src/tools/clippy/clippy_lints/src/redundant_clone.rs

   1 use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
   2 use clippy_utils::source::snippet_opt;
   3 use clippy_utils::ty::{has_drop, is_copy, is_type_diagnostic_item, walk_ptrs_ty_depth};
   4 use clippy_utils::{fn_has_unsatisfiable_preds, match_def_path, paths};
   5 use if_chain::if_chain;
   6 use rustc_data_structures::{fx::FxHashMap, transitive_relation::TransitiveRelation};
   7 use rustc_errors::Applicability;
   8 use rustc_hir::intravisit::FnKind;
   9 use rustc_hir::{def_id, Body, FnDecl, HirId};
  10 use rustc_index::bit_set::{BitSet, HybridBitSet};
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_middle::mir::{
  13     self, traversal,
  14     visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor as _},
  15     Mutability,
  16 };
  17 use rustc_middle::ty::{self, fold::TypeVisitor, Ty, TyCtxt};
  18 use rustc_mir_dataflow::{Analysis, AnalysisDomain, GenKill, GenKillAnalysis, ResultsCursor};
  19 use rustc_session::{declare_lint_pass, declare_tool_lint};
  20 use rustc_span::source_map::{BytePos, Span};
  21 use rustc_span::sym;
  22 use std::convert::TryFrom;
  23 use std::ops::ControlFlow;
  24
  25 macro_rules! unwrap_or_continue {
  26     ($x:expr) => {
  27         match $x {
  28             Some(x) => x,
  29             None => continue,
  30         }
  31     };
  32 }
  33
  34 declare_clippy_lint! {
  35     /// ### What it does
  36     /// Checks for a redundant `clone()` (and its relatives) which clones an owned
  37     /// value that is going to be dropped without further use.
  38     ///
  39     /// ### Why is this bad?
  40     /// It is not always possible for the compiler to eliminate useless
  41     /// allocations and deallocations generated by redundant `clone()`s.
  42     ///
  43     /// ### Known problems
  44     /// False-negatives: analysis performed by this lint is conservative and limited.
  45     ///
  46     /// ### Example
  47     /// ```rust
  48     /// # use std::path::Path;
  49     /// # #[derive(Clone)]
  50     /// # struct Foo;
  51     /// # impl Foo {
  52     /// #     fn new() -> Self { Foo {} }
  53     /// # }
  54     /// # fn call(x: Foo) {}
  55     /// {
  56     ///     let x = Foo::new();
  57     ///     call(x.clone());
  58     ///     call(x.clone()); // this can just pass `x`
  59     /// }
  60     ///
  61     /// ["lorem", "ipsum"].join(" ").to_string();
  62     ///
  63     /// Path::new("/a/b").join("c").to_path_buf();
  64     /// ```
  65     pub REDUNDANT_CLONE,
  66     perf,
  67     "`clone()` of an owned value that is going to be dropped immediately"
  68 }
  69
  70 declare_lint_pass!(RedundantClone => [REDUNDANT_CLONE]);
  71
  72 impl<'tcx> LateLintPass<'tcx> for RedundantClone {
  73     #[allow(clippy::too_many_lines)]
  74     fn check_fn(
  75         &mut self,
  76         cx: &LateContext<'tcx>,
  77         _: FnKind<'tcx>,
  78         _: &'tcx FnDecl<'_>,
  79         body: &'tcx Body<'_>,
  80         _: Span,
  81         _: HirId,
  82     ) {
  83         let def_id = cx.tcx.hir().body_owner_def_id(body.id());
  84
  85         // Building MIR for `fn`s with unsatisfiable preds results in ICE.
