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<'a, 'tcx> LateLintPass<'a, 'tcx> for RedundantClone {
70 #[allow(clippy::too_many_lines)]
73 cx: &LateContext<'a, '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 // `{ cloned = &arg; clone(move cloned); }` or `{ cloned = &arg; to_path_buf(cloned); }`
128 let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(cx, mir, arg, from_borrow, bb));
130 let loc = mir::Location {
132 statement_index: bbdata.statements.len(),
135 // `Local` to be cloned, and a local of `clone` call's destination
136 let (local, ret_local) = if from_borrow {
137 // `res = clone(arg)` can be turned into `res = move arg;`
138 // if `arg` is the only borrow of `cloned` at this point.
140 if cannot_move_out || !possible_borrower.only_borrowers(&[arg], cloned, loc) {
146 // `arg` is a reference as it is `.deref()`ed in the previous block.
147 // Look into the predecessor block and find out the source of deref.
149 let ps = &mir.predecessors()[bb];
153 let pred_terminator = mir[ps[0]].terminator();
155 // receiver of the `deref()` call
156 let (pred_arg, deref_clone_ret) = if_chain! {
157 if let Some((pred_fn_def_id, pred_arg, pred_arg_ty, res)) =
158 is_call_with_ref_arg(cx, mir, &pred_terminator.kind);
160 if match_def_path(cx, pred_fn_def_id, &paths::DEREF_TRAIT_METHOD);
161 if match_type(cx, pred_arg_ty, &paths::PATH_BUF)
162 || match_type(cx, pred_arg_ty, &paths::OS_STRING);
170 let (local, cannot_move_out) =
171 unwrap_or_continue!(find_stmt_assigns_to(cx, mir, pred_arg, true, ps[0]));
172 let loc = mir::Location {
174 statement_index: mir.basic_blocks()[bb].statements.len(),
177 // This can be turned into `res = move local` if `arg` and `cloned` are not borrowed
178 // at the last statement:
181 // pred_arg = &local;
182 // cloned = deref(pred_arg);
184 // StorageDead(pred_arg);
185 // res = to_path_buf(cloned);
187 if cannot_move_out || !possible_borrower.only_borrowers(&[arg, cloned], local, loc) {
191 (local, deref_clone_ret)
194 let is_temp = mir.local_kind(ret_local) == mir::LocalKind::Temp;
196 // 1. `local` can be moved out if it is not used later.
197 // 2. If `ret_local` is a temporary and is neither consumed nor mutated, we can remove this `clone`
199 let (used, consumed_or_mutated) = traversal::ReversePostorder::new(&mir, bb).skip(1).fold(
201 |(used, consumed), (tbb, tdata)| {
203 if (used && consumed) ||
205 tdata.terminator().successors().any(|s| *s == bb)
210 let mut vis = LocalUseVisitor {
211 used: (local, false),
212 consumed_or_mutated: (ret_local, false),
214 vis.visit_basic_block_data(tbb, tdata);
215 (used || vis.used.1, consumed || vis.consumed_or_mutated.1)
219 if !used || !consumed_or_mutated {
220 let span = terminator.source_info.span;
221 let scope = terminator.source_info.scope;
222 let node = mir.source_scopes[scope]
225 .assert_crate_local()
229 if let Some(snip) = snippet_opt(cx, span);
230 if let Some(dot) = snip.rfind('.');
232 let sugg_span = span.with_lo(
233 span.lo() + BytePos(u32::try_from(dot).unwrap())
235 let mut app = Applicability::MaybeIncorrect;
237 let mut call_snip = &snip[dot + 1..];
238 // Machine applicable when `call_snip` looks like `foobar()`
239 if call_snip.ends_with("()") {
240 call_snip = call_snip[..call_snip.len()-2].trim();
241 if call_snip.as_bytes().iter().all(|b| b.is_ascii_alphabetic() || *b == b'_') {
242 app = Applicability::MachineApplicable;
246 span_lint_hir_and_then(cx, REDUNDANT_CLONE, node, sugg_span, "redundant clone", |diag| {
247 diag.span_suggestion(
256 "cloned value is neither consumed nor mutated",
260 span.with_hi(span.lo() + BytePos(u32::try_from(dot).unwrap())),
261 "this value is dropped without further use",
266 span_lint_hir(cx, REDUNDANT_CLONE, node, span, "redundant clone");
274 /// If `kind` is `y = func(x: &T)` where `T: !Copy`, returns `(DefId of func, x, T, y)`.
