src/librustc_mir/transform/check_unsafety.rs

   1 use rustc_data_structures::fx::FxHashSet;
   2 use rustc_data_structures::indexed_vec::IndexVec;
   3 use rustc_data_structures::sync::Lrc;
   4
   5 use rustc::ty::query::Providers;
   6 use rustc::ty::{self, TyCtxt};
   7 use rustc::ty::cast::CastTy;
   8 use rustc::hir;
   9 use rustc::hir::Node;
  10 use rustc::hir::def_id::DefId;
  11 use rustc::lint::builtin::{SAFE_EXTERN_STATICS, SAFE_PACKED_BORROWS, UNUSED_UNSAFE};
  12 use rustc::mir::*;
  13 use rustc::mir::visit::{PlaceContext, Visitor, MutatingUseContext};
  14
  15 use syntax::symbol::{InternedString, sym};
  16
  17 use std::ops::Bound;
  18
  19 use crate::util;
  20
  21 pub struct UnsafetyChecker<'a, 'tcx> {
  22     body: &'a Body<'tcx>,
  23     const_context: bool,
  24     min_const_fn: bool,
  25     source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  26     violations: Vec<UnsafetyViolation>,
  27     source_info: SourceInfo,
  28     tcx: TyCtxt<'tcx>,
  29     param_env: ty::ParamEnv<'tcx>,
  30     /// Mark an `unsafe` block as used, so we don't lint it.
  31     used_unsafe: FxHashSet<hir::HirId>,
  32     inherited_blocks: Vec<(hir::HirId, bool)>,
  33 }
  34
  35 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
  36     fn new(
  37         const_context: bool,
  38         min_const_fn: bool,
  39         body: &'a Body<'tcx>,
  40         source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  41         tcx: TyCtxt<'tcx>,
  42         param_env: ty::ParamEnv<'tcx>,
  43     ) -> Self {
  44         // sanity check
  45         if min_const_fn {
  46             assert!(const_context);
  47         }
  48         Self {
  49             body,
  50             const_context,
  51             min_const_fn,
  52             source_scope_local_data,
  53             violations: vec![],
  54             source_info: SourceInfo {
  55                 span: body.span,
  56                 scope: OUTERMOST_SOURCE_SCOPE
  57             },
  58             tcx,
  59             param_env,
  60             used_unsafe: Default::default(),
  61             inherited_blocks: vec![],
  62         }
  63     }
  64 }
  65
  66 impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
  67     fn visit_terminator(&mut self,
  68                         terminator: &Terminator<'tcx>,
  69                         location: Location)
  70     {
  71         self.source_info = terminator.source_info;
  72         match terminator.kind {
  73             TerminatorKind::Goto { .. } |
  74             TerminatorKind::SwitchInt { .. } |
  75             TerminatorKind::Drop { .. } |
  76             TerminatorKind::Yield { .. } |
  77             TerminatorKind::Assert { .. } |
  78             TerminatorKind::DropAndReplace { .. } |
  79             TerminatorKind::GeneratorDrop |
  80             TerminatorKind::Resume |
  81             TerminatorKind::Abort |
  82             TerminatorKind::Return |
  83             TerminatorKind::Unreachable |
  84             TerminatorKind::FalseEdges { .. } |
  85             TerminatorKind::FalseUnwind { .. } => {
  86                 // safe (at least as emitted during MIR construction)
  87             }
  88
  89             TerminatorKind::Call { ref func, .. } => {
  90                 let func_ty = func.ty(self.body, self.tcx);
  91                 let sig = func_ty.fn_sig(self.tcx);
  92                 if let hir::Unsafety::Unsafe = sig.unsafety() {
  93                     self.require_unsafe("call to unsafe function",
  94                         "consult the function's documentation for information on how to avoid \
  95                          undefined behavior", UnsafetyViolationKind::GeneralAndConstFn)
  96                 }
  97             }
  98         }
  99         self.super_terminator(terminator, location);
 100     }
 101
 102     fn visit_statement(&mut self,
 103                        statement: &Statement<'tcx>,
 104                        location: Location)
 105     {
 106         self.source_info = statement.source_info;
 107         match statement.kind {
 108             StatementKind::Assign(..) |
 109             StatementKind::FakeRead(..) |
 110             StatementKind::SetDiscriminant { .. } |
 111             StatementKind::StorageLive(..) |
 112             StatementKind::StorageDead(..) |
 113             StatementKind::Retag { .. } |
 114             StatementKind::AscribeUserType(..) |
 115             StatementKind::Nop => {
 116                 // safe (at least as emitted during MIR construction)
 117             }
 118
 119             StatementKind::InlineAsm { .. } => {
 120                 self.require_unsafe("use of inline assembly",
 121                     "inline assembly is entirely unchecked and can cause undefined behavior",
