3 get_item_name, in_macro_or_desugar, snippet_with_applicability, span_lint, span_lint_and_sugg, walk_ptrs_ty,
5 use rustc::hir::def_id::DefId;
7 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
9 use rustc::{declare_lint_pass, declare_tool_lint};
10 use rustc_data_structures::fx::FxHashSet;
11 use rustc_errors::Applicability;
12 use syntax::ast::{LitKind, Name};
13 use syntax::source_map::{Span, Spanned};
14 use syntax::symbol::Symbol;
16 declare_clippy_lint! {
17 /// **What it does:** Checks for getting the length of something via `.len()`
18 /// just to compare to zero, and suggests using `.is_empty()` where applicable.
20 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster
21 /// than calculating their length. Notably, for slices, getting the length
22 /// requires a subtraction whereas `.is_empty()` is just a comparison. So it is
23 /// good to get into the habit of using `.is_empty()`, and having it is cheap.
24 /// Besides, it makes the intent clearer than a manual comparison.
26 /// **Known problems:** None.
42 /// if !y.is_empty() {
48 "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` could be used instead"
51 declare_clippy_lint! {
52 /// **What it does:** Checks for items that implement `.len()` but not
55 /// **Why is this bad?** It is good custom to have both methods, because for
56 /// some data structures, asking about the length will be a costly operation,
57 /// whereas `.is_empty()` can usually answer in constant time. Also it used to
58 /// lead to false positives on the [`len_zero`](#len_zero) lint – currently that
59 /// lint will ignore such entities.
61 /// **Known problems:** None.
66 /// pub fn len(&self) -> usize {
71 pub LEN_WITHOUT_IS_EMPTY,
73 "traits or impls with a public `len` method but no corresponding `is_empty` method"
76 declare_lint_pass!(LenZero => [LEN_ZERO, LEN_WITHOUT_IS_EMPTY]);
78 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
79 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
80 if in_macro_or_desugar(item.span) {
85 ItemKind::Trait(_, _, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
86 ItemKind::Impl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
91 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
92 if in_macro_or_desugar(expr.span) {
96 if let ExprKind::Binary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
99 check_cmp(cx, expr.span, left, right, "", 0); // len == 0
100 check_cmp(cx, expr.span, right, left, "", 0); // 0 == len
103 check_cmp(cx, expr.span, left, right, "!", 0); // len != 0
104 check_cmp(cx, expr.span, right, left, "!", 0); // 0 != len
107 check_cmp(cx, expr.span, left, right, "!", 0); // len > 0
108 check_cmp(cx, expr.span, right, left, "", 1); // 1 > len
111 check_cmp(cx, expr.span, left, right, "", 1); // len < 1
112 check_cmp(cx, expr.span, right, left, "!", 0); // 0 < len
114 BinOpKind::Ge => check_cmp(cx, expr.span, left, right, "!", 1), // len >= 1
115 BinOpKind::Le => check_cmp(cx, expr.span, right, left, "!", 1), // 1 <= len
122 fn check_trait_items(cx: &LateContext<'_, '_>, visited_trait: &Item, trait_items: &[TraitItemRef]) {
123 fn is_named_self(cx: &LateContext<'_, '_>, item: &TraitItemRef, name: Symbol) -> bool {
124 item.ident.name == name
125 && if let AssociatedItemKind::Method { has_self } = item.kind {
127 let did = cx.tcx.hir().local_def_id_from_hir_id(item.id.hir_id);
128 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
135 // fill the set with current and super traits
136 fn fill_trait_set(traitt: DefId, set: &mut FxHashSet<DefId>, cx: &LateContext<'_, '_>) {
137 if set.insert(traitt) {
138 for supertrait in rustc::traits::supertrait_def_ids(cx.tcx, traitt) {
139 fill_trait_set(supertrait, set, cx);
