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 use syntax::feature_gate::{emit_feature_err, GateIssue};
  27
  28 use std::ops::Bound;
  29
  30 use util;
  31
  32 pub struct UnsafetyChecker<'a, 'tcx: 'a> {
  33     mir: &'a Mir<'tcx>,
  34     min_const_fn: bool,
  35     source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  36     violations: Vec<UnsafetyViolation>,
  37     source_info: SourceInfo,
  38     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  39     param_env: ty::ParamEnv<'tcx>,
  40     /// mark an `unsafe` block as used, so we don't lint it
  41     used_unsafe: FxHashSet<ast::NodeId>,
  42     inherited_blocks: Vec<(ast::NodeId, bool)>,
  43 }
  44
  45 impl<'a, 'gcx, 'tcx> UnsafetyChecker<'a, 'tcx> {
  46     fn new(
  47         min_const_fn: bool,
  48         mir: &'a Mir<'tcx>,
  49         source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
  50         tcx: TyCtxt<'a, 'tcx, 'tcx>,
  51         param_env: ty::ParamEnv<'tcx>,
  52     ) -> Self {
  53         Self {
  54             mir,
  55             min_const_fn,
  56             source_scope_local_data,
  57             violations: vec![],
  58             source_info: SourceInfo {
  59                 span: mir.span,
  60                 scope: OUTERMOST_SOURCE_SCOPE
  61             },
  62             tcx,
  63             param_env,
  64             used_unsafe: Default::default(),
  65             inherited_blocks: vec![],
  66         }
  67     }
  68 }
  69
  70 impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
  71     fn visit_terminator(&mut self,
  72                         block: BasicBlock,
  73                         terminator: &Terminator<'tcx>,
  74                         location: Location)
  75     {
  76         self.source_info = terminator.source_info;
  77         match terminator.kind {
  78             TerminatorKind::Goto { .. } |
  79             TerminatorKind::SwitchInt { .. } |
  80             TerminatorKind::Drop { .. } |
  81             TerminatorKind::Yield { .. } |
  82             TerminatorKind::Assert { .. } |
  83             TerminatorKind::DropAndReplace { .. } |
  84             TerminatorKind::GeneratorDrop |
  85             TerminatorKind::Resume |
  86             TerminatorKind::Abort |
  87             TerminatorKind::Return |
  88             TerminatorKind::Unreachable |
  89             TerminatorKind::FalseEdges { .. } |
  90             TerminatorKind::FalseUnwind { .. } => {
  91                 // safe (at least as emitted during MIR construction)
  92             }
  93
  94             TerminatorKind::Call { ref func, .. } => {
  95                 let func_ty = func.ty(self.mir, self.tcx);
  96                 let sig = func_ty.fn_sig(self.tcx);
  97                 if let hir::Unsafety::Unsafe = sig.unsafety() {
  98                     self.require_unsafe("call to unsafe function",
  99                         "consult the function's documentation for information on how to avoid \
 100                          undefined behavior", UnsafetyViolationKind::GatedConstFnCall)
 101                 }
 102             }
 103         }
 104         self.super_terminator(block, terminator, location);
 105     }
 106
 107     fn visit_statement(&mut self,
 108                        block: BasicBlock,
 109                        statement: &Statement<'tcx>,
 110                        location: Location)
 111     {
 112         self.source_info = statement.source_info;
 113         match statement.kind {
 114             StatementKind::Assign(..) |
 115             StatementKind::FakeRead(..) |
 116             StatementKind::SetDiscriminant { .. } |
 117             StatementKind::StorageLive(..) |
 118             StatementKind::StorageDead(..) |
 119             StatementKind::Retag { .. } |
 120             StatementKind::EscapeToRaw { .. } |
 121             StatementKind::AscribeUserType(..) |
 122             StatementKind::Nop => {
 123                 // safe (at least as emitted during MIR construction)
 124             }
 125
 126             StatementKind::InlineAsm { .. } => {
 127                 self.require_unsafe("use of inline assembly",
 128                     "inline assembly is entirely unchecked and can cause undefined behavior",
 129                     UnsafetyViolationKind::General)
 130             },
 131         }
 132         self.super_statement(block, statement, location);