  86         if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
  87             return;
  88         }
  89
  90         let mir = cx.tcx.optimized_mir(def_id.to_def_id());
  91
  92         let possible_origin = {
  93             let mut vis = PossibleOriginVisitor::new(mir);
  94             vis.visit_body(mir);
  95             vis.into_map(cx)
  96         };
  97         let maybe_storage_live_result = MaybeStorageLive
  98             .into_engine(cx.tcx, mir)
  99             .pass_name("redundant_clone")
 100             .iterate_to_fixpoint()
 101             .into_results_cursor(mir);
 102         let mut possible_borrower = {
 103             let mut vis = PossibleBorrowerVisitor::new(cx, mir, possible_origin);
 104             vis.visit_body(mir);
 105             vis.into_map(cx, maybe_storage_live_result)
 106         };
 107
 108         for (bb, bbdata) in mir.basic_blocks().iter_enumerated() {
 109             let terminator = bbdata.terminator();
 110
 111             if terminator.source_info.span.from_expansion() {
 112                 continue;
 113             }
 114
 115             // Give up on loops
 116             if terminator.successors().any(|s| *s == bb) {
 117                 continue;
 118             }
 119
 120             let (fn_def_id, arg, arg_ty, clone_ret) =
 121                 unwrap_or_continue!(is_call_with_ref_arg(cx, mir, &terminator.kind));
 122
 123             let from_borrow = match_def_path(cx, fn_def_id, &paths::CLONE_TRAIT_METHOD)
 124                 || match_def_path(cx, fn_def_id, &paths::TO_OWNED_METHOD)
 125                 || (match_def_path(cx, fn_def_id, &paths::TO_STRING_METHOD)
 126                     && is_type_diagnostic_item(cx, arg_ty, sym::String));
 127
 128             let from_deref = !from_borrow
 129                 && (match_def_path(cx, fn_def_id, &paths::PATH_TO_PATH_BUF)
 130                     || match_def_path(cx, fn_def_id, &paths::OS_STR_TO_OS_STRING));
 131
 132             if !from_borrow && !from_deref {
 133                 continue;
 134             }
 135
 136             if let ty::Adt(def, _) = arg_ty.kind() {
 137                 if match_def_path(cx, def.did, &paths::MEM_MANUALLY_DROP) {
 138                     continue;
 139                 }
 140             }
 141
 142             // `{ arg = &cloned; clone(move arg); }` or `{ arg = &cloned; to_path_buf(arg); }`
 143             let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(cx, mir, arg, from_borrow, bb));
 144
 145             let loc = mir::Location {
 146                 block: bb,
 147                 statement_index: bbdata.statements.len(),
 148             };
 149
 150             // `Local` to be cloned, and a local of `clone` call's destination
 151             let (local, ret_local) = if from_borrow {
 152                 // `res = clone(arg)` can be turned into `res = move arg;`
 153                 // if `arg` is the only borrow of `cloned` at this point.
 154
 155                 if cannot_move_out || !possible_borrower.only_borrowers(&[arg], cloned, loc) {
 156                     continue;
 157                 }
 158
 159                 (cloned, clone_ret)
 160             } else {
 161                 // `arg` is a reference as it is `.deref()`ed in the previous block.
 162                 // Look into the predecessor block and find out the source of deref.
 163
 164                 let ps = &mir.predecessors()[bb];
 165                 if ps.len() != 1 {
 166                     continue;
 167                 }
 168                 let pred_terminator = mir[ps[0]].terminator();
 169
 170                 // receiver of the `deref()` call
 171                 let (pred_arg, deref_clone_ret) = if_chain! {
 172                     if let Some((pred_fn_def_id, pred_arg, pred_arg_ty, res)) =
 173                         is_call_with_ref_arg(cx, mir, &pred_terminator.kind);
 174                     if res == cloned;
 175                     if cx.tcx.is_diagnostic_item(sym::deref_method, pred_fn_def_id);
 176                     if is_type_diagnostic_item(cx, pred_arg_ty, sym::PathBuf)
 177                         || is_type_diagnostic_item(cx, pred_arg_ty, sym::OsString);
 178                     then {
 179                         (pred_arg, res)
 180                     } else {
 181                         continue;
 182                     }
 183                 };
 184
 185                 let (local, cannot_move_out) =
 186                     unwrap_or_continue!(find_stmt_assigns_to(cx, mir, pred_arg, true, ps[0]));
 187                 let loc = mir::Location {
 188                     block: bb,
 189                     statement_index: mir.basic_blocks()[bb].statements.len(),
 190                 };
 191
 192                 // This can be turned into `res = move local` if `arg` and `cloned` are not borrowed
 193                 // at the last statement:
 194                 //
 195                 // ```
 196                 // pred_arg = &local;
 197                 // cloned = deref(pred_arg);
 198                 // arg = &cloned;
 199                 // StorageDead(pred_arg);
 200                 // res = to_path_buf(cloned);
 201                 // ```
 202                 if cannot_move_out || !possible_borrower.only_borrowers(&[arg, cloned], local, loc) {
 203                     continue;
 204                 }
 205
 206                 (local, deref_clone_ret)
 207             };
 208
 209             let clone_usage = if local == ret_local {
 210                 CloneUsage {
 211                     cloned_used: false,
 212                     cloned_consume_or_mutate_loc: None,
 213                     clone_consumed_or_mutated: true,
 214                 }
 215             } else {
 216                 let clone_usage = visit_clone_usage(local, ret_local, mir, bb);
 217                 if clone_usage.cloned_used && clone_usage.clone_consumed_or_mutated {
 218                     // cloned value is used, and the clone is modified or moved
 219                     continue;
 220                 } else if let Some(loc) = clone_usage.cloned_consume_or_mutate_loc {
 221                     // cloned value is mutated, and the clone is alive.
