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             // Move to above list once mir construction uses it.
 109             StatementKind::Intrinsic(..) => unreachable!(),
 110         }
 111         self.super_statement(statement, location);
 112     }
 113
 114     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 115         match rvalue {
 116             Rvalue::Aggregate(box ref aggregate, _) => match aggregate {
 117                 &AggregateKind::Array(..) | &AggregateKind::Tuple => {}
 118                 &AggregateKind::Adt(adt_did, ..) => {
 119                     match self.tcx.layout_scalar_valid_range(adt_did) {
 120                         (Bound::Unbounded, Bound::Unbounded) => {}
 121                         _ => self.require_unsafe(
 122                             UnsafetyViolationKind::General,
 123                             UnsafetyViolationDetails::InitializingTypeWith,
 124                         ),
 125                     }
 126                 }
 127                 &AggregateKind::Closure(def_id, _) | &AggregateKind::Generator(def_id, _, _) => {
 128                     let UnsafetyCheckResult { violations, used_unsafe_blocks, .. } =
 129                         self.tcx.unsafety_check_result(def_id);
 130                     self.register_violations(violations, used_unsafe_blocks.iter().copied());
 131                 }
 132             },
 133             _ => {}
 134         }
 135         self.super_rvalue(rvalue, location);
 136     }
 137
 138     fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
 139         // On types with `scalar_valid_range`, prevent
 140         // * `&mut x.field`
 141         // * `x.field = y;`
 142         // * `&x.field` if `field`'s type has interior mutability
 143         // because either of these would allow modifying the layout constrained field and
 144         // insert values that violate the layout constraints.
 145         if context.is_mutating_use() || context.is_borrow() {
 146             self.check_mut_borrowing_layout_constrained_field(*place, context.is_mutating_use());
 147         }
 148
 149         // Some checks below need the extra meta info of the local declaration.
 150         let decl = &self.body.local_decls[place.local];
 151
 152         // Check the base local: it might be an unsafe-to-access static. We only check derefs of the
 153         // temporary holding the static pointer to avoid duplicate errors
 154         // <https://github.com/rust-lang/rust/pull/78068#issuecomment-731753506>.
 155         if decl.internal && place.projection.first() == Some(&ProjectionElem::Deref) {
 156             // If the projection root is an artificial local that we introduced when
 157             // desugaring `static`, give a more specific error message
 158             // (avoid the general "raw pointer" clause below, that would only be confusing).
 159             if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
 160                 if self.tcx.is_mutable_static(def_id) {
 161                     self.require_unsafe(
 162                         UnsafetyViolationKind::General,
 163                         UnsafetyViolationDetails::UseOfMutableStatic,
 164                     );
 165                     return;
 166                 } else if self.tcx.is_foreign_item(def_id) {
 167                     self.require_unsafe(
 168                         UnsafetyViolationKind::General,
 169                         UnsafetyViolationDetails::UseOfExternStatic,
 170                     );
 171                     return;
 172                 }
 173             }
 174         }
 175
 176         // Check for raw pointer `Deref`.
 177         for (base, proj) in place.iter_projections() {
 178             if proj == ProjectionElem::Deref {
 179                 let base_ty = base.ty(self.body, self.tcx).ty;
 180                 if base_ty.is_unsafe_ptr() {
 181                     self.require_unsafe(
 182                         UnsafetyViolationKind::General,
 183                         UnsafetyViolationDetails::DerefOfRawPointer,
 184                     )
 185                 }
 186             }
 187         }
 188
 189         // Check for union fields. For this we traverse right-to-left, as the last `Deref` changes
 190         // whether we *read* the union field or potentially *write* to it (if this place is being assigned to).
 191         let mut saw_deref = false;
 192         for (base, proj) in place.iter_projections().rev() {
 193             if proj == ProjectionElem::Deref {
 194                 saw_deref = true;
 195                 continue;
 196             }
 197
 198             let base_ty = base.ty(self.body, self.tcx).ty;
 199             if base_ty.is_union() {
 200                 // If we did not hit a `Deref` yet and the overall place use is an assignment, the
 201                 // rules are different.
 202                 let assign_to_field = !saw_deref
 203                     && matches!(
 204                         context,
 205                         PlaceContext::MutatingUse(
 206                             MutatingUseContext::Store
 207                                 | MutatingUseContext::Drop
 208                                 | MutatingUseContext::AsmOutput
 209                         )
 210                     );
 211                 // If this is just an assignment, determine if the assigned type needs dropping.
 212                 if assign_to_field {
 213                     // We have to check the actual type of the assignment, as that determines if the
 214                     // old value is being dropped.