 133     }
 134
 135     fn visit_rvalue(&mut self,
 136                     rvalue: &Rvalue<'tcx>,
 137                     location: Location)
 138     {
 139         if let &Rvalue::Aggregate(box ref aggregate, _) = rvalue {
 140             match aggregate {
 141                 &AggregateKind::Array(..) |
 142                 &AggregateKind::Tuple => {}
 143                 &AggregateKind::Adt(ref def, ..) => {
 144                     match self.tcx.layout_scalar_valid_range(def.did) {
 145                         (Bound::Unbounded, Bound::Unbounded) => {},
 146                         _ => self.require_unsafe(
 147                             "initializing type with `rustc_layout_scalar_valid_range` attr",
 148                             "initializing a layout restricted type's field with a value outside \
 149                             the valid range is undefined behavior",
 150                             UnsafetyViolationKind::GeneralAndConstFn,
 151                         ),
 152                     }
 153                 }
 154                 &AggregateKind::Closure(def_id, _) |
 155                 &AggregateKind::Generator(def_id, _, _) => {
 156                     let UnsafetyCheckResult {
 157                         violations, unsafe_blocks
 158                     } = self.tcx.unsafety_check_result(def_id);
 159                     self.register_violations(&violations, &unsafe_blocks);
 160                 }
 161             }
 162         }
 163         self.super_rvalue(rvalue, location);
 164     }
 165
 166     fn visit_place(&mut self,
 167                     place: &Place<'tcx>,
 168                     context: PlaceContext<'tcx>,
 169                     location: Location) {
 170         match place {
 171             &Place::Projection(box Projection {
 172                 ref base, ref elem
 173             }) => {
 174                 if context.is_borrow() {
 175                     if util::is_disaligned(self.tcx, self.mir, self.param_env, place) {
 176                         let source_info = self.source_info;
 177                         let lint_root =
 178                             self.source_scope_local_data[source_info.scope].lint_root;
 179                         self.register_violations(&[UnsafetyViolation {
 180                             source_info,
 181                             description: Symbol::intern("borrow of packed field").as_interned_str(),
 182                             details:
 183                                 Symbol::intern("fields of packed structs might be misaligned: \
 184                                                 dereferencing a misaligned pointer or even just \
 185                                                 creating a misaligned reference is undefined \
 186                                                 behavior")
 187                                     .as_interned_str(),
 188                             kind: UnsafetyViolationKind::BorrowPacked(lint_root)
 189                         }], &[]);
 190                     }
 191                 }
 192                 let is_borrow_of_interior_mut = context.is_borrow() && !base
 193                     .ty(self.mir, self.tcx)
 194                     .to_ty(self.tcx)
 195                     .is_freeze(self.tcx, self.param_env, self.source_info.span);
 196                 // prevent
 197                 // * `&mut x.field`
 198                 // * `x.field = y;`
 199                 // * `&x.field` if `field`'s type has interior mutability
 200                 // because either of these would allow modifying the layout constrained field and
 201                 // insert values that violate the layout constraints.
 202                 if context.is_mutating_use() || is_borrow_of_interior_mut {
 203                     self.check_mut_borrowing_layout_constrained_field(
 204                         place, context.is_mutating_use(),
 205                     );
 206                 }
 207                 let old_source_info = self.source_info;
 208                 if let &Place::Local(local) = base {
 209                     if self.mir.local_decls[local].internal {
 210                         // Internal locals are used in the `move_val_init` desugaring.
 211                         // We want to check unsafety against the source info of the
 212                         // desugaring, rather than the source info of the RHS.
