src/librustc_mir/transform/check_unsafety.rs

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