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::lint::in_external_macro;
   6 use rustc::ty::{self, Ty};
   7 use rustc_errors::Applicability;
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_hir::HirIdSet;
  11 use rustc_lint::{LateContext, LateLintPass, LintContext};
  12 use rustc_session::{declare_tool_lint, impl_lint_pass};
  13 use rustc_span::source_map::Span;
  14
  15 declare_clippy_lint! {
  16     /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  17     /// implementation of
  18     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  19     ///
  20     /// It detects both the case when a manual
  21     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  22     /// implementation is required and also when it can be created with
  23     /// `#[derive(Default)]`
  24     ///
  25     /// **Why is this bad?** The user might expect to be able to use
  26     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  27     /// type can be constructed without arguments.
  28     ///
  29     /// **Known problems:** Hopefully none.
  30     ///
  31     /// **Example:**
  32     ///
  33     /// ```ignore
  34     /// struct Foo(Bar);
  35     ///
  36     /// impl Foo {
  37     ///     fn new() -> Self {
  38     ///         Foo(Bar::new())
  39     ///     }
  40     /// }
  41     /// ```
  42     ///
  43     /// Instead, use:
  44     ///
  45     /// ```ignore
  46     /// struct Foo(Bar);
  47     ///
  48     /// impl Default for Foo {
  49     ///     fn default() -> Self {
  50     ///         Foo(Bar::new())
  51     ///     }
  52     /// }
  53     /// ```
  54     ///
  55     /// Or, if
  56     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  57     /// can be derived by `#[derive(Default)]`:
  58     ///
  59     /// ```rust
  60     /// struct Foo;
  61     ///
  62     /// impl Foo {
  63     ///     fn new() -> Self {
  64     ///         Foo
  65     ///     }
  66     /// }
  67     /// ```
  68     ///
  69     /// Instead, use:
  70     ///
  71     /// ```rust
  72     /// #[derive(Default)]
  73     /// struct Foo;
  74     ///
  75     /// impl Foo {
  76     ///     fn new() -> Self {
  77     ///         Foo
  78     ///     }
  79     /// }
  80     /// ```
  81     ///
  82     /// You can also have `new()` call `Default::default()`.
  83     pub NEW_WITHOUT_DEFAULT,
  84     style,
  85     "`fn new() -> Self` method without `Default` implementation"
  86 }
  87
  88 #[derive(Clone, Default)]
  89 pub struct NewWithoutDefault {
  90     impling_types: Option<HirIdSet>,
  91 }
  92
  93 impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);
  94
  95 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
  96     #[allow(clippy::too_many_lines)]
  97     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item<'_>) {
  98         if let hir::ItemKind::Impl {
  99             of_trait: None, items, ..
 100         } = item.kind
 101         {
 102             for assoc_item in items {
 103                 if let hir::AssocItemKind::Method { has_self: false } = assoc_item.kind {
 104                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
 105                     if in_external_macro(cx.sess(), impl_item.span) {
 106                         return;
 107                     }
 108                     if let hir::ImplItemKind::Method(ref sig, _) = impl_item.kind {
 109                         let name = impl_item.ident.name;
 110                         let id = impl_item.hir_id;
 111                         if sig.header.constness == hir::Constness::Const {
 112                             // can't be implemented by default
 113                             return;
 114                         }
 115                         if sig.header.unsafety == hir::Unsafety::Unsafe {
 116                             // can't be implemented for unsafe new
 117                             return;
 118                         }
 119                         if impl_item.generics.params.iter().any(|gen| match gen.kind {
 120                             hir::GenericParamKind::Type { .. } => true,
 121                             _ => false,
 122                         }) {
 123                             // when the result of `new()` depends on a type parameter we should not require
 124                             // an
 125                             // impl of `Default`
 126                             return;
 127                         }
 128                         if sig.decl.inputs.is_empty() && name == sym!(new) && cx.access_levels.is_reachable(id) {
 129                             let self_did = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
 130                             let self_ty = cx.tcx.type_of(self_did);
 131                             if_chain! {
 132                                 if same_tys(cx, self_ty, return_ty(cx, id));
 133                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 134                                 then {
 135                                     if self.impling_types.is_none() {
 136                                         let mut impls = HirIdSet::default();
 137                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 138                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 139                                                 if let Some(hir_id) = cx.tcx.hir().as_local_hir_id(ty_def.did) {
 140                                                     impls.insert(hir_id);
 141                                                 }
 142                                             }
 143                                         });
 144                                         self.impling_types = Some(impls);
 145                                     }
 146
 147                                     // Check if a Default implementation exists for the Self type, regardless of
 148                                     // generics
 149                                     if_chain! {
 150                                         if let Some(ref impling_types) = self.impling_types;
 151                                         if let Some(self_def) = cx.tcx.type_of(self_did).ty_adt_def();
 152                                         if self_def.did.is_local();
 153                                         then {
 154                                             let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_def.did.to_local());
 155                                             if impling_types.contains(&self_id) {
 156                                                 return;
 157                                             }
 158                                         }
 159                                     }
 160
 161                                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 162                                         span_lint_hir_and_then(
 163                                             cx,
 164                                             NEW_WITHOUT_DEFAULT,
 165                                             id,
 166                                             impl_item.span,
 167                                             &format!(
 168                                                 "you should consider deriving a `Default` implementation for `{}`",
 169                                                 self_ty
 170                                             ),
 171                                             |db| {
 172                                                 db.suggest_item_with_attr(
 173                                                     cx,
 174                                                     sp,
 175                                                     "try this",
 176                                                     "#[derive(Default)]",
 177                                                     Applicability::MaybeIncorrect,
 178                                                 );
 179                                             });
 180                                     } else {
 181                                         span_lint_hir_and_then(
 182                                             cx,
 183                                             NEW_WITHOUT_DEFAULT,
 184                                             id,
 185                                             impl_item.span,
 186                                             &format!(
 187                                                 "you should consider adding a `Default` implementation for `{}`",
 188                                                 self_ty
 189                                             ),
 190                                             |db| {
 191                                                 db.suggest_prepend_item(
 192                                                     cx,
 193                                                     item.span,
 194                                                     "try this",
 195                                                     &create_new_without_default_suggest_msg(self_ty),
 196                                                     Applicability::MaybeIncorrect,
 197                                                 );
 198                                             },
 199                                         );
 200                                     }
 201                                 }
 202                             }
 203                         }
 204                     }
 205                 }
 206             }
 207         }
 208     }
 209 }
 210
 211 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 212     #[rustfmt::skip]
 213     format!(
 214 "impl Default for {} {{
 215     fn default() -> Self {{
 216         Self::new()
 217     }}
 218 }}", ty)
 219 }
 220
 221 fn can_derive_default<'t, 'c>(ty: Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 222     match ty.kind {
 223         ty::Adt(adt_def, substs) if adt_def.is_struct() => {
 224             for field in adt_def.all_fields() {
 225                 let f_ty = field.ty(cx.tcx, substs);
 226                 if !implements_trait(cx, f_ty, default_trait_id, &[]) {
 227                     return None;
 228                 }
 229             }
 230             Some(cx.tcx.def_span(adt_def.did))
 231         },
 232         _ => None,
 233     }
 234 }