1 use rustc::hir::def_id::DefId;
3 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
4 use rustc::{declare_tool_lint, lint_array};
6 use std::collections::HashSet;
7 use syntax::ast::{Lit, LitKind, Name};
8 use syntax::source_map::{Span, Spanned};
9 use crate::utils::{get_item_name, in_macro, snippet, span_lint, span_lint_and_sugg, walk_ptrs_ty};
11 /// **What it does:** Checks for getting the length of something via `.len()`
12 /// just to compare to zero, and suggests using `.is_empty()` where applicable.
14 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster
15 /// than calculating their length. Notably, for slices, getting the length
16 /// requires a subtraction whereas `.is_empty()` is just a comparison. So it is
17 /// good to get into the habit of using `.is_empty()`, and having it is cheap.
18 /// Besides, it makes the intent clearer than a manual comparison.
20 /// **Known problems:** None.
24 /// if x.len() == 0 { .. }
25 /// if y.len() != 0 { .. }
29 /// if x.len().is_empty() { .. }
30 /// if !y.len().is_empty() { .. }
32 declare_clippy_lint! {
35 "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` \
36 could be used instead"
39 /// **What it does:** Checks for items that implement `.len()` but not
42 /// **Why is this bad?** It is good custom to have both methods, because for
43 /// some data structures, asking about the length will be a costly operation,
44 /// whereas `.is_empty()` can usually answer in constant time. Also it used to
45 /// lead to false positives on the [`len_zero`](#len_zero) lint – currently that
46 /// lint will ignore such entities.
48 /// **Known problems:** None.
53 /// pub fn len(&self) -> usize { .. }
56 declare_clippy_lint! {
57 pub LEN_WITHOUT_IS_EMPTY,
59 "traits or impls with a public `len` method but no corresponding `is_empty` method"
62 #[derive(Copy, Clone)]
65 impl LintPass for LenZero {
66 fn get_lints(&self) -> LintArray {
67 lint_array!(LEN_ZERO, LEN_WITHOUT_IS_EMPTY)
71 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
72 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
73 if in_macro(item.span) {
78 ItemKind::Trait(_, _, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
79 ItemKind::Impl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
84 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
85 if in_macro(expr.span) {
89 if let ExprKind::Binary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
92 check_cmp(cx, expr.span, left, right, "", 0); // len == 0
93 check_cmp(cx, expr.span, right, left, "", 0); // 0 == len
96 check_cmp(cx, expr.span, left, right, "!", 0); // len != 0
97 check_cmp(cx, expr.span, right, left, "!", 0); // 0 != len
100 check_cmp(cx, expr.span, left, right, "!", 0); // len > 0
101 check_cmp(cx, expr.span, right, left, "", 1); // 1 > len
104 check_cmp(cx, expr.span, left, right, "", 1); // len < 1
105 check_cmp(cx, expr.span, right, left, "!", 0); // 0 < len
107 BinOpKind::Ge => check_cmp(cx, expr.span, left, right, "!", 1), // len <= 1
108 BinOpKind::Le => check_cmp(cx, expr.span, right, left, "!", 1), // 1 >= len
115 fn check_trait_items(cx: &LateContext<'_, '_>, visited_trait: &Item, trait_items: &[TraitItemRef]) {
116 fn is_named_self(cx: &LateContext<'_, '_>, item: &TraitItemRef, name: &str) -> bool {
117 item.ident.name == name && if let AssociatedItemKind::Method { has_self } = item.kind {
119 let did = cx.tcx.hir.local_def_id(item.id.node_id);
120 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
127 // fill the set with current and super traits
128 fn fill_trait_set(traitt: DefId, set: &mut HashSet<DefId>, cx: &LateContext<'_, '_>) {
129 if set.insert(traitt) {
130 for supertrait in ::rustc::traits::supertrait_def_ids(cx.tcx, traitt) {
