compiler/rustc_mir_transform/src/check_unsafety.rs

   1 use rustc_data_structures::fx::FxHashMap;
   2 use rustc_errors::struct_span_err;
   3 use rustc_hir as hir;
   4 use rustc_hir::def_id::{DefId, LocalDefId};
   5 use rustc_hir::hir_id::HirId;
   6 use rustc_hir::intravisit;
   7 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
   8 use rustc_middle::ty::query::Providers;
   9 use rustc_middle::ty::{self, TyCtxt};
  10 use rustc_middle::{lint, mir::*};
  11 use rustc_session::lint::builtin::{UNSAFE_OP_IN_UNSAFE_FN, UNUSED_UNSAFE};
  12 use rustc_session::lint::Level;
  13
  14 use std::collections::hash_map;
  15 use std::ops::Bound;
  16
  17 pub struct UnsafetyChecker<'a, 'tcx> {
  18     body: &'a Body<'tcx>,
  19     body_did: LocalDefId,
  20     violations: Vec<UnsafetyViolation>,
  21     source_info: SourceInfo,
  22     tcx: TyCtxt<'tcx>,
  23     param_env: ty::ParamEnv<'tcx>,
  24
  25     /// Used `unsafe` blocks in this function. This is used for the "unused_unsafe" lint.
  26     ///
  27     /// The keys are the used `unsafe` blocks, the UnusedUnsafeKind indicates whether
  28     /// or not any of the usages happen at a place that doesn't allow `unsafe_op_in_unsafe_fn`.
  29     used_unsafe_blocks: FxHashMap<HirId, UsedUnsafeBlockData>,
  30 }
  31
  32 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
  33     fn new(
  34         body: &'a Body<'tcx>,
  35         body_did: LocalDefId,
  36         tcx: TyCtxt<'tcx>,
  37         param_env: ty::ParamEnv<'tcx>,
  38     ) -> Self {
  39         Self {
  40             body,
  41             body_did,
  42             violations: vec![],
  43             source_info: SourceInfo::outermost(body.span),
  44             tcx,
  45             param_env,
  46             used_unsafe_blocks: Default::default(),
  47         }
  48     }
  49 }
  50
  51 impl<'tcx> Visitor<'tcx> for UnsafetyChecker<'_, 'tcx> {
  52     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
  53         self.source_info = terminator.source_info;
  54         match terminator.kind {
  55             TerminatorKind::Goto { .. }
  56             | TerminatorKind::SwitchInt { .. }
  57             | TerminatorKind::Drop { .. }
  58             | TerminatorKind::Yield { .. }
  59             | TerminatorKind::Assert { .. }
  60             | TerminatorKind::DropAndReplace { .. }
  61             | TerminatorKind::GeneratorDrop
  62             | TerminatorKind::Resume
  63             | TerminatorKind::Abort
  64             | TerminatorKind::Return
  65             | TerminatorKind::Unreachable
  66             | TerminatorKind::FalseEdge { .. }
  67             | TerminatorKind::FalseUnwind { .. } => {
  68                 // safe (at least as emitted during MIR construction)
  69             }
  70
  71             TerminatorKind::Call { ref func, .. } => {
  72                 let func_ty = func.ty(self.body, self.tcx);
  73                 let func_id =
  74                     if let ty::FnDef(func_id, _) = func_ty.kind() { Some(func_id) } else { None };
  75                 let sig = func_ty.fn_sig(self.tcx);
  76                 if let hir::Unsafety::Unsafe = sig.unsafety() {
  77                     self.require_unsafe(
  78                         UnsafetyViolationKind::General,
  79                         UnsafetyViolationDetails::CallToUnsafeFunction,
  80                     )
  81                 }
  82
  83                 if let Some(func_id) = func_id {
  84                     self.check_target_features(*func_id);
  85                 }
  86             }
  87
  88             TerminatorKind::InlineAsm { .. } => self.require_unsafe(
  89                 UnsafetyViolationKind::General,
  90                 UnsafetyViolationDetails::UseOfInlineAssembly,
  91             ),
  92         }
  93         self.super_terminator(terminator, location);
  94     }
  95
  96     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
  97         self.source_info = statement.source_info;
  98         match statement.kind {
  99             StatementKind::Assign(..)
 100             | StatementKind::FakeRead(..)
