use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg, span_lint_and_then}; use clippy_utils::source::snippet_with_applicability; use clippy_utils::{get_item_name, get_parent_as_impl, is_lint_allowed}; use if_chain::if_chain; use rustc_ast::ast::LitKind; use rustc_errors::Applicability; use rustc_hir::def_id::DefIdSet; use rustc_hir::{ def_id::DefId, AssocItemKind, BinOpKind, Expr, ExprKind, FnRetTy, ImplItem, ImplItemKind, ImplicitSelfKind, Item, ItemKind, Mutability, Node, TraitItemRef, TyKind, }; use rustc_lint::{LateContext, LateLintPass}; use rustc_middle::ty::{self, AssocKind, FnSig, Ty}; use rustc_session::{declare_lint_pass, declare_tool_lint}; use rustc_span::{ source_map::{Span, Spanned, Symbol}, symbol::sym, }; declare_clippy_lint! { /// ### What it does /// Checks for getting the length of something via `.len()` /// just to compare to zero, and suggests using `.is_empty()` where applicable. /// /// ### 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 manual comparison in some contexts. /// /// ### Example /// ```ignore /// if x.len() == 0 { /// .. /// } /// if y.len() != 0 { /// .. /// } /// ``` /// instead use /// ```ignore /// if x.is_empty() { /// .. /// } /// if !y.is_empty() { /// .. /// } /// ``` #[clippy::version = "pre 1.29.0"] pub LEN_ZERO, style, "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` could be used instead" } declare_clippy_lint! { /// ### What it does /// Checks for items that implement `.len()` but not /// `.is_empty()`. /// /// ### 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. /// /// ### Example /// ```ignore /// impl X { /// pub fn len(&self) -> usize { /// .. /// } /// } /// ``` #[clippy::version = "pre 1.29.0"] pub LEN_WITHOUT_IS_EMPTY, style, "traits or impls with a public `len` method but no corresponding `is_empty` method" } declare_clippy_lint! { /// ### What it does /// Checks for comparing to an empty slice such as `""` or `[]`, /// and suggests using `.is_empty()` where applicable. /// /// ### Why is this bad? /// Some structures can answer `.is_empty()` much faster /// than checking for equality. 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 manual comparison in some contexts. /// /// ### Example /// /// ```ignore /// if s == "" { /// .. /// } /// /// if arr == [] { /// .. /// } /// ``` /// Use instead: /// ```ignore /// if s.is_empty() { /// .. /// } /// /// if arr.is_empty() { /// .. /// } /// ``` #[clippy::version = "1.49.0"] pub COMPARISON_TO_EMPTY, style, "checking `x == \"\"` or `x == []` (or similar) when `.is_empty()` could be used instead" } declare_lint_pass!(LenZero => [LEN_ZERO, LEN_WITHOUT_IS_EMPTY, COMPARISON_TO_EMPTY]); impl<'tcx> LateLintPass<'tcx> for LenZero { fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) { if item.span.from_expansion() { return; } if let ItemKind::Trait(_, _, _, _, trait_items) = item.kind { check_trait_items(cx, item, trait_items); } } fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) { if_chain! { if item.ident.name == sym::len; if let ImplItemKind::Fn(sig, _) = &item.kind; if sig.decl.implicit_self.has_implicit_self(); if cx.access_levels.is_exported(item.def_id); if matches!