2 has_drop, in_macro, is_copy, match_def_path, match_type, paths, snippet_opt, span_lint_node,
3 span_lint_node_and_then, walk_ptrs_ty_depth,
5 use if_chain::if_chain;
7 use rustc::hir::intravisit::FnKind;
8 use rustc::hir::{def_id, Body, FnDecl};
9 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
12 visit::{MutatingUseContext, PlaceContext, Visitor},
16 use rustc::{declare_tool_lint, lint_array};
17 use rustc_errors::Applicability;
18 use std::convert::TryFrom;
21 source_map::{BytePos, Span},
24 macro_rules! unwrap_or_continue {
33 /// **What it does:** Checks for a redudant `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 /// * Suggestions made by this lint could require NLL to be enabled.
42 /// * False-positive if there is a borrow preventing the value from moving out.
45 /// let x = String::new();
49 /// foo(x.clone()); // This lint suggests to remove this `clone()`
55 /// let x = Foo::new();
57 /// call(x.clone()); // this can just pass `x`
60 /// ["lorem", "ipsum"].join(" ").to_string()
62 /// Path::new("/a/b").join("c").to_path_buf()
64 declare_clippy_lint! {
67 "`clone()` of an owned value that is going to be dropped immediately"
70 pub struct RedundantClone;
72 impl LintPass for RedundantClone {
73 fn get_lints(&self) -> LintArray {
74 lint_array!(REDUNDANT_CLONE)
77 fn name(&self) -> &'static str {
82 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for RedundantClone {
83 #[allow(clippy::too_many_lines)]
86 cx: &LateContext<'a, 'tcx>,
93 let def_id = cx.tcx.hir().body_owner_def_id(body.id());
94 let mir = cx.tcx.optimized_mir(def_id);
96 for (bb, bbdata) in mir.basic_blocks().iter_enumerated() {
97 let terminator = bbdata.terminator();
99 if in_macro(terminator.source_info.span) {
104 if terminator.successors().any(|s| *s == bb) {
108 let (fn_def_id, arg, arg_ty, _) = unwrap_or_continue!(is_call_with_ref_arg(cx, mir, &terminator.kind));
110 let from_borrow = match_def_path(cx.tcx, fn_def_id, &paths::CLONE_TRAIT_METHOD)
111 || match_def_path(cx.tcx, fn_def_id, &paths::TO_OWNED_METHOD)
112 || (match_def_path(cx.tcx, fn_def_id, &paths::TO_STRING_METHOD)
113 && match_type(cx, arg_ty, &paths::STRING));
115 let from_deref = !from_borrow
116 && (match_def_path(cx.tcx, fn_def_id, &paths::PATH_TO_PATH_BUF)
117 || match_def_path(cx.tcx, fn_def_id, &paths::OS_STR_TO_OS_STRING));
119 if !from_borrow && !from_deref {
123 // _1 in MIR `{ _2 = &_1; clone(move _2); }` or `{ _2 = _1; to_path_buf(_2); } (from_deref)
124 // In case of `from_deref`, `arg` is already a reference since it is `deref`ed in the previous
126 let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(
131 bbdata.statements.iter()
134 if from_borrow && cannot_move_out {
138 // _1 in MIR `{ _2 = &_1; _3 = deref(move _2); } -> { _4 = _3; to_path_buf(move _4); }`
139 let referent = if from_deref {
140 let ps = mir.predecessors_for(bb);
144 let pred_terminator = mir[ps[0]].terminator();
146 let pred_arg = if_chain! {
147 if let Some((pred_fn_def_id, pred_arg, pred_arg_ty, Some(res))) =
148 is_call_with_ref_arg(cx, mir, &pred_terminator.kind);
149 if *res == mir::Place::Local(cloned);
150 if match_def_path(cx.tcx, pred_fn_def_id, &paths::DEREF_TRAIT_METHOD);
151 if match_type(cx, pred_arg_ty, &paths::PATH_BUF)
152 || match_type(cx, pred_arg_ty, &paths::OS_STRING);
160 let (local, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(
165 mir[ps[0]].statements.iter()
175 let used_later = traversal::ReversePostorder::new(&mir, bb).skip(1).any(|(tbb, tdata)| {
177 if tdata.terminator().successors().any(|s| *s == bb) {
181 let mut vis = LocalUseVisitor {
183 used_other_than_drop: false,
185 vis.visit_basic_block_data(tbb, tdata);
186 vis.used_other_than_drop
190 let span = terminator.source_info.span;
191 let node = if let mir::ClearCrossCrate::Set(scope_local_data) = &mir.source_scope_local_data {
192 scope_local_data[terminator.source_info.scope].lint_root
198 if let Some(snip) = snippet_opt(cx, span);
199 if let Some(dot) = snip.rfind('.');
201 let sugg_span = span.with_lo(
202 span.lo() + BytePos(u32::try_from(dot).unwrap())
205 span_lint_node_and_then(cx, REDUNDANT_CLONE, node, sugg_span, "redundant clone", |db| {
210 Applicability::MaybeIncorrect,
213 span.with_hi(span.lo() + BytePos(u32::try_from(dot).unwrap())),
214 "this value is dropped without further use",
218 span_lint_node(cx, REDUNDANT_CLONE, node, span, "redundant clone");
226 /// If `kind` is `y = func(x: &T)` where `T: !Copy`, returns `(DefId of func, x, T, y)`.