 122                     UnsafetyViolationKind::General)
 123             },
 124         }
 125         self.super_statement(statement, location);
 126     }
 127
 128     fn visit_rvalue(&mut self,
 129                     rvalue: &Rvalue<'tcx>,
 130                     location: Location)
 131     {
 132         match rvalue {
 133             Rvalue::Aggregate(box ref aggregate, _) => {
 134                 match aggregate {
 135                     &AggregateKind::Array(..) |
 136                     &AggregateKind::Tuple => {}
 137                     &AggregateKind::Adt(ref def, ..) => {
 138                         match self.tcx.layout_scalar_valid_range(def.did) {
 139                             (Bound::Unbounded, Bound::Unbounded) => {},
 140                             _ => self.require_unsafe(
 141                                 "initializing type with `rustc_layout_scalar_valid_range` attr",
 142                                 "initializing a layout restricted type's field with a value \
 143                                 outside the valid range is undefined behavior",
 144                                 UnsafetyViolationKind::GeneralAndConstFn,
 145                             ),
 146                         }
 147                     }
 148                     &AggregateKind::Closure(def_id, _) |
 149                     &AggregateKind::Generator(def_id, _, _) => {
 150                         let UnsafetyCheckResult {
 151                             violations, unsafe_blocks
 152                         } = self.tcx.unsafety_check_result(def_id);
 153                         self.register_violations(&violations, &unsafe_blocks);
 154                     }
 155                 }
 156             },
 157             // casting pointers to ints is unsafe in const fn because the const evaluator cannot
 158             // possibly know what the result of various operations like `address / 2` would be
 159             // pointers during const evaluation have no integral address, only an abstract one
 160             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty)
 161             if self.const_context && self.tcx.features().const_raw_ptr_to_usize_cast => {
 162                 let operand_ty = operand.ty(self.body, self.tcx);
 163                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 164                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 165                 match (cast_in, cast_out) {
 166                     (CastTy::Ptr(_), CastTy::Int(_)) |
 167                     (CastTy::FnPtr, CastTy::Int(_)) => {
 168                         self.register_violations(&[UnsafetyViolation {
 169                             source_info: self.source_info,
 170                             description: InternedString::intern("cast of pointer to int"),
 171                             details: InternedString::intern(
 172                                 "casting pointers to integers in constants"),
 173                             kind: UnsafetyViolationKind::General,
 174                         }], &[]);
 175                     },
 176                     _ => {},
 177                 }
 178             }
 179             // raw pointer and fn pointer operations are unsafe as it is not clear whether one
 180             // pointer would be "less" or "equal" to another, because we cannot know where llvm
 181             // or the linker will place various statics in memory. Without this information the
 182             // result of a comparison of addresses would differ between runtime and compile-time.
 183             Rvalue::BinaryOp(_, ref lhs, _)
 184             if self.const_context && self.tcx.features().const_compare_raw_pointers => {
 185                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).sty {
 186                     self.register_violations(&[UnsafetyViolation {
 187                         source_info: self.source_info,
 188                         description: InternedString::intern("pointer operation"),
 189                         details: InternedString::intern("operations on pointers in constants"),
 190                         kind: UnsafetyViolationKind::General,
 191                     }], &[]);
 192                 }
 193             }
 194             _ => {},
 195         }
 196         self.super_rvalue(rvalue, location);
 197     }
 198
 199     fn visit_place(&mut self,
 200                     place: &Place<'tcx>,
 201                     context: PlaceContext,
 202                     _location: Location) {
 203         place.iterate(|place_base, place_projections| {
 204             match place_base {
 205                 PlaceBase::Local(..) => {
 206                     // Locals are safe.
