src/librustc_mir/transform/check_unsafety.rs

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