rust-clippy/clippy_lints/src/len_zero.rs

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use rustc::lint::*;
use rustc::hir::def_id::DefId;
use rustc::ty;
use rustc::hir::*;
use syntax::ast::{Lit, LitKind, Name};
use syntax::codemap::{Span, Spanned};
use utils::{get_item_name, in_macro, snippet, span_lint, span_lint_and_sugg, walk_ptrs_ty};
/// **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 comparison.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// if x.len() == 0 { .. }
/// ```
declare_lint! {
pub LEN_ZERO,
Warn,
"checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` \
could be used instead"
}
/// **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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// impl X {
/// pub fn len(&self) -> usize { .. }
/// }
/// ```
declare_lint! {
pub LEN_WITHOUT_IS_EMPTY,
Warn,
"traits or impls with a public `len` method but no corresponding `is_empty` method"
}
#[derive(Copy,Clone)]
pub struct LenZero;
impl LintPass for LenZero {
fn get_lints(&self) -> LintArray {
lint_array!(LEN_ZERO, LEN_WITHOUT_IS_EMPTY)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
if in_macro(item.span) {
return;
}
match item.node {
ItemTrait(_, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
ItemImpl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
_ => (),
}
}
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
if in_macro(expr.span) {
return;
}
if let ExprBinary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
match cmp {
BiEq => check_cmp(cx, expr.span, left, right, ""),
BiGt | BiNe => check_cmp(cx, expr.span, left, right, "!"),
_ => (),
}
}
}
}
fn check_trait_items(cx: &LateContext, item: &Item, trait_items: &[TraitItemRef]) {
fn is_named_self(cx: &LateContext, item: &TraitItemRef, name: &str) -> bool {
item.name == name &&
if let AssociatedItemKind::Method { has_self } = item.kind {
has_self &&
{
let did = cx.tcx.hir.local_def_id(item.id.node_id);
cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
}
} else {
false
}
}
if !trait_items.iter().any(|i| is_named_self(cx, i, "is_empty")) {
if let Some(i) = trait_items.iter().find(|i| is_named_self(cx, i, "len")) {
if cx.access_levels.is_exported(i.id.node_id) {
span_lint(cx,
LEN_WITHOUT_IS_EMPTY,
item.span,
&format!("trait `{}` has a `len` method but no `is_empty` method", item.name));
}
}
}
}
fn check_impl_items(cx: &LateContext, item: &Item, impl_items: &[ImplItemRef]) {
fn is_named_self(cx: &LateContext, item: &ImplItemRef, name: &str) -> bool {
item.name == name &&
if let AssociatedItemKind::Method { has_self } = item.kind {
has_self &&
{
let did = cx.tcx.hir.local_def_id(item.id.node_id);
cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
}
} else {
false
}
}
let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
if cx.access_levels.is_exported(is_empty.id.node_id) {
return;
} else {
"a private"
}
} else {
"no corresponding"
};
if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
if cx.access_levels.is_exported(i.id.node_id) {
let def_id = cx.tcx.hir.local_def_id(item.id);
let ty = cx.tcx.type_of(def_id);
span_lint(cx,
LEN_WITHOUT_IS_EMPTY,
item.span,
&format!("item `{}` has a public `len` method but {} `is_empty` method", ty, is_empty));
}
}
}
fn check_cmp(cx: &LateContext, span: Span, left: &Expr, right: &Expr, op: &str) {
// check if we are in an is_empty() method
if let Some(name) = get_item_name(cx, left) {
if name == "is_empty" {
return;
}
}
match (&left.node, &right.node) {
(&ExprLit(ref lit), &ExprMethodCall(ref method, _, ref args)) |
(&ExprMethodCall(ref method, _, ref args), &ExprLit(ref lit)) => {
check_len_zero(cx, span, method.node, args, lit, op)
},
_ => (),
}
}
fn check_len_zero(cx: &LateContext, span: Span, name: Name, args: &[Expr], lit: &Lit, op: &str) {
if let Spanned { node: LitKind::Int(0, _), .. } = *lit {
if name == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
span_lint_and_sugg(cx, LEN_ZERO, span, "length comparison to zero",
"using `is_empty` is more concise:",
format!("{}{}.is_empty()", op, snippet(cx, args[0].span, "_")));
}
}
}
/// Check if this type has an `is_empty` method.
fn has_is_empty(cx: &LateContext, expr: &Expr) -> bool {
/// Get an `AssociatedItem` and return true if it matches `is_empty(self)`.
fn is_is_empty(cx: &LateContext, item: &ty::AssociatedItem) -> bool {
if let ty::AssociatedKind::Method = item.kind {
if item.name == "is_empty" {
let sig = cx.tcx.fn_sig(item.def_id);
let ty = sig.skip_binder();
ty.inputs().len() == 1
} else {
false
}
} else {
false
}
}
/// Check the inherent impl's items for an `is_empty(self)` method.
fn has_is_empty_impl(cx: &LateContext, id: DefId) -> bool {
cx.tcx
.inherent_impls(id)
.iter()
.any(|imp| cx.tcx.associated_items(*imp).any(|item| is_is_empty(cx, &item)))
}
let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
match ty.sty {
ty::TyDynamic(..) => {
cx.tcx
.associated_items(ty.ty_to_def_id().expect("trait impl not found"))
.any(|item| is_is_empty(cx, &item))
},
ty::TyProjection(_) => ty.ty_to_def_id().map_or(false, |id| has_is_empty_impl(cx, id)),
ty::TyAdt(id, _) => has_is_empty_impl(cx, id.did),
ty::TyArray(..) | ty::TySlice(..) | ty::TyStr => true,
_ => false,
}
}