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).kind {
 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         match place.base {
 204             PlaceBase::Local(..) => {
 205                 // Locals are safe.
 206             }
 207             PlaceBase::Static(box Static { kind: StaticKind::Promoted(_, _), .. }) => {
 208                 bug!("unsafety checking should happen before promotion")
 209             }
 210             PlaceBase::Static(box Static { kind: StaticKind::Static, def_id, .. }) => {
 211                 if self.tcx.is_mutable_static(def_id) {
 212                     self.require_unsafe("use of mutable static",
 213                         "mutable statics can be mutated by multiple threads: aliasing \
 214                          violations or data races will cause undefined behavior",
 215                          UnsafetyViolationKind::General);
 216                 } else if self.tcx.is_foreign_item(def_id) {
 217                     let source_info = self.source_info;
 218                     let lint_root =
 219                         self.source_scope_local_data[source_info.scope].lint_root;
 220                     self.register_violations(&[UnsafetyViolation {
 221                         source_info,
 222                         description: InternedString::intern("use of extern static"),
 223                         details: InternedString::intern(
 224                             "extern statics are not controlled by the Rust type system: \
 225                             invalid data, aliasing violations or data races will cause \
 226                             undefined behavior"),
 227                         kind: UnsafetyViolationKind::ExternStatic(lint_root)
 228                     }], &[]);
 229                 }
 230             }
 231         }
 232
 233         for (i, elem) in place.projection.iter().enumerate() {
 234             let proj_base = &place.projection[..i];
 235
 236             if context.is_borrow() {
 237                 if util::is_disaligned(self.tcx, self.body, self.param_env, place) {
 238                     let source_info = self.source_info;
 239                     let lint_root =
 240                         self.source_scope_local_data[source_info.scope].lint_root;
 241                     self.register_violations(&[UnsafetyViolation {
 242                         source_info,
 243                         description: InternedString::intern("borrow of packed field"),
 244                         details: InternedString::intern(
 245                             "fields of packed structs might be misaligned: dereferencing a \
 246                             misaligned pointer or even just creating a misaligned reference \
 247                             is undefined behavior"),
 248                         kind: UnsafetyViolationKind::BorrowPacked(lint_root)
 249                     }], &[]);
 250                 }
 251             }
 252             let is_borrow_of_interior_mut = context.is_borrow() &&
 253                 !Place::ty_from(&place.base, proj_base, self.body, self.tcx)
 254                 .ty
 255                 .is_freeze(self.tcx, self.param_env, self.source_info.span);
 256             // prevent
 257             // * `&mut x.field`
 258             // * `x.field = y;`
 259             // * `&x.field` if `field`'s type has interior mutability
 260             // because either of these would allow modifying the layout constrained field and
 261             // insert values that violate the layout constraints.
 262             if context.is_mutating_use() || is_borrow_of_interior_mut {
 263                 self.check_mut_borrowing_layout_constrained_field(
 264                     place, context.is_mutating_use(),
 265                 );
 266             }
 267             let old_source_info = self.source_info;
 268             if let (PlaceBase::Local(local), []) = (&place.base, proj_base) {
 269                 if self.body.local_decls[*local].internal {
 270                     // Internal locals are used in the `move_val_init` desugaring.
 271                     // We want to check unsafety against the source info of the
 272                     // desugaring, rather than the source info of the RHS.
