1 use crate::utils::{get_item_name, in_macro, snippet_with_applicability, span_lint, span_lint_and_sugg, walk_ptrs_ty};
2 use rustc::hir::def_id::DefId;
4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
6 use rustc::{declare_tool_lint, lint_array};
7 use rustc_data_structures::fx::FxHashSet;
8 use rustc_errors::Applicability;
9 use syntax::ast::{Lit, LitKind, Name};
10 use syntax::source_map::{Span, Spanned};
12 /// **What it does:** Checks for getting the length of something via `.len()`
13 /// just to compare to zero, and suggests using `.is_empty()` where applicable.
15 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster
16 /// than calculating their length. Notably, for slices, getting the length
17 /// requires a subtraction whereas `.is_empty()` is just a comparison. So it is
18 /// good to get into the habit of using `.is_empty()`, and having it is cheap.
19 /// Besides, it makes the intent clearer than a manual comparison.
21 /// **Known problems:** None.
37 /// if !y.is_empty() {
41 declare_clippy_lint! {
44 "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` \
45 could be used instead"
48 /// **What it does:** Checks for items that implement `.len()` but not
51 /// **Why is this bad?** It is good custom to have both methods, because for
52 /// some data structures, asking about the length will be a costly operation,
53 /// whereas `.is_empty()` can usually answer in constant time. Also it used to
54 /// lead to false positives on the [`len_zero`](#len_zero) lint – currently that
55 /// lint will ignore such entities.
57 /// **Known problems:** None.
62 /// pub fn len(&self) -> usize {
67 declare_clippy_lint! {
68 pub LEN_WITHOUT_IS_EMPTY,
70 "traits or impls with a public `len` method but no corresponding `is_empty` method"
73 #[derive(Copy, Clone)]
76 impl LintPass for LenZero {
77 fn get_lints(&self) -> LintArray {
78 lint_array!(LEN_ZERO, LEN_WITHOUT_IS_EMPTY)
81 fn name(&self) -> &'static str {
86 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
87 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
88 if in_macro(item.span) {
93 ItemKind::Trait(_, _, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
94 ItemKind::Impl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
99 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
100 if in_macro(expr.span) {
104 if let ExprKind::Binary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
107 check_cmp(cx, expr.span, left, right, "", 0); // len == 0
108 check_cmp(cx, expr.span, right, left, "", 0); // 0 == len
111 check_cmp(cx, expr.span, left, right, "!", 0); // len != 0
112 check_cmp(cx, expr.span, right, left, "!", 0); // 0 != len
115 check_cmp(cx, expr.span, left, right, "!", 0); // len > 0
116 check_cmp(cx, expr.span, right, left, "", 1); // 1 > len
119 check_cmp(cx, expr.span, left, right, "", 1); // len < 1
120 check_cmp(cx, expr.span, right, left, "!", 0); // 0 < len
122 BinOpKind::Ge => check_cmp(cx, expr.span, left, right, "!", 1), // len >= 1
123 BinOpKind::Le => check_cmp(cx, expr.span, right, left, "!", 1), // 1 <= len
130 fn check_trait_items(cx: &LateContext<'_, '_>, visited_trait: &Item, trait_items: &[TraitItemRef]) {
131 fn is_named_self(cx: &LateContext<'_, '_>, item: &TraitItemRef, name: &str) -> bool {
132 item.ident.name == name
133 && if let AssociatedItemKind::Method { has_self } = item.kind {
135 let did = cx.tcx.hir().local_def_id(item.id.node_id);
136 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
143 // fill the set with current and super traits
144 fn fill_trait_set(traitt: DefId, set: &mut FxHashSet<DefId>, cx: &LateContext<'_, '_>) {
145 if set.insert(traitt) {
146 for supertrait in rustc::traits::supertrait_def_ids(cx.tcx, traitt) {
