1 use crate::utils::{get_item_name, snippet_with_applicability, span_lint, span_lint_and_sugg, walk_ptrs_ty};
2 use rustc::declare_lint_pass;
3 use rustc::hir::def_id::DefId;
5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
7 use rustc_data_structures::fx::FxHashSet;
8 use rustc_errors::Applicability;
9 use rustc_session::declare_tool_lint;
10 use syntax::ast::{LitKind, Name};
11 use syntax::source_map::{Span, Spanned};
13 declare_clippy_lint! {
14 /// **What it does:** Checks for getting the length of something via `.len()`
15 /// just to compare to zero, and suggests using `.is_empty()` where applicable.
17 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster
18 /// than calculating their length. Notably, for slices, getting the length
19 /// requires a subtraction whereas `.is_empty()` is just a comparison. So it is
20 /// good to get into the habit of using `.is_empty()`, and having it is cheap.
21 /// Besides, it makes the intent clearer than a manual comparison.
23 /// **Known problems:** None.
39 /// if !y.is_empty() {
45 "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` could be used instead"
48 declare_clippy_lint! {
49 /// **What it does:** Checks for items that implement `.len()` but not
52 /// **Why is this bad?** It is good custom to have both methods, because for
53 /// some data structures, asking about the length will be a costly operation,
54 /// whereas `.is_empty()` can usually answer in constant time. Also it used to
55 /// lead to false positives on the [`len_zero`](#len_zero) lint – currently that
56 /// lint will ignore such entities.
58 /// **Known problems:** None.
63 /// pub fn len(&self) -> usize {
68 pub LEN_WITHOUT_IS_EMPTY,
70 "traits or impls with a public `len` method but no corresponding `is_empty` method"
73 declare_lint_pass!(LenZero => [LEN_ZERO, LEN_WITHOUT_IS_EMPTY]);
75 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
76 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
77 if item.span.from_expansion() {
82 ItemKind::Trait(_, _, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
83 ItemKind::Impl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
88 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
89 if expr.span.from_expansion() {
93 if let ExprKind::Binary(Spanned { node: cmp, .. }, ref left, ref right) = expr.kind {
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, "!", 0); // 0 != len
104 check_cmp(cx, expr.span, left, right, "!", 0); // len > 0
105 check_cmp(cx, expr.span, right, left, "", 1); // 1 > len
108 check_cmp(cx, expr.span, left, right, "", 1); // len < 1
109 check_cmp(cx, expr.span, right, left, "!", 0); // 0 < len
111 BinOpKind::Ge => check_cmp(cx, expr.span, left, right, "!", 1), // len >= 1
112 BinOpKind::Le => check_cmp(cx, expr.span, right, left, "!", 1), // 1 <= len
119 fn check_trait_items(cx: &LateContext<'_, '_>, visited_trait: &Item, trait_items: &[TraitItemRef]) {
120 fn is_named_self(cx: &LateContext<'_, '_>, item: &TraitItemRef, name: &str) -> bool {
121 item.ident.name.as_str() == name
122 && if let AssocItemKind::Method { has_self } = item.kind {
124 let did = cx.tcx.hir().local_def_id(item.id.hir_id);
125 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
132 // fill the set with current and super traits
133 fn fill_trait_set(traitt: DefId, set: &mut FxHashSet<DefId>, cx: &LateContext<'_, '_>) {
134 if set.insert(traitt) {
135 for supertrait in rustc::traits::supertrait_def_ids(cx.tcx, traitt) {
136 fill_trait_set(supertrait, set, cx);
141 if cx.access_levels.is_exported(visited_trait.hir_id) && trait_items.iter().any(|i| is_named_self(cx, i, "len")) {
142 let mut current_and_super_traits = FxHashSet::default();
143 let visited_trait_def_id = cx.tcx.hir().local_def_id(visited_trait.hir_id);
144 fill_trait_set(visited_trait_def_id, &mut current_and_super_traits, cx);
146 let is_empty_method_found = current_and_super_traits
148 .flat_map(|&i| cx.tcx.associated_items(i))
150 i.kind == ty::AssocKind::Method
151 && i.method_has_self_argument
152 && i.ident.name == sym!(is_empty)
153 && cx.tcx.fn_sig(i.def_id).inputs().skip_binder().len() == 1
156 if !is_empty_method_found {
159 LEN_WITHOUT_IS_EMPTY,
162 "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method",
163 visited_trait.ident.name
170 fn check_impl_items(cx: &LateContext<'_, '_>, item: &Item, impl_items: &[ImplItemRef]) {
171 fn is_named_self(cx: &LateContext<'_, '_>, item: &ImplItemRef, name: &str) -> bool {
172 item.ident.name.as_str() == name
173 && if let AssocItemKind::Method { has_self } = item.kind {
175 let did = cx.tcx.hir().local_def_id(item.id.hir_id);
176 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
183 let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
184 if cx.access_levels.is_exported(is_empty.id.hir_id) {
193 if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
194 if cx.access_levels.is_exported(i.id.hir_id) {
195 let def_id = cx.tcx.hir().local_def_id(item.hir_id);
196 let ty = cx.tcx.type_of(def_id);
200 LEN_WITHOUT_IS_EMPTY,
203 "item `{}` has a public `len` method but {} `is_empty` method",
211 fn check_cmp(cx: &LateContext<'_, '_>, span: Span, method: &Expr, lit: &Expr, op: &str, compare_to: u32) {
212 if let (&ExprKind::MethodCall(ref method_path, _, ref args), &ExprKind::Lit(ref lit)) = (&method.kind, &lit.kind) {
213 // check if we are in an is_empty() method
214 if let Some(name) = get_item_name(cx, method) {
215 if name.as_str() == "is_empty" {
220 check_len(cx, span, method_path.ident.name, args, &lit.node, op, compare_to)
225 cx: &LateContext<'_, '_>,
233 if let LitKind::Int(lit, _) = *lit {
234 // check if length is compared to the specified number
235 if lit != u128::from(compare_to) {
239 if method_name.as_str() == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
240 let mut applicability = Applicability::MachineApplicable;
245 &format!("length comparison to {}", if compare_to == 0 { "zero" } else { "one" }),
246 &format!("using `{}is_empty` is clearer and more explicit", op),
250 snippet_with_applicability(cx, args[0].span, "_", &mut applicability)
258 /// Checks if this type has an `is_empty` method.
259 fn has_is_empty(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
260 /// Gets an `AssocItem` and return true if it matches `is_empty(self)`.
261 fn is_is_empty(cx: &LateContext<'_, '_>, item: &ty::AssocItem) -> bool {
262 if let ty::AssocKind::Method = item.kind {
263 if item.ident.name.as_str() == "is_empty" {
264 let sig = cx.tcx.fn_sig(item.def_id);
265 let ty = sig.skip_binder();
266 ty.inputs().len() == 1
275 /// Checks the inherent impl's items for an `is_empty(self)` method.
276 fn has_is_empty_impl(cx: &LateContext<'_, '_>, id: DefId) -> bool {
280 .any(|imp| cx.tcx.associated_items(*imp).any(|item| is_is_empty(cx, &item)))
283 let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
285 ty::Dynamic(ref tt, ..) => {
286 if let Some(principal) = tt.principal() {
288 .associated_items(principal.def_id())
289 .any(|item| is_is_empty(cx, &item))
294 ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id),
295 ty::Adt(id, _) => has_is_empty_impl(cx, id.did),
296 ty::Array(..) | ty::Slice(..) | ty::Str => true,