src/librustc_mir/transform/check_unsafety.rs

   1 use rustc_data_structures::fx::FxHashSet;
   2 use rustc_data_structures::indexed_vec::IndexVec;
   3 use rustc_data_structures::sync::Lrc;
   4
   5 use rustc::ty::query::Providers;
   6 use rustc::ty::{self, TyCtxt};
   7 use rustc::hir;
   8 use rustc::hir::Node;
   9 use rustc::hir::def_id::DefId;
  10 use rustc::lint::builtin::{SAFE_EXTERN_STATICS, SAFE_PACKED_BORROWS, UNUSED_UNSAFE};
  11 use rustc::mir::*;
  12 use rustc::mir::visit::{PlaceContext, Visitor, MutatingUseContext};
  13
  14 use syntax::ast;
  15 use syntax::symbol::Symbol;
  16
  17 use std::ops::Bound;
  18
  19 use util;
  20
  21 pub struct UnsafetyChecker<'a, 'tcx: 'a> {
  22     mir: &'a Mir<'tcx>,
  23     min_const_fn: bool,
  24     source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  25     violations: Vec<UnsafetyViolation>,
  26     source_info: SourceInfo,
  27     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  28     param_env: ty::ParamEnv<'tcx>,
  29     /// mark an `unsafe` block as used, so we don't lint it
  30     used_unsafe: FxHashSet<ast::NodeId>,
  31     inherited_blocks: Vec<(ast::NodeId, bool)>,
  32 }
  33
  34 impl<'a, 'gcx, 'tcx> UnsafetyChecker<'a, 'tcx> {
  35     fn new(
  36         min_const_fn: bool,
  37         mir: &'a Mir<'tcx>,
  38         source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  39         tcx: TyCtxt<'a, 'tcx, 'tcx>,
  40         param_env: ty::ParamEnv<'tcx>,
  41     ) -> Self {
  42         Self {
  43             mir,
  44             min_const_fn,
  45             source_scope_local_data,
  46             violations: vec![],
  47             source_info: SourceInfo {
  48                 span: mir.span,
  49                 scope: OUTERMOST_SOURCE_SCOPE
  50             },
  51             tcx,
  52             param_env,
  53             used_unsafe: Default::default(),
  54             inherited_blocks: vec![],
  55         }
  56     }
  57 }
  58
  59 impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
  60     fn visit_terminator(&mut self,
  61                         block: BasicBlock,
  62                         terminator: &Terminator<'tcx>,
  63                         location: Location)
  64     {
  65         self.source_info = terminator.source_info;
  66         match terminator.kind {
  67             TerminatorKind::Goto { .. } |
  68             TerminatorKind::SwitchInt { .. } |
  69             TerminatorKind::Drop { .. } |
  70             TerminatorKind::Yield { .. } |
  71             TerminatorKind::Assert { .. } |
  72             TerminatorKind::DropAndReplace { .. } |
  73             TerminatorKind::GeneratorDrop |
  74             TerminatorKind::Resume |
  75             TerminatorKind::Abort |
  76             TerminatorKind::Return |
  77             TerminatorKind::Unreachable |
  78             TerminatorKind::FalseEdges { .. } |
  79             TerminatorKind::FalseUnwind { .. } => {
  80                 // safe (at least as emitted during MIR construction)
  81             }
  82
  83             TerminatorKind::Call { ref func, .. } => {
  84                 let func_ty = func.ty(self.mir, self.tcx);
  85                 let sig = func_ty.fn_sig(self.tcx);
  86                 if let hir::Unsafety::Unsafe = sig.unsafety() {
  87                     self.require_unsafe("call to unsafe function",
  88                         "consult the function's documentation for information on how to avoid \
  89                          undefined behavior", UnsafetyViolationKind::GeneralAndConstFn)
  90                 }
  91             }
  92         }
  93         self.super_terminator(block, terminator, location);
  94     }
  95
  96     fn visit_statement(&mut self,
  97                        block: BasicBlock,
  98                        statement: &Statement<'tcx>,
  99                        location: Location)
 100     {
 101         self.source_info = statement.source_info;
 102         match statement.kind {
 103             StatementKind::Assign(..) |
 104             StatementKind::FakeRead(..) |
 105             StatementKind::SetDiscriminant { .. } |
 106             StatementKind::StorageLive(..) |
 107             StatementKind::StorageDead(..) |
 108             StatementKind::Retag { .. } |
 109             StatementKind::AscribeUserType(..) |
 110             StatementKind::Nop => {
 111                 // safe (at least as emitted during MIR construction)
 112             }
 113
 114             StatementKind::InlineAsm { .. } => {
 115                 self.require_unsafe("use of inline assembly",