 273                     self.source_info = self.body.local_decls[*local].source_info;
 274                 }
 275             }
 276             let base_ty = Place::ty_from(&place.base, proj_base, self.body, self.tcx).ty;
 277             match base_ty.kind {
 278                 ty::RawPtr(..) => {
 279                     self.require_unsafe("dereference of raw pointer",
 280                         "raw pointers may be NULL, dangling or unaligned; they can violate \
 281                          aliasing rules and cause data races: all of these are undefined \
 282                          behavior", UnsafetyViolationKind::General)
 283                 }
 284                 ty::Adt(adt, _) => {
 285                     if adt.is_union() {
 286                         if context == PlaceContext::MutatingUse(MutatingUseContext::Store) ||
 287                             context == PlaceContext::MutatingUse(MutatingUseContext::Drop) ||
 288                             context == PlaceContext::MutatingUse(
 289                                 MutatingUseContext::AsmOutput
 290                             )
 291                         {
 292                             let elem_ty = match elem {
 293                                 ProjectionElem::Field(_, ty) => ty,
 294                                 _ => span_bug!(
 295                                     self.source_info.span,
 296                                     "non-field projection {:?} from union?",
 297                                     place)
 298                             };
 299                             if !elem_ty.is_copy_modulo_regions(
 300                                 self.tcx,
 301                                 self.param_env,
 302                                 self.source_info.span,
 303                             ) {
 304                                 self.require_unsafe(
 305                                     "assignment to non-`Copy` union field",
 306                                     "the previous content of the field will be dropped, which \
 307                                      causes undefined behavior if the field was not properly \
 308                                      initialized", UnsafetyViolationKind::GeneralAndConstFn)
 309                             } else {
 310                                 // write to non-move union, safe
 311                             }
 312                         } else {
 313                             self.require_unsafe("access to union field",
 314                                 "the field may not be properly initialized: using \
 315                                  uninitialized data will cause undefined behavior",
 316                                  UnsafetyViolationKind::GeneralAndConstFn)
 317                         }
 318                     }
 319                 }
 320                 _ => {}
 321             }
 322             self.source_info = old_source_info;
 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         place: &Place<'tcx>,
 408         is_mut_use: bool,
 409     ) {
 410         let mut cursor = &*place.projection;
 411         while let [proj_base @ .., elem] = cursor {
 412             cursor = proj_base;
 413
 414             match elem {
 415                 ProjectionElem::Field(..) => {
 416                     let ty =
 417                         Place::ty_from(&place.base, proj_base, &self.body.local_decls, self.tcx)
 418                             .ty;
 419                     match ty.kind {
 420                         ty::Adt(def, _) => match self.tcx.layout_scalar_valid_range(def.did) {
 421                             (Bound::Unbounded, Bound::Unbounded) => {},
 422                             _ => {
 423                                 let (description, details) = if is_mut_use {
 424                                     (
 425                                         "mutation of layout constrained field",
 426                                         "mutating layout constrained fields cannot statically be \
 427                                         checked for valid values",
 428                                     )
 429                                 } else {
 430                                     (
 431                                         "borrow of layout constrained field with interior \
 432                                         mutability",
 433                                         "references to fields of layout constrained fields \
 434                                         lose the constraints. Coupled with interior mutability, \
 435                                         the field can be changed to invalid values",
 436                                     )
 437                                 };
 438                                 let source_info = self.source_info;
 439                                 self.register_violations(&[UnsafetyViolation {
 440                                     source_info,
 441                                     description: InternedString::intern(description),
 442                                     details: InternedString::intern(details),
 443                                     kind: UnsafetyViolationKind::GeneralAndConstFn,
 444                                 }], &[]);
 445                             }
 446                         },
 447                         _ => {}
 448                     }
 449                 }
 450                 _ => {}
 451             }
 452         }
 453     }
 454 }
 455
 456 pub(crate) fn provide(providers: &mut Providers<'_>) {
 457     *providers = Providers {
 458         unsafety_check_result,
 459         unsafe_derive_on_repr_packed,
 460         ..*providers
 461     };
 462 }
 463
 464 struct UnusedUnsafeVisitor<'a> {
 465     used_unsafe: &'a FxHashSet<hir::HirId>,
 466     unsafe_blocks: &'a mut Vec<(hir::HirId, bool)>,
 467 }
 468
 469 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'a> {
 470     fn nested_visit_map<'this>(&'this mut self) ->
 471         hir::intravisit::NestedVisitorMap<'this, 'tcx>
 472     {
 473         hir::intravisit::NestedVisitorMap::None
 474     }
 475
 476     fn visit_block(&mut self, block: &'tcx hir::Block) {
 477         hir::intravisit::walk_block(self, block);
 478
 479         if let hir::UnsafeBlock(hir::UserProvided) = block.rules {
 480             self.unsafe_blocks.push((block.hir_id, self.used_unsafe.contains(&block.hir_id)));
 481         }
 482     }
 483 }
 484
 485 fn check_unused_unsafe(
 486     tcx: TyCtxt<'_>,
 487     def_id: DefId,
 488     used_unsafe: &FxHashSet<hir::HirId>,
 489     unsafe_blocks: &mut Vec<(hir::HirId, bool)>,
 490 ) {
 491     let body_id =
 492         tcx.hir().as_local_hir_id(def_id).and_then(|hir_id| {
 493             tcx.hir().maybe_body_owned_by(hir_id)
 494         });
 495
 496     let body_id = match body_id {
 497         Some(body) => body,
 498         None => {
 499             debug!("check_unused_unsafe({:?}) - no body found", def_id);
 500             return
 501         }
 502     };
 503     let body = tcx.hir().body(body_id);
 504     debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})",
 505            def_id, body, used_unsafe);
 506
 507     let mut visitor =  UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
 508     hir::intravisit::Visitor::visit_body(&mut visitor, body);
 509 }
 510
 511 fn unsafety_check_result(tcx: TyCtxt<'_>, def_id: DefId) -> UnsafetyCheckResult {
 512     debug!("unsafety_violations({:?})", def_id);
 513
 514     // N.B., this borrow is valid because all the consumers of
 515     // `mir_built` force this.