 213                         self.source_info = self.mir.local_decls[local].source_info;
 214                     }
 215                 }
 216                 let base_ty = base.ty(self.mir, self.tcx).to_ty(self.tcx);
 217                 match base_ty.sty {
 218                     ty::RawPtr(..) => {
 219                         self.require_unsafe("dereference of raw pointer",
 220                             "raw pointers may be NULL, dangling or unaligned; they can violate \
 221                              aliasing rules and cause data races: all of these are undefined \
 222                              behavior", UnsafetyViolationKind::General)
 223                     }
 224                     ty::Adt(adt, _) => {
 225                         if adt.is_union() {
 226                             if context == PlaceContext::MutatingUse(MutatingUseContext::Store) ||
 227                                 context == PlaceContext::MutatingUse(MutatingUseContext::Drop) ||
 228                                 context == PlaceContext::MutatingUse(
 229                                     MutatingUseContext::AsmOutput
 230                                 )
 231                             {
 232                                 let elem_ty = match elem {
 233                                     &ProjectionElem::Field(_, ty) => ty,
 234                                     _ => span_bug!(
 235                                         self.source_info.span,
 236                                         "non-field projection {:?} from union?",
 237                                         place)
 238                                 };
 239                                 if elem_ty.moves_by_default(self.tcx, self.param_env,
 240                                                             self.source_info.span) {
 241                                     self.require_unsafe(
 242                                         "assignment to non-`Copy` union field",
 243                                         "the previous content of the field will be dropped, which \
 244                                          causes undefined behavior if the field was not properly \
 245                                          initialized", UnsafetyViolationKind::General)
 246                                 } else {
 247                                     // write to non-move union, safe
 248                                 }
 249                             } else {
 250                                 self.require_unsafe("access to union field",
 251                                     "the field may not be properly initialized: using \
 252                                      uninitialized data will cause undefined behavior",
 253                                      UnsafetyViolationKind::General)
 254                             }
 255                         }
 256                     }
 257                     _ => {}
 258                 }
 259                 self.source_info = old_source_info;
 260             }
 261             &Place::Local(..) => {
 262                 // locals are safe
 263             }
 264             &Place::Promoted(_) => {
 265                 bug!("unsafety checking should happen before promotion")
 266             }
 267             &Place::Static(box Static { def_id, ty: _ }) => {
 268                 if self.tcx.is_static(def_id) == Some(hir::Mutability::MutMutable) {
 269                     self.require_unsafe("use of mutable static",
 270                         "mutable statics can be mutated by multiple threads: aliasing violations \
 271                          or data races will cause undefined behavior",
 272                          UnsafetyViolationKind::General);
 273                 } else if self.tcx.is_foreign_item(def_id) {
 274                     let source_info = self.source_info;
 275                     let lint_root =
 276                         self.source_scope_local_data[source_info.scope].lint_root;
 277                     self.register_violations(&[UnsafetyViolation {
 278                         source_info,
 279                         description: Symbol::intern("use of extern static").as_interned_str(),
 280                         details:
 281                             Symbol::intern("extern statics are not controlled by the Rust type \
 282                                             system: invalid data, aliasing violations or data \
 283                                             races will cause undefined behavior")
 284                                 .as_interned_str(),
 285                         kind: UnsafetyViolationKind::ExternStatic(lint_root)
 286                     }], &[]);
 287                 }
 288             }
 289         };
 290         self.super_place(place, context, location);
 291     }
 292 }
 293
 294 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
 295     fn require_unsafe(
 296         &mut self,
 297         description: &'static str,
 298         details: &'static str,