147 fill_trait_set(supertrait, set, cx);
152 if cx.access_levels.is_exported(visited_trait.id) && trait_items.iter().any(|i| is_named_self(cx, i, "len")) {
153 let mut current_and_super_traits = FxHashSet::default();
154 let visited_trait_def_id = cx.tcx.hir().local_def_id(visited_trait.id);
155 fill_trait_set(visited_trait_def_id, &mut current_and_super_traits, cx);
157 let is_empty_method_found = current_and_super_traits
159 .flat_map(|&i| cx.tcx.associated_items(i))
161 i.kind == ty::AssociatedKind::Method
162 && i.method_has_self_argument
163 && i.ident.name == "is_empty"
164 && cx.tcx.fn_sig(i.def_id).inputs().skip_binder().len() == 1
167 if !is_empty_method_found {
170 LEN_WITHOUT_IS_EMPTY,
173 "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method",
174 visited_trait.ident.name
181 fn check_impl_items(cx: &LateContext<'_, '_>, item: &Item, impl_items: &[ImplItemRef]) {
182 fn is_named_self(cx: &LateContext<'_, '_>, item: &ImplItemRef, name: &str) -> bool {
183 item.ident.name == name
184 && if let AssociatedItemKind::Method { has_self } = item.kind {
186 let did = cx.tcx.hir().local_def_id(item.id.node_id);
187 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
194 let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
195 if cx.access_levels.is_exported(is_empty.id.node_id) {
204 if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
205 if cx.access_levels.is_exported(i.id.node_id) {
206 let def_id = cx.tcx.hir().local_def_id(item.id);
207 let ty = cx.tcx.type_of(def_id);
211 LEN_WITHOUT_IS_EMPTY,
214 "item `{}` has a public `len` method but {} `is_empty` method",
222 fn check_cmp(cx: &LateContext<'_, '_>, span: Span, method: &Expr, lit: &Expr, op: &str, compare_to: u32) {
223 if let (&ExprKind::MethodCall(ref method_path, _, ref args), &ExprKind::Lit(ref lit)) = (&method.node, &lit.node) {
224 // check if we are in an is_empty() method
225 if let Some(name) = get_item_name(cx, method) {
226 if name == "is_empty" {
231 check_len(cx, span, method_path.ident.name, args, lit, op, compare_to)
236 cx: &LateContext<'_, '_>,
245 node: LitKind::Int(lit, _),
249 // check if length is compared to the specified number
250 if lit != u128::from(compare_to) {
254 if method_name == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
255 let mut applicability = Applicability::MachineApplicable;
260 &format!("length comparison to {}", if compare_to == 0 { "zero" } else { "one" }),
261 "using `is_empty` is clearer and more explicit",
265 snippet_with_applicability(cx, args[0].span, "_", &mut applicability)
273 /// Check if this type has an `is_empty` method.
274 fn has_is_empty(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
275 /// Get an `AssociatedItem` and return true if it matches `is_empty(self)`.
276 fn is_is_empty(cx: &LateContext<'_, '_>, item: &ty::AssociatedItem) -> bool {
277 if let ty::AssociatedKind::Method = item.kind {
278 if item.ident.name == "is_empty" {
279 let sig = cx.tcx.fn_sig(item.def_id);
280 let ty = sig.skip_binder();
281 ty.inputs().len() == 1
290 /// Check the inherent impl's items for an `is_empty(self)` method.
291 fn has_is_empty_impl(cx: &LateContext<'_, '_>, id: DefId) -> bool {
295 .any(|imp| cx.tcx.associated_items(*imp).any(|item| is_is_empty(cx, &item)))
298 let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
300 ty::Dynamic(ref tt, ..) => {
301 if let Some(principal) = tt.principal() {
303 .associated_items(principal.def_id())
304 .any(|item| is_is_empty(cx, &item))
309 ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id),
310 ty::Adt(id, _) => has_is_empty_impl(cx, id.did),
311 ty::Array(..) | ty::Slice(..) | ty::Str => true,