275 fn is_call_with_ref_arg<'tcx>(
276 cx: &LateContext<'_, 'tcx>,
277 mir: &'tcx mir::Body<'tcx>,
278 kind: &'tcx mir::TerminatorKind<'tcx>,
279 ) -> Option<(def_id::DefId, mir::Local, Ty<'tcx>, mir::Local)> {
281 if let mir::TerminatorKind::Call { func, args, destination, .. } = kind;
283 if let mir::Operand::Move(mir::Place { local, .. }) = &args[0];
284 if let ty::FnDef(def_id, _) = func.ty(&*mir, cx.tcx).kind;
285 if let (inner_ty, 1) = walk_ptrs_ty_depth(args[0].ty(&*mir, cx.tcx));
286 if !is_copy(cx, inner_ty);
288 Some((def_id, *local, inner_ty, destination.as_ref().map(|(dest, _)| dest)?.as_local()?))
295 type CannotMoveOut = bool;
297 /// Finds the first `to = (&)from`, and returns
298 /// ``Some((from, whether `from` cannot be moved out))``.
299 fn find_stmt_assigns_to<'tcx>(
300 cx: &LateContext<'_, 'tcx>,
301 mir: &mir::Body<'tcx>,
302 to_local: mir::Local,
305 ) -> Option<(mir::Local, CannotMoveOut)> {
306 let rvalue = mir.basic_blocks()[bb].statements.iter().rev().find_map(|stmt| {
307 if let mir::StatementKind::Assign(box (mir::Place { local, .. }, v)) = &stmt.kind {
308 return if *local == to_local { Some(v) } else { None };
314 match (by_ref, &*rvalue) {
315 (true, mir::Rvalue::Ref(_, _, place)) | (false, mir::Rvalue::Use(mir::Operand::Copy(place))) => {
316 base_local_and_movability(cx, mir, *place)
318 (false, mir::Rvalue::Ref(_, _, place)) => {
319 if let [mir::ProjectionElem::Deref] = place.as_ref().projection {
320 base_local_and_movability(cx, mir, *place)
329 /// Extracts and returns the undermost base `Local` of given `place`. Returns `place` itself
330 /// if it is already a `Local`.
332 /// Also reports whether given `place` cannot be moved out.
333 fn base_local_and_movability<'tcx>(
334 cx: &LateContext<'_, 'tcx>,
335 mir: &mir::Body<'tcx>,
336 place: mir::Place<'tcx>,
337 ) -> Option<(mir::Local, CannotMoveOut)> {
338 use rustc_middle::mir::PlaceRef;
340 // Dereference. You cannot move things out from a borrowed value.
341 let mut deref = false;
342 // Accessing a field of an ADT that has `Drop`. Moving the field out will cause E0509.
343 let mut field = false;
344 // If projection is a slice index then clone can be removed only if the
345 // underlying type implements Copy
346 let mut slice = false;
348 let PlaceRef { local, mut projection } = place.as_ref();
349 while let [base @ .., elem] = projection {
351 deref |= matches!(elem, mir::ProjectionElem::Deref);
352 field |= matches!(elem, mir::ProjectionElem::Field(..))
353 && has_drop(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
354 slice |= matches!(elem, mir::ProjectionElem::Index(..))
355 && !is_copy(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
358 Some((local, deref || field || slice))
361 struct LocalUseVisitor {
362 used: (mir::Local, bool),
363 consumed_or_mutated: (mir::Local, bool),
366 impl<'tcx> mir::visit::Visitor<'tcx> for LocalUseVisitor {
367 fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
368 let statements = &data.statements;
369 for (statement_index, statement) in statements.iter().enumerate() {
370 self.visit_statement(statement, mir::Location { block, statement_index });
373 self.visit_terminator(
377 statement_index: statements.len(),
382 fn visit_place(&mut self, place: &mir::Place<'tcx>, ctx: PlaceContext, _: mir::Location) {
383 let local = place.local;
385 if local == self.used.0
386 && !matches!(ctx, PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_))
391 if local == self.consumed_or_mutated.0 {
393 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
394 | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
395 self.consumed_or_mutated.1 = true;
403 /// Determines liveness of each local purely based on `StorageLive`/`Dead`.