 222                     if possible_borrower.is_alive_at(ret_local, loc) {
 223                         continue;
 224                     }
 225                 }
 226                 clone_usage
 227             };
 228
 229             let span = terminator.source_info.span;
 230             let scope = terminator.source_info.scope;
 231             let node = mir.source_scopes[scope]
 232                 .local_data
 233                 .as_ref()
 234                 .assert_crate_local()
 235                 .lint_root;
 236
 237             if_chain! {
 238                 if let Some(snip) = snippet_opt(cx, span);
 239                 if let Some(dot) = snip.rfind('.');
 240                 then {
 241                     let sugg_span = span.with_lo(
 242                         span.lo() + BytePos(u32::try_from(dot).unwrap())
 243                     );
 244                     let mut app = Applicability::MaybeIncorrect;
 245
 246                     let call_snip = &snip[dot + 1..];
 247                     // Machine applicable when `call_snip` looks like `foobar()`
 248                     if let Some(call_snip) = call_snip.strip_suffix("()").map(str::trim) {
 249                         if call_snip.as_bytes().iter().all(|b| b.is_ascii_alphabetic() || *b == b'_') {
 250                             app = Applicability::MachineApplicable;
 251                         }
 252                     }
 253
 254                     span_lint_hir_and_then(cx, REDUNDANT_CLONE, node, sugg_span, "redundant clone", |diag| {
 255                         diag.span_suggestion(
 256                             sugg_span,
 257                             "remove this",
 258                             String::new(),
 259                             app,
 260                         );
 261                         if clone_usage.cloned_used {
 262                             diag.span_note(
 263                                 span,
 264                                 "cloned value is neither consumed nor mutated",
 265                             );
 266                         } else {
 267                             diag.span_note(
 268                                 span.with_hi(span.lo() + BytePos(u32::try_from(dot).unwrap())),
 269                                 "this value is dropped without further use",
 270                             );
 271                         }
 272                     });
 273                 } else {
 274                     span_lint_hir(cx, REDUNDANT_CLONE, node, span, "redundant clone");
 275                 }
 276             }
 277         }
 278     }
 279 }
 280
 281 /// If `kind` is `y = func(x: &T)` where `T: !Copy`, returns `(DefId of func, x, T, y)`.
 282 fn is_call_with_ref_arg<'tcx>(
 283     cx: &LateContext<'tcx>,
 284     mir: &'tcx mir::Body<'tcx>,
 285     kind: &'tcx mir::TerminatorKind<'tcx>,
 286 ) -> Option<(def_id::DefId, mir::Local, Ty<'tcx>, mir::Local)> {
 287     if_chain! {
 288         if let mir::TerminatorKind::Call { func, args, destination, .. } = kind;
 289         if args.len() == 1;
 290         if let mir::Operand::Move(mir::Place { local, .. }) = &args[0];
 291         if let ty::FnDef(def_id, _) = *func.ty(&*mir, cx.tcx).kind();
 292         if let (inner_ty, 1) = walk_ptrs_ty_depth(args[0].ty(&*mir, cx.tcx));
 293         if !is_copy(cx, inner_ty);
 294         then {
 295             Some((def_id, *local, inner_ty, destination.as_ref().map(|(dest, _)| dest)?.as_local()?))
 296         } else {
 297             None
 298         }
 299     }
 300 }
 301
 302 type CannotMoveOut = bool;
 303
 304 /// Finds the first `to = (&)from`, and returns
 305 /// ``Some((from, whether `from` cannot be moved out))``.
