src/librustc_mir/transform/check_unsafety.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use rustc_data_structures::fx::FxHashSet;
  12 use rustc_data_structures::indexed_vec::IndexVec;
  13 use rustc_data_structures::sync::Lrc;
  14
  15 use rustc::ty::query::Providers;
  16 use rustc::ty::{self, TyCtxt};
  17 use rustc::hir;
  18 use rustc::hir::Node;
  19 use rustc::hir::def_id::DefId;
  20 use rustc::lint::builtin::{SAFE_EXTERN_STATICS, SAFE_PACKED_BORROWS, UNUSED_UNSAFE};
  21 use rustc::mir::*;
  22 use rustc::mir::visit::{PlaceContext, Visitor, MutatingUseContext};
  23
  24 use syntax::ast;
  25 use syntax::symbol::Symbol;
  26
  27 use std::ops::Bound;
  28
  29 use util;
  30
  31 pub struct UnsafetyChecker<'a, 'tcx: 'a> {
  32     mir: &'a Mir<'tcx>,
  33     min_const_fn: bool,
  34     source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  35     violations: Vec<UnsafetyViolation>,
  36     source_info: SourceInfo,
  37     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  38     param_env: ty::ParamEnv<'tcx>,
  39     /// mark an `unsafe` block as used, so we don't lint it
  40     used_unsafe: FxHashSet<ast::NodeId>,
  41     inherited_blocks: Vec<(ast::NodeId, bool)>,
  42 }
  43
  44 impl<'a, 'gcx, 'tcx> UnsafetyChecker<'a, 'tcx> {
  45     fn new(
  46         min_const_fn: bool,
  47         mir: &'a Mir<'tcx>,
  48         source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  49         tcx: TyCtxt<'a, 'tcx, 'tcx>,
  50         param_env: ty::ParamEnv<'tcx>,
  51     ) -> Self {
  52         Self {
  53             mir,
  54             min_const_fn,
  55             source_scope_local_data,
  56             violations: vec![],
  57             source_info: SourceInfo {
  58                 span: mir.span,
  59                 scope: OUTERMOST_SOURCE_SCOPE
  60             },
  61             tcx,
  62             param_env,
  63             used_unsafe: Default::default(),
  64             inherited_blocks: vec![],
  65         }
  66     }
  67 }
  68
  69 impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
  70     fn visit_terminator(&mut self,
  71                         block: BasicBlock,
  72                         terminator: &Terminator<'tcx>,
  73                         location: Location)
  74     {
  75         self.source_info = terminator.source_info;
  76         match terminator.kind {
  77             TerminatorKind::Goto { .. } |
  78             TerminatorKind::SwitchInt { .. } |
  79             TerminatorKind::Drop { .. } |
  80             TerminatorKind::Yield { .. } |
  81             TerminatorKind::Assert { .. } |
  82             TerminatorKind::DropAndReplace { .. } |
  83             TerminatorKind::GeneratorDrop |
  84             TerminatorKind::Resume |
  85             TerminatorKind::Abort |
  86             TerminatorKind::Return |
  87             TerminatorKind::Unreachable |
  88             TerminatorKind::FalseEdges { .. } |
  89             TerminatorKind::FalseUnwind { .. } => {
  90                 // safe (at least as emitted during MIR construction)
  91             }
  92
  93             TerminatorKind::Call { ref func, .. } => {
  94                 let func_ty = func.ty(self.mir, self.tcx);
  95                 let sig = func_ty.fn_sig(self.tcx);
  96                 if let hir::Unsafety::Unsafe = sig.unsafety() {
  97                     self.require_unsafe("call to unsafe function",
  98                         "consult the function's documentation for information on how to avoid \
  99                          undefined behavior", UnsafetyViolationKind::MinConstFn)
 100                 }
 101             }
 102         }
 103         self.super_terminator(block, terminator, location);
 104     }
 105
 106     fn visit_statement(&mut self,
 107                        block: BasicBlock,
 108                        statement: &Statement<'tcx>,
 109                        location: Location)
 110     {
 111         self.source_info = statement.source_info;
 112         match statement.kind {
 113             StatementKind::Assign(..) |
 114             StatementKind::FakeRead(..) |
 115             StatementKind::SetDiscriminant { .. } |
 116             StatementKind::StorageLive(..) |
 117             StatementKind::StorageDead(..) |
 118             StatementKind::Retag { .. } |