 299         kind: UnsafetyViolationKind,
 300     ) {
 301         let source_info = self.source_info;
 302         self.register_violations(&[UnsafetyViolation {
 303             source_info,
 304             description: Symbol::intern(description).as_interned_str(),
 305             details: Symbol::intern(details).as_interned_str(),
 306             kind,
 307         }], &[]);
 308     }
 309
 310     fn register_violations(&mut self,
 311                            violations: &[UnsafetyViolation],
 312                            unsafe_blocks: &[(ast::NodeId, bool)]) {
 313         let safety = self.source_scope_local_data[self.source_info.scope].safety;
 314         let within_unsafe = match (safety, self.min_const_fn) {
 315             // Erring on the safe side, pun intended
 316             (Safety::BuiltinUnsafe, true) |
 317             // mir building encodes const fn bodies as safe, even for `const unsafe fn`
 318             (Safety::FnUnsafe, true) => bug!("const unsafe fn body treated as inherently unsafe"),
 319             // `unsafe` blocks are required in safe code
 320             (Safety::Safe, _) => {
 321                 for violation in violations {
 322                     let mut violation = violation.clone();
 323                     match violation.kind {
 324                         UnsafetyViolationKind::GeneralAndConstFn |
 325                         UnsafetyViolationKind::General => {},
 326                         UnsafetyViolationKind::BorrowPacked(_) |
 327                         UnsafetyViolationKind::ExternStatic(_) => if self.min_const_fn {
 328                             // const fns don't need to be backwards compatible and can
 329                             // emit these violations as a hard error instead of a backwards
 330                             // compat lint
 331                             violation.kind = UnsafetyViolationKind::General;
 332                         },
 333                         UnsafetyViolationKind::GatedConstFnCall => {
 334                             // safe code can't call unsafe const fns, this `UnsafetyViolationKind`
 335                             // is only relevant for `Safety::ExplicitUnsafe` in `unsafe const fn`s
 336                             violation.kind = UnsafetyViolationKind::General;
 337                         }
 338                     }
 339                     if !self.violations.contains(&violation) {
 340                         self.violations.push(violation)
 341                     }
 342                 }
 343                 false
 344             }
 345             // regular `unsafe` function bodies allow unsafe without additional unsafe blocks
 346             (Safety::BuiltinUnsafe, false) | (Safety::FnUnsafe, false) => true,
 347             (Safety::ExplicitUnsafe(node_id), _) => {
 348                 // mark unsafe block as used if there are any unsafe operations inside
 349                 if !violations.is_empty() {
 350                     self.used_unsafe.insert(node_id);
 351                 }
 352                 // only some unsafety is allowed in const fn
 353                 if self.min_const_fn {
 354                     for violation in violations {
 355                         match violation.kind {
 356                             // these are allowed
 357                             UnsafetyViolationKind::GatedConstFnCall => {
 358                                 // if `#![feature(min_const_unsafe_fn)]` is active
 359                                 if !self.tcx.sess.features_untracked().min_const_unsafe_fn {
 360                                     if !self.violations.contains(&violation) {
 361                                         self.violations.push(violation.clone())
 362                                     }
 363                                 }
 364                             }
 365                             // these unsafe things are stable in const fn
 366                             UnsafetyViolationKind::GeneralAndConstFn => {},
 367                             UnsafetyViolationKind::General |
 368                             UnsafetyViolationKind::BorrowPacked(_) |
 369                             UnsafetyViolationKind::ExternStatic(_) => {
 370                                 let mut violation = violation.clone();
 371                                 // const fns don't need to be backwards compatible and can
 372                                 // emit these violations as a hard error instead of a backwards
 373                                 // compat lint
 374                                 violation.kind = UnsafetyViolationKind::General;
 375                                 if !self.violations.contains(&violation) {
 376                                     self.violations.push(violation)
 377                                 }
 378                             },
 379                         }
 380                     }
 381                 }
 382                 true
 383             }
 384         };