 215                     let assigned_ty = place.ty(&self.body.local_decls, self.tcx).ty;
 216                     if assigned_ty.needs_drop(self.tcx, self.param_env) {
 217                         // This would be unsafe, but should be outright impossible since we reject such unions.
 218                         self.tcx.sess.delay_span_bug(
 219                             self.source_info.span,
 220                             format!("union fields that need dropping should be impossible: {assigned_ty}")
 221                         );
 222                     }
 223                 } else {
 224                     self.require_unsafe(
 225                         UnsafetyViolationKind::General,
 226                         UnsafetyViolationDetails::AccessToUnionField,
 227                     )
 228                 }
 229             }
 230         }
 231     }
 232 }
 233
 234 impl<'tcx> UnsafetyChecker<'_, 'tcx> {
 235     fn require_unsafe(&mut self, kind: UnsafetyViolationKind, details: UnsafetyViolationDetails) {
 236         // Violations can turn out to be `UnsafeFn` during analysis, but they should not start out as such.
 237         assert_ne!(kind, UnsafetyViolationKind::UnsafeFn);
 238
 239         let source_info = self.source_info;
 240         let lint_root = self.body.source_scopes[self.source_info.scope]
 241             .local_data
 242             .as_ref()
 243             .assert_crate_local()
 244             .lint_root;
 245         self.register_violations(
 246             [&UnsafetyViolation { source_info, lint_root, kind, details }],
 247             [],
 248         );
 249     }
 250
 251     fn register_violations<'a>(
 252         &mut self,
 253         violations: impl IntoIterator<Item = &'a UnsafetyViolation>,
 254         new_used_unsafe_blocks: impl IntoIterator<Item = HirId>,
 255     ) {
 256         let safety = self.body.source_scopes[self.source_info.scope]
 257             .local_data
 258             .as_ref()
 259             .assert_crate_local()
 260             .safety;
 261         match safety {
 262             // `unsafe` blocks are required in safe code
 263             Safety::Safe => violations.into_iter().for_each(|&violation| {
 264                 match violation.kind {
 265                     UnsafetyViolationKind::General => {}
 266                     UnsafetyViolationKind::UnsafeFn => {
 267                         bug!("`UnsafetyViolationKind::UnsafeFn` in an `Safe` context")
 268                     }
 269                 }
 270                 if !self.violations.contains(&violation) {
 271                     self.violations.push(violation)
 272                 }
 273             }),
 274             // With the RFC 2585, no longer allow `unsafe` operations in `unsafe fn`s
 275             Safety::FnUnsafe => violations.into_iter().for_each(|&(mut violation)| {
 276                 violation.kind = UnsafetyViolationKind::UnsafeFn;
 277                 if !self.violations.contains(&violation) {
 278                     self.violations.push(violation)
 279                 }
 280             }),
 281             Safety::BuiltinUnsafe => {}
 282             Safety::ExplicitUnsafe(hir_id) => violations.into_iter().for_each(|_violation| {
 283                 self.used_unsafe_blocks.insert(hir_id);
 284             }),
 285         };
 286
 287         new_used_unsafe_blocks.into_iter().for_each(|hir_id| {
 288             self.used_unsafe_blocks.insert(hir_id);
 289         });
 290     }
 291     fn check_mut_borrowing_layout_constrained_field(
 292         &mut self,
 293         place: Place<'tcx>,
 294         is_mut_use: bool,
 295     ) {
 296         for (place_base, elem) in place.iter_projections().rev() {
 297             match elem {
 298                 // Modifications behind a dereference don't affect the value of
 299                 // the pointer.
 300                 ProjectionElem::Deref => return,
 301                 ProjectionElem::Field(..) => {
 302                     let ty = place_base.ty(&self.body.local_decls, self.tcx).ty;
 303                     if let ty::Adt(def, _) = ty.kind() {
 304                         if self.tcx.layout_scalar_valid_range(def.did())
 305                             != (Bound::Unbounded, Bound::Unbounded)
 306                         {
 307                             let details = if is_mut_use {
 308                                 UnsafetyViolationDetails::MutationOfLayoutConstrainedField
 309
 310                             // Check `is_freeze` as late as possible to avoid cycle errors
 311                             // with opaque types.
 312                             } else if !place
 313                                 .ty(self.body, self.tcx)
 314                                 .ty
 315                                 .is_freeze(self.tcx.at(self.source_info.span), self.param_env)
 316                             {
 317                                 UnsafetyViolationDetails::BorrowOfLayoutConstrainedField
 318                             } else {
 319                                 continue;
 320                             };
 321                             self.require_unsafe(UnsafetyViolationKind::General, details);
 322                         }
 323                     }
 324                 }
 325                 _ => {}
 326             }
 327         }
 328     }
 329
 330     /// Checks whether calling `func_did` needs an `unsafe` context or not, i.e. whether
 331     /// the called function has target features the calling function hasn't.