(sig.decl.output, FnRetTy::Return(_)); if let Some(imp) = get_parent_as_impl(cx.tcx, item.hir_id()); if imp.of_trait.is_none(); if let TyKind::Path(ty_path) = &imp.self_ty.kind; if let Some(ty_id) = cx.qpath_res(ty_path, imp.self_ty.hir_id).opt_def_id(); if let Some(local_id) = ty_id.as_local(); let ty_hir_id = cx.tcx.hir().local_def_id_to_hir_id(local_id); if !is_lint_allowed(cx, LEN_WITHOUT_IS_EMPTY, ty_hir_id); if let Some(output) = parse_len_output(cx, cx.tcx.fn_sig(item.def_id).skip_binder()); then { let (name, kind) = match cx.tcx.hir().find(ty_hir_id) { Some(Node::ForeignItem(x)) => (x.ident.name, "extern type"), Some(Node::Item(x)) => match x.kind { ItemKind::Struct(..) => (x.ident.name, "struct"), ItemKind::Enum(..) => (x.ident.name, "enum"), ItemKind::Union(..) => (x.ident.name, "union"), _ => (x.ident.name, "type"), } _ => return, }; check_for_is_empty(cx, sig.span, sig.decl.implicit_self, output, ty_id, name, kind) } } } fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) { if expr.span.from_expansion() { return; } if let ExprKind::Binary(Spanned { node: cmp, .. }, left, right) = expr.kind { match cmp { BinOpKind::Eq => { check_cmp(cx, expr.span, left, right, "", 0); // len == 0 check_cmp(cx, expr.span, right, left, "", 0); // 0 == len }, BinOpKind::Ne => { check_cmp(cx, expr.span, left, right, "!", 0); // len != 0 check_cmp(cx, expr.span, right, left, "!", 0); // 0 != len }, BinOpKind::Gt => { check_cmp(cx, expr.span, left, right, "!", 0); // len > 0 check_cmp(cx, expr.span, right, left, "", 1); // 1 > len }, BinOpKind::Lt => { check_cmp(cx, expr.span, left, right, "", 1); // len < 1 check_cmp(cx, expr.span, right, left, "!", 0); // 0 < len }, BinOpKind::Ge => check_cmp(cx, expr.span, left, right, "!", 1), // len >= 1 BinOpKind::Le => check_cmp(cx, expr.span, right, left, "!", 1), // 1 <= len _ => (), } } } } fn check_trait_items(cx: &LateContext<'_>, visited_trait: &Item<'_>, trait_items: &[TraitItemRef]) { fn is_named_self(cx: &LateContext<'_>, item: &TraitItemRef, name: Symbol) -> bool { item.ident.name == name && if let AssocItemKind::Fn { has_self } = item.kind { has_self && { cx.tcx.fn_sig(item.id.def_id).inputs().skip_binder().len() == 1 } } else { false } } // fill the set with current and super traits fn fill_trait_set(traitt: DefId, set: &mut DefIdSet, cx: &LateContext<'_>) { if set.insert(traitt) { for supertrait in rustc_trait_selection::traits::supertrait_def_ids(cx.tcx, traitt) { fill_trait_set(supertrait, set, cx); } } } if cx.access_levels.is_exported(visited_trait.def_id) && trait_items.iter().any(|i| is_named_self(cx, i, sym::len)) { let mut current_and_super_traits = DefIdSet::default(); fill_trait_set(visited_trait.def_id.to_def_id(), &mut current_and_super_traits, cx); let is_empty = sym!(is_empty); let is_empty_method_found = current_and_super_traits .iter() .flat_map(|&i| cx.tcx.associated_items(i).filter_by_name_unhygienic(is_empty)) .any(|i| { i.kind == ty::AssocKind::Fn && i.fn_has_self_parameter && cx.tcx.fn_sig(i.def_id).inputs().skip_binder().len() == 1 }); if !is_empty_method_found { span_lint( cx, LEN_WITHOUT_IS_EMPTY, visited_trait.span, &format!