 119             StatementKind::EscapeToRaw { .. } |
 120             StatementKind::AscribeUserType(..) |
 121             StatementKind::Nop => {
 122                 // safe (at least as emitted during MIR construction)
 123             }
 124
 125             StatementKind::InlineAsm { .. } => {
 126                 self.require_unsafe("use of inline assembly",
 127                     "inline assembly is entirely unchecked and can cause undefined behavior",
 128                     UnsafetyViolationKind::General)
 129             },
 130         }
 131         self.super_statement(block, statement, location);
 132     }
 133
 134     fn visit_rvalue(&mut self,
 135                     rvalue: &Rvalue<'tcx>,
 136                     location: Location)
 137     {
 138         if let &Rvalue::Aggregate(box ref aggregate, _) = rvalue {
 139             match aggregate {
 140                 &AggregateKind::Array(..) |
 141                 &AggregateKind::Tuple => {}
 142                 &AggregateKind::Adt(ref def, ..) => {
 143                     match self.tcx.layout_scalar_valid_range(def.did) {
 144                         (Bound::Unbounded, Bound::Unbounded) => {},
 145                         _ => self.require_unsafe(
 146                             "initializing type with `rustc_layout_scalar_valid_range` attr",
 147                             "initializing `NonZero` with a `0` violates layout constraints \
 148                             and is undefined behavior",
 149                             UnsafetyViolationKind::MinConstFn,
 150                         ),
 151                     }
 152                 }
 153                 &AggregateKind::Closure(def_id, _) |
 154                 &AggregateKind::Generator(def_id, _, _) => {
 155                     let UnsafetyCheckResult {
 156                         violations, unsafe_blocks
 157                     } = self.tcx.unsafety_check_result(def_id);
 158                     self.register_violations(&violations, &unsafe_blocks);
 159                 }
 160             }
 161         }
 162         self.super_rvalue(rvalue, location);
 163     }
 164
 165     fn visit_place(&mut self,
 166                     place: &Place<'tcx>,
 167                     context: PlaceContext<'tcx>,
 168                     location: Location) {
 169         match place {
 170             &Place::Projection(box Projection {
 171                 ref base, ref elem
 172             }) => {
 173                 if context.is_borrow() {
 174                     if util::is_disaligned(self.tcx, self.mir, self.param_env, place) {
 175                         let source_info = self.source_info;
 176                         let lint_root =
 177                             self.source_scope_local_data[source_info.scope].lint_root;
 178                         self.register_violations(&[UnsafetyViolation {
 179                             source_info,
 180                             description: Symbol::intern("borrow of packed field").as_interned_str(),
 181                             details:
 182                                 Symbol::intern("fields of packed structs might be misaligned: \
 183                                                 dereferencing a misaligned pointer or even just \
 184                                                 creating a misaligned reference is undefined \
 185                                                 behavior")
 186                                     .as_interned_str(),
 187                             kind: UnsafetyViolationKind::BorrowPacked(lint_root)
 188                         }], &[]);
 189                     }
 190                     if context.is_mutating_use() {
 191                         self.check_mut_borrowing_layout_constrained_field(place);
 192                     }
 193                 }
 194                 let old_source_info = self.source_info;
 195                 if let &Place::Local(local) = base {
 196                     if self.mir.local_decls[local].internal {
 197                         // Internal locals are used in the `move_val_init` desugaring.
 198                         // We want to check unsafety against the source info of the
 199                         // desugaring, rather than the source info of the RHS.