 101             | StatementKind::SetDiscriminant { .. }
 102             | StatementKind::Deinit(..)
 103             | StatementKind::StorageLive(..)
 104             | StatementKind::StorageDead(..)
 105             | StatementKind::Retag { .. }
 106             | StatementKind::AscribeUserType(..)
 107             | StatementKind::Coverage(..)
 108             | StatementKind::Nop => {
 109                 // safe (at least as emitted during MIR construction)
 110             }
 111
 112             StatementKind::CopyNonOverlapping(..) => unreachable!(),
 113         }
 114         self.super_statement(statement, location);
 115     }
 116
 117     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 118         match rvalue {
 119             Rvalue::Aggregate(box ref aggregate, _) => match aggregate {
 120                 &AggregateKind::Array(..) | &AggregateKind::Tuple => {}
 121                 &AggregateKind::Adt(adt_did, ..) => {
 122                     match self.tcx.layout_scalar_valid_range(adt_did) {
 123                         (Bound::Unbounded, Bound::Unbounded) => {}
 124                         _ => self.require_unsafe(
 125                             UnsafetyViolationKind::General,
 126                             UnsafetyViolationDetails::InitializingTypeWith,
 127                         ),
 128                     }
 129                 }
 130                 &AggregateKind::Closure(def_id, _) | &AggregateKind::Generator(def_id, _, _) => {
 131                     let UnsafetyCheckResult { violations, used_unsafe_blocks, .. } =
 132                         self.tcx.unsafety_check_result(def_id.expect_local());
 133                     self.register_violations(
 134                         violations,
 135                         used_unsafe_blocks.iter().map(|(&h, &d)| (h, d)),
 136                     );
 137                 }
 138             },
 139             _ => {}
 140         }
 141         self.super_rvalue(rvalue, location);
 142     }
 143
 144     fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
 145         // On types with `scalar_valid_range`, prevent
 146         // * `&mut x.field`
 147         // * `x.field = y;`
 148         // * `&x.field` if `field`'s type has interior mutability
 149         // because either of these would allow modifying the layout constrained field and
 150         // insert values that violate the layout constraints.
 151         if context.is_mutating_use() || context.is_borrow() {
 152             self.check_mut_borrowing_layout_constrained_field(*place, context.is_mutating_use());
 153         }
 154
 155         // Some checks below need the extra meta info of the local declaration.
 156         let decl = &self.body.local_decls[place.local];
 157
 158         // Check the base local: it might be an unsafe-to-access static. We only check derefs of the
 159         // temporary holding the static pointer to avoid duplicate errors
 160         // <https://github.com/rust-lang/rust/pull/78068#issuecomment-731753506>.
 161         if decl.internal && place.projection.first() == Some(&ProjectionElem::Deref) {
 162             // If the projection root is an artificial local that we introduced when
 163             // desugaring `static`, give a more specific error message
 164             // (avoid the general "raw pointer" clause below, that would only be confusing).
 165             if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
 166                 if self.tcx.is_mutable_static(def_id) {
 167                     self.require_unsafe(
 168                         UnsafetyViolationKind::General,
 169                         UnsafetyViolationDetails::UseOfMutableStatic,
 170                     );
 171                     return;
 172                 } else if self.tcx.is_foreign_item(def_id) {
 173                     self.require_unsafe(
 174                         UnsafetyViolationKind::General,
 175                         UnsafetyViolationDetails::UseOfExternStatic,
 176                     );
 177                     return;
 178                 }
 179             }
 180         }
 181
 182         // Check for raw pointer `Deref`.
 183         for (base, proj) in place.iter_projections() {
 184             if proj == ProjectionElem::Deref {
 185                 let base_ty = base.ty(self.body, self.tcx).ty;
 186                 if base_ty.is_unsafe_ptr() {
 187                     self.require_unsafe(
 188                         UnsafetyViolationKind::General,
 189                         UnsafetyViolationDetails::DerefOfRawPointer,
 190                     )
 191                 }
 192             }
 193         }
 194
 195         // Check for union fields. For this we traverse right-to-left, as the last `Deref` changes
 196         // whether we *read* the union field or potentially *write* to it (if this place is being assigned to).