 116                     "inline assembly is entirely unchecked and can cause undefined behavior",
 117                     UnsafetyViolationKind::General)
 118             },
 119         }
 120         self.super_statement(block, statement, location);
 121     }
 122
 123     fn visit_rvalue(&mut self,
 124                     rvalue: &Rvalue<'tcx>,
 125                     location: Location)
 126     {
 127         if let &Rvalue::Aggregate(box ref aggregate, _) = rvalue {
 128             match aggregate {
 129                 &AggregateKind::Array(..) |
 130                 &AggregateKind::Tuple => {}
 131                 &AggregateKind::Adt(ref def, ..) => {
 132                     match self.tcx.layout_scalar_valid_range(def.did) {
 133                         (Bound::Unbounded, Bound::Unbounded) => {},
 134                         _ => self.require_unsafe(
 135                             "initializing type with `rustc_layout_scalar_valid_range` attr",
 136                             "initializing a layout restricted type's field with a value outside \
 137                             the valid range is undefined behavior",
 138                             UnsafetyViolationKind::GeneralAndConstFn,
 139                         ),
 140                     }
 141                 }
 142                 &AggregateKind::Closure(def_id, _) |
 143                 &AggregateKind::Generator(def_id, _, _) => {
 144                     let UnsafetyCheckResult {
 145                         violations, unsafe_blocks
 146                     } = self.tcx.unsafety_check_result(def_id);
 147                     self.register_violations(&violations, &unsafe_blocks);
 148                 }
 149             }
 150         }
 151         self.super_rvalue(rvalue, location);
 152     }
 153
 154     fn visit_place(&mut self,
 155                     place: &Place<'tcx>,
 156                     context: PlaceContext<'tcx>,
 157                     location: Location) {
 158         match place {
 159             &Place::Projection(box Projection {
 160                 ref base, ref elem
 161             }) => {
 162                 if context.is_borrow() {
 163                     if util::is_disaligned(self.tcx, self.mir, self.param_env, place) {
 164                         let source_info = self.source_info;
 165                         let lint_root =
 166                             self.source_scope_local_data[source_info.scope].lint_root;
 167                         self.register_violations(&[UnsafetyViolation {
 168                             source_info,
 169                             description: Symbol::intern("borrow of packed field").as_interned_str(),
 170                             details:
 171                                 Symbol::intern("fields of packed structs might be misaligned: \
 172                                                 dereferencing a misaligned pointer or even just \
 173                                                 creating a misaligned reference is undefined \
 174                                                 behavior")
 175                                     .as_interned_str(),
 176                             kind: UnsafetyViolationKind::BorrowPacked(lint_root)
 177                         }], &[]);
 178                     }
 179                 }
 180                 let is_borrow_of_interior_mut = context.is_borrow() && !base
 181                     .ty(self.mir, self.tcx)
 182                     .to_ty(self.tcx)
 183                     .is_freeze(self.tcx, self.param_env, self.source_info.span);
 184                 // prevent
 185                 // * `&mut x.field`
 186                 // * `x.field = y;`
 187                 // * `&x.field` if `field`'s type has interior mutability
 188                 // because either of these would allow modifying the layout constrained field and
 189                 // insert values that violate the layout constraints.
 190                 if context.is_mutating_use() || is_borrow_of_interior_mut {
 191                     self.check_mut_borrowing_layout_constrained_field(
 192                         place, context.is_mutating_use(),
 193                     );
 194                 }
 195                 let old_source_info = self.source_info;
 196                 if let &Place::Local(local) = base {
 197                     if self.mir.local_decls[local].internal {
 198                         // Internal locals are used in the `move_val_init` desugaring.
 199                         // We want to check unsafety against the source info of the
 200                         // desugaring, rather than the source info of the RHS.
