src/tools/clippy/clippy_lints/src/derive.rs

   1 use rustc::lint::*;
   2 use rustc::ty::{self, Ty};
   3 use rustc::hir::*;
   4 use syntax::codemap::Span;
   5 use utils::paths;
   6 use utils::{is_automatically_derived, is_copy, match_path, span_lint_and_then};
   7
   8 /// **What it does:** Checks for deriving `Hash` but implementing `PartialEq`
   9 /// explicitly.
  10 ///
  11 /// **Why is this bad?** The implementation of these traits must agree (for
  12 /// example for use with `HashMap`) so it’s probably a bad idea to use a
  13 /// default-generated `Hash` implementation with an explicitly defined
  14 /// `PartialEq`. In particular, the following must hold for any type:
  15 ///
  16 /// ```rust
  17 /// k1 == k2 ⇒ hash(k1) == hash(k2)
  18 /// ```
  19 ///
  20 /// **Known problems:** None.
  21 ///
  22 /// **Example:**
  23 /// ```rust
  24 /// #[derive(Hash)]
  25 /// struct Foo;
  26 ///
  27 /// impl PartialEq for Foo {
  28 ///     ...
  29 /// }
  30 /// ```
  31 declare_lint! {
  32     pub DERIVE_HASH_XOR_EQ,
  33     Warn,
  34     "deriving `Hash` but implementing `PartialEq` explicitly"
  35 }
  36
  37 /// **What it does:** Checks for explicit `Clone` implementations for `Copy`
  38 /// types.
  39 ///
  40 /// **Why is this bad?** To avoid surprising behaviour, these traits should
  41 /// agree and the behaviour of `Copy` cannot be overridden. In almost all
  42 /// situations a `Copy` type should have a `Clone` implementation that does
  43 /// nothing more than copy the object, which is what `#[derive(Copy, Clone)]`
  44 /// gets you.
  45 ///
  46 /// **Known problems:** None.
  47 ///
  48 /// **Example:**
  49 /// ```rust
  50 /// #[derive(Copy)]
  51 /// struct Foo;
  52 ///
  53 /// impl Clone for Foo {
  54 ///     ..
  55 /// }
  56 /// ```
  57 declare_lint! {
  58     pub EXPL_IMPL_CLONE_ON_COPY,
  59     Warn,
  60     "implementing `Clone` explicitly on `Copy` types"
  61 }
  62
  63 pub struct Derive;
  64
  65 impl LintPass for Derive {
  66     fn get_lints(&self) -> LintArray {
  67         lint_array!(EXPL_IMPL_CLONE_ON_COPY, DERIVE_HASH_XOR_EQ)
  68     }
  69 }
  70
  71 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Derive {
  72     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
  73         if let ItemImpl(_, _, _, _, Some(ref trait_ref), _, _) = item.node {
  74             let ty = cx.tcx.type_of(cx.tcx.hir.local_def_id(item.id));
  75             let is_automatically_derived = is_automatically_derived(&*item.attrs);
  76
  77             check_hash_peq(cx, item.span, trait_ref, ty, is_automatically_derived);
  78
  79             if !is_automatically_derived {
  80                 check_copy_clone(cx, item, trait_ref, ty);
  81             }
  82         }
  83     }
  84 }
  85
  86 /// Implementation of the `DERIVE_HASH_XOR_EQ` lint.
  87 fn check_hash_peq<'a, 'tcx>(
  88     cx: &LateContext<'a, 'tcx>,
  89     span: Span,
  90     trait_ref: &TraitRef,
  91     ty: Ty<'tcx>,
  92     hash_is_automatically_derived: bool,
  93 ) {
  94     if_chain! {
  95         if match_path(&trait_ref.path, &paths::HASH);
  96         if let Some(peq_trait_def_id) = cx.tcx.lang_items().eq_trait();
  97         then {
  98             // Look for the PartialEq implementations for `ty`
  99             cx.tcx.for_each_relevant_impl(peq_trait_def_id, ty, |impl_id| {
 100                 let peq_is_automatically_derived = is_automatically_derived(&cx.tcx.get_attrs(impl_id));
 101
 102                 if peq_is_automatically_derived == hash_is_automatically_derived {
 103                     return;
 104                 }
 105
 106                 let trait_ref = cx.tcx.impl_trait_ref(impl_id).expect("must be a trait implementation");
 107
 108                 // Only care about `impl PartialEq<Foo> for Foo`
 109                 // For `impl PartialEq<B> for A, input_types is [A, B]
 110                 if trait_ref.substs.type_at(1) == ty {
 111                     let mess = if peq_is_automatically_derived {
 112                         "you are implementing `Hash` explicitly but have derived `PartialEq`"
 113                     } else {
 114                         "you are deriving `Hash` but have implemented `PartialEq` explicitly"
 115                     };
 116
 117                     span_lint_and_then(
 118                         cx, DERIVE_HASH_XOR_EQ, span,
 119                         mess,
 120                         |db| {
 121                         if let Some(node_id) = cx.tcx.hir.as_local_node_id(impl_id) {
 122                             db.span_note(
 123                                 cx.tcx.hir.span(node_id),
 124                                 "`PartialEq` implemented here"
 125                             );
 126                         }
 127                     });
 128                 }
 129             });
 130         }
 131     }
 132 }
 133
 134 /// Implementation of the `EXPL_IMPL_CLONE_ON_COPY` lint.
 135 fn check_copy_clone<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, item: &Item, trait_ref: &TraitRef, ty: Ty<'tcx>) {
 136     if match_path(&trait_ref.path, &paths::CLONE_TRAIT) {
 137         if !is_copy(cx, ty) {
 138             return;
 139         }
 140
 141         match ty.sty {
 142             ty::TyAdt(def, _) if def.is_union() => return,
 143
 144             // Some types are not Clone by default but could be cloned “by hand” if necessary
 145             ty::TyAdt(def, substs) => for variant in &def.variants {
 146                 for field in &variant.fields {
 147                     if let ty::TyFnDef(..) = field.ty(cx.tcx, substs).sty {
 148                         return;
 149                     }
 150                 }
 151             },
 152             _ => (),
 153         }
 154
 155         span_lint_and_then(
 156             cx,
 157             EXPL_IMPL_CLONE_ON_COPY,
 158             item.span,
 159             "you are implementing `Clone` explicitly on a `Copy` type",
 160             |db| {
 161                 db.span_note(item.span, "consider deriving `Clone` or removing `Copy`");
 162             },
 163         );
 164     }
 165 }