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
94 .find_by_hir_id(cx.tcx.hir().get_parent_node_by_hir_id(hir_id))
96 if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
103 // Allow `Borrow` or functions to be taken by value
104 let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
105 let whitelisted_traits = [
106 need!(cx.tcx.lang_items().fn_trait()),
107 need!(cx.tcx.lang_items().fn_once_trait()),
108 need!(cx.tcx.lang_items().fn_mut_trait()),
109 need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
112 let sized_trait = need!(cx.tcx.lang_items().sized_trait());
114 let fn_def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
116 let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
117 .filter(|p| !p.is_global())
119 if let ty::Predicate::Trait(poly_trait_ref) = pred {
120 if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
129 .collect::<Vec<_>>();
131 // Collect moved variables and spans which will need dereferencings from the
133 let MovedVariablesCtxt {
138 let mut ctx = MovedVariablesCtxt::new(cx);
139 let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
140 euv::ExprUseVisitor::new(
153 let fn_sig = cx.tcx.fn_sig(fn_def_id);
154 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
156 for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
157 // All spans generated from a proc-macro invocation are the same...
158 if span == input.span {
164 if let PatKind::Binding(.., ident, _) = arg.pat.node {
165 if ident.as_str() == "self" {
172 // * Exclude a type that is specifically bounded by `Borrow`.
173 // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
174 // `serde::Serialize`)
175 let (implements_borrow_trait, all_borrowable_trait) = {
178 .filter(|t| t.skip_binder().self_ty() == ty)
179 .collect::<Vec<_>>();
182 preds.iter().any(|t| t.def_id() == borrow_trait),
184 && preds.iter().all(|t| {
192 .collect::<Vec<_>>();
193 implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
200 if !ty.is_mutable_pointer();
202 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
203 if !implements_borrow_trait;
204 if !all_borrowable_trait;
206 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
207 if !moved_vars.contains(&canonical_id);
209 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
213 // Dereference suggestion
214 let sugg = |db: &mut DiagnosticBuilder<'_>| {
215 if let ty::Adt(def, ..) = ty.sty {
216 if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
217 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
218 db.span_help(span, "consider marking this type as Copy");
223 let deref_span = spans_need_deref.get(&canonical_id);
225 if match_type(cx, ty, &paths::VEC);
226 if let Some(clone_spans) =
227 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
228 if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
229 if let Some(elem_ty) = path.segments.iter()
230 .find(|seg| seg.ident.name == sym!(Vec))
231 .and_then(|ps| ps.args.as_ref())
232 .map(|params| params.args.iter().find_map(|arg| match arg {
233 GenericArg::Type(ty) => Some(ty),
237 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
240 "consider changing the type to",
242 Applicability::Unspecified,
245 for (span, suggestion) in clone_spans {
248 &snippet_opt(cx, span)
250 "change the call to".into(),
251 |x| Cow::from(format!("change `{}` to", x)),
254 Applicability::Unspecified,
258 // cannot be destructured, no need for `*` suggestion
259 assert!(deref_span.is_none());
264 if match_type(cx, ty, &paths::STRING) {
265 if let Some(clone_spans) =
266 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
269 "consider changing the type to",
271 Applicability::Unspecified,
274 for (span, suggestion) in clone_spans {
277 &snippet_opt(cx, span)
279 "change the call to".into(),
280 |x| Cow::from(format!("change `{}` to", x))
283 Applicability::Unspecified,
287 assert!(deref_span.is_none());
292 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
294 // Suggests adding `*` to dereference the added reference.
295 if let Some(deref_span) = deref_span {
300 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
302 spans.sort_by_key(|&(span, _)| span);
304 multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
309 NEEDLESS_PASS_BY_VALUE,
311 "this argument is passed by value, but not consumed in the function body",
320 /// Functions marked with these attributes must have the exact signature.
321 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
322 attrs.iter().any(|attr| {
323 [sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
325 .any(|&allow| attr.check_name(allow))
329 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
330 cx: &'a LateContext<'a, 'tcx>,
331 moved_vars: FxHashSet<HirId>,
332 /// Spans which need to be prefixed with `*` for dereferencing the
333 /// suggested additional reference.
334 spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
337 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
338 fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
341 moved_vars: FxHashSet::default(),
342 spans_need_deref: FxHashMap::default(),
346 fn move_common(&mut self, _consume_id: HirId, _span: Span, cmt: &mc::cmt_<'tcx>) {
347 let cmt = unwrap_downcast_or_interior(cmt);
349 if let mc::Categorization::Local(vid) = cmt.cat {
350 self.moved_vars.insert(vid);
354 fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
355 let cmt = unwrap_downcast_or_interior(cmt);
357 if let mc::Categorization::Local(vid) = cmt.cat {
358 let mut id = matched_pat.hir_id;
360 let parent = self.cx.tcx.hir().get_parent_node_by_hir_id(id);
367 if let Some(node) = self.cx.tcx.hir().find_by_hir_id(id) {
370 // `match` and `if let`
371 if let ExprKind::Match(ref c, ..) = e.node {
372 self.spans_need_deref
374 .or_insert_with(FxHashSet::default)
382 if let StmtKind::Local(ref local) = s.node;
384 self.spans_need_deref
386 .or_insert_with(FxHashSet::default)
390 .expect("`let` stmt without init aren't caught by match_pat"));
403 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
404 fn consume(&mut self, consume_id: HirId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
405 if let euv::ConsumeMode::Move(_) = mode {
406 self.move_common(consume_id, consume_span, cmt);
410 fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
411 if let euv::MatchMode::MovingMatch = mode {
412 self.move_common(matched_pat.hir_id, matched_pat.span, cmt);
414 self.non_moving_pat(matched_pat, cmt);
418 fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
419 if let euv::ConsumeMode::Move(_) = mode {
420 self.move_common(consume_pat.hir_id, consume_pat.span, cmt);
435 fn mutate(&mut self, _: HirId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
437 fn decl_without_init(&mut self, _: HirId, _: Span) {}
440 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
443 mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
446 _ => return (*cmt).clone(),