227 fn is_call_with_ref_arg<'tcx>(
228 cx: &LateContext<'_, 'tcx>,
229 mir: &'tcx mir::Mir<'tcx>,
230 kind: &'tcx mir::TerminatorKind<'tcx>,
231 ) -> Option<(def_id::DefId, mir::Local, ty::Ty<'tcx>, Option<&'tcx mir::Place<'tcx>>)> {
233 if let TerminatorKind::Call { func, args, destination, .. } = kind;
235 if let mir::Operand::Move(mir::Place::Local(local)) = &args[0];
236 if let ty::FnDef(def_id, _) = func.ty(&*mir, cx.tcx).sty;
237 if let (inner_ty, 1) = walk_ptrs_ty_depth(args[0].ty(&*mir, cx.tcx));
238 if !is_copy(cx, inner_ty);
240 Some((def_id, *local, inner_ty, destination.as_ref().map(|(dest, _)| dest)))
247 type CannotMoveOut = bool;
249 /// Finds the first `to = (&)from`, and returns
250 /// ``Some((from, [`true` if `from` cannot be moved out]))``.
251 fn find_stmt_assigns_to<'a, 'tcx: 'a>(
252 cx: &LateContext<'_, 'tcx>,
253 mir: &mir::Mir<'tcx>,
256 stmts: impl DoubleEndedIterator<Item = &'a mir::Statement<'tcx>>,
257 ) -> Option<(mir::Local, CannotMoveOut)> {
261 if let mir::StatementKind::Assign(mir::Place::Local(local), v) = &stmt.kind {
271 if let mir::Rvalue::Ref(_, _, ref place) = **v {
272 return base_local_and_movability(cx, mir, place);
274 } else if let mir::Rvalue::Use(mir::Operand::Copy(ref place)) = **v {
275 return base_local_and_movability(cx, mir, place);
281 /// Extracts and returns the undermost base `Local` of given `place`. Returns `place` itself
282 /// if it is already a `Local`.
284 /// Also reports whether given `place` cannot be moved out.
285 fn base_local_and_movability<'tcx>(
286 cx: &LateContext<'_, 'tcx>,
287 mir: &mir::Mir<'tcx>,
288 mut place: &mir::Place<'tcx>,
289 ) -> Option<(mir::Local, CannotMoveOut)> {
290 use rustc::mir::Place::*;
292 // Dereference. You cannot move things out from a borrowed value.
293 let mut deref = false;
294 // Accessing a field of an ADT that has `Drop`. Moving the field out will cause E0509.
295 let mut field = false;
299 Local(local) => return Some((*local, deref || field)),
300 Projection(proj) => {
302 deref = deref || matches!(proj.elem, mir::ProjectionElem::Deref);
303 if !field && matches!(proj.elem, mir::ProjectionElem::Field(..)) {
304 field = has_drop(cx, place.ty(&mir.local_decls, cx.tcx).to_ty(cx.tcx));
312 struct LocalUseVisitor {
314 used_other_than_drop: bool,
317 impl<'tcx> mir::visit::Visitor<'tcx> for LocalUseVisitor {
318 fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
319 let statements = &data.statements;
320 for (statement_index, statement) in statements.iter().enumerate() {
321 self.visit_statement(block, statement, mir::Location { block, statement_index });
323 // Once flagged, skip remaining statements
324 if self.used_other_than_drop {
329 self.visit_terminator(
334 statement_index: statements.len(),
339 fn visit_local(&mut self, local: &mir::Local, ctx: PlaceContext<'tcx>, _: mir::Location) {
341 PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_) => return,
345 if *local == self.local {
346 self.used_other_than_drop = true;