 207                 }
 208                 PlaceBase::Static(box Static { kind: StaticKind::Promoted(_), .. }) => {
 209                     bug!("unsafety checking should happen before promotion")
 210                 }
 211                 PlaceBase::Static(box Static { kind: StaticKind::Static(def_id), .. }) => {
 212                     if self.tcx.is_mutable_static(*def_id) {
 213                         self.require_unsafe("use of mutable static",
 214                             "mutable statics can be mutated by multiple threads: aliasing \
 215                              violations or data races will cause undefined behavior",
 216                              UnsafetyViolationKind::General);
 217                     } else if self.tcx.is_foreign_item(*def_id) {
 218                         let source_info = self.source_info;
 219                         let lint_root =
 220                             self.source_scope_local_data[source_info.scope].lint_root;
 221                         self.register_violations(&[UnsafetyViolation {
 222                             source_info,
 223                             description: InternedString::intern("use of extern static"),
 224                             details: InternedString::intern(
 225                                 "extern statics are not controlled by the Rust type system: \
 226                                 invalid data, aliasing violations or data races will cause \
 227                                 undefined behavior"),
 228                             kind: UnsafetyViolationKind::ExternStatic(lint_root)
 229                         }], &[]);
 230                     }
 231                 }
 232             }
 233
 234             for proj in place_projections {
 235                 if context.is_borrow() {
 236                     if util::is_disaligned(self.tcx, self.body, self.param_env, place) {
 237                         let source_info = self.source_info;
 238                         let lint_root =
 239                             self.source_scope_local_data[source_info.scope].lint_root;
 240                         self.register_violations(&[UnsafetyViolation {
 241                             source_info,
 242                             description: InternedString::intern("borrow of packed field"),
 243                             details: InternedString::intern(
 244                                 "fields of packed structs might be misaligned: dereferencing a \
 245                                 misaligned pointer or even just creating a misaligned reference \
 246                                 is undefined behavior"),
 247                             kind: UnsafetyViolationKind::BorrowPacked(lint_root)
 248                         }], &[]);
 249                     }
 250                 }
 251                 let is_borrow_of_interior_mut = context.is_borrow() && !proj.base
 252                     .ty(self.body, self.tcx)
 253                     .ty
 254                     .is_freeze(self.tcx, self.param_env, self.source_info.span);
 255                 // prevent
 256                 // * `&mut x.field`
 257                 // * `x.field = y;`
 258                 // * `&x.field` if `field`'s type has interior mutability
 259                 // because either of these would allow modifying the layout constrained field and
 260                 // insert values that violate the layout constraints.
 261                 if context.is_mutating_use() || is_borrow_of_interior_mut {
 262                     self.check_mut_borrowing_layout_constrained_field(
 263                         place, context.is_mutating_use(),
 264                     );
 265                 }
 266                 let old_source_info = self.source_info;
 267                 if let Place::Base(PlaceBase::Local(local)) = proj.base {
 268                     if self.body.local_decls[local].internal {
 269                         // Internal locals are used in the `move_val_init` desugaring.
 270                         // We want to check unsafety against the source info of the
 271                         // desugaring, rather than the source info of the RHS.
