1 use crate::utils::ptr::get_spans;
3 get_trait_def_id, implements_trait, in_macro_or_desugar, is_copy, is_self, match_type, multispan_sugg, paths,
4 snippet, snippet_opt, span_lint_and_then,
6 use if_chain::if_chain;
8 use rustc::hir::intravisit::FnKind;
10 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
11 use rustc::middle::expr_use_visitor as euv;
12 use rustc::middle::mem_categorization as mc;
14 use rustc::ty::{self, RegionKind, TypeFoldable};
15 use rustc::{declare_lint_pass, declare_tool_lint};
16 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
17 use rustc_errors::Applicability;
18 use rustc_target::spec::abi::Abi;
20 use syntax::ast::Attribute;
21 use syntax::errors::DiagnosticBuilder;
24 declare_clippy_lint! {
25 /// **What it does:** Checks for functions taking arguments by value, but not
26 /// consuming them in its
29 /// **Why is this bad?** Taking arguments by reference is more flexible and can
31 /// unnecessary allocations.
33 /// **Known problems:**
34 /// * This lint suggests taking an argument by reference,
35 /// however sometimes it is better to let users decide the argument type
36 /// (by using `Borrow` trait, for example), depending on how the function is used.
40 /// fn foo(v: Vec<i32>) {
41 /// assert_eq!(v.len(), 42);
44 /// fn foo(v: &[i32]) {
45 /// assert_eq!(v.len(), 42);
48 pub NEEDLESS_PASS_BY_VALUE,
50 "functions taking arguments by value, but not consuming them in its body"
53 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
65 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
66 #[allow(clippy::too_many_lines)]
69 cx: &LateContext<'a, 'tcx>,
76 if in_macro_or_desugar(span) {
81 FnKind::ItemFn(.., header, _, attrs) => {
82 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
86 FnKind::Method(..) => (),
90 // Exclude non-inherent impls
91 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
92 if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
99 // Allow `Borrow` or functions to be taken by value
100 let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
101 let whitelisted_traits = [
102 need!(cx.tcx.lang_items().fn_trait()),
103 need!(cx.tcx.lang_items().fn_once_trait()),
104 need!(cx.tcx.lang_items().fn_mut_trait()),
105 need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
108 let sized_trait = need!(cx.tcx.lang_items().sized_trait());
110 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
112 let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
113 .filter(|p| !p.is_global())
115 if let ty::Predicate::Trait(poly_trait_ref) = pred {
116 if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
125 .collect::<Vec<_>>();
127 // Collect moved variables and spans which will need dereferencings from the
129 let MovedVariablesCtxt {
134 let mut ctx = MovedVariablesCtxt::new(cx);
135 let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
136 euv::ExprUseVisitor::new(
149 let fn_sig = cx.tcx.fn_sig(fn_def_id);
150 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
152 for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
153 // All spans generated from a proc-macro invocation are the same...
154 if span == input.span {
160 if let PatKind::Binding(.., ident, _) = arg.pat.node {
161 if ident.as_str() == "self" {
168 // * Exclude a type that is specifically bounded by `Borrow`.