 516     let body = &tcx.mir_built(def_id).borrow();
 517
 518     let source_scope_local_data = match body.source_scope_local_data {
 519         ClearCrossCrate::Set(ref data) => data,
 520         ClearCrossCrate::Clear => {
 521             debug!("unsafety_violations: {:?} - remote, skipping", def_id);
 522             return UnsafetyCheckResult {
 523                 violations: Lrc::new([]),
 524                 unsafe_blocks: Lrc::new([])
 525             }
 526         }
 527     };
 528
 529     let param_env = tcx.param_env(def_id);
 530
 531     let id = tcx.hir().as_local_hir_id(def_id).unwrap();
 532     let (const_context, min_const_fn) = match tcx.hir().body_owner_kind(id) {
 533         hir::BodyOwnerKind::Closure => (false, false),
 534         hir::BodyOwnerKind::Fn => (tcx.is_const_fn(def_id), tcx.is_min_const_fn(def_id)),
 535         hir::BodyOwnerKind::Const |
 536         hir::BodyOwnerKind::Static(_) => (true, false),
 537     };
 538     let mut checker = UnsafetyChecker::new(
 539         const_context, min_const_fn,
 540         body, source_scope_local_data, tcx, param_env);
 541     checker.visit_body(body);
 542
 543     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
 544     UnsafetyCheckResult {
 545         violations: checker.violations.into(),
 546         unsafe_blocks: checker.inherited_blocks.into()
 547     }
 548 }
 549
 550 fn unsafe_derive_on_repr_packed(tcx: TyCtxt<'_>, def_id: DefId) {
 551     let lint_hir_id = tcx.hir().as_local_hir_id(def_id).unwrap_or_else(||
 552         bug!("checking unsafety for non-local def id {:?}", def_id));
 553
 554     // FIXME: when we make this a hard error, this should have its
 555     // own error code.
 556     let message = if tcx.generics_of(def_id).own_requires_monomorphization() {
 557         "`#[derive]` can't be used on a `#[repr(packed)]` struct with \
 558          type or const parameters (error E0133)".to_string()
 559     } else {
 560         "`#[derive]` can't be used on a `#[repr(packed)]` struct that \
 561          does not derive Copy (error E0133)".to_string()
 562     };
 563     tcx.lint_hir(SAFE_PACKED_BORROWS,
 564                  lint_hir_id,
 565                  tcx.def_span(def_id),
 566                  &message);
 567 }
 568
 569 /// Returns the `HirId` for an enclosing scope that is also `unsafe`.
 570 fn is_enclosed(
 571     tcx: TyCtxt<'_>,
 572     used_unsafe: &FxHashSet<hir::HirId>,
 573     id: hir::HirId,
 574 ) -> Option<(String, hir::HirId)> {
 575     let parent_id = tcx.hir().get_parent_node(id);
 576     if parent_id != id {
 577         if used_unsafe.contains(&parent_id) {
 578             Some(("block".to_string(), parent_id))
 579         } else if let Some(Node::Item(&hir::Item {
 580             node: hir::ItemKind::Fn(_, header, _, _),
 581             ..