 272                         self.source_info = self.body.local_decls[local].source_info;
 273                     }
 274                 }
 275                 let base_ty = proj.base.ty(self.body, self.tcx).ty;
 276                 match base_ty.sty {
 277                     ty::RawPtr(..) => {
 278                         self.require_unsafe("dereference of raw pointer",
 279                             "raw pointers may be NULL, dangling or unaligned; they can violate \
 280                              aliasing rules and cause data races: all of these are undefined \
 281                              behavior", UnsafetyViolationKind::General)
 282                     }
 283                     ty::Adt(adt, _) => {
 284                         if adt.is_union() {
 285                             if context == PlaceContext::MutatingUse(MutatingUseContext::Store) ||
 286                                 context == PlaceContext::MutatingUse(MutatingUseContext::Drop) ||
 287                                 context == PlaceContext::MutatingUse(
 288                                     MutatingUseContext::AsmOutput
 289                                 )
 290                             {
 291                                 let elem_ty = match proj.elem {
 292                                     ProjectionElem::Field(_, ty) => ty,
 293                                     _ => span_bug!(
 294                                         self.source_info.span,
 295                                         "non-field projection {:?} from union?",
 296                                         place)
 297                                 };
 298                                 if !elem_ty.is_copy_modulo_regions(
 299                                     self.tcx,
 300                                     self.param_env,
 301                                     self.source_info.span,
 302                                 ) {
 303                                     self.require_unsafe(
 304                                         "assignment to non-`Copy` union field",
 305                                         "the previous content of the field will be dropped, which \
 306                                          causes undefined behavior if the field was not properly \
 307                                          initialized", UnsafetyViolationKind::General)
 308                                 } else {
 309                                     // write to non-move union, safe
 310                                 }
 311                             } else {
 312                                 self.require_unsafe("access to union field",
 313                                     "the field may not be properly initialized: using \
 314                                      uninitialized data will cause undefined behavior",
 315                                      UnsafetyViolationKind::General)
 316                             }
 317                         }
 318                     }
 319                     _ => {}
 320                 }
 321                 self.source_info = old_source_info;
 322             }
 323         });
 324     }
 325 }
 326
 327 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
 328     fn require_unsafe(
 329         &mut self,
 330         description: &'static str,
 331         details: &'static str,
 332         kind: UnsafetyViolationKind,
 333     ) {
 334         let source_info = self.source_info;
 335         self.register_violations(&[UnsafetyViolation {
 336             source_info,
 337             description: InternedString::intern(description),
 338             details: InternedString::intern(details),
 339             kind,
 340         }], &[]);
 341     }
 342
 343     fn register_violations(&mut self,
 344                            violations: &[UnsafetyViolation],
 345                            unsafe_blocks: &[(hir::HirId, bool)]) {
 346         let safety = self.source_scope_local_data[self.source_info.scope].safety;
 347         let within_unsafe = match safety {
 348             // `unsafe` blocks are required in safe code
 349             Safety::Safe => {
 350                 for violation in violations {
 351                     let mut violation = violation.clone();
 352                     match violation.kind {
 353                         UnsafetyViolationKind::GeneralAndConstFn |
 354                         UnsafetyViolationKind::General => {},
 355                         UnsafetyViolationKind::BorrowPacked(_) |
 356                         UnsafetyViolationKind::ExternStatic(_) => if self.min_const_fn {
 357                             // const fns don't need to be backwards compatible and can
 358                             // emit these violations as a hard error instead of a backwards
 359                             // compat lint
 360                             violation.kind = UnsafetyViolationKind::General;
 361                         },
 362                     }
 363                     if !self.violations.contains(&violation) {
 364                         self.violations.push(violation)
 365                     }
 366                 }
 367                 false
 368             }
 369             // `unsafe` function bodies allow unsafe without additional unsafe blocks
 370             Safety::BuiltinUnsafe | Safety::FnUnsafe => true,
 371             Safety::ExplicitUnsafe(hir_id) => {
 372                 // mark unsafe block as used if there are any unsafe operations inside
 373                 if !violations.is_empty() {
 374                     self.used_unsafe.insert(hir_id);
 375                 }
 376                 // only some unsafety is allowed in const fn
 377                 if self.min_const_fn {
 378                     for violation in violations {
 379                         match violation.kind {
 380                             // these unsafe things are stable in const fn