 385         self.inherited_blocks.extend(unsafe_blocks.iter().map(|&(node_id, is_used)| {
 386             (node_id, is_used && !within_unsafe)
 387         }));
 388     }
 389     fn check_mut_borrowing_layout_constrained_field(
 390         &mut self,
 391         mut place: &Place<'tcx>,
 392         is_mut_use: bool,
 393     ) {
 394         while let &Place::Projection(box Projection {
 395             ref base, ref elem
 396         }) = place {
 397             match *elem {
 398                 ProjectionElem::Field(..) => {
 399                     let ty = base.ty(&self.mir.local_decls, self.tcx).to_ty(self.tcx);
 400                     match ty.sty {
 401                         ty::Adt(def, _) => match self.tcx.layout_scalar_valid_range(def.did) {
 402                             (Bound::Unbounded, Bound::Unbounded) => {},
 403                             _ => {
 404                                 let (description, details) = if is_mut_use {
 405                                     (
 406                                         "mutation of layout constrained field",
 407                                         "mutating layout constrained fields cannot statically be \
 408                                         checked for valid values",
 409                                     )
 410                                 } else {
 411                                     (
 412                                         "borrow of layout constrained field with interior \
 413                                         mutability",
 414                                         "references to fields of layout constrained fields \
 415                                         lose the constraints. Coupled with interior mutability, \
 416                                         the field can be changed to invalid values",
 417                                     )
 418                                 };
 419                                 let source_info = self.source_info;
 420                                 self.register_violations(&[UnsafetyViolation {
 421                                     source_info,
 422                                     description: Symbol::intern(description).as_interned_str(),
 423                                     details: Symbol::intern(details).as_interned_str(),
 424                                     kind: UnsafetyViolationKind::GeneralAndConstFn,
 425                                 }], &[]);
 426                             }
 427                         },
 428                         _ => {}
 429                     }
 430                 }
 431                 _ => {}
 432             }
 433             place = base;
 434         }
 435     }
 436 }
 437
 438 pub(crate) fn provide(providers: &mut Providers) {
 439     *providers = Providers {
 440         unsafety_check_result,
 441         unsafe_derive_on_repr_packed,
 442         ..*providers
 443     };
 444 }
 445
 446 struct UnusedUnsafeVisitor<'a> {
 447     used_unsafe: &'a FxHashSet<ast::NodeId>,
 448     unsafe_blocks: &'a mut Vec<(ast::NodeId, bool)>,
 449 }
 450
 451 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'a> {
 452     fn nested_visit_map<'this>(&'this mut self) ->
 453         hir::intravisit::NestedVisitorMap<'this, 'tcx>
 454     {
 455         hir::intravisit::NestedVisitorMap::None
 456     }
 457
 458     fn visit_block(&mut self, block: &'tcx hir::Block) {
 459         hir::intravisit::walk_block(self, block);
 460
 461         if let hir::UnsafeBlock(hir::UserProvided) = block.rules {
 462             self.unsafe_blocks.push((block.id, self.used_unsafe.contains(&block.id)));
 463         }
 464     }
 465 }
 466
 467 fn check_unused_unsafe<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 468                                  def_id: DefId,
 469                                  used_unsafe: &FxHashSet<ast::NodeId>,
 470                                  unsafe_blocks: &'a mut Vec<(ast::NodeId, bool)>)
 471 {
 472     let body_id =
 473         tcx.hir.as_local_node_id(def_id).and_then(|node_id| {
 474             tcx.hir.maybe_body_owned_by(node_id)
 475         });
 476
 477     let body_id = match body_id {
 478         Some(body) => body,
 479         None => {
 480             debug!("check_unused_unsafe({:?}) - no body found", def_id);
 481             return
 482         }
 483     };
 484     let body = tcx.hir.body(body_id);
 485     debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})",
 486            def_id, body, used_unsafe);
 487
 488     let mut visitor =  UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
 489     hir::intravisit::Visitor::visit_body(&mut visitor, body);
 490 }
 491
 492 fn unsafety_check_result<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId)
 493                                    -> UnsafetyCheckResult
 494 {
 495     debug!("unsafety_violations({:?})", def_id);
 496
 497     // NB: this borrow is valid because all the consumers of
 498     // `mir_built` force this.