 582         })) = tcx.hir().find(parent_id) {
 583             match header.unsafety {
 584                 hir::Unsafety::Unsafe => Some(("fn".to_string(), parent_id)),
 585                 hir::Unsafety::Normal => None,
 586             }
 587         } else {
 588             is_enclosed(tcx, used_unsafe, parent_id)
 589         }
 590     } else {
 591         None
 592     }
 593 }
 594
 595 fn report_unused_unsafe(tcx: TyCtxt<'_>, used_unsafe: &FxHashSet<hir::HirId>, id: hir::HirId) {
 596     let span = tcx.sess.source_map().def_span(tcx.hir().span(id));
 597     let msg = "unnecessary `unsafe` block";
 598     let mut db = tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, msg);
 599     db.span_label(span, msg);
 600     if let Some((kind, id)) = is_enclosed(tcx, used_unsafe, id) {
 601         db.span_label(tcx.sess.source_map().def_span(tcx.hir().span(id)),
 602                       format!("because it's nested under this `unsafe` {}", kind));
 603     }
 604     db.emit();
 605 }
 606
 607 fn builtin_derive_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
 608     debug!("builtin_derive_def_id({:?})", def_id);
 609     if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
 610         if tcx.has_attr(impl_def_id, sym::automatically_derived) {
 611             debug!("builtin_derive_def_id({:?}) - is {:?}", def_id, impl_def_id);
 612             Some(impl_def_id)
 613         } else {
 614             debug!("builtin_derive_def_id({:?}) - not automatically derived", def_id);
 615             None
 616         }
 617     } else {
 618         debug!("builtin_derive_def_id({:?}) - not a method", def_id);
 619         None
 620     }
 621 }
 622
 623 pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: DefId) {
 624     debug!("check_unsafety({:?})", def_id);
 625
 626     // closures are handled by their parent fn.
 627     if tcx.is_closure(def_id) {
 628         return;
 629     }
 630
 631     let UnsafetyCheckResult {
 632         violations,
 633         unsafe_blocks
 634     } = tcx.unsafety_check_result(def_id);
 635
 636     for &UnsafetyViolation {
 637         source_info, description, details, kind
 638     } in violations.iter() {
 639         // Report an error.
 640         match kind {
 641             UnsafetyViolationKind::GeneralAndConstFn |
 642             UnsafetyViolationKind::General => {
 643                 struct_span_err!(
 644                     tcx.sess, source_info.span, E0133,
 645                     "{} is unsafe and requires unsafe function or block", description)
 646                     .span_label(source_info.span, &description.as_str()[..])
 647                     .note(&details.as_str()[..])
 648                     .emit();
 649             }
 650             UnsafetyViolationKind::ExternStatic(lint_hir_id) => {
 651                 tcx.lint_node_note(SAFE_EXTERN_STATICS,
 652                               lint_hir_id,
 653                               source_info.span,
 654                               &format!("{} is unsafe and requires unsafe function or block \
 655                                         (error E0133)", &description.as_str()[..]),
 656                               &details.as_str()[..]);
 657             }
 658             UnsafetyViolationKind::BorrowPacked(lint_hir_id) => {
 659                 if let Some(impl_def_id) = builtin_derive_def_id(tcx, def_id) {
 660                     tcx.unsafe_derive_on_repr_packed(impl_def_id);
 661                 } else {
 662                     tcx.lint_node_note(SAFE_PACKED_BORROWS,
 663                                   lint_hir_id,
 664                                   source_info.span,
 665                                   &format!("{} is unsafe and requires unsafe function or block \
 666                                             (error E0133)", &description.as_str()[..]),
 667                                   &details.as_str()[..]);
 668                 }
 669             }
 670         }
 671     }
 672
 673     let mut unsafe_blocks: Vec<_> = unsafe_blocks.into_iter().collect();
 674     unsafe_blocks.sort_by_cached_key(|(hir_id, _)| tcx.hir().hir_to_node_id(*hir_id));
 675     let used_unsafe: FxHashSet<_> = unsafe_blocks.iter()
 676         .flat_map(|&&(id, used)| if used { Some(id) } else { None })
 677         .collect();
 678     for &(block_id, is_used) in unsafe_blocks {
 679         if !is_used {
 680             report_unused_unsafe(tcx, &used_unsafe, block_id);
 681         }
 682     }
 683 }