 381                             UnsafetyViolationKind::GeneralAndConstFn => {},
 382                             // these things are forbidden in const fns
 383                             UnsafetyViolationKind::General |
 384                             UnsafetyViolationKind::BorrowPacked(_) |
 385                             UnsafetyViolationKind::ExternStatic(_) => {
 386                                 let mut violation = violation.clone();
 387                                 // const fns don't need to be backwards compatible and can
 388                                 // emit these violations as a hard error instead of a backwards
 389                                 // compat lint
 390                                 violation.kind = UnsafetyViolationKind::General;
 391                                 if !self.violations.contains(&violation) {
 392                                     self.violations.push(violation)
 393                                 }
 394                             },
 395                         }
 396                     }
 397                 }
 398                 true
 399             }
 400         };
 401         self.inherited_blocks.extend(unsafe_blocks.iter().map(|&(hir_id, is_used)| {
 402             (hir_id, is_used && !within_unsafe)
 403         }));
 404     }
 405     fn check_mut_borrowing_layout_constrained_field(
 406         &mut self,
 407         mut place: &Place<'tcx>,
 408         is_mut_use: bool,
 409     ) {
 410         while let &Place::Projection(box Projection {
 411             ref base, ref elem
 412         }) = place {
 413             match *elem {
 414                 ProjectionElem::Field(..) => {
 415                     let ty = base.ty(&self.body.local_decls, self.tcx).ty;
 416                     match ty.sty {
 417                         ty::Adt(def, _) => match self.tcx.layout_scalar_valid_range(def.did) {
 418                             (Bound::Unbounded, Bound::Unbounded) => {},
 419                             _ => {
 420                                 let (description, details) = if is_mut_use {
 421                                     (
 422                                         "mutation of layout constrained field",
 423                                         "mutating layout constrained fields cannot statically be \
 424                                         checked for valid values",
 425                                     )
 426                                 } else {
 427                                     (
 428                                         "borrow of layout constrained field with interior \
 429                                         mutability",
 430                                         "references to fields of layout constrained fields \
 431                                         lose the constraints. Coupled with interior mutability, \
 432                                         the field can be changed to invalid values",
 433                                     )
 434                                 };
 435                                 let source_info = self.source_info;
 436                                 self.register_violations(&[UnsafetyViolation {
 437                                     source_info,
 438                                     description: InternedString::intern(description),
 439                                     details: InternedString::intern(details),
 440                                     kind: UnsafetyViolationKind::GeneralAndConstFn,
 441                                 }], &[]);
 442                             }
 443                         },
 444                         _ => {}
 445                     }
 446                 }
 447                 _ => {}
 448             }
 449             place = base;
 450         }
 451     }
 452 }
 453
 454 pub(crate) fn provide(providers: &mut Providers<'_>) {
 455     *providers = Providers {
 456         unsafety_check_result,
 457         unsafe_derive_on_repr_packed,
 458         ..*providers
 459     };
 460 }
 461
 462 struct UnusedUnsafeVisitor<'a> {
 463     used_unsafe: &'a FxHashSet<hir::HirId>,
 464     unsafe_blocks: &'a mut Vec<(hir::HirId, bool)>,
 465 }
 466
 467 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'a> {
 468     fn nested_visit_map<'this>(&'this mut self) ->
 469         hir::intravisit::NestedVisitorMap<'this, 'tcx>
 470     {
 471         hir::intravisit::NestedVisitorMap::None
 472     }
 473
 474     fn visit_block(&mut self, block: &'tcx hir::Block) {
 475         hir::intravisit::walk_block(self, block);
 476
 477         if let hir::UnsafeBlock(hir::UserProvided) = block.rules {
 478             self.unsafe_blocks.push((block.hir_id, self.used_unsafe.contains(&block.hir_id)));
 479         }
 480     }
 481 }
 482
 483 fn check_unused_unsafe<'a, 'tcx>(
 484     tcx: TyCtxt<'tcx>,
 485     def_id: DefId,
 486     used_unsafe: &FxHashSet<hir::HirId>,
 487     unsafe_blocks: &'a mut Vec<(hir::HirId, bool)>,
 488 ) {
 489     let body_id =
 490         tcx.hir().as_local_hir_id(def_id).and_then(|hir_id| {
 491             tcx.hir().maybe_body_owned_by(hir_id)
 492         });
 493
 494     let body_id = match body_id {
 495         Some(body) => body,
 496         None => {
 497             debug!("check_unused_unsafe({:?}) - no body found", def_id);
 498             return
 499         }
 500     };
 501     let body = tcx.hir().body(body_id);
 502     debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})",
 503            def_id, body, used_unsafe);
 504
 505     let mut visitor =  UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
 506     hir::intravisit::Visitor::visit_body(&mut visitor, body);
 507 }
 508
 509 fn unsafety_check_result<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> UnsafetyCheckResult {
 510     debug!("unsafety_violations({:?})", def_id);
 511
 512     // N.B., this borrow is valid because all the consumers of
 513     // `mir_built` force this.