 332     fn check_target_features(&mut self, func_did: DefId) {
 333         // Unsafety isn't required on wasm targets. For more information see
 334         // the corresponding check in typeck/src/collect.rs
 335         if self.tcx.sess.target.options.is_like_wasm {
 336             return;
 337         }
 338
 339         let callee_features = &self.tcx.codegen_fn_attrs(func_did).target_features;
 340         // The body might be a constant, so it doesn't have codegen attributes.
 341         let self_features = &self.tcx.body_codegen_attrs(self.body_did.to_def_id()).target_features;
 342
 343         // Is `callee_features` a subset of `calling_features`?
 344         if !callee_features.iter().all(|feature| self_features.contains(feature)) {
 345             self.require_unsafe(
 346                 UnsafetyViolationKind::General,
 347                 UnsafetyViolationDetails::CallToFunctionWith,
 348             )
 349         }
 350     }
 351 }
 352
 353 pub(crate) fn provide(providers: &mut Providers) {
 354     *providers = Providers {
 355         unsafety_check_result: |tcx, def_id| {
 356             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 357                 tcx.unsafety_check_result_for_const_arg(def)
 358             } else {
 359                 unsafety_check_result(tcx, ty::WithOptConstParam::unknown(def_id))
 360             }
 361         },
 362         unsafety_check_result_for_const_arg: |tcx, (did, param_did)| {
 363             unsafety_check_result(
 364                 tcx,
 365                 ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 366             )
 367         },
 368         ..*providers
 369     };
 370 }
 371
 372 /// Context information for [`UnusedUnsafeVisitor`] traversal,
 373 /// saves (innermost) relevant context
 374 #[derive(Copy, Clone, Debug)]
 375 enum Context {
 376     Safe,
 377     /// in an `unsafe fn`
 378     UnsafeFn(HirId),
 379     /// in a *used* `unsafe` block
 380     /// (i.e. a block without unused-unsafe warning)
 381     UnsafeBlock(HirId),
 382 }
 383
 384 struct UnusedUnsafeVisitor<'a, 'tcx> {
 385     tcx: TyCtxt<'tcx>,
 386     used_unsafe_blocks: &'a FxHashSet<HirId>,
 387     context: Context,
 388     unused_unsafes: &'a mut Vec<(HirId, UnusedUnsafe)>,
 389 }
 390
 391 impl<'tcx> intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'_, 'tcx> {
 392     fn visit_block(&mut self, block: &'tcx hir::Block<'tcx>) {
 393         if let hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::UserProvided) = block.rules {
 394             let used = match self.tcx.lint_level_at_node(UNUSED_UNSAFE, block.hir_id) {
 395                 (Level::Allow, _) => true,
 396                 _ => self.used_unsafe_blocks.contains(&block.hir_id),
 397             };
 398             let unused_unsafe = match (self.context, used) {
 399                 (_, false) => UnusedUnsafe::Unused,
 400                 (Context::Safe, true) | (Context::UnsafeFn(_), true) => {
 401                     let previous_context = self.context;
 402                     self.context = Context::UnsafeBlock(block.hir_id);
 403                     intravisit::walk_block(self, block);
 404                     self.context = previous_context;
 405                     return;
 406                 }
 407                 (Context::UnsafeBlock(hir_id), true) => UnusedUnsafe::InUnsafeBlock(hir_id),
 408             };
 409             self.unused_unsafes.push((block.hir_id, unused_unsafe));
 410         }
 411         intravisit::walk_block(self, block);
 412     }
 413
 414     fn visit_fn(
 415         &mut self,
 416         fk: intravisit::FnKind<'tcx>,
 417         _fd: &'tcx hir::FnDecl<'tcx>,
 418         b: hir::BodyId,
 419         _s: rustc_span::Span,
 420         _id: HirId,
 421     ) {
 422         if matches!(fk, intravisit::FnKind::Closure) {
 423             self.visit_body(self.tcx.hir().body(b))
 424         }
 425     }
 426 }
 427
 428 fn check_unused_unsafe(
 429     tcx: TyCtxt<'_>,
 430     def_id: LocalDefId,
 431     used_unsafe_blocks: &FxHashSet<HirId>,
 432 ) -> Vec<(HirId, UnusedUnsafe)> {
 433     let body_id = tcx.hir().maybe_body_owned_by(def_id);
 434
 435     let Some(body_id) = body_id else {
 436         debug!("check_unused_unsafe({:?}) - no body found", def_id);
 437         return vec![];
 438     };
 439
 440     let body = tcx.hir().body(body_id);
 441     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 442     let context = match tcx.hir().fn_sig_by_hir_id(hir_id) {
 443         Some(sig) if sig.header.unsafety == hir::Unsafety::Unsafe => Context::UnsafeFn(hir_id),
 444         _ => Context::Safe,
 445     };
 446
 447     debug!(
 448         "check_unused_unsafe({:?}, context={:?}, body={:?}, used_unsafe_blocks={:?})",
 449         def_id, body, context, used_unsafe_blocks
 450     );
 451
 452     let mut unused_unsafes = vec![];
 453
 454     let mut visitor = UnusedUnsafeVisitor {
 455         tcx,
 456         used_unsafe_blocks,
 457         context,
 458         unused_unsafes: &mut unused_unsafes,
 459     };
 460     intravisit::Visitor::visit_body(&mut visitor, body);
 461
 462     unused_unsafes
 463 }
 464
 465 fn unsafety_check_result<'tcx>(
 466     tcx: TyCtxt<'tcx>,
 467     def: ty::WithOptConstParam<LocalDefId>,
 468 ) -> &'tcx UnsafetyCheckResult {
 469     debug!("unsafety_violations({:?})", def);
 470
 471     // N.B., this borrow is valid because all the consumers of
 472     // `mir_built` force this.