 201                         self.source_info = self.mir.local_decls[local].source_info;
 202                     }
 203                 }
 204                 let base_ty = base.ty(self.mir, self.tcx).to_ty(self.tcx);
 205                 match base_ty.sty {
 206                     ty::RawPtr(..) => {
 207                         self.require_unsafe("dereference of raw pointer",
 208                             "raw pointers may be NULL, dangling or unaligned; they can violate \
 209                              aliasing rules and cause data races: all of these are undefined \
 210                              behavior", UnsafetyViolationKind::General)
 211                     }
 212                     ty::Adt(adt, _) => {
 213                         if adt.is_union() {
 214                             if context == PlaceContext::MutatingUse(MutatingUseContext::Store) ||
 215                                 context == PlaceContext::MutatingUse(MutatingUseContext::Drop) ||
 216                                 context == PlaceContext::MutatingUse(
 217                                     MutatingUseContext::AsmOutput
 218                                 )
 219                             {
 220                                 let elem_ty = match elem {
 221                                     &ProjectionElem::Field(_, ty) => ty,
 222                                     _ => span_bug!(
 223                                         self.source_info.span,
 224                                         "non-field projection {:?} from union?",
 225                                         place)
 226                                 };
 227                                 if !elem_ty.is_copy_modulo_regions(
 228                                     self.tcx,
 229                                     self.param_env,
 230                                     self.source_info.span,
 231                                 ) {
 232                                     self.require_unsafe(
 233                                         "assignment to non-`Copy` union field",
 234                                         "the previous content of the field will be dropped, which \
 235                                          causes undefined behavior if the field was not properly \
 236                                          initialized", UnsafetyViolationKind::General)
 237                                 } else {
 238                                     // write to non-move union, safe
 239                                 }
 240                             } else {
 241                                 self.require_unsafe("access to union field",
 242                                     "the field may not be properly initialized: using \
 243                                      uninitialized data will cause undefined behavior",
 244                                      UnsafetyViolationKind::General)
 245                             }
 246                         }
 247                     }
 248                     _ => {}
 249                 }
 250                 self.source_info = old_source_info;
 251             }
 252             &Place::Local(..) => {
 253                 // locals are safe
 254             }
 255             &Place::Promoted(_) => {
 256                 bug!("unsafety checking should happen before promotion")
 257             }
 258             &Place::Static(box Static { def_id, ty: _ }) => {
 259                 if self.tcx.is_static(def_id) == Some(hir::Mutability::MutMutable) {
 260                     self.require_unsafe("use of mutable static",
 261                         "mutable statics can be mutated by multiple threads: aliasing violations \
 262                          or data races will cause undefined behavior",
 263                          UnsafetyViolationKind::General);
 264                 } else if self.tcx.is_foreign_item(def_id) {
 265                     let source_info = self.source_info;
 266                     let lint_root =
 267                         self.source_scope_local_data[source_info.scope].lint_root;
 268                     self.register_violations(&[UnsafetyViolation {
 269                         source_info,
 270                         description: Symbol::intern("use of extern static").as_interned_str(),
 271                         details:
 272                             Symbol::intern("extern statics are not controlled by the Rust type \
 273                                             system: invalid data, aliasing violations or data \
 274                                             races will cause undefined behavior")
 275                                 .as_interned_str(),
 276                         kind: UnsafetyViolationKind::ExternStatic(lint_root)
 277                     }], &[]);
 278                 }
 279             }
 280         };
 281         self.super_place(place, context, location);
 282     }
 283 }
 284
 285 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
 286     fn require_unsafe(
 287         &mut self,
 288         description: &'static str,
 289         details: &'static str,
 290         kind: UnsafetyViolationKind,
 291     ) {
 292         let source_info = self.source_info;
 293         self.register_violations(&[UnsafetyViolation {
 294             source_info,
 295             description: Symbol::intern(description).as_interned_str(),
 296             details: Symbol::intern(details).as_interned_str(),