 306 fn find_stmt_assigns_to<'tcx>(
 307     cx: &LateContext<'tcx>,
 308     mir: &mir::Body<'tcx>,
 309     to_local: mir::Local,
 310     by_ref: bool,
 311     bb: mir::BasicBlock,
 312 ) -> Option<(mir::Local, CannotMoveOut)> {
 313     let rvalue = mir.basic_blocks()[bb].statements.iter().rev().find_map(|stmt| {
 314         if let mir::StatementKind::Assign(box (mir::Place { local, .. }, v)) = &stmt.kind {
 315             return if *local == to_local { Some(v) } else { None };
 316         }
 317
 318         None
 319     })?;
 320
 321     match (by_ref, &*rvalue) {
 322         (true, mir::Rvalue::Ref(_, _, place)) | (false, mir::Rvalue::Use(mir::Operand::Copy(place))) => {
 323             Some(base_local_and_movability(cx, mir, *place))
 324         },
 325         (false, mir::Rvalue::Ref(_, _, place)) => {
 326             if let [mir::ProjectionElem::Deref] = place.as_ref().projection {
 327                 Some(base_local_and_movability(cx, mir, *place))
 328             } else {
 329                 None
 330             }
 331         },
 332         _ => None,
 333     }
 334 }
 335
 336 /// Extracts and returns the undermost base `Local` of given `place`. Returns `place` itself
 337 /// if it is already a `Local`.
 338 ///
 339 /// Also reports whether given `place` cannot be moved out.
 340 fn base_local_and_movability<'tcx>(
 341     cx: &LateContext<'tcx>,
 342     mir: &mir::Body<'tcx>,
 343     place: mir::Place<'tcx>,
 344 ) -> (mir::Local, CannotMoveOut) {
 345     use rustc_middle::mir::PlaceRef;
 346
 347     // Dereference. You cannot move things out from a borrowed value.
 348     let mut deref = false;
 349     // Accessing a field of an ADT that has `Drop`. Moving the field out will cause E0509.
 350     let mut field = false;
 351     // If projection is a slice index then clone can be removed only if the
 352     // underlying type implements Copy
 353     let mut slice = false;
 354
 355     let PlaceRef { local, mut projection } = place.as_ref();
 356     while let [base @ .., elem] = projection {
 357         projection = base;
 358         deref |= matches!(elem, mir::ProjectionElem::Deref);
 359         field |= matches!(elem, mir::ProjectionElem::Field(..))
 360             && has_drop(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
 361         slice |= matches!(elem, mir::ProjectionElem::Index(..))
 362             && !is_copy(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
 363     }
 364
 365     (local, deref || field || slice)
 366 }
 367
 368 #[derive(Default)]
 369 struct CloneUsage {
 370     /// Whether the cloned value is used after the clone.
 371     cloned_used: bool,
 372     /// The first location where the cloned value is consumed or mutated, if any.
 373     cloned_consume_or_mutate_loc: Option<mir::Location>,
 374     /// Whether the clone value is mutated.