 197         let mut saw_deref = false;
 198         for (base, proj) in place.iter_projections().rev() {
 199             if proj == ProjectionElem::Deref {
 200                 saw_deref = true;
 201                 continue;
 202             }
 203
 204             let base_ty = base.ty(self.body, self.tcx).ty;
 205             if base_ty.is_union() {
 206                 // If we did not hit a `Deref` yet and the overall place use is an assignment, the
 207                 // rules are different.
 208                 let assign_to_field = !saw_deref
 209                     && matches!(
 210                         context,
 211                         PlaceContext::MutatingUse(
 212                             MutatingUseContext::Store
 213                                 | MutatingUseContext::Drop
 214                                 | MutatingUseContext::AsmOutput
 215                         )
 216                     );
 217                 // If this is just an assignment, determine if the assigned type needs dropping.
 218                 if assign_to_field {
 219                     // We have to check the actual type of the assignment, as that determines if the
 220                     // old value is being dropped.
 221                     let assigned_ty = place.ty(&self.body.local_decls, self.tcx).ty;
 222                     if assigned_ty.needs_drop(self.tcx, self.param_env) {
 223                         // This would be unsafe, but should be outright impossible since we reject such unions.
 224                         self.tcx.sess.delay_span_bug(
 225                             self.source_info.span,
 226                             format!("union fields that need dropping should be impossible: {assigned_ty}")
 227                         );
 228                     }
 229                 } else {
 230                     self.require_unsafe(
 231                         UnsafetyViolationKind::General,
 232                         UnsafetyViolationDetails::AccessToUnionField,
 233                     )
 234                 }
 235             }
 236         }
 237     }
 238 }
 239
 240 impl<'tcx> UnsafetyChecker<'_, 'tcx> {
 241     fn require_unsafe(&mut self, kind: UnsafetyViolationKind, details: UnsafetyViolationDetails) {
 242         // Violations can turn out to be `UnsafeFn` during analysis, but they should not start out as such.
 243         assert_ne!(kind, UnsafetyViolationKind::UnsafeFn);
 244
 245         let source_info = self.source_info;
 246         let lint_root = self.body.source_scopes[self.source_info.scope]
 247             .local_data
 248             .as_ref()
 249             .assert_crate_local()
 250             .lint_root;
 251         self.register_violations(
 252             [&UnsafetyViolation { source_info, lint_root, kind, details }],
 253             [],
 254         );
 255     }
 256
 257     fn register_violations<'a>(
 258         &mut self,
 259         violations: impl IntoIterator<Item = &'a UnsafetyViolation>,
 260         new_used_unsafe_blocks: impl IntoIterator<Item = (HirId, UsedUnsafeBlockData)>,
 261     ) {
 262         use UsedUnsafeBlockData::{AllAllowedInUnsafeFn, SomeDisallowedInUnsafeFn};
 263
 264         let update_entry = |this: &mut Self, hir_id, new_usage| {
 265             match this.used_unsafe_blocks.entry(hir_id) {
 266                 hash_map::Entry::Occupied(mut entry) => {
 267                     if new_usage == SomeDisallowedInUnsafeFn {
 268                         *entry.get_mut() = SomeDisallowedInUnsafeFn;
 269                     }
 270                 }
 271                 hash_map::Entry::Vacant(entry) => {
 272                     entry.insert(new_usage);
 273                 }
 274             };
 275         };
 276         let safety = self.body.source_scopes[self.source_info.scope]
 277             .local_data
 278             .as_ref()
 279             .assert_crate_local()
 280             .safety;
 281         match safety {
 282             // `unsafe` blocks are required in safe code
 283             Safety::Safe => violations.into_iter().for_each(|&violation| {
 284                 match violation.kind {
 285                     UnsafetyViolationKind::General => {}
 286                     UnsafetyViolationKind::UnsafeFn => {
 287                         bug!("`UnsafetyViolationKind::UnsafeFn` in an `Safe` context")
 288                     }
 289                 }
 290                 if !self.violations.contains(&violation) {
 291                     self.violations.push(violation)
 292                 }
 293             }),
 294             // With the RFC 2585, no longer allow `unsafe` operations in `unsafe fn`s
 295             Safety::FnUnsafe => violations.into_iter().for_each(|&(mut violation)| {
 296                 violation.kind = UnsafetyViolationKind::UnsafeFn;
 297                 if !self.violations.contains(&violation) {
 298                     self.violations.push(violation)
 299                 }
 300             }),
 301             Safety::BuiltinUnsafe => {}
 302             Safety::ExplicitUnsafe(hir_id) => violations.into_iter().for_each(|violation| {
 303                 update_entry(
 304                     self,
 305                     hir_id,
 306                     match self.tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, violation.lint_root).0
 307                     {
 308                         Level::Allow => AllAllowedInUnsafeFn(violation.lint_root),
 309                         _ => SomeDisallowedInUnsafeFn,
 310                     },
 311                 )
 312             }),
 313         };
 314
 315         new_used_unsafe_blocks
 316             .into_iter()
 317             .for_each(|(hir_id, usage_data)| update_entry(self, hir_id, usage_data));
 318     }
 319     fn check_mut_borrowing_layout_constrained_field(
 320         &mut self,
 321         place: Place<'tcx>,
 322         is_mut_use: bool,
 323     ) {
 324         for (place_base, elem) in place.iter_projections().rev() {
 325             match elem {
 326                 // Modifications behind a dereference don't affect the value of
 327                 // the pointer.