 200                         self.source_info = self.mir.local_decls[local].source_info;
 201                     }
 202                 }
 203                 let base_ty = base.ty(self.mir, self.tcx).to_ty(self.tcx);
 204                 match base_ty.sty {
 205                     ty::RawPtr(..) => {
 206                         self.require_unsafe("dereference of raw pointer",
 207                             "raw pointers may be NULL, dangling or unaligned; they can violate \
 208                              aliasing rules and cause data races: all of these are undefined \
 209                              behavior", UnsafetyViolationKind::General)
 210                     }
 211                     ty::Adt(adt, _) => {
 212                         if adt.is_union() {
 213                             if context == PlaceContext::MutatingUse(MutatingUseContext::Store) ||
 214                                 context == PlaceContext::MutatingUse(MutatingUseContext::Drop) ||
 215                                 context == PlaceContext::MutatingUse(
 216                                     MutatingUseContext::AsmOutput
 217                                 )
 218                             {
 219                                 let elem_ty = match elem {
 220                                     &ProjectionElem::Field(_, ty) => ty,
 221                                     _ => span_bug!(
 222                                         self.source_info.span,
 223                                         "non-field projection {:?} from union?",
 224                                         place)
 225                                 };
 226                                 if elem_ty.moves_by_default(self.tcx, self.param_env,
 227                                                             self.source_info.span) {
 228                                     self.require_unsafe(
 229                                         "assignment to non-`Copy` union field",
 230                                         "the previous content of the field will be dropped, which \
 231                                          causes undefined behavior if the field was not properly \
 232                                          initialized", UnsafetyViolationKind::General)
 233                                 } else {
 234                                     // write to non-move union, safe
 235                                 }
 236                             } else {
 237                                 self.require_unsafe("access to union field",
 238                                     "the field may not be properly initialized: using \
 239                                      uninitialized data will cause undefined behavior",
 240                                      UnsafetyViolationKind::General)
 241                             }
 242                         }
 243                     }
 244                     _ => {}
 245                 }
 246                 self.source_info = old_source_info;
 247             }
 248             &Place::Local(..) => {
 249                 // locals are safe
 250             }
 251             &Place::Promoted(_) => {
 252                 bug!("unsafety checking should happen before promotion")
 253             }
 254             &Place::Static(box Static { def_id, ty: _ }) => {
 255                 if self.tcx.is_static(def_id) == Some(hir::Mutability::MutMutable) {
 256                     self.require_unsafe("use of mutable static",
 257                         "mutable statics can be mutated by multiple threads: aliasing violations \
 258                          or data races will cause undefined behavior",
 259                          UnsafetyViolationKind::General);
 260                 } else if self.tcx.is_foreign_item(def_id) {
 261                     let source_info = self.source_info;
 262                     let lint_root =
 263                         self.source_scope_local_data[source_info.scope].lint_root;
 264                     self.register_violations(&[UnsafetyViolation {
 265                         source_info,
 266                         description: Symbol::intern("use of extern static").as_interned_str(),
 267                         details:
 268                             Symbol::intern("extern statics are not controlled by the Rust type \
 269                                             system: invalid data, aliasing violations or data \
 270                                             races will cause undefined behavior")
 271                                 .as_interned_str(),
 272                         kind: UnsafetyViolationKind::ExternStatic(lint_root)
 273                     }], &[]);
 274                 }
 275             }
 276         };
 277         self.super_place(place, context, location);
 278     }
 279 }
 280
 281 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
 282     fn require_unsafe(
 283         &mut self,
 284         description: &'static str,
 285         details: &'static str,
 286         kind: UnsafetyViolationKind,
 287     ) {
 288         let source_info = self.source_info;
