminatorKind::FalseUnwind { .. }
 613             | TerminatorKind::GeneratorDrop
 614             | TerminatorKind::Goto { .. }
 615             | TerminatorKind::Resume
 616             | TerminatorKind::Return
 617             | TerminatorKind::SwitchInt { .. }
 618             | TerminatorKind::Unreachable
 619             | TerminatorKind::Yield { .. } => {}
 620         }
 621     }
 622 }
 623
 624 fn error_min_const_fn_violation(tcx: TyCtxt<'_>, span: Span, msg: Cow<'_, str>) {
 625     struct_span_err!(tcx.sess, span, E0723, "{}", msg)
 626         .note(
 627             "see issue #57563 <https://github.com/rust-lang/rust/issues/57563> \
 628              for more information",
 629         )
 630         .help("add `#![feature(const_fn)]` to the crate attributes to enable")
 631         .emit();
 632 }
 633
 634 fn check_short_circuiting_in_const_local(ccx: &ConstCx<'_, 'tcx>) {
 635     let body = ccx.body;
 636
 637     if body.control_flow_destroyed.is_empty() {
 638         return;
 639     }
 640
 641     let mut locals = body.vars_iter();
 642     if let Some(local) = locals.next() {
 643         let span = body.local_decls[local].source_info.span;
 644         let mut error = ccx.tcx.sess.struct_span_err(
 645             span,
 646             &format!(
 647                 "new features like let bindings are not permitted in {}s \
 648                 which also use short circuiting operators",
 649                 ccx.const_kind(),
 650             ),
 651         );
 652         for (span, kind) in body.control_flow_destroyed.iter() {
 653             error.span_note(
 654                 *span,
 655                 &format!(
 656                     "use of {} here does not actually short circuit due to \
 657                      the const evaluator presently not being able to do control flow. \
 658                      See issue #49146 <https://github.com/rust-lang/rust/issues/49146> \
 659                      for more information.",
 660                     kind
 661                 ),
 662             );
 663         }
 664         for local in locals {
 665             let span = body.local_decls[local].source_info.span;
 666             error.span_note(span, "more locals are defined here");
 667         }
 668         error.emit();
 669     }
 670 }
 671
 672 fn check_return_ty_is_sync(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, hir_id: HirId) {
 673     let ty = body.return_ty();
 674     tcx.infer_ctxt().enter(|infcx| {
 675         let cause = traits::ObligationCause::new(body.span, hir_id, traits::SharedStatic);
 676         let mut fulfillment_cx = traits::FulfillmentContext::new();
 677         let sync_def_id = tcx.require_lang_item(lang_items::SyncTraitLangItem, Some(body.span));
 678         fulfillment_cx.register_bound(&infcx, ty::ParamEnv::empty(), ty, sync_def_id, cause);
 679         if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
 680             infcx.report_fulfillment_errors(&err, None, false);
 681         }
 682     });
 683 }
 684
 685 fn place_as_reborrow(
 686     tcx: TyCtxt<'tcx>,
 687     body: &Body<'tcx>,
 688     place: Place<'tcx>,
 689 ) -> Option<&'a [PlaceElem<'tcx>]> {
 690     place.projection.split_last().and_then(|(outermost, inner)| {
 691         if outermost != &ProjectionElem::Deref {
 692             return None;
 693         }
 694
 695         // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
 696         // that points to the allocation for the static. Don't treat these as reborrows.
 697         if body.local_decls[place.local].is_ref_to_static() {
 698             return None;
 699         }
 700
 701         // Ensure the type being derefed is a reference and not a raw pointer.
 702         //
 703         // This is sufficient to prevent an access to a `static mut` from being marked as a
 704         // reborrow, even if the check above were to disappear.
 705         let inner_ty = Place::ty_from(place.local, inner, body, tcx).ty;
 706         match inner_ty.kind {
 707             ty::Ref(..) => Some(inner),
 708             _ => None,
 709         }
 710     })
 711 }