 297             kind,
 298         }], &[]);
 299     }
 300
 301     fn register_violations(&mut self,
 302                            violations: &[UnsafetyViolation],
 303                            unsafe_blocks: &[(ast::NodeId, bool)]) {
 304         let safety = self.source_scope_local_data[self.source_info.scope].safety;
 305         let within_unsafe = match safety {
 306             // `unsafe` blocks are required in safe code
 307             Safety::Safe => {
 308                 for violation in violations {
 309                     let mut violation = violation.clone();
 310                     match violation.kind {
 311                         UnsafetyViolationKind::GeneralAndConstFn |
 312                         UnsafetyViolationKind::General => {},
 313                         UnsafetyViolationKind::BorrowPacked(_) |
 314                         UnsafetyViolationKind::ExternStatic(_) => if self.min_const_fn {
 315                             // const fns don't need to be backwards compatible and can
 316                             // emit these violations as a hard error instead of a backwards
 317                             // compat lint
 318                             violation.kind = UnsafetyViolationKind::General;
 319                         },
 320                     }
 321                     if !self.violations.contains(&violation) {
 322                         self.violations.push(violation)
 323                     }
 324                 }
 325                 false
 326             }
 327             // `unsafe` function bodies allow unsafe without additional unsafe blocks
 328             Safety::BuiltinUnsafe | Safety::FnUnsafe => true,
 329             Safety::ExplicitUnsafe(node_id) => {
 330                 // mark unsafe block as used if there are any unsafe operations inside
 331                 if !violations.is_empty() {
 332                     self.used_unsafe.insert(node_id);
 333                 }
 334                 // only some unsafety is allowed in const fn
 335                 if self.min_const_fn {
 336                     for violation in violations {
 337                         match violation.kind {
 338                             // these unsafe things are stable in const fn
 339                             UnsafetyViolationKind::GeneralAndConstFn => {},
 340                             // these things are forbidden in const fns
 341                             UnsafetyViolationKind::General |
 342                             UnsafetyViolationKind::BorrowPacked(_) |
 343                             UnsafetyViolationKind::ExternStatic(_) => {
 344                                 let mut violation = violation.clone();
 345                                 // const fns don't need to be backwards compatible and can
 346                                 // emit these violations as a hard error instead of a backwards
 347                                 // compat lint
 348                                 violation.kind = UnsafetyViolationKind::General;
 349                                 if !self.violations.contains(&violation) {
 350                                     self.violations.push(violation)
 351                                 }
 352                             },
 353                         }
 354                     }
 355                 }
 356                 true
 357             }
 358         };
 359         self.inherited_blocks.extend(unsafe_blocks.iter().map(|&(node_id, is_used)| {
 360             (node_id, is_used && !within_unsafe)
 361         }));
 362     }
 363     fn check_mut_borrowing_layout_constrained_field(
 364         &mut self,
 365         mut place: &Place<'tcx>,
 366         is_mut_use: bool,
 367     ) {
 368         while let &Place::Projection(box Projection {
 369             ref base, ref elem
 370         }) = place {
 371             match *elem {
 372                 ProjectionElem::Field(..) => {
 373                     let ty = base.ty(&self.mir.local_decls, self.tcx).to_ty(self.tcx);
 374                     match ty.sty {
 375                         ty::Adt(def, _) => match self.tcx.layout_scalar_valid_range(def.did) {
 376                             (Bound::Unbounded, Bound::Unbounded) => {},
 377                             _ => {
 378                                 let (description, details) = if is_mut_use {
 379                                     (
 380                                         "mutation of layout constrained field",
 381                                         "mutating layout constrained fields cannot statically be \
 382                                         checked for valid values",
 383                                     )
 384                                 } else {
 385                                     (
 386                                         "borrow of layout constrained field with interior \
 387                                         mutability",
 388                                         "references to fields of layout constrained fields \
 389                                         lose the constraints. Coupled with interior mutability, \
 390                                         the field can be changed to invalid values",
 391                                     )
 392                                 };
 393                                 let source_info = self.source_info;
