clippy_lints/src/new_without_default.rs

   1 use rustc::hir::def_id::DefId;
   2 use rustc::hir;
   3 use rustc::lint::*;
   4 use rustc::ty::{self, Ty};
   5 use syntax::codemap::Span;
   6 use utils::paths;
   7 use utils::{get_trait_def_id, implements_trait, in_external_macro, return_ty, same_tys, span_lint_and_then};
   8 use utils::sugg::DiagnosticBuilderExt;
   9
  10 /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  11 /// implementation of
  12 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  13 ///
  14 /// **Why is this bad?** The user might expect to be able to use
  15 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  16 /// type can be constructed without arguments.
  17 ///
  18 /// **Known problems:** Hopefully none.
  19 ///
  20 /// **Example:**
  21 ///
  22 /// ```rust,ignore
  23 /// struct Foo(Bar);
  24 ///
  25 /// impl Foo {
  26 ///     fn new() -> Self {
  27 ///         Foo(Bar::new())
  28 ///     }
  29 /// }
  30 /// ```
  31 ///
  32 /// Instead, use:
  33 ///
  34 /// ```rust
  35 /// struct Foo(Bar);
  36 ///
  37 /// impl Default for Foo {
  38 ///     fn default() -> Self {
  39 ///         Foo(Bar::new())
  40 ///     }
  41 /// }
  42 /// ```
  43 ///
  44 /// You can also have `new()` call `Default::default()`.
  45 declare_lint! {
  46     pub NEW_WITHOUT_DEFAULT,
  47     Warn,
  48     "`fn new() -> Self` method without `Default` implementation"
  49 }
  50
  51 /// **What it does:** Checks for types with a `fn new() -> Self` method
  52 /// and no implementation of
  53 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html),
  54 /// where the `Default` can be derived by `#[derive(Default)]`.
  55 ///
  56 /// **Why is this bad?** The user might expect to be able to use
  57 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  58 /// type can be constructed without arguments.
  59 ///
  60 /// **Known problems:** Hopefully none.
  61 ///
  62 /// **Example:**
  63 ///
  64 /// ```rust,ignore
  65 /// struct Foo;
  66 ///
  67 /// impl Foo {
  68 ///     fn new() -> Self {
  69 ///         Foo
  70 ///     }
  71 /// }
  72 /// ```
  73 ///
  74 /// Just prepend `#[derive(Default)]` before the `struct` definition.
  75 declare_lint! {
  76     pub NEW_WITHOUT_DEFAULT_DERIVE,
  77     Warn,
  78     "`fn new() -> Self` without `#[derive]`able `Default` implementation"
  79 }
  80
  81 #[derive(Copy, Clone)]
  82 pub struct NewWithoutDefault;
  83
  84 impl LintPass for NewWithoutDefault {
  85     fn get_lints(&self) -> LintArray {
  86         lint_array!(NEW_WITHOUT_DEFAULT, NEW_WITHOUT_DEFAULT_DERIVE)
  87     }
  88 }
  89
  90 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
  91     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item) {
  92         if let hir::ItemImpl(_, _, _, _, None, _, ref items) = item.node {
  93             for assoc_item in items {
  94                 if let hir::AssociatedItemKind::Method { has_self: false } = assoc_item.kind {
  95                     let impl_item = cx.tcx.hir.impl_item(assoc_item.id);
  96                     if in_external_macro(cx, impl_item.span) {
  97                         return;
  98                     }
  99                     if let hir::ImplItemKind::Method(ref sig, _) = impl_item.node {
 100                         let name = impl_item.name;
 101                         let span = impl_item.span;
 102                         let id = impl_item.id;
 103                         let decl = &sig.decl;
 104                         if sig.constness == hir::Constness::Const {
 105                             // can't be implemented by default
 106                             return;
 107                         }
 108                         if !impl_item.generics
 109                             .ty_params
 110                             .is_empty()
 111                         {
 112                             // when the result of `new()` depends on a type parameter we should not require
 113                             // an
 114                             // impl of `Default`
 115                             return;
 116                         }
 117                         if decl.inputs.is_empty() && name == "new" && cx.access_levels.is_reachable(id) {
 118                             let self_ty = cx.tcx
 119                                 .type_of(cx.tcx.hir.local_def_id(cx.tcx.hir.get_parent(id)));
 120                             if_chain! {
 121                                 if same_tys(cx, self_ty, return_ty(cx, id));
 122                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 123                                 if !implements_trait(cx, self_ty, default_trait_id, &[]);
 124                                 then {
 125                                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 126                                         span_lint_and_then(
 127                                             cx,
 128                                             NEW_WITHOUT_DEFAULT_DERIVE,
 129                                             span,
 130                                             &format!("you should consider deriving a `Default` implementation for `{}`", self_ty),
 131                                             |db| {
 132                                                 db.suggest_item_with_attr(cx, sp, "try this", "#[derive(Default)]");
 133                                             });
 134                                     } else {
 135                                         span_lint_and_then(
 136                                             cx,
 137                                             NEW_WITHOUT_DEFAULT,
 138                                             span,
 139                                             &format!("you should consider adding a `Default` implementation for `{}`", self_ty),
 140                                             |db| {
 141                                                 db.suggest_prepend_item(
 142                                                     cx,
 143                                                     span,
 144                                                     "try this",
 145                                                     &create_new_without_default_suggest_msg(self_ty),
 146                                                 );
 147                                             },
 148                                         );
 149                                     }
 150                                 }
 151                             }
 152                         }
 153                     }
 154                 }
 155             }
 156         }
 157     }
 158 }
 159
 160 fn create_new_without_default_suggest_msg(ty: Ty) -> String {
 161     #[rustfmt_skip]
 162     format!(
 163 "impl Default for {} {{
 164     fn default() -> Self {{
 165         Self::new()
 166     }}
 167 }}", ty)
 168 }
 169
 170 fn can_derive_default<'t, 'c>(ty: Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 171     match ty.sty {
 172         ty::TyAdt(adt_def, substs) if adt_def.is_struct() => {
 173             for field in adt_def.all_fields() {
 174                 let f_ty = field.ty(cx.tcx, substs);
 175                 if !implements_trait(cx, f_ty, default_trait_id, &[]) {
 176                     return None;
 177                 }
 178             }
 179             Some(cx.tcx.def_span(adt_def.did))
 180         },
 181         _ => None,
 182     }
 183 }