2 use rustc::hir::def_id::DefId;
3 use rustc::ty::{self, MethodTraitItemId, ImplOrTraitItemId};
5 use syntax::ast::{Lit, LitKind, Name};
6 use syntax::codemap::{Span, Spanned};
8 use utils::{get_item_name, in_macro, snippet, span_lint, span_lint_and_then, walk_ptrs_ty};
10 /// **What it does:** This lint checks for getting the length of something via `.len()` just to compare to zero, and suggests using `.is_empty()` where applicable.
12 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster than calculating their length. So it is good to get into the habit of using `.is_empty()`, and having it is cheap. Besides, it makes the intent clearer than a comparison.
14 /// **Known problems:** None
16 /// **Example:** `if x.len() == 0 { .. }`
19 "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` \
20 could be used instead"
23 /// **What it does:** This lint checks for items that implement `.len()` but not `.is_empty()`.
25 /// **Why is this bad?** It is good custom to have both methods, because for some data structures, asking about the length will be a costly operation, whereas `.is_empty()` can usually answer in constant time. Also it used to lead to false positives on the [`len_zero`](#len_zero) lint – currently that lint will ignore such entities.
27 /// **Known problems:** None
32 /// fn len(&self) -> usize { .. }
36 pub LEN_WITHOUT_IS_EMPTY, Warn,
37 "traits and impls that have `.len()` but not `.is_empty()`"
43 impl LintPass for LenZero {
44 fn get_lints(&self) -> LintArray {
45 lint_array!(LEN_ZERO, LEN_WITHOUT_IS_EMPTY)
49 impl LateLintPass for LenZero {
50 fn check_item(&mut self, cx: &LateContext, item: &Item) {
51 if in_macro(cx, item.span) {
56 ItemTrait(_, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
57 ItemImpl(_, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
62 fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
63 if in_macro(cx, expr.span) {
67 if let ExprBinary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
69 BiEq => check_cmp(cx, expr.span, left, right, ""),
70 BiGt | BiNe => check_cmp(cx, expr.span, left, right, "!"),
77 fn check_trait_items(cx: &LateContext, item: &Item, trait_items: &[TraitItem]) {
78 fn is_named_self(item: &TraitItem, name: &str) -> bool {
79 item.name.as_str() == name &&
80 if let MethodTraitItem(ref sig, _) = item.node {
87 if !trait_items.iter().any(|i| is_named_self(i, "is_empty")) {
88 for i in trait_items {
89 if is_named_self(i, "len") {
93 &format!("trait `{}` has a `.len(_: &Self)` method, but no `.is_empty(_: &Self)` method. \
101 fn check_impl_items(cx: &LateContext, item: &Item, impl_items: &[ImplItem]) {
102 fn is_named_self(item: &ImplItem, name: &str) -> bool {
103 item.name.as_str() == name &&
104 if let ImplItemKind::Method(ref sig, _) = item.node {
111 if !impl_items.iter().any(|i| is_named_self(i, "is_empty")) {
112 for i in impl_items {
113 if is_named_self(i, "len") {
114 let ty = cx.tcx.node_id_to_type(item.id);
118 LEN_WITHOUT_IS_EMPTY,
124 &format!("item `{}` has a `.len(_: &Self)` method, but no `.is_empty(_: &Self)` method. \
125 Consider adding one",
133 fn is_self_sig(sig: &MethodSig) -> bool {
134 if let SelfStatic = sig.explicit_self.node {
137 sig.decl.inputs.len() == 1
141 fn check_cmp(cx: &LateContext, span: Span, left: &Expr, right: &Expr, op: &str) {
142 // check if we are in an is_empty() method
143 if let Some(name) = get_item_name(cx, left) {
144 if name.as_str() == "is_empty" {
148 match (&left.node, &right.node) {
149 (&ExprLit(ref lit), &ExprMethodCall(ref method, _, ref args)) |
150 (&ExprMethodCall(ref method, _, ref args), &ExprLit(ref lit)) => {
151 check_len_zero(cx, span, &method.node, args, lit, op)
157 fn check_len_zero(cx: &LateContext, span: Span, name: &Name, args: &[P<Expr>], lit: &Lit, op: &str) {
158 if let Spanned { node: LitKind::Int(0, _), .. } = *lit {
159 if name.as_str() == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
160 span_lint_and_then(cx, LEN_ZERO, span, "length comparison to zero", |db| {
161 db.span_suggestion(span,
162 "consider using `is_empty`",
163 format!("{}{}.is_empty()", op, snippet(cx, args[0].span, "_")));
169 /// Check if this type has an `is_empty` method.
170 fn has_is_empty(cx: &LateContext, expr: &Expr) -> bool {
171 /// Get an `ImplOrTraitItem` and return true if it matches `is_empty(self)`.
172 fn is_is_empty(cx: &LateContext, id: &ImplOrTraitItemId) -> bool {
173 if let MethodTraitItemId(def_id) = *id {
174 if let ty::MethodTraitItem(ref method) = cx.tcx.impl_or_trait_item(def_id) {
175 method.name.as_str() == "is_empty" && method.fty.sig.skip_binder().inputs.len() == 1
184 /// Check the inherent impl's items for an `is_empty(self)` method.
185 fn has_is_empty_impl(cx: &LateContext, id: &DefId) -> bool {
186 let impl_items = cx.tcx.impl_items.borrow();
187 cx.tcx.inherent_impls.borrow().get(id).map_or(false, |ids| {
188 ids.iter().any(|iid| impl_items.get(iid).map_or(false, |iids| iids.iter().any(|i| is_is_empty(cx, i))))
192 let ty = &walk_ptrs_ty(&cx.tcx.expr_ty(expr));
198 .get(&ty.ty_to_def_id().expect("trait impl not found"))
199 .map_or(false, |ids| ids.iter().any(|i| is_is_empty(cx, i)))
201 ty::TyProjection(_) => ty.ty_to_def_id().map_or(false, |id| has_is_empty_impl(cx, &id)),
202 ty::TyEnum(ref id, _) |
203 ty::TyStruct(ref id, _) => has_is_empty_impl(cx, &id.did),
204 ty::TyArray(..) | ty::TyStr => true,