 394                                 self.register_violations(&[UnsafetyViolation {
 395                                     source_info,
 396                                     description: Symbol::intern(description).as_interned_str(),
 397                                     details: Symbol::intern(details).as_interned_str(),
 398                                     kind: UnsafetyViolationKind::GeneralAndConstFn,
 399                                 }], &[]);
 400                             }
 401                         },
 402                         _ => {}
 403                     }
 404                 }
 405                 _ => {}
 406             }
 407             place = base;
 408         }
 409     }
 410 }
 411
 412 pub(crate) fn provide(providers: &mut Providers) {
 413     *providers = Providers {
 414         unsafety_check_result,
 415         unsafe_derive_on_repr_packed,
 416         ..*providers
 417     };
 418 }
 419
 420 struct UnusedUnsafeVisitor<'a> {
 421     used_unsafe: &'a FxHashSet<ast::NodeId>,
 422     unsafe_blocks: &'a mut Vec<(ast::NodeId, bool)>,
 423 }
 424
 425 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'a> {
 426     fn nested_visit_map<'this>(&'this mut self) ->
 427         hir::intravisit::NestedVisitorMap<'this, 'tcx>
 428     {
 429         hir::intravisit::NestedVisitorMap::None
 430     }
 431
 432     fn visit_block(&mut self, block: &'tcx hir::Block) {
 433         hir::intravisit::walk_block(self, block);
 434
 435         if let hir::UnsafeBlock(hir::UserProvided) = block.rules {
 436             self.unsafe_blocks.push((block.id, self.used_unsafe.contains(&block.id)));
 437         }
 438     }
 439 }
 440
 441 fn check_unused_unsafe<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 442                                  def_id: DefId,
 443                                  used_unsafe: &FxHashSet<ast::NodeId>,
 444                                  unsafe_blocks: &'a mut Vec<(ast::NodeId, bool)>)
 445 {
 446     let body_id =
 447         tcx.hir().as_local_node_id(def_id).and_then(|node_id| {
 448             tcx.hir().maybe_body_owned_by(node_id)
 449         });
 450
 451     let body_id = match body_id {
 452         Some(body) => body,
 453         None => {
 454             debug!("check_unused_unsafe({:?}) - no body found", def_id);
 455             return
 456         }
 457     };
 458     let body = tcx.hir().body(body_id);
 459     debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})",
 460            def_id, body, used_unsafe);
 461
 462     let mut visitor =  UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
 463     hir::intravisit::Visitor::visit_body(&mut visitor, body);
 464 }
 465
 466 fn unsafety_check_result<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId)
 467                                    -> UnsafetyCheckResult
 468 {
 469     debug!("unsafety_violations({:?})", def_id);
 470
 471     // N.B., this borrow is valid because all the consumers of
 472     // `mir_built` force this.
 473     let mir = &tcx.mir_built(def_id).borrow();
 474
 475     let source_scope_local_data = match mir.source_scope_local_data {
 476         ClearCrossCrate::Set(ref data) => data,
 477         ClearCrossCrate::Clear => {
 478             debug!("unsafety_violations: {:?} - remote, skipping", def_id);
 479             return UnsafetyCheckResult {
 480                 violations: Lrc::new([]),
 481                 unsafe_blocks: Lrc::new([])
 482             }
 483         }
 484     };
 485
 486     let param_env = tcx.param_env(def_id);
 487     let mut checker = UnsafetyChecker::new(
 488         tcx.is_min_const_fn(def_id),
 489         mir, source_scope_local_data, tcx, param_env);
 490     checker.visit_mir(mir);
 491
 492     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
 493     UnsafetyCheckResult {
 494         violations: checker.violations.into(),
 495         unsafe_blocks: checker.inherited_blocks.into()
 496     }
 497 }
 498
 499 fn unsafe_derive_on_repr_packed<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 500     let lint_node_id = match tcx.hir().as_local_node_id(def_id) {
 501         Some(node_id) => node_id,
 502         None => bug!("checking unsafety for non-local def id {:?}", def_id)
 503     };
 504
 505     // FIXME: when we make this a hard error, this should have its
 506     // own error code.
 507     let message = if tcx.generics_of(def_id).own_counts().types != 0 {
 508         "#[derive] can't be used on a #[repr(packed)] struct with \
 509          type parameters (error E0133)".to_string()
 510     } else {
 511         "#[derive] can't be used on a #[repr(packed)] struct that \
 512          does not derive Copy (error E0133)".to_string()
 513     };
 514     tcx.lint_node(SAFE_PACKED_BORROWS,
 515                   lint_node_id,
 516                   tcx.def_span(def_id),
 517                   &message);
 518 }
 519
 520 /// Return the NodeId for an enclosing scope that is also `unsafe`
 521 fn is_enclosed(tcx: TyCtxt,
 522                used_unsafe: &FxHashSet<ast::NodeId>,
 523                id: ast::NodeId) -> Option<(String, ast::NodeId)> {
 524     let parent_id = tcx.hir().get_parent_node(id);
 525     if parent_id != id {
 526         if used_unsafe.contains(&parent_id) {
 527             Some(("block".to_string(), parent_id))
 528         } else if let Some(Node::Item(&hir::Item {
 529             node: hir::ItemKind::Fn(_, header, _, _),
 530             ..