( "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method", visited_trait.ident.name ), ); } } } #[derive(Debug, Clone, Copy)] enum LenOutput<'tcx> { Integral, Option(DefId), Result(DefId, Ty<'tcx>), } fn parse_len_output<'tcx>(cx: &LateContext<'_>, sig: FnSig<'tcx>) -> Option> { match *sig.output().kind() { ty::Int(_) | ty::Uint(_) => Some(LenOutput::Integral), ty::Adt(adt, subs) if cx.tcx.is_diagnostic_item(sym::Option, adt.did) => { subs.type_at(0).is_integral().then(|| LenOutput::Option(adt.did)) }, ty::Adt(adt, subs) if cx.tcx.is_diagnostic_item(sym::Result, adt.did) => subs .type_at(0) .is_integral() .then(|| LenOutput::Result(adt.did, subs.type_at(1))), _ => None, } } impl LenOutput<'_> { fn matches_is_empty_output(self, ty: Ty<'_>) -> bool { match (self, ty.kind()) { (_, &ty::Bool) => true, (Self::Option(id), &ty::Adt(adt, subs)) if id == adt.did => subs.type_at(0).is_bool(), (Self::Result(id, err_ty), &ty::Adt(adt, subs)) if id == adt.did => { subs.type_at(0).is_bool() && subs.type_at(1) == err_ty }, _ => false, } } fn expected_sig(self, self_kind: ImplicitSelfKind) -> String { let self_ref = match self_kind { ImplicitSelfKind::ImmRef => "&", ImplicitSelfKind::MutRef => "&mut ", _ => "", }; match self { Self::Integral => format!("expected signature: `({}self) -> bool`", self_ref), Self::Option(_) => format!( "expected signature: `({}self) -> bool` or `({}self) -> Option", self_ref, self_ref ), Self::Result(..) => format!( "expected signature: `({}self) -> bool` or `({}self) -> Result", self_ref, self_ref ), } } } /// Checks if the given signature matches the expectations for `is_empty` fn check_is_empty_sig(sig: FnSig<'_>, self_kind: ImplicitSelfKind, len_output: LenOutput<'_>) -> bool { match &**sig.inputs_and_output { [arg, res] if len_output.matches_is_empty_output(res) => { matches!( (arg.kind(), self_kind), (ty::Ref(_, _, Mutability::Not), ImplicitSelfKind::ImmRef) | (ty::Ref(_, _, Mutability::Mut), ImplicitSelfKind::MutRef) ) || (!arg.is_ref() && matches!(self_kind, ImplicitSelfKind::Imm | ImplicitSelfKind::Mut)) }, _ => false, } } /// Checks if the given type has an `is_empty` method with the appropriate signature. fn check_for_is_empty( cx: &LateContext<'_>, span: Span, self_kind: ImplicitSelfKind, output: LenOutput<'_>, impl_ty: DefId, item_name: Symbol, item_kind: &str, ) { let is_empty = Symbol::intern("is_empty"); let is_empty = cx .tcx .inherent_impls(impl_ty) .iter() .flat_map(|&id| cx.tcx.associated_items(id).filter_by_name_unhygienic(is_empty)) .find(|item| item.kind == AssocKind::Fn); let (msg, is_empty_span, self_kind) = match is_empty { None => ( format!( "{} `{}` has a public `len` method, but no `is_empty` method", item_kind, item_name.as_str(), ), None, None, ), Some(is_empty) if !cx.access_levels.is_exported(is_empty.def_id.expect_local()) => ( format!( "{} `{}` has a public `len` method, but a private `is_empty` method", item_kind, item_name.as_str(), ), Some(cx.tcx.def_span(is_empty.def_id)), None, ), Some(is_empty) if !(is_empty.fn_has_self_parameter && check_is_empty_sig(cx.tcx.fn_sig(is_empty.def_id).skip_binder(), self_kind, output)) => { ( format!