 375     clone_consumed_or_mutated: bool,
 376 }
 377 fn visit_clone_usage(cloned: mir::Local, clone: mir::Local, mir: &mir::Body<'_>, bb: mir::BasicBlock) -> CloneUsage {
 378     struct V {
 379         cloned: mir::Local,
 380         clone: mir::Local,
 381         result: CloneUsage,
 382     }
 383     impl<'tcx> mir::visit::Visitor<'tcx> for V {
 384         fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
 385             let statements = &data.statements;
 386             for (statement_index, statement) in statements.iter().enumerate() {
 387                 self.visit_statement(statement, mir::Location { block, statement_index });
 388             }
 389
 390             self.visit_terminator(
 391                 data.terminator(),
 392                 mir::Location {
 393                     block,
 394                     statement_index: statements.len(),
 395                 },
 396             );
 397         }
 398
 399         fn visit_place(&mut self, place: &mir::Place<'tcx>, ctx: PlaceContext, loc: mir::Location) {
 400             let local = place.local;
 401
 402             if local == self.cloned
 403                 && !matches!(
 404                     ctx,
 405                     PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_)
 406                 )
 407             {
 408                 self.result.cloned_used = true;
 409                 self.result.cloned_consume_or_mutate_loc = self.result.cloned_consume_or_mutate_loc.or_else(|| {
 410                     matches!(
 411                         ctx,
 412                         PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
 413                             | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 414                     )
 415                     .then(|| loc)
 416                 });
 417             } else if local == self.clone {
 418                 match ctx {
 419                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
 420                     | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
 421                         self.result.clone_consumed_or_mutated = true;
 422                     },
 423                     _ => {},
 424                 }
 425             }
 426         }
 427     }
 428
 429     let init = CloneUsage {
 430         cloned_used: false,
 431         cloned_consume_or_mutate_loc: None,
 432         // Consider non-temporary clones consumed.
 433         // TODO: Actually check for mutation of non-temporaries.
 434         clone_consumed_or_mutated: mir.local_kind(clone) != mir::LocalKind::Temp,
 435     };
 436     traversal::ReversePostorder::new(mir, bb)
 437         .skip(1)
 438         .fold(init, |usage, (tbb, tdata)| {
 439             // Short-circuit
 440             if (usage.cloned_used && usage.clone_consumed_or_mutated) ||
 441                 // Give up on loops
 442                 tdata.terminator().successors().any(|s| *s == bb)
 443             {
 444                 return CloneUsage {
 445                     cloned_used: true,
 446                     clone_consumed_or_mutated: true,
 447                     ..usage
 448                 };
 449             }
 450
 451             let mut v = V {
 452                 cloned,
 453                 clone,
 454                 result: usage,
 455             };
 456             v.visit_basic_block_data(tbb, tdata);
 457             v.result
 458         })
 459 }
 460
 461 /// Determines liveness of each local purely based on `StorageLive`/`Dead`.
 462 #[derive(Copy, Clone)]
 463 struct MaybeStorageLive;
 464
 465 impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
 466     type Domain = BitSet<mir::Local>;
 467     const NAME: &'static str = "maybe_storage_live";
 468
 469     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
 470         // bottom = dead
 471         BitSet::new_empty(body.local_decls.len())
 472     }
 473
 474     fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain) {
 475         for arg in body.args_iter() {
 476             state.insert(arg);
 477         }
 478     }
 479 }
 480
 481 impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
 482     type Idx = mir::Local;
 483
 484     fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
 485         match stmt.kind {
 486             mir::StatementKind::StorageLive(l) => trans.gen(l),
 487             mir::StatementKind::StorageDead(l) => trans.kill(l),
 488             _ => (),
 489         }
 490     }
 491
 492     fn terminator_effect(
 493         &self,
 494         _trans: &mut impl GenKill<Self::Idx>,
 495         _terminator: &mir::Terminator<'tcx>,
 496         _loc: mir::Location,
 497     ) {
 498     }
 499
 500     fn call_return_effect(
 501         &self,
 502         _in_out: &mut impl GenKill<Self::Idx>,
 503         _block: mir::BasicBlock,
 504         _func: &mir::Operand<'tcx>,
 505         _args: &[mir::Operand<'tcx>],
 506         _return_place: mir::Place<'tcx>,
 507     ) {
 508         // Nothing to do when a call returns successfully
 509     }
 510 }
 511
 512 /// Collects the possible borrowers of each local.
 513 /// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
 514 /// possible borrowers of `a`.