 328                 ProjectionElem::Deref => return,
 329                 ProjectionElem::Field(..) => {
 330                     let ty = place_base.ty(&self.body.local_decls, self.tcx).ty;
 331                     if let ty::Adt(def, _) = ty.kind() {
 332                         if self.tcx.layout_scalar_valid_range(def.did())
 333                             != (Bound::Unbounded, Bound::Unbounded)
 334                         {
 335                             let details = if is_mut_use {
 336                                 UnsafetyViolationDetails::MutationOfLayoutConstrainedField
 337
 338                             // Check `is_freeze` as late as possible to avoid cycle errors
 339                             // with opaque types.
 340                             } else if !place
 341                                 .ty(self.body, self.tcx)
 342                                 .ty
 343                                 .is_freeze(self.tcx.at(self.source_info.span), self.param_env)
 344                             {
 345                                 UnsafetyViolationDetails::BorrowOfLayoutConstrainedField
 346                             } else {
 347                                 continue;
 348                             };
 349                             self.require_unsafe(UnsafetyViolationKind::General, details);
 350                         }
 351                     }
 352                 }
 353                 _ => {}
 354             }
 355         }
 356     }
 357
 358     /// Checks whether calling `func_did` needs an `unsafe` context or not, i.e. whether
 359     /// the called function has target features the calling function hasn't.
 360     fn check_target_features(&mut self, func_did: DefId) {
 361         // Unsafety isn't required on wasm targets. For more information see
 362         // the corresponding check in typeck/src/collect.rs
 363         if self.tcx.sess.target.options.is_like_wasm {
 364             return;
 365         }
 366
 367         let callee_features = &self.tcx.codegen_fn_attrs(func_did).target_features;
 368         // The body might be a constant, so it doesn't have codegen attributes.
 369         let self_features = &self.tcx.body_codegen_attrs(self.body_did.to_def_id()).target_features;
 370
 371         // Is `callee_features` a subset of `calling_features`?
 372         if !callee_features.iter().all(|feature| self_features.contains(feature)) {
 373             self.require_unsafe(
 374                 UnsafetyViolationKind::General,
 375                 UnsafetyViolationDetails::CallToFunctionWith,
 376             )
 377         }
 378     }
 379 }
 380
 381 pub(crate) fn provide(providers: &mut Providers) {
 382     *providers = Providers {
 383         unsafety_check_result: |tcx, def_id| {
 384             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 385                 tcx.unsafety_check_result_for_const_arg(def)
 386             } else {
 387                 unsafety_check_result(tcx, ty::WithOptConstParam::unknown(def_id))
 388             }
 389         },
 390         unsafety_check_result_for_const_arg: |tcx, (did, param_did)| {
 391             unsafety_check_result(
 392                 tcx,
 393                 ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 394             )
 395         },
 396         ..*providers
 397     };
 398 }
 399
 400 /// Context information for [`UnusedUnsafeVisitor`] traversal,
 401 /// saves (innermost) relevant context
 402 #[derive(Copy, Clone, Debug)]
 403 enum Context {
 404     Safe,
 405     /// in an `unsafe fn`
 406     UnsafeFn(HirId),
 407     /// in a *used* `unsafe` block
 408     /// (i.e. a block without unused-unsafe warning)
 409     UnsafeBlock(HirId),
 410 }
 411
 412 struct UnusedUnsafeVisitor<'a, 'tcx> {
