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