 289         self.register_violations(&[UnsafetyViolation {
 290             source_info,
 291             description: Symbol::intern(description).as_interned_str(),
 292             details: Symbol::intern(details).as_interned_str(),
 293             kind,
 294         }], &[]);
 295     }
 296
 297     fn register_violations(&mut self,
 298                            violations: &[UnsafetyViolation],
 299                            unsafe_blocks: &[(ast::NodeId, bool)]) {
 300         let safety = self.source_scope_local_data[self.source_info.scope].safety;
 301         let within_unsafe = match (safety, self.min_const_fn) {
 302             // Erring on the safe side, pun intended
 303             (Safety::BuiltinUnsafe, true) |
 304             // mir building encodes const fn bodies as safe, even for `const unsafe fn`
 305             (Safety::FnUnsafe, true) => bug!("const unsafe fn body treated as inherently unsafe"),
 306             // `unsafe` blocks are required in safe code
 307             (Safety::Safe, _) => {
 308                 for violation in violations {
 309                     let mut violation = violation.clone();
 310                     if self.min_const_fn {
 311                         // overwrite unsafety violation in const fn with a single hard error kind
 312                         violation.kind = UnsafetyViolationKind::MinConstFn;
 313                     } else if let UnsafetyViolationKind::MinConstFn = violation.kind {
 314                         // outside of const fns we treat `MinConstFn` and `General` the same
 315                         violation.kind = UnsafetyViolationKind::General;
 316                     }
 317                     if !self.violations.contains(&violation) {
 318                         self.violations.push(violation)
 319                     }
 320                 }
 321                 false
 322             }
 323             // regular `unsafe` function bodies allow unsafe without additional unsafe blocks
 324             (Safety::BuiltinUnsafe, false) | (Safety::FnUnsafe, false) => true,
 325             (Safety::ExplicitUnsafe(node_id), _) => {
 326                 // mark unsafe block as used if there are any unsafe operations inside
 327                 if !violations.is_empty() {
 328                     self.used_unsafe.insert(node_id);
 329                 }
 330                 // only some unsafety is allowed in const fn
 331                 if self.min_const_fn {
 332                     for violation in violations {
 333                         match violation.kind {
 334                             // these are allowed
 335                             UnsafetyViolationKind::MinConstFn
 336                                 // if `#![feature(min_const_unsafe_fn)]` is active
 337                                 if self.tcx.sess.features_untracked().min_const_unsafe_fn => {},
 338                             _ => {
 339                                 let mut violation = violation.clone();
 340                                 // overwrite unsafety violation in const fn with a hard error
 341                                 violation.kind = UnsafetyViolationKind::MinConstFn;
 342                                 if !self.violations.contains(&violation) {
 343                                     self.violations.push(violation)
 344                                 }
 345                             },
 346                         }
 347                     }
 348                 }
 349                 true
 350             }
 351         };
 352         self.inherited_blocks.extend(unsafe_blocks.iter().map(|&(node_id, is_used)| {
 353             (node_id, is_used && !within_unsafe)
 354         }));
 355     }
 356     fn check_mut_borrowing_layout_constrained_field(
 357         &mut self,
 358         mut place: &Place<'tcx>,
 359     ) {
 360         while let &Place::Projection(box Projection {
 361             ref base, ref elem
 362         }) = place {
 363             match *elem {
 364                 ProjectionElem::Field(..) => {
 365                     let ty = base.ty(&self.mir.local_decls, self.tcx).to_ty(self.tcx);
 366                     match ty.sty {
 367                         ty::Adt(def, _) => match self.tcx.layout_scalar_valid_range(def.did) {
 368                             (Bound::Unbounded, Bound::Unbounded) => {},
 369                             _ => {
 370                                 let source_info = self.source_info;
 371                                 self.register_violations(&[UnsafetyViolation {
 372                                     source_info,
 373                                     description: Symbol::intern(
 374                                         "borrow of layout constrained field",
 375                                     ).as_interned_str(),
 376                                     details:
 377                                         Symbol::intern(
 378                                             "references to fields of layout constrained fields \
 379                                             lose the constraints",
 380                                         ).as_interned_str(),
