2 fn_has_unsatisfiable_preds, has_drop, is_copy, is_type_diagnostic_item, match_def_path, match_type, paths,
3 snippet_opt, span_lint_hir, span_lint_hir_and_then, walk_ptrs_ty_depth,
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::{
14 visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor as _},
16 use rustc_middle::ty::{self, fold::TypeVisitor, Ty};
17 use rustc_mir::dataflow::BottomValue;
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 std::convert::TryFrom;
23 macro_rules! unwrap_or_continue {
32 declare_clippy_lint! {
33 /// **What it does:** Checks for a redundant `clone()` (and its relatives) which clones an owned
34 /// value that is going to be dropped without further use.
36 /// **Why is this bad?** It is not always possible for the compiler to eliminate useless
37 /// allocations and deallocations generated by redundant `clone()`s.
39 /// **Known problems:**
41 /// False-negatives: analysis performed by this lint is conservative and limited.
45 /// # use std::path::Path;
46 /// # #[derive(Clone)]
49 /// # fn new() -> Self { Foo {} }
51 /// # fn call(x: Foo) {}
53 /// let x = Foo::new();
55 /// call(x.clone()); // this can just pass `x`
58 /// ["lorem", "ipsum"].join(" ").to_string();
60 /// Path::new("/a/b").join("c").to_path_buf();
64 "`clone()` of an owned value that is going to be dropped immediately"
67 declare_lint_pass!(RedundantClone => [REDUNDANT_CLONE]);
69 impl<'tcx> LateLintPass<'tcx> for RedundantClone {
70 #[allow(clippy::too_many_lines)]
73 cx: &LateContext<'tcx>,
80 let def_id = cx.tcx.hir().body_owner_def_id(body.id());
82 // Building MIR for `fn`s with unsatisfiable preds results in ICE.
83 if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
87 let mir = cx.tcx.optimized_mir(def_id.to_def_id());
89 let maybe_storage_live_result = MaybeStorageLive
90 .into_engine(cx.tcx, mir, def_id.to_def_id())
91 .iterate_to_fixpoint()
92 .into_results_cursor(mir);
93 let mut possible_borrower = {
94 let mut vis = PossibleBorrowerVisitor::new(cx, mir);
96 vis.into_map(cx, maybe_storage_live_result)
99 for (bb, bbdata) in mir.basic_blocks().iter_enumerated() {
100 let terminator = bbdata.terminator();
102 if terminator.source_info.span.from_expansion() {
107 if terminator.successors().any(|s| *s == bb) {
111 let (fn_def_id, arg, arg_ty, clone_ret) =
112 unwrap_or_continue!(is_call_with_ref_arg(cx, mir, &terminator.kind));
114 let from_borrow = match_def_path(cx, fn_def_id, &paths::CLONE_TRAIT_METHOD)
115 || match_def_path(cx, fn_def_id, &paths::TO_OWNED_METHOD)
116 || (match_def_path(cx, fn_def_id, &paths::TO_STRING_METHOD)
117 && is_type_diagnostic_item(cx, arg_ty, sym!(string_type)));
119 let from_deref = !from_borrow
120 && (match_def_path(cx, fn_def_id, &paths::PATH_TO_PATH_BUF)
121 || match_def_path(cx, fn_def_id, &paths::OS_STR_TO_OS_STRING));
123 if !from_borrow && !from_deref {
127 if let ty::Adt(ref def, _) = arg_ty.kind {
128 if match_def_path(cx, def.did, &paths::MEM_MANUALLY_DROP) {
133 // `{ cloned = &arg; clone(move cloned); }` or `{ cloned = &arg; to_path_buf(cloned); }`
134 let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(cx, mir, arg, from_borrow, bb));
136 let loc = mir::Location {
138 statement_index: bbdata.statements.len(),
141 // `Local` to be cloned, and a local of `clone` call's destination
142 let (local, ret_local) = if from_borrow {
143 // `res = clone(arg)` can be turned into `res = move arg;`
144 // if `arg` is the only borrow of `cloned` at this point.
146 if cannot_move_out || !possible_borrower.only_borrowers(&[arg], cloned, loc) {
152 // `arg` is a reference as it is `.deref()`ed in the previous block.
153 // Look into the predecessor block and find out the source of deref.