 515 struct PossibleBorrowerVisitor<'a, 'tcx> {
 516     possible_borrower: TransitiveRelation<mir::Local>,
 517     body: &'a mir::Body<'tcx>,
 518     cx: &'a LateContext<'tcx>,
 519     possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
 520 }
 521
 522 impl<'a, 'tcx> PossibleBorrowerVisitor<'a, 'tcx> {
 523     fn new(
 524         cx: &'a LateContext<'tcx>,
 525         body: &'a mir::Body<'tcx>,
 526         possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
 527     ) -> Self {
 528         Self {
 529             possible_borrower: TransitiveRelation::default(),
 530             cx,
 531             body,
 532             possible_origin,
 533         }
 534     }
 535
 536     fn into_map(
 537         self,
 538         cx: &LateContext<'tcx>,
 539         maybe_live: ResultsCursor<'tcx, 'tcx, MaybeStorageLive>,
 540     ) -> PossibleBorrowerMap<'a, 'tcx> {
 541         let mut map = FxHashMap::default();
 542         for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
 543             if is_copy(cx, self.body.local_decls[row].ty) {
 544                 continue;
 545             }
 546
 547             let borrowers = self.possible_borrower.reachable_from(&row);
 548             if !borrowers.is_empty() {
 549                 let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
 550                 for &c in borrowers {
 551                     if c != mir::Local::from_usize(0) {
 552                         bs.insert(c);
 553                     }
 554                 }
 555
 556                 if !bs.is_empty() {
 557                     map.insert(row, bs);
 558                 }
 559             }
 560         }
 561
 562         let bs = BitSet::new_empty(self.body.local_decls.len());
 563         PossibleBorrowerMap {
 564             map,
 565             maybe_live,
 566             bitset: (bs.clone(), bs),
 567         }
 568     }
 569 }
 570
 571 impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
 572     fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
 573         let lhs = place.local;
 574         match rvalue {
 575             mir::Rvalue::Ref(_, _, borrowed) => {
 576                 self.possible_borrower.add(borrowed.local, lhs);
 577             },
 578             other => {
 579                 if ContainsRegion(self.cx.tcx)
 580                     .visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty)
 581                     .is_continue()
 582                 {
 583                     return;
 584                 }
 585                 rvalue_locals(other, |rhs| {
 586                     if lhs != rhs {
 587                         self.possible_borrower.add(rhs, lhs);
 588                     }
 589                 });
 590             },
 591         }
 592     }
 593
 594     fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
 595         if let mir::TerminatorKind::Call {
 596             args,
 597             destination: Some((mir::Place { local: dest, .. }, _)),
 598             ..
 599         } = &terminator.kind
 600         {
 601             // TODO add doc
 602             // If the call returns something with lifetimes,
 603             // let's conservatively assume the returned value contains lifetime of all the arguments.
 604             // For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.
 605
 606             let mut immutable_borrowers = vec![];
 607             let mut mutable_borrowers = vec![];
 608
 609             for op in args {
 610                 match op {
 611                     mir::Operand::Copy(p) | mir::Operand::Move(p) => {
 612                         if let ty::Ref(_, _, Mutability::Mut) = self.body.local_decls[p.local].ty.kind() {
 613                             mutable_borrowers.push(p.local);
 614                         } else {
 615                             immutable_borrowers.push(p.local);
 616                         }
 617                     },
 618                     mir::Operand::Constant(..) => (),
 619                 }
 620             }
 621
 622             let mut mutable_variables: Vec<mir::Local> = mutable_borrowers
 623                 .iter()
 624                 .filter_map(|r| self.possible_origin.get(r))
 625                 .flat_map(HybridBitSet::iter)
 626                 .collect();
 627
 628             if ContainsRegion(self.cx.tcx)
 629                 .visit_ty(self.body.local_decls[*dest].ty)
 630                 .is_break()
 631             {
 632                 mutable_variables.push(*dest);
 633             }
 634
 635             for y in mutable_variables {
 636                 for x in &immutable_borrowers {
 637                     self.possible_borrower.add(*x, y);
 638                 }
 639                 for x in &mutable_borrowers {
 640                     self.possible_borrower.add(*x, y);
 641                 }
 642             }
 643         }
 644     }
 645 }
 646
 647 /// Collect possible borrowed for every `&mut` local.