 381                                     kind: UnsafetyViolationKind::MinConstFn,
 382                                 }], &[]);
 383                             }
 384                         },
 385                         _ => {}
 386                     }
 387                 }
 388                 _ => {}
 389             }
 390             place = base;
 391         }
 392     }
 393 }
 394
 395 pub(crate) fn provide(providers: &mut Providers) {
 396     *providers = Providers {
 397         unsafety_check_result,
 398         unsafe_derive_on_repr_packed,
 399         ..*providers
 400     };
 401 }
 402
 403 struct UnusedUnsafeVisitor<'a> {
 404     used_unsafe: &'a FxHashSet<ast::NodeId>,
 405     unsafe_blocks: &'a mut Vec<(ast::NodeId, bool)>,
 406 }
 407
 408 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'a> {
 409     fn nested_visit_map<'this>(&'this mut self) ->
 410         hir::intravisit::NestedVisitorMap<'this, 'tcx>
 411     {
 412         hir::intravisit::NestedVisitorMap::None
 413     }
 414
 415     fn visit_block(&mut self, block: &'tcx hir::Block) {
 416         hir::intravisit::walk_block(self, block);
 417
 418         if let hir::UnsafeBlock(hir::UserProvided) = block.rules {
 419             self.unsafe_blocks.push((block.id, self.used_unsafe.contains(&block.id)));
 420         }
 421     }
 422 }
 423
 424 fn check_unused_unsafe<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 425                                  def_id: DefId,
 426                                  used_unsafe: &FxHashSet<ast::NodeId>,
 427                                  unsafe_blocks: &'a mut Vec<(ast::NodeId, bool)>)
 428 {
 429     let body_id =
 430         tcx.hir.as_local_node_id(def_id).and_then(|node_id| {
 431             tcx.hir.maybe_body_owned_by(node_id)
 432         });
 433
 434     let body_id = match body_id {
 435         Some(body) => body,
 436         None => {
 437             debug!("check_unused_unsafe({:?}) - no body found", def_id);
 438             return
 439         }
 440     };
 441     let body = tcx.hir.body(body_id);
 442     debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})",
 443            def_id, body, used_unsafe);
 444
 445     let mut visitor =  UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
 446     hir::intravisit::Visitor::visit_body(&mut visitor, body);
 447 }
 448
 449 fn unsafety_check_result<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId)
 450                                    -> UnsafetyCheckResult
 451 {
 452     debug!("unsafety_violations({:?})", def_id);
 453
 454     // NB: this borrow is valid because all the consumers of
 455     // `mir_built` force this.
 456     let mir = &tcx.mir_built(def_id).borrow();
 457
 458     let source_scope_local_data = match mir.source_scope_local_data {
 459         ClearCrossCrate::Set(ref data) => data,
 460         ClearCrossCrate::Clear => {
 461             debug!("unsafety_violations: {:?} - remote, skipping", def_id);
 462             return UnsafetyCheckResult {
 463                 violations: Lrc::new([]),
 464                 unsafe_blocks: Lrc::new([])
 465             }
 466         }
 467     };
 468
 469     let param_env = tcx.param_env(def_id);
 470     let mut checker = UnsafetyChecker::new(
 471         tcx.is_const_fn(def_id) && tcx.is_min_const_fn(def_id),
 472         mir, source_scope_local_data, tcx, param_env);
 473     checker.visit_mir(mir);
 474
 475     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
 476     UnsafetyCheckResult {
 477         violations: checker.violations.into(),
 478         unsafe_blocks: checker.inherited_blocks.into()
 479     }
 480 }
 481
 482 fn unsafe_derive_on_repr_packed<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 483     let lint_node_id = match tcx.hir.as_local_node_id(def_id) {
 484         Some(node_id) => node_id,
 485         None => bug!("checking unsafety for non-local def id {:?}", def_id)
 486     };
 487
 488     // FIXME: when we make this a hard error, this should have its
 489     // own error code.
 490     let message = if tcx.generics_of(def_id).own_counts().types != 0 {
 491         "#[derive] can't be used on a #[repr(packed)] struct with \
 492          type parameters (error E0133)".to_string()
 493     } else {
 494         "#[derive] can't be used on a #[repr(packed)] struct that \
 495          does not derive Copy (error E0133)".to_string()
 496     };
 497     tcx.lint_node(SAFE_PACKED_BORROWS,
 498                   lint_node_id,
 499                   tcx.def_span(def_id),
 500                   &message);
 501 }
 502
 503 /// Return the NodeId for an enclosing scope that is also `unsafe`
 504 fn is_enclosed(tcx: TyCtxt,
 505                used_unsafe: &FxHashSet<ast::NodeId>,
 506                id: ast::NodeId) -> Option<(String, ast::NodeId)> {
 507     let parent_id = tcx.hir.get_parent_node(id);
 508     if parent_id != id {
 509         if used_unsafe.contains(&parent_id) {
 510             Some(("block".to_string(), parent_id))
 511         } else if let Some(Node::Item(&hir::Item {
 512             node: hir::ItemKind::Fn(_, header, _, _),
 513             ..