 413     tcx: TyCtxt<'tcx>,
 414     used_unsafe_blocks: &'a FxHashMap<HirId, UsedUnsafeBlockData>,
 415     context: Context,
 416     unused_unsafes: &'a mut Vec<(HirId, UnusedUnsafe)>,
 417 }
 418
 419 impl<'tcx> intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'_, 'tcx> {
 420     fn visit_block(&mut self, block: &'tcx hir::Block<'tcx>) {
 421         use UsedUnsafeBlockData::{AllAllowedInUnsafeFn, SomeDisallowedInUnsafeFn};
 422
 423         if let hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::UserProvided) = block.rules {
 424             let used = match self.tcx.lint_level_at_node(UNUSED_UNSAFE, block.hir_id) {
 425                 (Level::Allow, _) => Some(SomeDisallowedInUnsafeFn),
 426                 _ => self.used_unsafe_blocks.get(&block.hir_id).copied(),
 427             };
 428             let unused_unsafe = match (self.context, used) {
 429                 (_, None) => UnusedUnsafe::Unused,
 430                 (Context::Safe, Some(_))
 431                 | (Context::UnsafeFn(_), Some(SomeDisallowedInUnsafeFn)) => {
 432                     let previous_context = self.context;
 433                     self.context = Context::UnsafeBlock(block.hir_id);
 434                     intravisit::walk_block(self, block);
 435                     self.context = previous_context;
 436                     return;
 437                 }
 438                 (Context::UnsafeFn(hir_id), Some(AllAllowedInUnsafeFn(lint_root))) => {
 439                     UnusedUnsafe::InUnsafeFn(hir_id, lint_root)
 440                 }
 441                 (Context::UnsafeBlock(hir_id), Some(_)) => UnusedUnsafe::InUnsafeBlock(hir_id),
 442             };
 443             self.unused_unsafes.push((block.hir_id, unused_unsafe));
 444         }
 445         intravisit::walk_block(self, block);
 446     }
 447
 448     fn visit_fn(
 449         &mut self,
 450         fk: intravisit::FnKind<'tcx>,
 451         _fd: &'tcx hir::FnDecl<'tcx>,
 452         b: hir::BodyId,
 453         _s: rustc_span::Span,
 454         _id: HirId,
 455     ) {
 456         if matches!(fk, intravisit::FnKind::Closure) {
 457             self.visit_body(self.tcx.hir().body(b))
 458         }
 459     }
 460 }
 461
 462 fn check_unused_unsafe(
 463     tcx: TyCtxt<'_>,
 464     def_id: LocalDefId,
 465     used_unsafe_blocks: &FxHashMap<HirId, UsedUnsafeBlockData>,
 466 ) -> Vec<(HirId, UnusedUnsafe)> {
 467     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 468     let body_id = tcx.hir().maybe_body_owned_by(hir_id);
 469
 470     let Some(body_id) = body_id else {
 471         debug!("check_unused_unsafe({:?}) - no body found", def_id);
 472         return vec![];
 473     };
 474     let body = tcx.hir().body(body_id);
 475
 476     let context = match tcx.hir().fn_sig_by_hir_id(hir_id) {
 477         Some(sig) if sig.header.unsafety == hir::Unsafety::Unsafe => Context::UnsafeFn(hir_id),
 478         _ => Context::Safe,
 479     };
 480
 481     debug!(
 482         "check_unused_unsafe({:?}, context={:?}, body={:?}, used_unsafe_blocks={:?})",
 483         def_id, body, context, used_unsafe_blocks
 484     );
 485
 486     let mut unused_unsafes = vec![];
 487
 488     let mut visitor = UnusedUnsafeVisitor {
 489         tcx,
 490         used_unsafe_blocks,
 491         context,
 492         unused_unsafes: &mut unused_unsafes,
 493     };
 494     intravisit::Visitor::visit_body(&mut visitor, body);
 495
 496     unused_unsafes
 497 }
 498
 499 fn unsafety_check_result<'tcx>(
 500     tcx: TyCtxt<'tcx>,
 501     def: ty::WithOptConstParam<LocalDefId>,
 502 ) -> &'tcx UnsafetyCheckResult {
 503     debug!("unsafety_violations({:?})", def);
 504
 505     // N.B., this borrow is valid because all the consumers of
 506     // `mir_built` force this.