155 let ps = &mir.predecessors()[bb];
159 let pred_terminator = mir[ps[0]].terminator();
161 // receiver of the `deref()` call
162 let (pred_arg, deref_clone_ret) = if_chain! {
163 if let Some((pred_fn_def_id, pred_arg, pred_arg_ty, res)) =
164 is_call_with_ref_arg(cx, mir, &pred_terminator.kind);
166 if match_def_path(cx, pred_fn_def_id, &paths::DEREF_TRAIT_METHOD);
167 if match_type(cx, pred_arg_ty, &paths::PATH_BUF)
168 || match_type(cx, pred_arg_ty, &paths::OS_STRING);
176 let (local, cannot_move_out) =
177 unwrap_or_continue!(find_stmt_assigns_to(cx, mir, pred_arg, true, ps[0]));
178 let loc = mir::Location {
180 statement_index: mir.basic_blocks()[bb].statements.len(),
183 // This can be turned into `res = move local` if `arg` and `cloned` are not borrowed
184 // at the last statement:
187 // pred_arg = &local;
188 // cloned = deref(pred_arg);
190 // StorageDead(pred_arg);
191 // res = to_path_buf(cloned);
193 if cannot_move_out || !possible_borrower.only_borrowers(&[arg, cloned], local, loc) {
197 (local, deref_clone_ret)
200 let is_temp = mir.local_kind(ret_local) == mir::LocalKind::Temp;
202 // 1. `local` can be moved out if it is not used later.
203 // 2. If `ret_local` is a temporary and is neither consumed nor mutated, we can remove this `clone`
205 let (used, consumed_or_mutated) = traversal::ReversePostorder::new(&mir, bb).skip(1).fold(
207 |(used, consumed), (tbb, tdata)| {
209 if (used && consumed) ||
211 tdata.terminator().successors().any(|s| *s == bb)
216 let mut vis = LocalUseVisitor {
217 used: (local, false),
218 consumed_or_mutated: (ret_local, false),
220 vis.visit_basic_block_data(tbb, tdata);
221 (used || vis.used.1, consumed || vis.consumed_or_mutated.1)
225 if !used || !consumed_or_mutated {
226 let span = terminator.source_info.span;
227 let scope = terminator.source_info.scope;
228 let node = mir.source_scopes[scope]
231 .assert_crate_local()
235 if let Some(snip) = snippet_opt(cx, span);
236 if let Some(dot) = snip.rfind('.');
238 let sugg_span = span.with_lo(
239 span.lo() + BytePos(u32::try_from(dot).unwrap())
241 let mut app = Applicability::MaybeIncorrect;
243 let mut call_snip = &snip[dot + 1..];
244 // Machine applicable when `call_snip` looks like `foobar()`
245 if call_snip.ends_with("()") {
246 call_snip = call_snip[..call_snip.len()-2].trim();
247 if call_snip.as_bytes().iter().all(|b| b.is_ascii_alphabetic() || *b == b'_') {
248 app = Applicability::MachineApplicable;
252 span_lint_hir_and_then(cx, REDUNDANT_CLONE, node, sugg_span, "redundant clone", |diag| {
253 diag.span_suggestion(
262 "cloned value is neither consumed nor mutated",
266 span.with_hi(span.lo() + BytePos(u32::try_from(dot).unwrap())),
267 "this value is dropped without further use",
272 span_lint_hir(cx, REDUNDANT_CLONE, node, span, "redundant clone");
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)> {
287 if let mir::TerminatorKind::Call { func, args, destination, .. } = kind;
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);
294 Some((def_id, *local, inner_ty, destination.as_ref().map(|(dest, _)| dest)?.as_local()?))
301 type CannotMoveOut = bool;
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,
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 };
320 match (by_ref, &*rvalue) {
321 (true, mir::Rvalue::Ref(_, _, place)) | (false, mir::Rvalue::Use(mir::Operand::Copy(place))) => {
322 base_local_and_movability(cx, mir, *place)
324 (false, mir::Rvalue::Ref(_, _, place)) => {
325 if let [mir::ProjectionElem::Deref] = place.as_ref().projection {
326 base_local_and_movability(cx, mir, *place)
335 /// Extracts and returns the undermost base `Local` of given `place`. Returns `place` itself
336 /// if it is already a `Local`.
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 ) -> Option<(mir::Local, CannotMoveOut)> {
344 use rustc_middle::mir::PlaceRef;
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;
354 let PlaceRef { local, mut projection } = place.as_ref();
355 while let [base @ .., elem] = projection {
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);
364 Some((local, deref || field || slice))
367 struct LocalUseVisitor {
368 used: (mir::Local, bool),
369 consumed_or_mutated: (mir::Local, bool),
372 impl<'tcx> mir::visit::Visitor<'tcx> for LocalUseVisitor {
373 fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
374 let statements = &data.statements;
375 for (statement_index, statement) in statements.iter().enumerate() {
376 self.visit_statement(statement, mir::Location { block, statement_index });
379 self.visit_terminator(
383 statement_index: statements.len(),
388 fn visit_place(&mut self, place: &mir::Place<'tcx>, ctx: PlaceContext, _: mir::Location) {
389 let local = place.local;
391 if local == self.used.0
392 && !matches!(ctx, PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_))
397 if local == self.consumed_or_mutated.0 {
399 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
400 | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
401 self.consumed_or_mutated.1 = true;
409 /// Determines liveness of each local purely based on `StorageLive`/`Dead`.
