compiler/rustc_mir_transform/src/check_unsafety.rs

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