144 if cx.access_levels.is_exported(visited_trait.hir_id) && trait_items.iter().any(|i| is_named_self(cx, i, *sym::len))
146 let mut current_and_super_traits = FxHashSet::default();
147 let visited_trait_def_id = cx.tcx.hir().local_def_id_from_hir_id(visited_trait.hir_id);
148 fill_trait_set(visited_trait_def_id, &mut current_and_super_traits, cx);
150 let is_empty_method_found = current_and_super_traits
152 .flat_map(|&i| cx.tcx.associated_items(i))
154 i.kind == ty::AssociatedKind::Method
155 && i.method_has_self_argument
156 && i.ident.name == *sym::is_empty
157 && cx.tcx.fn_sig(i.def_id).inputs().skip_binder().len() == 1
160 if !is_empty_method_found {
163 LEN_WITHOUT_IS_EMPTY,
166 "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method",
167 visited_trait.ident.name
174 fn check_impl_items(cx: &LateContext<'_, '_>, item: &Item, impl_items: &[ImplItemRef]) {
175 fn is_named_self(cx: &LateContext<'_, '_>, item: &ImplItemRef, name: Symbol) -> bool {
176 item.ident.name == name
177 && if let AssociatedItemKind::Method { has_self } = item.kind {
179 let did = cx.tcx.hir().local_def_id_from_hir_id(item.id.hir_id);
180 cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
187 let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, *sym::is_empty)) {
188 if cx.access_levels.is_exported(is_empty.id.hir_id) {
197 if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, *sym::len)) {
198 if cx.access_levels.is_exported(i.id.hir_id) {
199 let def_id = cx.tcx.hir().local_def_id_from_hir_id(item.hir_id);
200 let ty = cx.tcx.type_of(def_id);
204 LEN_WITHOUT_IS_EMPTY,
207 "item `{}` has a public `len` method but {} `is_empty` method",
215 fn check_cmp(cx: &LateContext<'_, '_>, span: Span, method: &Expr, lit: &Expr, op: &str, compare_to: u32) {
216 if let (&ExprKind::MethodCall(ref method_path, _, ref args), &ExprKind::Lit(ref lit)) = (&method.node, &lit.node) {
217 // check if we are in an is_empty() method
218 if let Some(name) = get_item_name(cx, method) {
219 if name == *sym::is_empty {
224 check_len(cx, span, method_path.ident.name, args, &lit.node, op, compare_to)
229 cx: &LateContext<'_, '_>,
237 if let LitKind::Int(lit, _) = *lit {
238 // check if length is compared to the specified number
239 if lit != u128::from(compare_to) {
243 if method_name == *sym::len && args.len() == 1 && has_is_empty(cx, &args[0]) {
244 let mut applicability = Applicability::MachineApplicable;
249 &format!("length comparison to {}", if compare_to == 0 { "zero" } else { "one" }),
250 "using `is_empty` is clearer and more explicit",
254 snippet_with_applicability(cx, args[0].span, "_", &mut applicability)
262 /// Checks if this type has an `is_empty` method.
263 fn has_is_empty(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
264 /// Gets an `AssociatedItem` and return true if it matches `is_empty(self)`.
265 fn is_is_empty(cx: &LateContext<'_, '_>, item: &ty::AssociatedItem) -> bool {
266 if let ty::AssociatedKind::Method = item.kind {
267 if item.ident.name == *sym::is_empty {
268 let sig = cx.tcx.fn_sig(item.def_id);
269 let ty = sig.skip_binder();
270 ty.inputs().len() == 1
279 /// Checks the inherent impl's items for an `is_empty(self)` method.
280 fn has_is_empty_impl(cx: &LateContext<'_, '_>, id: DefId) -> bool {
284 .any(|imp| cx.tcx.associated_items(*imp).any(|item| is_is_empty(cx, &item)))
287 let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
289 ty::Dynamic(ref tt, ..) => {
290 if let Some(principal) = tt.principal() {
292 .associated_items(principal.def_id())
293 .any(|item| is_is_empty(cx, &item))
298 ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id),
299 ty::Adt(id, _) => has_is_empty_impl(cx, id.did),
300 ty::Array(..) | ty::Slice(..) | ty::Str => true,