1 use crate::utils::ptr::get_spans;
3 get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths, snippet,
4 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_tool_lint, lint_array};
16 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
17 use rustc_errors::Applicability;
18 use rustc_target::spec::abi::Abi;
20 use syntax::ast::NodeId;
21 use syntax::errors::DiagnosticBuilder;
24 /// **What it does:** Checks for functions taking arguments by value, but not
25 /// consuming them in its
28 /// **Why is this bad?** Taking arguments by reference is more flexible and can
30 /// unnecessary allocations.
32 /// **Known problems:**
33 /// * This lint suggests taking an argument by reference,
34 /// however sometimes it is better to let users decide the argument type
35 /// (by using `Borrow` trait, for example), depending on how the function is used.
39 /// fn foo(v: Vec<i32>) {
40 /// assert_eq!(v.len(), 42);
43 /// fn foo(v: &[i32]) {
44 /// assert_eq!(v.len(), 42);
47 declare_clippy_lint! {
48 pub NEEDLESS_PASS_BY_VALUE,
50 "functions taking arguments by value, but not consuming them in its body"
53 pub struct NeedlessPassByValue;
55 impl LintPass for NeedlessPassByValue {
56 fn get_lints(&self) -> LintArray {
57 lint_array![NEEDLESS_PASS_BY_VALUE]
60 fn name(&self) -> &'static str {
75 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
76 #[allow(clippy::too_many_lines)]
79 cx: &LateContext<'a, 'tcx>,
91 FnKind::ItemFn(.., header, _, attrs) => {
92 if header.abi != Abi::Rust {
96 if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
101 FnKind::Method(..) => (),
105 // Exclude non-inherent impls
106 if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(node_id)) {
107 if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
114 // Allow `Borrow` or functions to be taken by value
115 let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
116 let whitelisted_traits = [
117 need!(cx.tcx.lang_items().fn_trait()),
118 need!(cx.tcx.lang_items().fn_once_trait()),
119 need!(cx.tcx.lang_items().fn_mut_trait()),
120 need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
123 let sized_trait = need!(cx.tcx.lang_items().sized_trait());
125 let fn_def_id = cx.tcx.hir().local_def_id(node_id);
127 let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
128 .filter(|p| !p.is_global())
130 if let ty::Predicate::Trait(poly_trait_ref) = pred {
131 if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
140 .collect::<Vec<_>>();
142 // Collect moved variables and spans which will need dereferencings from the
144 let MovedVariablesCtxt {
149 let mut ctx = MovedVariablesCtxt::new(cx);
150 let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
151 euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
156 let fn_sig = cx.tcx.fn_sig(fn_def_id);
157 let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
159 for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
160 // All spans generated from a proc-macro invocation are the same...
161 if span == input.span {
167 if let PatKind::Binding(.., ident, _) = arg.pat.node {
168 if ident.as_str() == "self" {
175 // * Exclude a type that is specifically bounded by `Borrow`.
176 // * Exclude a type whose reference also fulfills its bound. (e.g. `std::convert::AsRef`,
177 // `serde::Serialize`)
178 let (implements_borrow_trait, all_borrowable_trait) = {
181 .filter(|t| t.skip_binder().self_ty() == ty)
182 .collect::<Vec<_>>();
185 preds.iter().any(|t| t.def_id() == borrow_trait),
187 && preds.iter().all(|t| {
195 .collect::<Vec<_>>();
196 implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
203 if !ty.is_mutable_pointer();
205 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
206 if !implements_borrow_trait;
207 if !all_borrowable_trait;
209 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
210 if !moved_vars.contains(&canonical_id);
212 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
216 // Dereference suggestion
217 let sugg = |db: &mut DiagnosticBuilder<'_>| {
218 if let ty::Adt(def, ..) = ty.sty {
219 if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
220 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
221 db.span_help(span, "consider marking this type as Copy");
226 let deref_span = spans_need_deref.get(&canonical_id);
228 if match_type(cx, ty, &paths::VEC);
229 if let Some(clone_spans) =
230 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
231 if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
232 if let Some(elem_ty) = path.segments.iter()
233 .find(|seg| seg.ident.name == "Vec")
234 .and_then(|ps| ps.args.as_ref())
235 .map(|params| params.args.iter().find_map(|arg| match arg {
236 GenericArg::Type(ty) => Some(ty),
240 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
243 "consider changing the type to",
245 Applicability::Unspecified,
248 for (span, suggestion) in clone_spans {
251 &snippet_opt(cx, span)
253 "change the call to".into(),
254 |x| Cow::from(format!("change `{}` to", x)),
257 Applicability::Unspecified,
261 // cannot be destructured, no need for `*` suggestion
262 assert!(deref_span.is_none());
267 if match_type(cx, ty, &paths::STRING) {
268 if let Some(clone_spans) =
269 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
272 "consider changing the type to",
274 Applicability::Unspecified,
277 for (span, suggestion) in clone_spans {
280 &snippet_opt(cx, span)
282 "change the call to".into(),
283 |x| Cow::from(format!("change `{}` to", x))
286 Applicability::Unspecified,
290 assert!(deref_span.is_none());
295 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
297 // Suggests adding `*` to dereference the added reference.
298 if let Some(deref_span) = deref_span {
303 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
305 spans.sort_by_key(|&(span, _)| span);
307 multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
312 NEEDLESS_PASS_BY_VALUE,
314 "this argument is passed by value, but not consumed in the function body",
323 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
324 cx: &'a LateContext<'a, 'tcx>,
325 moved_vars: FxHashSet<NodeId>,
326 /// Spans which need to be prefixed with `*` for dereferencing the
327 /// suggested additional reference.
328 spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
331 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
332 fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
335 moved_vars: FxHashSet::default(),
336 spans_need_deref: FxHashMap::default(),
340 fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
341 let cmt = unwrap_downcast_or_interior(cmt);
343 if let mc::Categorization::Local(vid) = cmt.cat {
344 self.moved_vars.insert(vid);
348 fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
349 let cmt = unwrap_downcast_or_interior(cmt);
351 if let mc::Categorization::Local(vid) = cmt.cat {
352 let mut id = matched_pat.id;
354 let parent = self.cx.tcx.hir().get_parent_node(id);
361 if let Some(node) = self.cx.tcx.hir().find(id) {
364 // `match` and `if let`
365 if let ExprKind::Match(ref c, ..) = e.node {
366 self.spans_need_deref
368 .or_insert_with(FxHashSet::default)
376 if let StmtKind::Local(ref local) = s.node;
378 self.spans_need_deref
380 .or_insert_with(FxHashSet::default)
384 .expect("`let` stmt without init aren't caught by match_pat"));
397 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
398 fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
399 if let euv::ConsumeMode::Move(_) = mode {
400 self.move_common(consume_id, consume_span, cmt);
404 fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
405 if let euv::MatchMode::MovingMatch = mode {
406 self.move_common(matched_pat.id, matched_pat.span, cmt);
408 self.non_moving_pat(matched_pat, cmt);
412 fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
413 if let euv::ConsumeMode::Move(_) = mode {
414 self.move_common(consume_pat.id, consume_pat.span, cmt);
429 fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
431 fn decl_without_init(&mut self, _: NodeId, _: Span) {}
434 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
437 mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
440 _ => return (*cmt).clone(),