3 use rustc::hir::intravisit::FnKind;
4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
5 use rustc::{declare_tool_lint, lint_array};
6 use if_chain::if_chain;
7 use rustc::ty::{self, RegionKind, TypeFoldable};
9 use rustc::middle::expr_use_visitor as euv;
10 use rustc::middle::mem_categorization as mc;
11 use rustc_target::spec::abi::Abi;
12 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
13 use syntax::ast::NodeId;
15 use syntax::errors::DiagnosticBuilder;
16 use crate::utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
17 snippet, snippet_opt, span_lint_and_then};
18 use crate::utils::ptr::get_spans;
21 /// **What it does:** Checks for functions taking arguments by value, but not
22 /// consuming them in its
25 /// **Why is this bad?** Taking arguments by reference is more flexible and can
27 /// unnecessary allocations.
29 /// **Known problems:**
30 /// * This lint suggests taking an argument by reference,
31 /// however sometimes it is better to let users decide the argument type
32 /// (by using `Borrow` trait, for example), depending on how the function is used.
36 /// fn foo(v: Vec<i32>) {
37 /// assert_eq!(v.len(), 42);
40 /// fn foo(v: &[i32]) {
41 /// assert_eq!(v.len(), 42);
44 declare_clippy_lint! {
45 pub NEEDLESS_PASS_BY_VALUE,
47 "functions taking arguments by value, but not consuming them in its body"
50 pub struct NeedlessPassByValue;
52 impl LintPass for NeedlessPassByValue {
53 fn get_lints(&self) -> LintArray {
54 lint_array![NEEDLESS_PASS_BY_VALUE]
59 ($e: expr) => { if let Some(x) = $e { x } else { return; } };
62 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
65 cx: &LateContext<'a, 'tcx>,
77 FnKind::ItemFn(.., header, _, attrs) => {
78 if header.abi != Abi::Rust {
82 if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
87 FnKind::Method(..) => (),
91 // Exclude non-inherent impls
92 if let Some(Node::Item(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
93 if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
100 // Allow `Borrow` or functions to be taken by value
101 let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
102 let whitelisted_traits = [
103 need!(cx.tcx.lang_items().fn_trait()),
104 need!(cx.tcx.lang_items().fn_once_trait()),
105 need!(cx.tcx.lang_items().fn_mut_trait()),
106 need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT))
109 let sized_trait = need!(cx.tcx.lang_items().sized_trait());
111 let fn_def_id = cx.tcx.hir.local_def_id(node_id);
113 let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
114 .filter(|p| !p.is_global())
116 if let ty::Predicate::Trait(poly_trait_ref) = pred {
117 if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
125 .collect::<Vec<_>>();
127 // Collect moved variables and spans which will need dereferencings from the
129 let MovedVariablesCtxt {
134 let mut ctx = MovedVariablesCtxt::new(cx);
135 let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
136 euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
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
146 .zip(fn_sig.inputs())
147 .zip(&body.arguments)
150 // All spans generated from a proc-macro invocation are the same...
151 if span == input.span {
157 if let PatKind::Binding(_, _, ident, ..) = arg.pat.node {
158 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.
166 // (e.g. `std::convert::AsRef`, `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),
175 !preds.is_empty() && preds.iter().all(|t| {
178 cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
184 .collect::<Vec<_>>(),
192 if !ty.is_mutable_pointer();
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.node;
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::TyKind::Adt(def, ..) = ty.sty {
208 if let Some(span) = cx.tcx.hir.span_if_local(def.did) {
209 if cx.param_env.can_type_implement_copy(cx.tcx, 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 match_type(cx, ty, &paths::VEC);
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.node;
221 if let Some(elem_ty) = path.segments.iter()
222 .find(|seg| seg.ident.name == "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),
226 GenericArg::Lifetime(_) => None,
229 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
230 db.span_suggestion(input.span,
231 "consider changing the type to",
234 for (span, suggestion) in clone_spans {
237 &snippet_opt(cx, span)
239 "change the call to".into(),
240 |x| Cow::from(format!("change `{}` to", x)),
246 // cannot be destructured, no need for `*` suggestion
247 assert!(deref_span.is_none());
252 if match_type(cx, ty, &paths::STRING) {
253 if let Some(clone_spans) =
254 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
255 db.span_suggestion(input.span, "consider changing the type to", "&str".to_string());
257 for (span, suggestion) in clone_spans {
260 &snippet_opt(cx, span)
262 "change the call to".into(),
263 |x| Cow::from(format!("change `{}` to", x))
269 assert!(deref_span.is_none());
274 let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
276 // Suggests adding `*` to dereference the added reference.
277 if let Some(deref_span) = deref_span {
282 .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
284 spans.sort_by_key(|&(span, _)| span);
286 multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
291 NEEDLESS_PASS_BY_VALUE,
293 "this argument is passed by value, but not consumed in the function body",
302 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
303 cx: &'a LateContext<'a, 'tcx>,
304 moved_vars: FxHashSet<NodeId>,
305 /// Spans which need to be prefixed with `*` for dereferencing the
306 /// suggested additional reference.
307 spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
310 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
311 fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
314 moved_vars: FxHashSet::default(),
315 spans_need_deref: FxHashMap::default(),
319 fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
320 let cmt = unwrap_downcast_or_interior(cmt);
322 if let mc::Categorization::Local(vid) = cmt.cat {
323 self.moved_vars.insert(vid);
327 fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
328 let cmt = unwrap_downcast_or_interior(cmt);
330 if let mc::Categorization::Local(vid) = cmt.cat {
331 let mut id = matched_pat.id;
333 let parent = self.cx.tcx.hir.get_parent_node(id);
340 if let Some(node) = self.cx.tcx.hir.find(id) {
343 // `match` and `if let`
344 if let ExprKind::Match(ref c, ..) = e.node {
345 self.spans_need_deref
347 .or_insert_with(FxHashSet::default)
355 if let StmtKind::Decl(ref decl, _) = s.node;
356 if let DeclKind::Local(ref local) = decl.node;
358 self.spans_need_deref
360 .or_insert_with(FxHashSet::default)
364 .expect("`let` stmt without init aren't caught by match_pat"));
377 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
378 fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
379 if let euv::ConsumeMode::Move(_) = mode {
380 self.move_common(consume_id, consume_span, cmt);
384 fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
385 if let euv::MatchMode::MovingMatch = mode {
386 self.move_common(matched_pat.id, matched_pat.span, cmt);
388 self.non_moving_pat(matched_pat, cmt);
392 fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
393 if let euv::ConsumeMode::Move(_) = mode {
394 self.move_common(consume_pat.id, consume_pat.span, cmt);
398 fn borrow(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: ty::Region<'_>, _: ty::BorrowKind, _: euv::LoanCause) {}
400 fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
402 fn decl_without_init(&mut self, _: NodeId, _: Span) {}
406 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
409 mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
412 _ => return (*cmt).clone(),