 507     let body = &tcx.mir_built(def).borrow();
 508
 509     let param_env = tcx.param_env(def.did);
 510
 511     let mut checker = UnsafetyChecker::new(body, def.did, tcx, param_env);
 512     checker.visit_body(&body);
 513
 514     let unused_unsafes = (!tcx.is_closure(def.did.to_def_id()))
 515         .then(|| check_unused_unsafe(tcx, def.did, &checker.used_unsafe_blocks));
 516
 517     tcx.arena.alloc(UnsafetyCheckResult {
 518         violations: checker.violations,
 519         used_unsafe_blocks: checker.used_unsafe_blocks,
 520         unused_unsafes,
 521     })
 522 }
 523
 524 fn report_unused_unsafe(tcx: TyCtxt<'_>, kind: UnusedUnsafe, id: HirId) {
 525     let span = tcx.sess.source_map().guess_head_span(tcx.hir().span(id));
 526     tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, |lint| {
 527         let msg = "unnecessary `unsafe` block";
 528         let mut db = lint.build(msg);
 529         db.span_label(span, msg);
 530         match kind {
 531             UnusedUnsafe::Unused => {}
 532             UnusedUnsafe::InUnsafeBlock(id) => {
 533                 db.span_label(
 534                     tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
 535                     "because it's nested under this `unsafe` block",
 536                 );
 537             }
 538             UnusedUnsafe::InUnsafeFn(id, usage_lint_root) => {
 539                 db.span_label(
 540                     tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
 541                     "because it's nested under this `unsafe` fn",
 542                 )
 543                 .note(
 544                     "this `unsafe` block does contain unsafe operations, \
 545                     but those are already allowed in an `unsafe fn`",
 546                 );
 547                 let (level, source) =
 548                     tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, usage_lint_root);
 549                 assert_eq!(level, Level::Allow);
 550                 lint::explain_lint_level_source(
 551                     UNSAFE_OP_IN_UNSAFE_FN,
 552                     Level::Allow,
 553                     source,
 554                     &mut db,
 555                 );
 556             }
 557         }
 558
 559         db.emit();
 560     });
 561 }
 562
 563 pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 564     debug!("check_unsafety({:?})", def_id);
 565
 566     // closures are handled by their parent fn.
 567     if tcx.is_closure(def_id.to_def_id()) {
 568         return;
 569     }
 570
 571     let UnsafetyCheckResult { violations, unused_unsafes, .. } = tcx.unsafety_check_result(def_id);
 572
 573     for &UnsafetyViolation { source_info, lint_root, kind, details } in violations.iter() {
 574         let (description, note) = details.description_and_note();
 575
 576         // Report an error.
 577         let unsafe_fn_msg =
 578             if unsafe_op_in_unsafe_fn_allowed(tcx, lint_root) { " function or" } else { "" };
 579
 580         match kind {
 581             UnsafetyViolationKind::General => {
 582                 // once
 583                 struct_span_err!(
 584                     tcx.sess,
 585                     source_info.span,
 586                     E0133,
 587                     "{} is unsafe and requires unsafe{} block",
 588                     description,
 589                     unsafe_fn_msg,
 590                 )
 591                 .span_label(source_info.span, description)
 592                 .note(note)
 593                 .emit();
 594             }
 595             UnsafetyViolationKind::UnsafeFn => tcx.struct_span_lint_hir(
 596                 UNSAFE_OP_IN_UNSAFE_FN,
 597                 lint_root,
 598                 source_info.span,
 599                 |lint| {
 600                     lint.build(&format!(
 601                         "{} is unsafe and requires unsafe block (error E0133)",
 602                         description,
 603                     ))
 604                     .span_label(source_info.span, description)
 605                     .note(note)
 606                     .emit();
 607                 },
 608             ),
 609         }
 610     }
 611
 612     for &(block_id, kind) in unused_unsafes.as_ref().unwrap() {
 613         report_unused_unsafe(tcx, kind, block_id);
 614     }
 615 }
 616
 617 fn unsafe_op_in_unsafe_fn_allowed(tcx: TyCtxt<'_>, id: HirId) -> bool {
 618     tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, id).0 == Level::Allow
 619 }