 499     let mir = &tcx.mir_built(def_id).borrow();
 500
 501     let source_scope_local_data = match mir.source_scope_local_data {
 502         ClearCrossCrate::Set(ref data) => data,
 503         ClearCrossCrate::Clear => {
 504             debug!("unsafety_violations: {:?} - remote, skipping", def_id);
 505             return UnsafetyCheckResult {
 506                 violations: Lrc::new([]),
 507                 unsafe_blocks: Lrc::new([])
 508             }
 509         }
 510     };
 511
 512     let param_env = tcx.param_env(def_id);
 513     let mut checker = UnsafetyChecker::new(
 514         tcx.is_const_fn(def_id) && tcx.is_min_const_fn(def_id),
 515         mir, source_scope_local_data, tcx, param_env);
 516     checker.visit_mir(mir);
 517
 518     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);
 519     UnsafetyCheckResult {
 520         violations: checker.violations.into(),
 521         unsafe_blocks: checker.inherited_blocks.into()
 522     }
 523 }
 524
 525 fn unsafe_derive_on_repr_packed<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 526     let lint_node_id = match tcx.hir.as_local_node_id(def_id) {
 527         Some(node_id) => node_id,
 528         None => bug!("checking unsafety for non-local def id {:?}", def_id)
 529     };
 530
 531     // FIXME: when we make this a hard error, this should have its
 532     // own error code.
 533     let message = if tcx.generics_of(def_id).own_counts().types != 0 {
 534         "#[derive] can't be used on a #[repr(packed)] struct with \
 535          type parameters (error E0133)".to_string()
 536     } else {
 537         "#[derive] can't be used on a #[repr(packed)] struct that \
 538          does not derive Copy (error E0133)".to_string()
 539     };
 540     tcx.lint_node(SAFE_PACKED_BORROWS,
 541                   lint_node_id,
 542                   tcx.def_span(def_id),
 543                   &message);
 544 }
 545
 546 /// Return the NodeId for an enclosing scope that is also `unsafe`
 547 fn is_enclosed(tcx: TyCtxt,
 548                used_unsafe: &FxHashSet<ast::NodeId>,
 549                id: ast::NodeId) -> Option<(String, ast::NodeId)> {
 550     let parent_id = tcx.hir.get_parent_node(id);
 551     if parent_id != id {
 552         if used_unsafe.contains(&parent_id) {
 553             Some(("block".to_string(), parent_id))
 554         } else if let Some(Node::Item(&hir::Item {
 555             node: hir::ItemKind::Fn(_, header, _, _),
 556             ..
 557         })) = tcx.hir.find(parent_id) {
 558             match header.unsafety {
 559                 hir::Unsafety::Unsafe => Some(("fn".to_string(), parent_id)),
 560                 hir::Unsafety::Normal => None,
 561             }
 562         } else {
 563             is_enclosed(tcx, used_unsafe, parent_id)
 564         }
 565     } else {
 566         None
 567     }
 568 }
 569
 570 fn report_unused_unsafe(tcx: TyCtxt, used_unsafe: &FxHashSet<ast::NodeId>, id: ast::NodeId) {
 571     let span = tcx.sess.source_map().def_span(tcx.hir.span(id));
 572     let msg = "unnecessary `unsafe` block";
 573     let mut db = tcx.struct_span_lint_node(UNUSED_UNSAFE, id, span, msg);
 574     db.span_label(span, msg);
 575     if let Some((kind, id)) = is_enclosed(tcx, used_unsafe, id) {
 576         db.span_label(tcx.sess.source_map().def_span(tcx.hir.span(id)),
 577                       format!("because it's nested under this `unsafe` {}", kind));
 578     }
 579     db.emit();
 580 }
 581
 582 fn builtin_derive_def_id<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> Option<DefId> {
 583     debug!("builtin_derive_def_id({:?})", def_id);
 584     if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
 585         if tcx.has_attr(impl_def_id, "automatically_derived") {
 586             debug!("builtin_derive_def_id({:?}) - is {:?}", def_id, impl_def_id);
 587             Some(impl_def_id)
 588         } else {
 589             debug!("builtin_derive_def_id({:?}) - not automatically derived", def_id);
 590             None
 591         }
 592     } else {
 593         debug!("builtin_derive_def_id({:?}) - not a method", def_id);
 594         None
 595     }
 596 }
 597
 598 pub fn check_unsafety<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 599     debug!("check_unsafety({:?})", def_id);
 600
 601     // closures are handled by their parent fn.