131 fill_trait_set(supertrait, set, cx);
136 if cx.access_levels.is_exported(visited_trait.id) && trait_items.iter().any(|i| is_named_self(cx, i, "len")) {
137 let mut current_and_super_traits = HashSet::new();
138 let visited_trait_def_id = cx.tcx.hir.local_def_id(visited_trait.id);
139 fill_trait_set(visited_trait_def_id, &mut current_and_super_traits, cx);
141 let is_empty_method_found = current_and_super_traits
143 .flat_map(|&i| cx.tcx.associated_items(i))
145 i.kind == ty::AssociatedKind::Method && i.method_has_self_argument && i.ident.name == "is_empty"
146 && cx.tcx.fn_sig(i.def_id).inputs().skip_binder().len() == 1
149 if !is_empty_method_found {
152 LEN_WITHOUT_IS_EMPTY,
155 "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method",
163 fn check_impl_items(cx: &LateContext<'_, '_>, item: &Item, impl_items: &[ImplItemRef]) {
164 fn is_named_self(cx: &LateContext<'_, '_>, item: &ImplItemRef, name: &str) -> bool {
165 item.ident.name == name && if let AssociatedItemKind::Method { has_self } = item.kind {
167 let did = cx.tcx.hir.local_def_id(item.id.node_id);
168 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
175 let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
176 if cx.access_levels.is_exported(is_empty.id.node_id) {
185 if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
186 if cx.access_levels.is_exported(i.id.node_id) {
187 let def_id = cx.tcx.hir.local_def_id(item.id);
188 let ty = cx.tcx.type_of(def_id);
192 LEN_WITHOUT_IS_EMPTY,
195 "item `{}` has a public `len` method but {} `is_empty` method",
203 fn check_cmp(cx: &LateContext<'_, '_>, span: Span, method: &Expr, lit: &Expr, op: &str, compare_to: u32) {
204 if let (&ExprKind::MethodCall(ref method_path, _, ref args), &ExprKind::Lit(ref lit)) = (&method.node, &lit.node) {
205 // check if we are in an is_empty() method
206 if let Some(name) = get_item_name(cx, method) {
207 if name == "is_empty" {
212 check_len(cx, span, method_path.ident.name, args, lit, op, compare_to)
216 fn check_len(cx: &LateContext<'_, '_>, span: Span, method_name: Name, args: &[Expr], lit: &Lit, op: &str, compare_to: u32) {
218 node: LitKind::Int(lit, _),
222 // check if length is compared to the specified number
223 if lit != u128::from(compare_to) {
227 if method_name == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
232 &format!("length comparison to {}", if compare_to == 0 { "zero" } else { "one" }),
233 "using `is_empty` is more concise",
234 format!("{}{}.is_empty()", op, snippet(cx, args[0].span, "_")),
240 /// Check if this type has an `is_empty` method.
241 fn has_is_empty(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
242 /// Get an `AssociatedItem` and return true if it matches `is_empty(self)`.
243 fn is_is_empty(cx: &LateContext<'_, '_>, item: &ty::AssociatedItem) -> bool {
244 if let ty::AssociatedKind::Method = item.kind {
245 if item.ident.name == "is_empty" {
246 let sig = cx.tcx.fn_sig(item.def_id);
247 let ty = sig.skip_binder();
248 ty.inputs().len() == 1
257 /// Check the inherent impl's items for an `is_empty(self)` method.
258 fn has_is_empty_impl(cx: &LateContext<'_, '_>, id: DefId) -> bool {
259 cx.tcx.inherent_impls(id).iter().any(|imp| {
261 .associated_items(*imp)
262 .any(|item| is_is_empty(cx, &item))
266 let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
268 ty::Dynamic(ref tt, ..) => cx.tcx
269 .associated_items(tt.principal().expect("trait impl not found").def_id())
270 .any(|item| is_is_empty(cx, &item)),
271 ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id),
272 ty::Adt(id, _) => has_is_empty_impl(cx, id.did),
273 ty::Array(..) | ty::Slice(..) | ty::Str => true,