169 // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
170 // `serde::Serialize`)
171 let (implements_borrow_trait, all_borrowable_trait) = {
174 .filter(|t| t.skip_binder().self_ty() == ty)
175 .collect::<Vec<_>>();
178 preds.iter().any(|t| t.def_id() == borrow_trait),
180 && preds.iter().all(|t| {
188 .collect::<Vec<_>>();
189 implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
196 if !ty.is_mutable_ptr();
198 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
199 if !implements_borrow_trait;
200 if !all_borrowable_trait;
202 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
203 if !moved_vars.contains(&canonical_id);
205 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
209 // Dereference suggestion
210 let sugg = |db: &mut DiagnosticBuilder<'_>| {
211 if let ty::Adt(def, ..) = ty.sty {
212 if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
213 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
214 db.span_help(span, "consider marking this type as Copy");
219 let deref_span = spans_need_deref.get(&canonical_id);
221 if match_type(cx, ty, &paths::VEC);
222 if let Some(clone_spans) =
223 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
224 if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
225 if let Some(elem_ty) = path.segments.iter()
226 .find(|seg| seg.ident.name == sym!(Vec))
227 .and_then(|ps| ps.args.as_ref())
228 .map(|params| params.args.iter().find_map(|arg| match arg {
229 GenericArg::Type(ty) => Some(ty),
233 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
236 "consider changing the type to",
238 Applicability::Unspecified,
241 for (span, suggestion) in clone_spans {
244 &snippet_opt(cx, span)
246 "change the call to".into(),
247 |x| Cow::from(format!("change `{}` to", x)),
250 Applicability::Unspecified,
254 // cannot be destructured, no need for `*` suggestion
255 assert!(deref_span.is_none());
260 if match_type(cx, ty, &paths::STRING) {
261 if let Some(clone_spans) =
262 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
265 "consider changing the type to",
267 Applicability::Unspecified,
270 for (span, suggestion) in clone_spans {
273 &snippet_opt(cx, span)
275 "change the call to".into(),
276 |x| Cow::from(format!("change `{}` to", x))
279 Applicability::Unspecified,
283 assert!(deref_span.is_none());
288 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
290 // Suggests adding `*` to dereference the added reference.
291 if let Some(deref_span) = deref_span {
296 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
298 spans.sort_by_key(|&(span, _)| span);
300 multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
305 NEEDLESS_PASS_BY_VALUE,
307 "this argument is passed by value, but not consumed in the function body",
316 /// Functions marked with these attributes must have the exact signature.
317 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
318 attrs.iter().any(|attr| {
319 [sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
321 .any(|&allow| attr.check_name(allow))
325 struct MovedVariablesCtxt<'a, 'tcx> {
326 cx: &'a LateContext<'a, 'tcx>,
327 moved_vars: FxHashSet<HirId>,
328 /// Spans which need to be prefixed with `*` for dereferencing the
329 /// suggested additional reference.
330 spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
333 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
334 fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
337 moved_vars: FxHashSet::default(),
338 spans_need_deref: FxHashMap::default(),
342 fn move_common(&mut self, _consume_id: HirId, _span: Span, cmt: &mc::cmt_<'tcx>) {
343 let cmt = unwrap_downcast_or_interior(cmt);
345 if let mc::Categorization::Local(vid) = cmt.cat {
346 self.moved_vars.insert(vid);
350 fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
351 let cmt = unwrap_downcast_or_interior(cmt);
353 if let mc::Categorization::Local(vid) = cmt.cat {
354 let mut id = matched_pat.hir_id;
356 let parent = self.cx.tcx.hir().get_parent_node(id);
363 if let Some(node) = self.cx.tcx.hir().find(id) {
366 // `match` and `if let`
367 if let ExprKind::Match(ref c, ..) = e.node {
368 self.spans_need_deref
370 .or_insert_with(FxHashSet::default)
378 if let StmtKind::Local(ref local) = s.node;
380 self.spans_need_deref
382 .or_insert_with(FxHashSet::default)
386 .expect("`let` stmt without init aren't caught by match_pat"));
399 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
400 fn consume(&mut self, consume_id: HirId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
401 if let euv::ConsumeMode::Move(_) = mode {
402 self.move_common(consume_id, consume_span, cmt);
406 fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
407 if let euv::MatchMode::MovingMatch = mode {
408 self.move_common(matched_pat.hir_id, matched_pat.span, cmt);
410 self.non_moving_pat(matched_pat, cmt);
414 fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
415 if let euv::ConsumeMode::Move(_) = mode {
416 self.move_common(consume_pat.hir_id, consume_pat.span, cmt);
431 fn mutate(&mut self, _: HirId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
433 fn decl_without_init(&mut self, _: HirId, _: Span) {}
436 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
439 mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
442 _ => return (*cmt).clone(),