1 use crate::utils::ptr::get_spans;
3 get_trait_def_id, implements_trait, is_copy, is_self, is_type_diagnostic_item, multispan_sugg, paths, snippet,
4 snippet_opt, span_lint_and_then,
6 use if_chain::if_chain;
7 use rustc_ast::ast::Attribute;
8 use rustc_data_structures::fx::{FxHashMap, 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_infer::infer::TyCtxtInferExt;
13 use rustc_lint::{LateContext, LateLintPass};
14 use rustc_middle::ty::{self, TypeFoldable};
15 use rustc_session::{declare_lint_pass, declare_tool_lint};
16 use rustc_span::symbol::kw;
17 use rustc_span::{sym, Span};
18 use rustc_target::spec::abi::Abi;
19 use rustc_trait_selection::traits;
20 use rustc_trait_selection::traits::misc::can_type_implement_copy;
21 use rustc_typeck::expr_use_visitor as euv;
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);
46 /// fn foo(v: &[i32]) {
47 /// assert_eq!(v.len(), 42);
50 pub NEEDLESS_PASS_BY_VALUE,
52 "functions taking arguments by value, but not consuming them in its body"
55 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
67 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
68 #[allow(clippy::too_many_lines)]
71 cx: &LateContext<'tcx>,
73 decl: &'tcx FnDecl<'_>,
78 if span.from_expansion() {
83 FnKind::ItemFn(.., header, _, attrs) => {
84 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
88 FnKind::Method(..) => (),
89 FnKind::Closure(..) => return,
92 // Exclude non-inherent impls
93 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
96 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
102 // Allow `Borrow` or functions to be taken by value
103 let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
104 let allowed_traits = [
105 need!(cx.tcx.lang_items().fn_trait()),
106 need!(cx.tcx.lang_items().fn_once_trait()),
107 need!(cx.tcx.lang_items().fn_mut_trait()),
108 need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
111 let sized_trait = need!(cx.tcx.lang_items().sized_trait());
113 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
115 let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
116 .filter(|p| !p.is_global())
117 .filter_map(|obligation| {
118 // Note that we do not want to deal with qualified predicates here.
119 match obligation.predicate.kind().no_bound_vars() {
120 Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
124 .collect::<Vec<_>>();
126 // Collect moved variables and spans which will need dereferencings from the
128 let MovedVariablesCtxt {
133 let mut ctx = MovedVariablesCtxt::default();
134 cx.tcx.infer_ctxt().enter(|infcx| {
135 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
141 let fn_sig = cx.tcx.fn_sig(fn_def_id);
142 let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
144 for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
145 // All spans generated from a proc-macro invocation are the same...
146 if span == input.span {
152 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
153 if ident.name == kw::SelfLower {
160 // * Exclude a type that is specifically bounded by `Borrow`.
161 // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
162 // `serde::Serialize`)
163 let (implements_borrow_trait, all_borrowable_trait) = {
164 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
167 preds.iter().any(|t| t.def_id() == borrow_trait),
168 !preds.is_empty() && {
169 let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
170 preds.iter().all(|t| {
171 let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
172 implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
180 if !ty.is_mutable_ptr();
182 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
183 if !implements_borrow_trait;
184 if !all_borrowable_trait;
186 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
187 if !moved_vars.contains(&canonical_id);
189 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
193 // Dereference suggestion
194 let sugg = |diag: &mut DiagnosticBuilder<'_>| {
195 if let ty::Adt(def, ..) = ty.kind() {
196 if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
197 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
198 diag.span_help(span, "consider marking this type as `Copy`");
203 let deref_span = spans_need_deref.get(&canonical_id);
205 if is_type_diagnostic_item(cx, ty, sym::vec_type);
206 if let Some(clone_spans) =
207 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
208 if let TyKind::Path(QPath::Resolved(_, ref path)) = input.kind;
209 if let Some(elem_ty) = path.segments.iter()
210 .find(|seg| seg.ident.name == sym::Vec)
211 .and_then(|ps| ps.args.as_ref())
212 .map(|params| params.args.iter().find_map(|arg| match arg {
213 GenericArg::Type(ty) => Some(ty),
217 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
218 diag.span_suggestion(
220 "consider changing the type to",
222 Applicability::Unspecified,
225 for (span, suggestion) in clone_spans {
226 diag.span_suggestion(
228 &snippet_opt(cx, span)
230 "change the call to".into(),
231 |x| Cow::from(format!("change `{}` to", x)),
234 Applicability::Unspecified,
238 // cannot be destructured, no need for `*` suggestion
239 assert!(deref_span.is_none());
244 if is_type_diagnostic_item(cx, ty, sym::string_type) {
245 if let Some(clone_spans) =
246 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
247 diag.span_suggestion(
249 "consider changing the type to",
251 Applicability::Unspecified,
254 for (span, suggestion) in clone_spans {
255 diag.span_suggestion(
257 &snippet_opt(cx, span)
259 "change the call to".into(),
260 |x| Cow::from(format!("change `{}` to", x))
263 Applicability::Unspecified,
267 assert!(deref_span.is_none());
272 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
274 // Suggests adding `*` to dereference the added reference.
275 if let Some(deref_span) = deref_span {
280 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
282 spans.sort_by_key(|&(span, _)| span);
284 multispan_sugg(diag, "consider taking a reference instead", spans);
289 NEEDLESS_PASS_BY_VALUE,
291 "this argument is passed by value, but not consumed in the function body",
300 /// Functions marked with these attributes must have the exact signature.
301 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
302 attrs.iter().any(|attr| {
303 [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
305 .any(|&allow| attr.has_name(allow))
310 struct MovedVariablesCtxt {
311 moved_vars: FxHashSet<HirId>,
312 /// Spans which need to be prefixed with `*` for dereferencing the
313 /// suggested additional reference.
314 spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
317 impl MovedVariablesCtxt {
318 fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
319 if let euv::PlaceBase::Local(vid) = cmt.place.base {
320 self.moved_vars.insert(vid);
325 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
326 fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId, mode: euv::ConsumeMode) {
327 if let euv::ConsumeMode::Move = mode {
328 self.move_common(cmt);
332 fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
334 fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}