clippy_lints/src/new_without_default.rs

   1 use rustc::hir::def_id::DefId;
   2 use rustc::hir;
   3 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass, in_external_macro, LintContext};
   4 use rustc::{declare_tool_lint, lint_array};
   5 use if_chain::if_chain;
   6 use rustc::ty::{self, Ty};
   7 use syntax::source_map::Span;
   8 use crate::utils::paths;
   9 use crate::utils::{get_trait_def_id, implements_trait, return_ty, same_tys, span_lint_and_then};
  10 use crate::utils::sugg::DiagnosticBuilderExt;
  11
  12 /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  13 /// implementation of
  14 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  15 ///
  16 /// **Why is this bad?** The user might expect to be able to use
  17 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  18 /// type can be constructed without arguments.
  19 ///
  20 /// **Known problems:** Hopefully none.
  21 ///
  22 /// **Example:**
  23 ///
  24 /// ```rust
  25 /// struct Foo(Bar);
  26 ///
  27 /// impl Foo {
  28 ///     fn new() -> Self {
  29 ///         Foo(Bar::new())
  30 ///     }
  31 /// }
  32 /// ```
  33 ///
  34 /// Instead, use:
  35 ///
  36 /// ```rust
  37 /// struct Foo(Bar);
  38 ///
  39 /// impl Default for Foo {
  40 ///     fn default() -> Self {
  41 ///         Foo(Bar::new())
  42 ///     }
  43 /// }
  44 /// ```
  45 ///
  46 /// You can also have `new()` call `Default::default()`.
  47 declare_clippy_lint! {
  48     pub NEW_WITHOUT_DEFAULT,
  49     style,
  50     "`fn new() -> Self` method without `Default` implementation"
  51 }
  52
  53 /// **What it does:** Checks for types with a `fn new() -> Self` method
  54 /// and no implementation of
  55 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html),
  56 /// where the `Default` can be derived by `#[derive(Default)]`.
  57 ///
  58 /// **Why is this bad?** The user might expect to be able to use
  59 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  60 /// type can be constructed without arguments.
  61 ///
  62 /// **Known problems:** Hopefully none.
  63 ///
  64 /// **Example:**
  65 ///
  66 /// ```rust
  67 /// struct Foo;
  68 ///
  69 /// impl Foo {
  70 ///     fn new() -> Self {
  71 ///         Foo
  72 ///     }
  73 /// }
  74 /// ```
  75 ///
  76 /// Just prepend `#[derive(Default)]` before the `struct` definition.
  77 declare_clippy_lint! {
  78     pub NEW_WITHOUT_DEFAULT_DERIVE,
  79     style,
  80     "`fn new() -> Self` without `#[derive]`able `Default` implementation"
  81 }
  82
  83 #[derive(Copy, Clone)]
  84 pub struct NewWithoutDefault;
  85
  86 impl LintPass for NewWithoutDefault {
  87     fn get_lints(&self) -> LintArray {
  88         lint_array!(NEW_WITHOUT_DEFAULT, NEW_WITHOUT_DEFAULT_DERIVE)
  89     }
  90 }
  91
  92 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
  93     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item) {
  94         if let hir::ItemKind::Impl(_, _, _, _, None, _, ref items) = item.node {
  95             for assoc_item in items {
  96                 if let hir::AssociatedItemKind::Method { has_self: false } = assoc_item.kind {
  97                     let impl_item = cx.tcx.hir.impl_item(assoc_item.id);
  98                     if in_external_macro(cx.sess(), impl_item.span) {
  99                         return;
 100                     }
 101                     if let hir::ImplItemKind::Method(ref sig, _) = impl_item.node {
 102                         let name = impl_item.ident.name;
 103                         let id = impl_item.id;
 104                         if sig.header.constness == hir::Constness::Const {
 105                             // can't be implemented by default
 106                             return;
 107                         }
 108                         if impl_item.generics.params.iter().any(|gen| match gen.kind {
 109                             hir::GenericParamKind::Type { .. } => true,
 110                             _ => false
 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 sig.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                                             impl_item.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                                             impl_item.span,
 139                                             &format!("you should consider adding a `Default` implementation for `{}`", self_ty),
 140                                             |db| {
 141                                                 db.suggest_prepend_item(
 142                                                     cx,
 143                                                     item.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::Adt(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 }