 514     let body = &tcx.mir_built(def_id).borrow();
 515
 516     let source_scope_local_data = match body.source_scope_local_data {
 517         ClearCrossCrate::Set(ref data) => data,
 518         ClearCrossCrate::Clear => {
 519             debug!("unsafety_violations: {:?} - remote, skipping", def_id);
 520             return UnsafetyCheckResult {
 521                 violations: Lrc::new([]),
 522                 unsafe_blocks: Lrc::new([])
 523             }
 524         }
 525     };
 526
 527     let param_env = tcx.param_env(def_id);
 528
 529     let id = tcx.hir().as_local_hir_id(def_id).unwrap();
 530     let (const_context, min_const_fn) = match tcx.hir().body_owner_kind(id) {
 531         hir::BodyOwnerKind::Closure => (false, false),
 532         hir::BodyOwnerKind::Fn => (tcx.is_const_fn(def_id), tcx.is_min_const_fn(def_id)),
 533         hir::BodyOwnerKind::Const |
 534         hir::BodyOwnerKind::Static(_) => (true, false),
 535     };
 536     let mut checker = UnsafetyChecker::new(
 537         const_context, min_const_fn,
 538         body, source_scope_local_data, tcx, param_env);
 539     checker.visit_body(body);
 540
 541     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
 542     UnsafetyCheckResult {
 543         violations: checker.violations.into(),
 544         unsafe_blocks: checker.inherited_blocks.into()
 545     }
 546 }
 547
 548 fn unsafe_derive_on_repr_packed<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) {
 549     let lint_hir_id = tcx.hir().as_local_hir_id(def_id).unwrap_or_else(||
 550         bug!("checking unsafety for non-local def id {:?}", def_id));
 551
 552     // FIXME: when we make this a hard error, this should have its
 553     // own error code.
 554     let message = if tcx.generics_of(def_id).own_requires_monomorphization() {
 555         "#[derive] can't be used on a #[repr(packed)] struct with \
 556          type or const parameters (error E0133)".to_string()
 557     } else {
 558         "#[derive] can't be used on a #[repr(packed)] struct that \
 559          does not derive Copy (error E0133)".to_string()
 560     };
 561     tcx.lint_hir(SAFE_PACKED_BORROWS,
 562                  lint_hir_id,
 563                  tcx.def_span(def_id),
 564                  &message);
 565 }
 566
 567 /// Returns the `HirId` for an enclosing scope that is also `unsafe`.
 568 fn is_enclosed(
 569     tcx: TyCtxt<'_>,
 570     used_unsafe: &FxHashSet<hir::HirId>,
 571     id: hir::HirId,
 572 ) -> Option<(String, hir::HirId)> {
 573     let parent_id = tcx.hir().get_parent_node(id);
 574     if parent_id != id {
 575         if used_unsafe.contains(&parent_id) {
 576             Some(("block".to_string(), parent_id))
 577         } else if let Some(Node::Item(&hir::Item {
 578             node: hir::ItemKind::Fn(_, header, _, _),
 579             ..
