src/librustc_mir/transform/check_unsafety.rs

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