 602     if tcx.is_closure(def_id) {
 603         return;
 604     }
 605
 606     let UnsafetyCheckResult {
 607         violations,
 608         unsafe_blocks
 609     } = tcx.unsafety_check_result(def_id);
 610
 611     for &UnsafetyViolation {
 612         source_info, description, details, kind
 613     } in violations.iter() {
 614         // Report an error.
 615         match kind {
 616             UnsafetyViolationKind::General if tcx.is_min_const_fn(def_id) => {
 617                 tcx.sess.struct_span_err(
 618                     source_info.span,
 619                     &format!("{} is unsafe and unsafe operations \
 620                             are not allowed in const fn", description))
 621                     .span_label(source_info.span, &description.as_str()[..])
 622                     .note(&details.as_str()[..])
 623                     .emit();
 624             }
 625             UnsafetyViolationKind::GeneralAndConstFn |
 626             UnsafetyViolationKind::General => {
 627                 struct_span_err!(
 628                     tcx.sess, source_info.span, E0133,
 629                     "{} is unsafe and requires unsafe function or block", description)
 630                     .span_label(source_info.span, &description.as_str()[..])
 631                     .note(&details.as_str()[..])
 632                     .emit();
 633             }
 634             UnsafetyViolationKind::GatedConstFnCall => {
 635                 emit_feature_err(
 636                     &tcx.sess.parse_sess,
 637                     "min_const_unsafe_fn",
 638                     source_info.span,
 639                     GateIssue::Language,
 640                     "calls to `const unsafe fn` in const fns are unstable",
 641                 );
 642
 643             }
 644             UnsafetyViolationKind::ExternStatic(lint_node_id) => {
 645                 tcx.lint_node_note(SAFE_EXTERN_STATICS,
 646                               lint_node_id,
 647                               source_info.span,
 648                               &format!("{} is unsafe and requires unsafe function or block \
 649                                         (error E0133)", &description.as_str()[..]),
 650                               &details.as_str()[..]);
 651             }
 652             UnsafetyViolationKind::BorrowPacked(lint_node_id) => {
 653                 if let Some(impl_def_id) = builtin_derive_def_id(tcx, def_id) {
 654                     tcx.unsafe_derive_on_repr_packed(impl_def_id);
 655                 } else {
 656                     tcx.lint_node_note(SAFE_PACKED_BORROWS,
 657                                   lint_node_id,
 658                                   source_info.span,
 659                                   &format!("{} is unsafe and requires unsafe function or block \
 660                                             (error E0133)", &description.as_str()[..]),
 661                                   &details.as_str()[..]);
 662                 }
 663             }
 664         }
 665     }
 666
 667     let mut unsafe_blocks: Vec<_> = unsafe_blocks.into_iter().collect();
 668     unsafe_blocks.sort();
 669     let used_unsafe: FxHashSet<_> = unsafe_blocks.iter()
 670         .flat_map(|&&(id, used)| if used { Some(id) } else { None })
 671         .collect();
 672     for &(block_id, is_used) in unsafe_blocks {
 673         if !is_used {
 674             report_unused_unsafe(tcx, &used_unsafe, block_id);
 675         }
 676     }
 677 }