1 use crate::utils::ptr::get_spans;
3 get_trait_def_id, implements_trait, is_copy, is_self, is_type_diagnostic_item, match_type, multispan_sugg, paths,
4 snippet, snippet_opt, span_lint_and_then,
6 use if_chain::if_chain;
8 use rustc::declare_lint_pass;
9 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
11 use rustc::traits::misc::can_type_implement_copy;
12 use rustc::ty::{self, RegionKind, TypeFoldable};
13 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
14 use rustc_errors::Applicability;
15 use rustc_hir::intravisit::FnKind;
17 use rustc_session::declare_tool_lint;
18 use rustc_span::{Span, Symbol};
19 use rustc_target::spec::abi::Abi;
20 use rustc_typeck::expr_use_visitor as euv;
22 use syntax::ast::Attribute;
23 use syntax::errors::DiagnosticBuilder;
25 declare_clippy_lint! {
26 /// **What it does:** Checks for functions taking arguments by value, but not
27 /// consuming them in its
30 /// **Why is this bad?** Taking arguments by reference is more flexible and can
32 /// unnecessary allocations.
34 /// **Known problems:**
35 /// * This lint suggests taking an argument by reference,
36 /// however sometimes it is better to let users decide the argument type
37 /// (by using `Borrow` trait, for example), depending on how the function is used.
41 /// fn foo(v: Vec<i32>) {
42 /// assert_eq!(v.len(), 42);
48 /// fn foo(v: &[i32]) {
49 /// assert_eq!(v.len(), 42);
52 pub NEEDLESS_PASS_BY_VALUE,
54 "functions taking arguments by value, but not consuming them in its body"
57 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
69 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
70 #[allow(clippy::too_many_lines)]
73 cx: &LateContext<'a, 'tcx>,
75 decl: &'tcx FnDecl<'_>,
80 if span.from_expansion() {
85 FnKind::ItemFn(.., header, _, attrs) => {
86 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
90 FnKind::Method(..) => (),
94 // Exclude non-inherent impls
95 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
96 if matches!(item.kind, 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(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::default();
139 cx.tcx.infer_ctxt().enter(|infcx| {
140 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.tables).consume_body(body);
145 let fn_sig = cx.tcx.fn_sig(fn_def_id);
146 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
148 for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
149 // All spans generated from a proc-macro invocation are the same...
150 if span == input.span {
156 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
157 if ident.as_str() == "self" {
164 // * Exclude a type that is specifically bounded by `Borrow`.
165 // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
166 // `serde::Serialize`)
167 let (implements_borrow_trait, all_borrowable_trait) = {
170 .filter(|t| t.skip_binder().self_ty() == ty)
171 .collect::<Vec<_>>();
174 preds.iter().any(|t| t.def_id() == borrow_trait),
176 && preds.iter().all(|t| {
184 .collect::<Vec<_>>();
185 implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
192 if !ty.is_mutable_ptr();
194 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
195 if !implements_borrow_trait;
196 if !all_borrowable_trait;
198 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
199 if !moved_vars.contains(&canonical_id);
201 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
205 // Dereference suggestion
206 let sugg = |db: &mut DiagnosticBuilder<'_>| {
207 if let ty::Adt(def, ..) = ty.kind {
208 if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
209 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
210 db.span_help(span, "consider marking this type as `Copy`");
215 let deref_span = spans_need_deref.get(&canonical_id);
217 if is_type_diagnostic_item(cx, ty, Symbol::intern("vec_type"));
218 if let Some(clone_spans) =
219 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
220 if let TyKind::Path(QPath::Resolved(_, ref path)) = input.kind;
221 if let Some(elem_ty) = path.segments.iter()
222 .find(|seg| seg.ident.name == sym!(Vec))
223 .and_then(|ps| ps.args.as_ref())
224 .map(|params| params.args.iter().find_map(|arg| match arg {
225 GenericArg::Type(ty) => Some(ty),
229 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
232 "consider changing the type to",
234 Applicability::Unspecified,
237 for (span, suggestion) in clone_spans {
240 &snippet_opt(cx, span)
242 "change the call to".into(),
243 |x| Cow::from(format!("change `{}` to", x)),
246 Applicability::Unspecified,
250 // cannot be destructured, no need for `*` suggestion
251 assert!(deref_span.is_none());
256 if match_type(cx, ty, &paths::STRING) {
257 if let Some(clone_spans) =
258 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
261 "consider changing the type to",
263 Applicability::Unspecified,
266 for (span, suggestion) in clone_spans {
269 &snippet_opt(cx, span)
271 "change the call to".into(),
272 |x| Cow::from(format!("change `{}` to", x))
275 Applicability::Unspecified,
279 assert!(deref_span.is_none());
284 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
286 // Suggests adding `*` to dereference the added reference.
287 if let Some(deref_span) = deref_span {
292 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
294 spans.sort_by_key(|&(span, _)| span);
296 multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
301 NEEDLESS_PASS_BY_VALUE,
303 "this argument is passed by value, but not consumed in the function body",
312 /// Functions marked with these attributes must have the exact signature.
313 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
314 attrs.iter().any(|attr| {
315 [sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
317 .any(|&allow| attr.check_name(allow))
322 struct MovedVariablesCtxt {
323 moved_vars: FxHashSet<HirId>,
324 /// Spans which need to be prefixed with `*` for dereferencing the
325 /// suggested additional reference.
326 spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
329 impl MovedVariablesCtxt {
330 fn move_common(&mut self, cmt: &euv::Place<'_>) {
331 if let euv::PlaceBase::Local(vid) = cmt.base {
332 self.moved_vars.insert(vid);
337 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
338 fn consume(&mut self, cmt: &euv::Place<'tcx>, mode: euv::ConsumeMode) {
339 if let euv::ConsumeMode::Move = mode {
340 self.move_common(cmt);
344 fn borrow(&mut self, _: &euv::Place<'tcx>, _: ty::BorrowKind) {}
346 fn mutate(&mut self, _: &euv::Place<'tcx>) {}