404 #[derive(Copy, Clone)]
405 struct MaybeStorageLive;
407 impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
408 type Idx = mir::Local;
409 const NAME: &'static str = "maybe_storage_live";
411 fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
412 body.local_decls.len()
415 fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut BitSet<Self::Idx>) {
416 for arg in body.args_iter() {
422 impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
423 fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
425 mir::StatementKind::StorageLive(l) => trans.gen(l),
426 mir::StatementKind::StorageDead(l) => trans.kill(l),
431 fn terminator_effect(
433 _trans: &mut impl GenKill<Self::Idx>,
434 _terminator: &mir::Terminator<'tcx>,
439 fn call_return_effect(
441 _in_out: &mut impl GenKill<Self::Idx>,
442 _block: mir::BasicBlock,
443 _func: &mir::Operand<'tcx>,
444 _args: &[mir::Operand<'tcx>],
445 _return_place: mir::Place<'tcx>,
447 // Nothing to do when a call returns successfully
451 impl BottomValue for MaybeStorageLive {
453 const BOTTOM_VALUE: bool = false;
456 /// Collects the possible borrowers of each local.
457 /// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
458 /// possible borrowers of `a`.
459 struct PossibleBorrowerVisitor<'a, 'tcx> {
460 possible_borrower: TransitiveRelation<mir::Local>,
461 body: &'a mir::Body<'tcx>,
462 cx: &'a LateContext<'a, 'tcx>,
465 impl<'a, 'tcx> PossibleBorrowerVisitor<'a, 'tcx> {
466 fn new(cx: &'a LateContext<'a, 'tcx>, body: &'a mir::Body<'tcx>) -> Self {
468 possible_borrower: TransitiveRelation::default(),
476 cx: &LateContext<'a, 'tcx>,
477 maybe_live: ResultsCursor<'tcx, 'tcx, MaybeStorageLive>,
478 ) -> PossibleBorrowerMap<'a, 'tcx> {
479 let mut map = FxHashMap::default();
480 for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
481 if is_copy(cx, self.body.local_decls[row].ty) {
485 let borrowers = self.possible_borrower.reachable_from(&row);
486 if !borrowers.is_empty() {
487 let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
488 for &c in borrowers {
489 if c != mir::Local::from_usize(0) {
500 let bs = BitSet::new_empty(self.body.local_decls.len());
501 PossibleBorrowerMap {
504 bitset: (bs.clone(), bs),
509 impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
510 fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
511 let lhs = place.local;
513 mir::Rvalue::Ref(_, _, borrowed) => {
514 self.possible_borrower.add(borrowed.local, lhs);
517 if !ContainsRegion.visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty) {
520 rvalue_locals(other, |rhs| {
522 self.possible_borrower.add(rhs, lhs);
529 fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
530 if let mir::TerminatorKind::Call {
532 destination: Some((mir::Place { local: dest, .. }, _)),
536 // If the call returns something with lifetimes,
537 // let's conservatively assume the returned value contains lifetime of all the arguments.
538 // For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.
539 if !ContainsRegion.visit_ty(&self.body.local_decls[*dest].ty) {
545 mir::Operand::Copy(p) | mir::Operand::Move(p) => {
546 self.possible_borrower.add(p.local, *dest);
555 struct ContainsRegion;
557 impl TypeVisitor<'_> for ContainsRegion {
558 fn visit_region(&mut self, _: ty::Region<'_>) -> bool {
563 fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
564 use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};
566 let mut visit_op = |op: &mir::Operand<'_>| match op {
567 mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
572 Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
573 Aggregate(_, ops) => ops.iter().for_each(visit_op),
574 BinaryOp(_, lhs, rhs) | CheckedBinaryOp(_, lhs, rhs) => {
582 /// Result of `PossibleBorrowerVisitor`.
583 struct PossibleBorrowerMap<'a, 'tcx> {
584 /// Mapping `Local -> its possible borrowers`
585 map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
586 maybe_live: ResultsCursor<'a, 'tcx, MaybeStorageLive>,
587 // Caches to avoid allocation of `BitSet` on every query
588 bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
591 impl PossibleBorrowerMap<'_, '_> {
592 /// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
593 fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
594 self.maybe_live.seek_after_primary_effect(at);
596 self.bitset.0.clear();
597 let maybe_live = &mut self.maybe_live;
598 if let Some(bitset) = self.map.get(&borrowed) {
599 for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
600 self.bitset.0.insert(b);
606 self.bitset.1.clear();
608 self.bitset.1.insert(*b);
611 self.bitset.0 == self.bitset.1