src/tools/clippy/clippy_lints/src/len_zero.rs

   1 use rustc::lint::*;
   2 use rustc::hir::def_id::DefId;
   3 use rustc::ty;
   4 use rustc::hir::*;
   5 use std::collections::HashSet;
   6 use syntax::ast::{Lit, LitKind, Name};
   7 use syntax::codemap::{Span, Spanned};
   8 use utils::{get_item_name, in_macro, snippet, span_lint, span_lint_and_sugg, walk_ptrs_ty};
   9
  10 /// **What it does:** Checks for getting the length of something via `.len()`
  11 /// just to compare to zero, and suggests using `.is_empty()` where applicable.
  12 ///
  13 /// **Why is this bad?** Some structures can answer `.is_empty()` much faster
  14 /// than calculating their length. Notably, for slices, getting the length
  15 /// requires a subtraction whereas `.is_empty()` is just a comparison. So it is
  16 /// good to get into the habit of using `.is_empty()`, and having it is cheap.
  17 /// Besides, it makes the intent clearer than a manual comparison.
  18 ///
  19 /// **Known problems:** None.
  20 ///
  21 /// **Example:**
  22 /// ```rust
  23 /// if x.len() == 0 { .. }
  24 /// ```
  25 declare_lint! {
  26     pub LEN_ZERO,
  27     Warn,
  28     "checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` \
  29      could be used instead"
  30 }
  31
  32 /// **What it does:** Checks for items that implement `.len()` but not
  33 /// `.is_empty()`.
  34 ///
  35 /// **Why is this bad?** It is good custom to have both methods, because for
  36 /// some data structures, asking about the length will be a costly operation,
  37 /// whereas `.is_empty()` can usually answer in constant time. Also it used to
  38 /// lead to false positives on the [`len_zero`](#len_zero) lint – currently that
  39 /// lint will ignore such entities.
  40 ///
  41 /// **Known problems:** None.
  42 ///
  43 /// **Example:**
  44 /// ```rust
  45 /// impl X {
  46 ///     pub fn len(&self) -> usize { .. }
  47 /// }
  48 /// ```
  49 declare_lint! {
  50     pub LEN_WITHOUT_IS_EMPTY,
  51     Warn,
  52     "traits or impls with a public `len` method but no corresponding `is_empty` method"
  53 }
  54
  55 #[derive(Copy, Clone)]
  56 pub struct LenZero;
  57
  58 impl LintPass for LenZero {
  59     fn get_lints(&self) -> LintArray {
  60         lint_array!(LEN_ZERO, LEN_WITHOUT_IS_EMPTY)
  61     }
  62 }
  63
  64 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LenZero {
  65     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
  66         if in_macro(item.span) {
  67             return;
  68         }
  69
  70         match item.node {
  71             ItemTrait(_, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
  72             ItemImpl(_, _, _, _, None, _, ref impl_items) => check_impl_items(cx, item, impl_items),
  73             _ => (),
  74         }
  75     }
  76
  77     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  78         if in_macro(expr.span) {
  79             return;
  80         }
  81
  82         if let ExprBinary(Spanned { node: cmp, .. }, ref left, ref right) = expr.node {
  83             match cmp {
  84                 BiEq => check_cmp(cx, expr.span, left, right, ""),
  85                 BiGt | BiNe => check_cmp(cx, expr.span, left, right, "!"),
  86                 _ => (),
  87             }
  88         }
  89     }
  90 }
  91
  92 fn check_trait_items(cx: &LateContext, visited_trait: &Item, trait_items: &[TraitItemRef]) {
  93     fn is_named_self(cx: &LateContext, item: &TraitItemRef, name: &str) -> bool {
  94         item.name == name &&
  95             if let AssociatedItemKind::Method { has_self } = item.kind {
  96                 has_self &&
  97                     {
  98                         let did = cx.tcx.hir.local_def_id(item.id.node_id);
  99                         cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
 100                     }
 101             } else {
 102                 false
 103             }
 104     }
 105
 106     // fill the set with current and super traits
 107     fn fill_trait_set<'a, 'b: 'a>(traitt: &'b Item, set: &'a mut HashSet<&'b Item>, cx: &'b LateContext) {
 108         if set.insert(traitt) {
 109             if let ItemTrait(.., ref ty_param_bounds, _) = traitt.node {
 110                 for ty_param_bound in ty_param_bounds {
 111                     if let TraitTyParamBound(ref poly_trait_ref, _) = *ty_param_bound {
 112                         let super_trait_node_id = cx.tcx
 113                             .hir
 114                             .as_local_node_id(poly_trait_ref.trait_ref.path.def.def_id())
 115                             .expect("the DefId is local, the NodeId should be available");
 116                         let super_trait = cx.tcx.hir.expect_item(super_trait_node_id);
 117                         fill_trait_set(super_trait, set, cx);
 118                     }
 119                 }
 120             }
 121         }
 122     }
 123
 124     if cx.access_levels.is_exported(visited_trait.id) && trait_items.iter().any(|i| is_named_self(cx, i, "len")) {
 125         let mut current_and_super_traits = HashSet::new();
 126         fill_trait_set(visited_trait, &mut current_and_super_traits, cx);
 127