 514         })) = tcx.hir.find(parent_id) {
 515             match header.unsafety {
 516                 hir::Unsafety::Unsafe => Some(("fn".to_string(), parent_id)),
 517                 hir::Unsafety::Normal => None,
 518             }
 519         } else {
 520             is_enclosed(tcx, used_unsafe, parent_id)
 521         }
 522     } else {
 523         None
 524     }
 525 }
 526
 527 fn report_unused_unsafe(tcx: TyCtxt, used_unsafe: &FxHashSet<ast::NodeId>, id: ast::NodeId) {
 528     let span = tcx.sess.source_map().def_span(tcx.hir.span(id));
 529     let msg = "unnecessary `unsafe` block";
 530     let mut db = tcx.struct_span_lint_node(UNUSED_UNSAFE, id, span, msg);
 531     db.span_label(span, msg);
 532     if let Some((kind, id)) = is_enclosed(tcx, used_unsafe, id) {
 533         db.span_label(tcx.sess.source_map().def_span(tcx.hir.span(id)),
 534                       format!("because it's nested under this `unsafe` {}", kind));
 535     }
 536     db.emit();
 537 }
 538
 539 fn builtin_derive_def_id<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> Option<DefId> {
 540     debug!("builtin_derive_def_id({:?})", def_id);
 541     if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
 542         if tcx.has_attr(impl_def_id, "automatically_derived") {
 543             debug!("builtin_derive_def_id({:?}) - is {:?}", def_id, impl_def_id);
 544             Some(impl_def_id)
 545         } else {
 546             debug!("builtin_derive_def_id({:?}) - not automatically derived", def_id);
 547             None
 548         }
 549     } else {
 550         debug!("builtin_derive_def_id({:?}) - not a method", def_id);
 551         None
 552     }
 553 }
 554
 555 pub fn check_unsafety<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 556     debug!("check_unsafety({:?})", def_id);
 557
 558     // closures are handled by their parent fn.
 559     if tcx.is_closure(def_id) {
 560         return;
 561     }
 562
 563     let UnsafetyCheckResult {
 564         violations,
 565         unsafe_blocks
 566     } = tcx.unsafety_check_result(def_id);
 567
 568     for &UnsafetyViolation {
 569         source_info, description, details, kind
 570     } in violations.iter() {
 571         // Report an error.
 572         match kind {
 573             UnsafetyViolationKind::General => {
 574                 struct_span_err!(
 575                     tcx.sess, source_info.span, E0133,
 576                     "{} is unsafe and requires unsafe function or block", description)
 577                     .span_label(source_info.span, &description.as_str()[..])
 578                     .note(&details.as_str()[..])
 579                     .emit();
 580             }
 581             UnsafetyViolationKind::MinConstFn => {
 582                 tcx.sess.struct_span_err(
 583                     source_info.span,
 584                     &format!("{} is unsafe and unsafe operations \
 585                             are not allowed in const fn", description))
 586                     .span_label(source_info.span, &description.as_str()[..])
 587                     .note(&details.as_str()[..])
 588                     .emit();
 589             }
 590             UnsafetyViolationKind::ExternStatic(lint_node_id) => {
 591                 tcx.lint_node_note(SAFE_EXTERN_STATICS,
 592                               lint_node_id,
 593                               source_info.span,
 594                               &format!("{} is unsafe and requires unsafe function or block \
 595                                         (error E0133)", &description.as_str()[..]),
 596                               &details.as_str()[..]);
 597             }
 598             UnsafetyViolationKind::BorrowPacked(lint_node_id) => {
 599                 if let Some(impl_def_id) = builtin_derive_def_id(tcx, def_id) {
 600                     tcx.unsafe_derive_on_repr_packed(impl_def_id);
 601                 } else {
 602                     tcx.lint_node_note(SAFE_PACKED_BORROWS,
 603                                   lint_node_id,
 604                                   source_info.span,
 605                                   &format!("{} is unsafe and requires unsafe function or block \
 606                                             (error E0133)", &description.as_str()[..]),
 607                                   &details.as_str()[..]);
 608                 }
 609             }
 610         }
 611     }
 612
 613     let mut unsafe_blocks: Vec<_> = unsafe_blocks.into_iter().collect();
 614     unsafe_blocks.sort();
 615     let used_unsafe: FxHashSet<_> = unsafe_blocks.iter()
 616         .flat_map(|&&(id, used)| if used { Some(id) } else { None })
 617         .collect();
 618     for &(block_id, is_used) in unsafe_blocks {
 619         if !is_used {
 620             report_unused_unsafe(tcx, &used_unsafe, block_id);
 621         }
 622     }
 623 }