( "{} `{}` has a public `len` method, but the `is_empty` method has an unexpected signature", item_kind, item_name.as_str(), ), Some(cx.tcx.def_span(is_empty.def_id)), Some(self_kind), ) }, Some(_) => return, }; span_lint_and_then(cx, LEN_WITHOUT_IS_EMPTY, span, &msg, |db| { if let Some(span) = is_empty_span { db.span_note(span, "`is_empty` defined here"); } if let Some(self_kind) = self_kind { db.note(&output.expected_sig(self_kind)); } }); } fn check_cmp(cx: &LateContext<'_>, span: Span, method: &Expr<'_>, lit: &Expr<'_>, op: &str, compare_to: u32) { if let (&ExprKind::MethodCall(method_path, args, _), &ExprKind::Lit(ref lit)) = (&method.kind, &lit.kind) { // check if we are in an is_empty() method if let Some(name) = get_item_name(cx, method) { if name.as_str() == "is_empty" { return; } } check_len(cx, span, method_path.ident.name, args, &lit.node, op, compare_to); } else { check_empty_expr(cx, span, method, lit, op); } } fn check_len( cx: &LateContext<'_>, span: Span, method_name: Symbol, args: &[Expr<'_>], lit: &LitKind, op: &str, compare_to: u32, ) { if let LitKind::Int(lit, _) = *lit { // check if length is compared to the specified number if lit != u128::from(compare_to) { return; } if method_name == sym::len && args.len() == 1 && has_is_empty(cx, &args[0]) { let mut applicability = Applicability::MachineApplicable; span_lint_and_sugg( cx, LEN_ZERO, span, &format!("length comparison to {}", if compare_to == 0 { "zero" } else { "one" }), &format!("using `{}is_empty` is clearer and more explicit", op), format!( "{}{}.is_empty()", op, snippet_with_applicability(cx, args[0].span, "_", &mut applicability) ), applicability, ); } } } fn check_empty_expr(cx: &LateContext<'_>, span: Span, lit1: &Expr<'_>, lit2: &Expr<'_>, op: &str) { if (is_empty_array(lit2) || is_empty_string(lit2)) && has_is_empty(cx, lit1) { let mut applicability = Applicability::MachineApplicable; span_lint_and_sugg( cx, COMPARISON_TO_EMPTY, span, "comparison to empty slice", &format!("using `{}is_empty` is clearer and more explicit", op), format!( "{}{}.is_empty()", op, snippet_with_applicability(cx, lit1.span, "_", &mut applicability) ), applicability, ); } } fn is_empty_string(expr: &Expr<'_>) -> bool { if let ExprKind::Lit(ref lit) = expr.kind { if let LitKind::Str(lit, _) = lit.node { let lit = lit.as_str(); return lit.is_empty(); } } false } fn is_empty_array(expr: &Expr<'_>) -> bool { if let ExprKind::Array(arr) = expr.kind { return arr.is_empty(); } false } /// Checks if this type has an `is_empty` method. fn has_is_empty(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool { /// Gets an `AssocItem` and return true if it matches `is_empty(self)`. fn is_is_empty(cx: &LateContext<'_>, item: &ty::AssocItem) -> bool { if item.kind == ty::AssocKind::Fn { let sig = cx.tcx.fn_sig(item.def_id); let ty = sig.skip_binder(); ty.inputs().len() == 1 } else { false } } /// Checks the inherent impl's items for an `is_empty(self)` method. fn has_is_empty_impl(cx: &LateContext<'_>, id: DefId) -> bool { let is_empty = sym!(is_empty); cx.tcx.inherent_impls(id).iter().any(|imp| { cx.tcx .associated_items(*imp) .filter_by_name_unhygienic(is_empty) .any(|item| is_is_empty(cx, item)) }) } let ty = &cx.typeck_results().expr_ty(expr).peel_refs(); match ty.kind() { ty::Dynamic(tt, ..) => tt.principal().map_or(false, |principal| { let is_empty = sym!(is_empty); cx.tcx .associated_items(principal.def_id()) .filter_by_name_unhygienic(is_empty) .any(|item| is_is_empty(cx, item)) }), ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id), ty::Adt(id, _) => has_is_empty_impl(cx, id.did), ty::Array(..) | ty::Slice(..) | ty::Str => true, _ => false, } }