1 use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
2 use clippy_utils::ptr::get_spans;
3 use clippy_utils::source::{snippet, snippet_opt};
4 use clippy_utils::ty::{implements_trait, is_copy, is_type_diagnostic_item};
5 use clippy_utils::{get_trait_def_id, is_self, paths};
6 use if_chain::if_chain;
7 use rustc_ast::ast::Attribute;
8 use rustc_data_structures::fx::FxHashSet;
9 use rustc_errors::{Applicability, DiagnosticBuilder};
10 use rustc_hir::intravisit::FnKind;
11 use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
12 use rustc_hir::{HirIdMap, HirIdSet};
13 use rustc_infer::infer::TyCtxtInferExt;
14 use rustc_lint::{LateContext, LateLintPass};
15 use rustc_middle::mir::FakeReadCause;
16 use rustc_middle::ty::{self, TypeFoldable};
17 use rustc_session::{declare_lint_pass, declare_tool_lint};
18 use rustc_span::symbol::kw;
19 use rustc_span::{sym, Span};
20 use rustc_target::spec::abi::Abi;
21 use rustc_trait_selection::traits;
22 use rustc_trait_selection::traits::misc::can_type_implement_copy;
23 use rustc_typeck::expr_use_visitor as euv;
26 declare_clippy_lint! {
28 /// Checks for functions taking arguments by value, but not
29 /// consuming them in its
32 /// ### Why is this bad?
33 /// Taking arguments by reference is more flexible and can
35 /// unnecessary allocations.
37 /// ### Known problems
38 /// * This lint suggests taking an argument by reference,
39 /// however sometimes it is better to let users decide the argument type
40 /// (by using `Borrow` trait, for example), depending on how the function is used.
44 /// fn foo(v: Vec<i32>) {
45 /// assert_eq!(v.len(), 42);
50 /// fn foo(v: &[i32]) {
51 /// assert_eq!(v.len(), 42);
54 pub NEEDLESS_PASS_BY_VALUE,
56 "functions taking arguments by value, but not consuming them in its body"
59 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
71 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
72 #[allow(clippy::too_many_lines)]
75 cx: &LateContext<'tcx>,
77 decl: &'tcx FnDecl<'_>,
82 if span.from_expansion() {
87 FnKind::ItemFn(.., header, _) => {
88 let attrs = cx.tcx.hir().attrs(hir_id);
89 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
93 FnKind::Method(..) => (),
94 FnKind::Closure => return,
97 // Exclude non-inherent impls
98 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
101 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
107 // Allow `Borrow` or functions to be taken by value
108 let allowed_traits = [
109 need!(cx.tcx.lang_items().fn_trait()),
110 need!(cx.tcx.lang_items().fn_once_trait()),
111 need!(cx.tcx.lang_items().fn_mut_trait()),
112 need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
115 let sized_trait = need!(cx.tcx.lang_items().sized_trait());
117 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
119 let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
120 .filter(|p| !p.is_global())
121 .filter_map(|obligation| {
122 // Note that we do not want to deal with qualified predicates here.
123 match obligation.predicate.kind().no_bound_vars() {
124 Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
128 .collect::<Vec<_>>();
130 // Collect moved variables and spans which will need dereferencings from the
132 let MovedVariablesCtxt {
137 let mut ctx = MovedVariablesCtxt::default();
138 cx.tcx.infer_ctxt().enter(|infcx| {
139 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
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.name == kw::SelfLower {
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) = {
168 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
171 preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
172 !preds.is_empty() && {
173 let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
174 preds.iter().all(|t| {
175 let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
176 implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
184 if !ty.is_mutable_ptr();
186 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
187 if !implements_borrow_trait;
188 if !all_borrowable_trait;
190 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
191 if !moved_vars.contains(&canonical_id);
193 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
197 // Dereference suggestion
198 let sugg = |diag: &mut DiagnosticBuilder<'_>| {
199 if let ty::Adt(def, ..) = ty.kind() {
200 if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
201 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
202 diag.span_help(span, "consider marking this type as `Copy`");
207 let deref_span = spans_need_deref.get(&canonical_id);
209 if is_type_diagnostic_item(cx, ty, sym::vec_type);
210 if let Some(clone_spans) =
211 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
212 if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
213 if let Some(elem_ty) = path.segments.iter()
214 .find(|seg| seg.ident.name == sym::Vec)
215 .and_then(|ps| ps.args.as_ref())
216 .map(|params| params.args.iter().find_map(|arg| match arg {
217 GenericArg::Type(ty) => Some(ty),
221 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
222 diag.span_suggestion(
224 "consider changing the type to",
226 Applicability::Unspecified,
229 for (span, suggestion) in clone_spans {
230 diag.span_suggestion(
232 &snippet_opt(cx, span)
234 "change the call to".into(),
235 |x| Cow::from(format!("change `{}` to", x)),
238 Applicability::Unspecified,
242 // cannot be destructured, no need for `*` suggestion
243 assert!(deref_span.is_none());
248 if is_type_diagnostic_item(cx, ty, sym::string_type) {
249 if let Some(clone_spans) =
250 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
251 diag.span_suggestion(
253 "consider changing the type to",
255 Applicability::Unspecified,
258 for (span, suggestion) in clone_spans {
259 diag.span_suggestion(
261 &snippet_opt(cx, span)
263 "change the call to".into(),
264 |x| Cow::from(format!("change `{}` to", x))
267 Applicability::Unspecified,
271 assert!(deref_span.is_none());
276 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
278 // Suggests adding `*` to dereference the added reference.
279 if let Some(deref_span) = deref_span {
284 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
286 spans.sort_by_key(|&(span, _)| span);
288 multispan_sugg(diag, "consider taking a reference instead", spans);
293 NEEDLESS_PASS_BY_VALUE,
295 "this argument is passed by value, but not consumed in the function body",
304 /// Functions marked with these attributes must have the exact signature.
305 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
306 attrs.iter().any(|attr| {
307 [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
309 .any(|&allow| attr.has_name(allow))
314 struct MovedVariablesCtxt {
315 moved_vars: HirIdSet,
316 /// Spans which need to be prefixed with `*` for dereferencing the
317 /// suggested additional reference.
318 spans_need_deref: HirIdMap<FxHashSet<Span>>,
321 impl MovedVariablesCtxt {
322 fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
323 if let euv::PlaceBase::Local(vid) = cmt.place.base {
324 self.moved_vars.insert(vid);
329 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
330 fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
331 self.move_common(cmt);
334 fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
336 fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
338 fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}