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 declare_clippy_lint! {
13 /// **What it does:** Checks for getting the length of something via `.len()`
14 /// just to compare to zero, and suggests using `.is_empty()` where applicable.
16 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster
17 /// than calculating their length. Notably, for slices, getting the length
18 /// requires a subtraction whereas `.is_empty()` is just a comparison. So it is
19 /// good to get into the habit of using `.is_empty()`, and having it is cheap.
20 /// Besides, it makes the intent clearer than a manual comparison.
22 /// **Known problems:** None.
38 /// if !y.is_empty() {
44 "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` could be used instead"
47 declare_clippy_lint! {
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 pub LEN_WITHOUT_IS_EMPTY,
69 "traits or impls with a public `len` method but no corresponding `is_empty` method"
72 #[derive(Copy, Clone)]
75 impl LintPass for LenZero {
76 fn get_lints(&self) -> LintArray {
77 lint_array!(LEN_ZERO, LEN_WITHOUT_IS_EMPTY)
80 fn name(&self) -> &'static str {
85 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
86 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
87 if in_macro(item.span) {
92 ItemKind::Trait(_, _, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
93 ItemKind::Impl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
98 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
99 if in_macro(expr.span) {
103 if let ExprKind::Binary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
106 check_cmp(cx, expr.span, left, right, "", 0); // len == 0
107 check_cmp(cx, expr.span, right, left, "", 0); // 0 == len
110 check_cmp(cx, expr.span, left, right, "!", 0); // len != 0
111 check_cmp(cx, expr.span, right, left, "!", 0); // 0 != len
114 check_cmp(cx, expr.span, left, right, "!", 0); // len > 0
115 check_cmp(cx, expr.span, right, left, "", 1); // 1 > len
118 check_cmp(cx, expr.span, left, right, "", 1); // len < 1
119 check_cmp(cx, expr.span, right, left, "!", 0); // 0 < len
121 BinOpKind::Ge => check_cmp(cx, expr.span, left, right, "!", 1), // len >= 1
122 BinOpKind::Le => check_cmp(cx, expr.span, right, left, "!", 1), // 1 <= len
129 fn check_trait_items(cx: &LateContext<'_, '_>, visited_trait: &Item, trait_items: &[TraitItemRef]) {
130 fn is_named_self(cx: &LateContext<'_, '_>, item: &TraitItemRef, name: &str) -> bool {
131 item.ident.name == name
132 && if let AssociatedItemKind::Method { has_self } = item.kind {
134 let did = cx.tcx.hir().local_def_id_from_hir_id(item.id.hir_id);
135 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
142 // fill the set with current and super traits
143 fn fill_trait_set(traitt: DefId, set: &mut FxHashSet<DefId>, cx: &LateContext<'_, '_>) {
144 if set.insert(traitt) {
145 for supertrait in rustc::traits::supertrait_def_ids(cx.tcx, traitt) {
146 fill_trait_set(supertrait, set, cx);
151 if cx.access_levels.is_exported(visited_trait.hir_id) && trait_items.iter().any(|i| is_named_self(cx, i, "len")) {
152 let mut current_and_super_traits = FxHashSet::default();
153 let visited_trait_def_id = cx.tcx.hir().local_def_id_from_hir_id(visited_trait.hir_id);
154 fill_trait_set(visited_trait_def_id, &mut current_and_super_traits, cx);
156 let is_empty_method_found = current_and_super_traits
158 .flat_map(|&i| cx.tcx.associated_items(i))
160 i.kind == ty::AssociatedKind::Method
161 && i.method_has_self_argument
162 && i.ident.name == "is_empty"
163 && cx.tcx.fn_sig(i.def_id).inputs().skip_binder().len() == 1
166 if !is_empty_method_found {
169 LEN_WITHOUT_IS_EMPTY,
172 "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method",
173 visited_trait.ident.name
180 fn check_impl_items(cx: &LateContext<'_, '_>, item: &Item, impl_items: &[ImplItemRef]) {
181 fn is_named_self(cx: &LateContext<'_, '_>, item: &ImplItemRef, name: &str) -> bool {
182 item.ident.name == name
183 && if let AssociatedItemKind::Method { has_self } = item.kind {
185 let did = cx.tcx.hir().local_def_id_from_hir_id(item.id.hir_id);
186 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
193 let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
194 if cx.access_levels.is_exported(is_empty.id.hir_id) {
203 if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
204 if cx.access_levels.is_exported(i.id.hir_id) {
205 let def_id = cx.tcx.hir().local_def_id_from_hir_id(item.hir_id);
206 let ty = cx.tcx.type_of(def_id);
210 LEN_WITHOUT_IS_EMPTY,
213 "item `{}` has a public `len` method but {} `is_empty` method",
221 fn check_cmp(cx: &LateContext<'_, '_>, span: Span, method: &Expr, lit: &Expr, op: &str, compare_to: u32) {
222 if let (&ExprKind::MethodCall(ref method_path, _, ref args), &ExprKind::Lit(ref lit)) = (&method.node, &lit.node) {
223 // check if we are in an is_empty() method
224 if let Some(name) = get_item_name(cx, method) {
225 if name == "is_empty" {
230 check_len(cx, span, method_path.ident.name, args, lit, op, compare_to)
235 cx: &LateContext<'_, '_>,
244 node: LitKind::Int(lit, _),
248 // check if length is compared to the specified number
249 if lit != u128::from(compare_to) {
253 if method_name == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
254 let mut applicability = Applicability::MachineApplicable;
259 &format!("length comparison to {}", if compare_to == 0 { "zero" } else { "one" }),
260 "using `is_empty` is clearer and more explicit",
264 snippet_with_applicability(cx, args[0].span, "_", &mut applicability)
272 /// Checks if this type has an `is_empty` method.
273 fn has_is_empty(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
274 /// Gets an `AssociatedItem` and return true if it matches `is_empty(self)`.
275 fn is_is_empty(cx: &LateContext<'_, '_>, item: &ty::AssociatedItem) -> bool {
276 if let ty::AssociatedKind::Method = item.kind {
277 if item.ident.name == "is_empty" {
278 let sig = cx.tcx.fn_sig(item.def_id);
279 let ty = sig.skip_binder();
280 ty.inputs().len() == 1
289 /// Checks the inherent impl's items for an `is_empty(self)` method.
290 fn has_is_empty_impl(cx: &LateContext<'_, '_>, id: DefId) -> bool {
294 .any(|imp| cx.tcx.associated_items(*imp).any(|item| is_is_empty(cx, &item)))
297 let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
299 ty::Dynamic(ref tt, ..) => {
300 if let Some(principal) = tt.principal() {
302 .associated_items(principal.def_id())
303 .any(|item| is_is_empty(cx, &item))
308 ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id),
309 ty::Adt(id, _) => has_is_empty_impl(cx, id.did),
310 ty::Array(..) | ty::Slice(..) | ty::Str => true,