410 #[derive(Copy, Clone)]
411 struct MaybeStorageLive;
413 impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
414 type Idx = mir::Local;
415 const NAME: &'static str = "maybe_storage_live";
417 fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
418 body.local_decls.len()
421 fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut BitSet<Self::Idx>) {
422 for arg in body.args_iter() {
428 impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
429 fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
431 mir::StatementKind::StorageLive(l) => trans.gen(l),
432 mir::StatementKind::StorageDead(l) => trans.kill(l),
437 fn terminator_effect(
439 _trans: &mut impl GenKill<Self::Idx>,
440 _terminator: &mir::Terminator<'tcx>,
445 fn call_return_effect(
447 _in_out: &mut impl GenKill<Self::Idx>,
448 _block: mir::BasicBlock,
449 _func: &mir::Operand<'tcx>,
450 _args: &[mir::Operand<'tcx>],
451 _return_place: mir::Place<'tcx>,
453 // Nothing to do when a call returns successfully
457 impl BottomValue for MaybeStorageLive {
459 const BOTTOM_VALUE: bool = false;
462 /// Collects the possible borrowers of each local.
463 /// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
464 /// possible borrowers of `a`.
465 struct PossibleBorrowerVisitor<'a, 'tcx> {
466 possible_borrower: TransitiveRelation<mir::Local>,
467 body: &'a mir::Body<'tcx>,
468 cx: &'a LateContext<'tcx>,
471 impl<'a, 'tcx> PossibleBorrowerVisitor<'a, 'tcx> {
472 fn new(cx: &'a LateContext<'tcx>, body: &'a mir::Body<'tcx>) -> Self {
474 possible_borrower: TransitiveRelation::default(),
482 cx: &LateContext<'tcx>,
483 maybe_live: ResultsCursor<'tcx, 'tcx, MaybeStorageLive>,
484 ) -> PossibleBorrowerMap<'a, 'tcx> {
485 let mut map = FxHashMap::default();
486 for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
487 if is_copy(cx, self.body.local_decls[row].ty) {
491 let borrowers = self.possible_borrower.reachable_from(&row);
492 if !borrowers.is_empty() {
493 let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
494 for &c in borrowers {
495 if c != mir::Local::from_usize(0) {
506 let bs = BitSet::new_empty(self.body.local_decls.len());
507 PossibleBorrowerMap {
510 bitset: (bs.clone(), bs),
515 impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
516 fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
517 let lhs = place.local;
519 mir::Rvalue::Ref(_, _, borrowed) => {
520 self.possible_borrower.add(borrowed.local, lhs);
523 if !ContainsRegion.visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty) {
526 rvalue_locals(other, |rhs| {
528 self.possible_borrower.add(rhs, lhs);
535 fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
536 if let mir::TerminatorKind::Call {
538 destination: Some((mir::Place { local: dest, .. }, _)),
542 // If the call returns something with lifetimes,
543 // let's conservatively assume the returned value contains lifetime of all the arguments.
544 // For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.
545 if !ContainsRegion.visit_ty(&self.body.local_decls[*dest].ty) {
551 mir::Operand::Copy(p) | mir::Operand::Move(p) => {
552 self.possible_borrower.add(p.local, *dest);
561 struct ContainsRegion;
563 impl TypeVisitor<'_> for ContainsRegion {
564 fn visit_region(&mut self, _: ty::Region<'_>) -> bool {
569 fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
570 use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};
572 let mut visit_op = |op: &mir::Operand<'_>| match op {
573 mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
578 Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
579 Aggregate(_, ops) => ops.iter().for_each(visit_op),
580 BinaryOp(_, lhs, rhs) | CheckedBinaryOp(_, lhs, rhs) => {
588 /// Result of `PossibleBorrowerVisitor`.
589 struct PossibleBorrowerMap<'a, 'tcx> {
590 /// Mapping `Local -> its possible borrowers`
591 map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
592 maybe_live: ResultsCursor<'a, 'tcx, MaybeStorageLive>,
593 // Caches to avoid allocation of `BitSet` on every query
594 bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
597 impl PossibleBorrowerMap<'_, '_> {
598 /// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
599 fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
600 self.maybe_live.seek_after_primary_effect(at);
602 self.bitset.0.clear();
603 let maybe_live = &mut self.maybe_live;
604 if let Some(bitset) = self.map.get(&borrowed) {
605 for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
606 self.bitset.0.insert(b);
612 self.bitset.1.clear();
614 self.bitset.1.insert(*b);
617 self.bitset.0 == self.bitset.1