 473     let body = &tcx.mir_built(def).borrow();
 474
 475     let param_env = tcx.param_env(def.did);
 476
 477     let mut checker = UnsafetyChecker::new(body, def.did, tcx, param_env);
 478     checker.visit_body(&body);
 479
 480     let unused_unsafes = (!tcx.is_closure(def.did.to_def_id()))
 481         .then(|| check_unused_unsafe(tcx, def.did, &checker.used_unsafe_blocks));
 482
 483     tcx.arena.alloc(UnsafetyCheckResult {
 484         violations: checker.violations,
 485         used_unsafe_blocks: checker.used_unsafe_blocks,
 486         unused_unsafes,
 487     })
 488 }
 489
 490 fn report_unused_unsafe(tcx: TyCtxt<'_>, kind: UnusedUnsafe, id: HirId) {
 491     let span = tcx.sess.source_map().guess_head_span(tcx.hir().span(id));
 492     tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, |lint| {
 493         let msg = "unnecessary `unsafe` block";
 494         let mut db = lint.build(msg);
 495         db.span_label(span, msg);
 496         match kind {
 497             UnusedUnsafe::Unused => {}
 498             UnusedUnsafe::InUnsafeBlock(id) => {
 499                 db.span_label(
 500                     tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
 501                     "because it's nested under this `unsafe` block",
 502                 );
 503             }
 504         }
 505
 506         db.emit();
 507     });
 508 }
 509
 510 pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 511     debug!("check_unsafety({:?})", def_id);
 512
 513     // closures are handled by their parent fn.
 514     if tcx.is_closure(def_id.to_def_id()) {
 515         return;
 516     }
 517
 518     let UnsafetyCheckResult { violations, unused_unsafes, .. } = tcx.unsafety_check_result(def_id);
 519
 520     for &UnsafetyViolation { source_info, lint_root, kind, details } in violations.iter() {
 521         let (description, note) = details.description_and_note();
 522
 523         // Report an error.
 524         let unsafe_fn_msg =
 525             if unsafe_op_in_unsafe_fn_allowed(tcx, lint_root) { " function or" } else { "" };
 526
 527         match kind {
 528             UnsafetyViolationKind::General => {
 529                 // once
 530                 struct_span_err!(
 531                     tcx.sess,
 532                     source_info.span,
 533                     E0133,
 534                     "{} is unsafe and requires unsafe{} block",
 535                     description,
 536                     unsafe_fn_msg,
 537                 )
 538                 .span_label(source_info.span, description)
 539                 .note(note)
 540                 .emit();
 541             }
 542             UnsafetyViolationKind::UnsafeFn => tcx.struct_span_lint_hir(
 543                 UNSAFE_OP_IN_UNSAFE_FN,
 544                 lint_root,
 545                 source_info.span,
 546                 |lint| {
 547                     lint.build(&format!(
 548                         "{} is unsafe and requires unsafe block (error E0133)",
 549                         description,
 550                     ))
 551                     .span_label(source_info.span, description)
 552                     .note(note)
 553                     .emit();
 554                 },
 555             ),
 556         }
 557     }
 558
 559     for &(block_id, kind) in unused_unsafes.as_ref().unwrap() {
 560         report_unused_unsafe(tcx, kind, block_id);
 561     }
 562 }
 563
 564 fn unsafe_op_in_unsafe_fn_allowed(tcx: TyCtxt<'_>, id: HirId) -> bool {
 565     tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, id).0 == Level::Allow
 566 }