clippy_lints/src/new_without_default.rs

   1 use crate::utils::paths;
   2 use crate::utils::sugg::DiagnosticBuilderExt;
   3 use crate::utils::{get_trait_def_id, implements_trait, return_ty, same_tys, span_lint_hir_and_then};
   4 use if_chain::if_chain;
   5 use rustc::hir;
   6 use rustc::hir::def_id::DefId;
   7 use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintContext, LintPass};
   8 use rustc::ty::{self, Ty};
   9 use rustc::util::nodemap::HirIdSet;
  10 use rustc::{declare_tool_lint, impl_lint_pass};
  11 use rustc_errors::Applicability;
  12 use syntax::source_map::Span;
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  16     /// implementation of
  17     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  18     ///
  19     /// It detects both the case when a manual
  20     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  21     /// implementation is required and also when it can be created with
  22     /// `#[derive(Default)]`
  23     ///
  24     /// **Why is this bad?** The user might expect to be able to use
  25     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  26     /// type can be constructed without arguments.
  27     ///
  28     /// **Known problems:** Hopefully none.
  29     ///
  30     /// **Example:**
  31     ///
  32     /// ```ignore
  33     /// struct Foo(Bar);
  34     ///
  35     /// impl Foo {
  36     ///     fn new() -> Self {
  37     ///         Foo(Bar::new())
  38     ///     }
  39     /// }
  40     /// ```
  41     ///
  42     /// Instead, use:
  43     ///
  44     /// ```ignore
  45     /// struct Foo(Bar);
  46     ///
  47     /// impl Default for Foo {
  48     ///     fn default() -> Self {
  49     ///         Foo(Bar::new())
  50     ///     }
  51     /// }
  52     /// ```
  53     ///
  54     /// Or, if
  55     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  56     /// can be derived by `#[derive(Default)]`:
  57     ///
  58     /// ```rust
  59     /// struct Foo;
  60     ///
  61     /// impl Foo {
  62     ///     fn new() -> Self {
  63     ///         Foo
  64     ///     }
  65     /// }
  66     /// ```
  67     ///
  68     /// Instead, use:
  69     ///
  70     /// ```rust
  71     /// #[derive(Default)]
  72     /// struct Foo;
  73     ///
  74     /// impl Foo {
  75     ///     fn new() -> Self {
  76     ///         Foo
  77     ///     }
  78     /// }
  79     /// ```
  80     ///
  81     /// You can also have `new()` call `Default::default()`.
  82     pub NEW_WITHOUT_DEFAULT,
  83     style,
  84     "`fn new() -> Self` method without `Default` implementation"
  85 }
  86
  87 #[derive(Clone, Default)]
  88 pub struct NewWithoutDefault {
  89     impling_types: Option<HirIdSet>,
  90 }
  91
  92 impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);
  93
  94 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
  95     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item) {
  96         if let hir::ItemKind::Impl(_, _, _, _, None, _, ref items) = item.node {
  97             for assoc_item in items {
  98                 if let hir::AssocItemKind::Method { has_self: false } = assoc_item.kind {
  99                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
 100                     if in_external_macro(cx.sess(), impl_item.span) {
 101                         return;
 102                     }
 103                     if let hir::ImplItemKind::Method(ref sig, _) = impl_item.node {
 104                         let name = impl_item.ident.name;
 105                         let id = impl_item.hir_id;
 106                         if sig.header.constness == hir::Constness::Const {
 107                             // can't be implemented by default
 108                             return;
 109                         }
 110                         if sig.header.unsafety == hir::Unsafety::Unsafe {
 111                             // can't be implemented for unsafe new
 112                             return;
 113                         }
 114                         if impl_item.generics.params.iter().any(|gen| match gen.kind {
 115                             hir::GenericParamKind::Type { .. } => true,
 116                             _ => false,
 117                         }) {
 118                             // when the result of `new()` depends on a type parameter we should not require
 119                             // an
 120                             // impl of `Default`
 121                             return;
 122                         }
 123                         if sig.decl.inputs.is_empty() && name == sym!(new) && cx.access_levels.is_reachable(id) {
 124                             let self_did = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
 125                             let self_ty = cx.tcx.type_of(self_did);
 126                             if_chain! {
 127                                 if same_tys(cx, self_ty, return_ty(cx, id));
 128                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 129                                 then {
 130                                     if self.impling_types.is_none() {
 131                                         let mut impls = HirIdSet::default();
 132                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 133                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 134                                                 if let Some(hir_id) = cx.tcx.hir().as_local_hir_id(ty_def.did) {
 135                                                     impls.insert(hir_id);
 136                                                 }
 137                                             }
 138                                         });
 139                                         self.impling_types = Some(impls);
 140                                     }
 141
 142                                     // Check if a Default implementation exists for the Self type, regardless of
 143                                     // generics
 144                                     if_chain! {
 145                                         if let Some(ref impling_types) = self.impling_types;
 146                                         if let Some(self_def) = cx.tcx.type_of(self_did).ty_adt_def();
 147                                         if self_def.did.is_local();
 148                                         then {
 149                                             let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_def.did.to_local());
 150                                             if impling_types.contains(&self_id) {
 151                                                 return;
 152                                             }
 153                                         }
 154                                     }
 155
 156                                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 157                                         span_lint_hir_and_then(
 158                                             cx,
 159                                             NEW_WITHOUT_DEFAULT,
 160                                             id,
 161                                             impl_item.span,
 162                                             &format!(
 163                                                 "you should consider deriving a `Default` implementation for `{}`",
 164                                                 self_ty
 165                                             ),
 166                                             |db| {
 167                                                 db.suggest_item_with_attr(
 168                                                     cx,
 169                                                     sp,
 170                                                     "try this",
 171                                                     "#[derive(Default)]",
 172                                                     Applicability::MaybeIncorrect,
 173                                                 );
 174                                             });
 175                                     } else {
 176                                         span_lint_hir_and_then(
 177                                             cx,
 178                                             NEW_WITHOUT_DEFAULT,
 179                                             id,
 180                                             impl_item.span,
 181                                             &format!(
 182                                                 "you should consider adding a `Default` implementation for `{}`",
 183                                                 self_ty
 184                                             ),
 185                                             |db| {
 186                                                 db.suggest_prepend_item(
 187                                                     cx,
 188                                                     item.span,
 189                                                     "try this",
 190                                                     &create_new_without_default_suggest_msg(self_ty),
 191                                                     Applicability::MaybeIncorrect,
 192                                                 );
 193                                             },
 194                                         );
 195                                     }
 196                                 }
 197                             }
 198                         }
 199                     }
 200                 }
 201             }
 202         }
 203     }
 204 }
 205
 206 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 207     #[rustfmt::skip]
 208     format!(
 209 "impl Default for {} {{
 210     fn default() -> Self {{
 211         Self::new()
 212     }}
 213 }}", ty)
 214 }
 215
 216 fn can_derive_default<'t, 'c>(ty: Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 217     match ty.sty {
 218         ty::Adt(adt_def, substs) if adt_def.is_struct() => {
 219             for field in adt_def.all_fields() {
 220                 let f_ty = field.ty(cx.tcx, substs);
 221                 if !implements_trait(cx, f_ty, default_trait_id, &[]) {
 222                     return None;
 223                 }
 224             }
 225             Some(cx.tcx.def_span(adt_def.did))
 226         },
 227         _ => None,
 228     }
 229 }