 128         let is_empty_method_found = current_and_super_traits
 129             .iter()
 130             .flat_map(|i| match i.node {
 131                 ItemTrait(.., ref trait_items) => trait_items.iter(),
 132                 _ => bug!("should only handle traits"),
 133             })
 134             .any(|i| is_named_self(cx, i, "is_empty"));
 135
 136         if !is_empty_method_found {
 137             span_lint(
 138                 cx,
 139                 LEN_WITHOUT_IS_EMPTY,
 140                 visited_trait.span,
 141                 &format!(
 142                     "trait `{}` has a `len` method but no (possibly inherited) `is_empty` method",
 143                     visited_trait.name
 144                 ),
 145             );
 146         }
 147     }
 148 }
 149
 150 fn check_impl_items(cx: &LateContext, item: &Item, impl_items: &[ImplItemRef]) {
 151     fn is_named_self(cx: &LateContext, item: &ImplItemRef, name: &str) -> bool {
 152         item.name == name &&
 153             if let AssociatedItemKind::Method { has_self } = item.kind {
 154                 has_self &&
 155                     {
 156                         let did = cx.tcx.hir.local_def_id(item.id.node_id);
 157                         cx.tcx.fn_sig(did).inputs().skip_binder().len() == 1
 158                     }
 159             } else {
 160                 false
 161             }
 162     }
 163
 164     let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
 165         if cx.access_levels.is_exported(is_empty.id.node_id) {
 166             return;
 167         } else {
 168             "a private"
 169         }
 170     } else {
 171         "no corresponding"
 172     };
 173
 174     if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
 175         if cx.access_levels.is_exported(i.id.node_id) {
 176             let def_id = cx.tcx.hir.local_def_id(item.id);
 177             let ty = cx.tcx.type_of(def_id);
 178
 179             span_lint(
 180                 cx,
 181                 LEN_WITHOUT_IS_EMPTY,
 182                 item.span,
 183                 &format!("item `{}` has a public `len` method but {} `is_empty` method", ty, is_empty),
 184             );
 185         }
 186     }
 187 }
 188
 189 fn check_cmp(cx: &LateContext, span: Span, left: &Expr, right: &Expr, op: &str) {
 190     // check if we are in an is_empty() method
 191     if let Some(name) = get_item_name(cx, left) {
 192         if name == "is_empty" {
 193             return;
 194         }
 195     }
 196     match (&left.node, &right.node) {
 197         (&ExprLit(ref lit), &ExprMethodCall(ref method_path, _, ref args)) |
 198         (&ExprMethodCall(ref method_path, _, ref args), &ExprLit(ref lit)) => {
 199             check_len_zero(cx, span, method_path.name, args, lit, op)
 200         },
 201         _ => (),
 202     }
 203 }
 204
 205 fn check_len_zero(cx: &LateContext, span: Span, name: Name, args: &[Expr], lit: &Lit, op: &str) {
 206     if let Spanned { node: LitKind::Int(0, _), .. } = *lit {
 207         if name == "len" && args.len() == 1 && has_is_empty(cx, &args[0]) {
 208             span_lint_and_sugg(
 209                 cx,
 210                 LEN_ZERO,
 211                 span,
 212                 "length comparison to zero",
 213                 "using `is_empty` is more concise",
 214                 format!("{}{}.is_empty()", op, snippet(cx, args[0].span, "_")),
 215             );
 216         }
 217     }
 218 }
 219
 220 /// Check if this type has an `is_empty` method.
 221 fn has_is_empty(cx: &LateContext, expr: &Expr) -> bool {
 222     /// Get an `AssociatedItem` and return true if it matches `is_empty(self)`.
 223     fn is_is_empty(cx: &LateContext, item: &ty::AssociatedItem) -> bool {
 224         if let ty::AssociatedKind::Method = item.kind {
 225             if item.name == "is_empty" {
 226                 let sig = cx.tcx.fn_sig(item.def_id);
 227                 let ty = sig.skip_binder();
 228                 ty.inputs().len() == 1
 229             } else {
 230                 false
 231             }
 232         } else {
 233             false
 234         }
 235     }
 236
 237     /// Check the inherent impl's items for an `is_empty(self)` method.
 238     fn has_is_empty_impl(cx: &LateContext, id: DefId) -> bool {
 239         cx.tcx.inherent_impls(id).iter().any(|imp| {
 240             cx.tcx.associated_items(*imp).any(
 241                 |item| is_is_empty(cx, &item),
 242             )
 243         })
 244     }
 245
 246     let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
 247     match ty.sty {
 248         ty::TyDynamic(..) => {
 249             cx.tcx
 250                 .associated_items(ty.ty_to_def_id().expect("trait impl not found"))
 251                 .any(|item| is_is_empty(cx, &item))
 252         },
 253         ty::TyProjection(_) => {
 254             ty.ty_to_def_id().map_or(
 255                 false,
 256                 |id| has_is_empty_impl(cx, id),
 257             )
 258         },
 259         ty::TyAdt(id, _) => has_is_empty_impl(cx, id.did),
 260         ty::TyArray(..) | ty::TySlice(..) | ty::TyStr => true,
 261         _ => false,
 262     }
 263 }