 648 /// For exampel, `_1 = &mut _2` generate _1: {_2,...}
 649 /// Known Problems: not sure all borrowed are tracked
 650 struct PossibleOriginVisitor<'a, 'tcx> {
 651     possible_origin: TransitiveRelation<mir::Local>,
 652     body: &'a mir::Body<'tcx>,
 653 }
 654
 655 impl<'a, 'tcx> PossibleOriginVisitor<'a, 'tcx> {
 656     fn new(body: &'a mir::Body<'tcx>) -> Self {
 657         Self {
 658             possible_origin: TransitiveRelation::default(),
 659             body,
 660         }
 661     }
 662
 663     fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, HybridBitSet<mir::Local>> {
 664         let mut map = FxHashMap::default();
 665         for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
 666             if is_copy(cx, self.body.local_decls[row].ty) {
 667                 continue;
 668             }
 669
 670             let borrowers = self.possible_origin.reachable_from(&row);
 671             if !borrowers.is_empty() {
 672                 let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
 673                 for &c in borrowers {
 674                     if c != mir::Local::from_usize(0) {
 675                         bs.insert(c);
 676                     }
 677                 }
 678
 679                 if !bs.is_empty() {
 680                     map.insert(row, bs);
 681                 }
 682             }
 683         }
 684         map
 685     }
 686 }
 687
 688 impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleOriginVisitor<'a, 'tcx> {
 689     fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
 690         let lhs = place.local;
 691         match rvalue {
 692             // Only consider `&mut`, which can modify origin place
 693             mir::Rvalue::Ref(_, rustc_middle::mir::BorrowKind::Mut { .. }, borrowed) |
 694             // _2: &mut _;
 695             // _3 = move _2
 696             mir::Rvalue::Use(mir::Operand::Move(borrowed))  |
 697             // _3 = move _2 as &mut _;
 698             mir::Rvalue::Cast(_, mir::Operand::Move(borrowed), _)
 699                 => {
 700                 self.possible_origin.add(lhs, borrowed.local);
 701             },
 702             _ => {},
 703         }
 704     }
 705 }
 706
 707 struct ContainsRegion<'tcx>(TyCtxt<'tcx>);
 708
 709 impl<'tcx> TypeVisitor<'tcx> for ContainsRegion<'tcx> {
 710     type BreakTy = ();
 711     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
 712         Some(self.0)
 713     }
 714
 715     fn visit_region(&mut self, _: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
 716         ControlFlow::BREAK
 717     }
 718 }
 719
 720 fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
 721     use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};
 722
 723     let mut visit_op = |op: &mir::Operand<'_>| match op {
 724         mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
 725         mir::Operand::Constant(..) => (),
 726     };
 727
 728     match rvalue {
 729         Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
 730         Aggregate(_, ops) => ops.iter().for_each(visit_op),
 731         BinaryOp(_, box (lhs, rhs)) | CheckedBinaryOp(_, box (lhs, rhs)) => {
 732             visit_op(lhs);
 733             visit_op(rhs);
 734         },
 735         _ => (),
 736     }
 737 }
 738
 739 /// Result of `PossibleBorrowerVisitor`.
 740 struct PossibleBorrowerMap<'a, 'tcx> {
 741     /// Mapping `Local -> its possible borrowers`
 742     map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
 743     maybe_live: ResultsCursor<'a, 'tcx, MaybeStorageLive>,
 744     // Caches to avoid allocation of `BitSet` on every query
 745     bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
 746 }
 747
 748 impl PossibleBorrowerMap<'_, '_> {
 749     /// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
 750     fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
 751         self.maybe_live.seek_after_primary_effect(at);
 752
 753         self.bitset.0.clear();
 754         let maybe_live = &mut self.maybe_live;
 755         if let Some(bitset) = self.map.get(&borrowed) {
 756             for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
 757                 self.bitset.0.insert(b);
 758             }
 759         } else {
 760             return false;
 761         }
 762
 763         self.bitset.1.clear();
 764         for b in borrowers {
 765             self.bitset.1.insert(*b);
 766         }
 767
 768         self.bitset.0 == self.bitset.1
 769     }
 770
 771     fn is_alive_at(&mut self, local: mir::Local, at: mir::Location) -> bool {
 772         self.maybe_live.seek_after_primary_effect(at);
 773         self.maybe_live.contains(local)
 774     }
 775 }