 580         })) = tcx.hir().find_by_hir_id(parent_id) {
 581             match header.unsafety {
 582                 hir::Unsafety::Unsafe => Some(("fn".to_string(), parent_id)),
 583                 hir::Unsafety::Normal => None,
 584             }
 585         } else {
 586             is_enclosed(tcx, used_unsafe, parent_id)
 587         }
 588     } else {
 589         None
 590     }
 591 }
 592
 593 fn report_unused_unsafe(tcx: TyCtxt<'_>, used_unsafe: &FxHashSet<hir::HirId>, id: hir::HirId) {
 594     let span = tcx.sess.source_map().def_span(tcx.hir().span(id));
 595     let msg = "unnecessary `unsafe` block";
 596     let mut db = tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, msg);
 597     db.span_label(span, msg);
 598     if let Some((kind, id)) = is_enclosed(tcx, used_unsafe, id) {
 599         db.span_label(tcx.sess.source_map().def_span(tcx.hir().span(id)),
 600                       format!("because it's nested under this `unsafe` {}", kind));
 601     }
 602     db.emit();
 603 }
 604
 605 fn builtin_derive_def_id<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Option<DefId> {
 606     debug!("builtin_derive_def_id({:?})", def_id);
 607     if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
 608         if tcx.has_attr(impl_def_id, sym::automatically_derived) {
 609             debug!("builtin_derive_def_id({:?}) - is {:?}", def_id, impl_def_id);
 610             Some(impl_def_id)
 611         } else {
 612             debug!("builtin_derive_def_id({:?}) - not automatically derived", def_id);
 613             None
 614         }
 615     } else {
 616         debug!("builtin_derive_def_id({:?}) - not a method", def_id);
 617         None
 618     }
 619 }
 620
 621 pub fn check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) {
 622     debug!("check_unsafety({:?})", def_id);
 623
 624     // closures are handled by their parent fn.
 625     if tcx.is_closure(def_id) {
 626         return;
 627     }
 628
 629     let UnsafetyCheckResult {
 630         violations,
 631         unsafe_blocks
 632     } = tcx.unsafety_check_result(def_id);
 633
 634     for &UnsafetyViolation {
 635         source_info, description, details, kind
 636     } in violations.iter() {
 637         // Report an error.
 638         match kind {
 639             UnsafetyViolationKind::GeneralAndConstFn |
 640             UnsafetyViolationKind::General => {
 641                 struct_span_err!(
 642                     tcx.sess, source_info.span, E0133,
 643                     "{} is unsafe and requires unsafe function or block", description)
 644                     .span_label(source_info.span, &description.as_str()[..])
 645                     .note(&details.as_str()[..])
 646                     .emit();
 647             }
 648             UnsafetyViolationKind::ExternStatic(lint_hir_id) => {
 649                 tcx.lint_node_note(SAFE_EXTERN_STATICS,
 650                               lint_hir_id,
 651                               source_info.span,
 652                               &format!("{} is unsafe and requires unsafe function or block \
 653                                         (error E0133)", &description.as_str()[..]),
 654                               &details.as_str()[..]);
 655             }
 656             UnsafetyViolationKind::BorrowPacked(lint_hir_id) => {
 657                 if let Some(impl_def_id) = builtin_derive_def_id(tcx, def_id) {
 658                     tcx.unsafe_derive_on_repr_packed(impl_def_id);
 659                 } else {
 660                     tcx.lint_node_note(SAFE_PACKED_BORROWS,
 661                                   lint_hir_id,
 662                                   source_info.span,
 663                                   &format!("{} is unsafe and requires unsafe function or block \
 664                                             (error E0133)", &description.as_str()[..]),
 665                                   &details.as_str()[..]);
 666                 }
 667             }
 668         }
 669     }
 670
 671     let mut unsafe_blocks: Vec<_> = unsafe_blocks.into_iter().collect();
 672     unsafe_blocks.sort_by_cached_key(|(hir_id, _)| tcx.hir().hir_to_node_id(*hir_id));
 673     let used_unsafe: FxHashSet<_> = unsafe_blocks.iter()
 674         .flat_map(|&&(id, used)| if used { Some(id) } else { None })
 675         .collect();
 676     for &(block_id, is_used) in unsafe_blocks {
 677         if !is_used {
 678             report_unused_unsafe(tcx, &used_unsafe, block_id);
 679         }
 680     }
 681 }