 531         })) = tcx.hir().find(parent_id) {
 532             match header.unsafety {
 533                 hir::Unsafety::Unsafe => Some(("fn".to_string(), parent_id)),
 534                 hir::Unsafety::Normal => None,
 535             }
 536         } else {
 537             is_enclosed(tcx, used_unsafe, parent_id)
 538         }
 539     } else {
 540         None
 541     }
 542 }
 543
 544 fn report_unused_unsafe(tcx: TyCtxt, used_unsafe: &FxHashSet<ast::NodeId>, id: ast::NodeId) {
 545     let span = tcx.sess.source_map().def_span(tcx.hir().span(id));
 546     let msg = "unnecessary `unsafe` block";
 547     let mut db = tcx.struct_span_lint_node(UNUSED_UNSAFE, id, span, msg);
 548     db.span_label(span, msg);
 549     if let Some((kind, id)) = is_enclosed(tcx, used_unsafe, id) {
 550         db.span_label(tcx.sess.source_map().def_span(tcx.hir().span(id)),
 551                       format!("because it's nested under this `unsafe` {}", kind));
 552     }
 553     db.emit();
 554 }
 555
 556 fn builtin_derive_def_id<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> Option<DefId> {
 557     debug!("builtin_derive_def_id({:?})", def_id);
 558     if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
 559         if tcx.has_attr(impl_def_id, "automatically_derived") {
 560             debug!("builtin_derive_def_id({:?}) - is {:?}", def_id, impl_def_id);
 561             Some(impl_def_id)
 562         } else {
 563             debug!("builtin_derive_def_id({:?}) - not automatically derived", def_id);
 564             None
 565         }
 566     } else {
 567         debug!("builtin_derive_def_id({:?}) - not a method", def_id);
 568         None
 569     }
 570 }
 571
 572 pub fn check_unsafety<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 573     debug!("check_unsafety({:?})", def_id);
 574
 575     // closures are handled by their parent fn.
 576     if tcx.is_closure(def_id) {
 577         return;
 578     }
 579
 580     let UnsafetyCheckResult {
 581         violations,
 582         unsafe_blocks
 583     } = tcx.unsafety_check_result(def_id);
 584
 585     for &UnsafetyViolation {
 586         source_info, description, details, kind
 587     } in violations.iter() {
 588         // Report an error.
 589         match kind {
 590             UnsafetyViolationKind::GeneralAndConstFn |
 591             UnsafetyViolationKind::General => {
 592                 struct_span_err!(
 593                     tcx.sess, source_info.span, E0133,
 594                     "{} is unsafe and requires unsafe function or block", description)
 595                     .span_label(source_info.span, &description.as_str()[..])
 596                     .note(&details.as_str()[..])
 597                     .emit();
 598             }
 599             UnsafetyViolationKind::ExternStatic(lint_node_id) => {
 600                 tcx.lint_node_note(SAFE_EXTERN_STATICS,
 601                               lint_node_id,
 602                               source_info.span,
 603                               &format!("{} is unsafe and requires unsafe function or block \
 604                                         (error E0133)", &description.as_str()[..]),
 605                               &details.as_str()[..]);
 606             }
 607             UnsafetyViolationKind::BorrowPacked(lint_node_id) => {
 608                 if let Some(impl_def_id) = builtin_derive_def_id(tcx, def_id) {
 609                     tcx.unsafe_derive_on_repr_packed(impl_def_id);
 610                 } else {
 611                     tcx.lint_node_note(SAFE_PACKED_BORROWS,
 612                                   lint_node_id,
 613                                   source_info.span,
 614                                   &format!("{} is unsafe and requires unsafe function or block \
 615                                             (error E0133)", &description.as_str()[..]),
 616                                   &details.as_str()[..]);
 617                 }
 618             }
 619         }
 620     }
 621
 622     let mut unsafe_blocks: Vec<_> = unsafe_blocks.into_iter().collect();
 623     unsafe_blocks.sort();
 624     let used_unsafe: FxHashSet<_> = unsafe_blocks.iter()
 625         .flat_map(|&&(id, used)| if used { Some(id) } else { None })
 626         .collect();
 627     for &(block_id, is_used) in unsafe_blocks {
 628         if !is_used {
 629             report_unused_unsafe(tcx, &used_unsafe, block_id);
 630         }
 631     }
 632 }