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_node_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::NodeSet;
  10 use rustc::{declare_tool_lint, lint_array};
  11 use rustc_errors::Applicability;
  12 use syntax::source_map::Span;
  13
  14 /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  15 /// implementation of
  16 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  17 ///
  18 /// It detects both the case when a manual
  19 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  20 /// implementation is required and also when it can be created with
  21 /// `#[derive(Default)]`
  22 ///
  23 /// **Why is this bad?** The user might expect to be able to use
  24 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  25 /// type can be constructed without arguments.
  26 ///
  27 /// **Known problems:** Hopefully none.
  28 ///
  29 /// **Example:**
  30 ///
  31 /// ```rust
  32 /// struct Foo(Bar);
  33 ///
  34 /// impl Foo {
  35 ///     fn new() -> Self {
  36 ///         Foo(Bar::new())
  37 ///     }
  38 /// }
  39 /// ```
  40 ///
  41 /// Instead, use:
  42 ///
  43 /// ```rust
  44 /// struct Foo(Bar);
  45 ///
  46 /// impl Default for Foo {
  47 ///     fn default() -> Self {
  48 ///         Foo(Bar::new())
  49 ///     }
  50 /// }
  51 /// ```
  52 ///
  53 /// Or, if
  54 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  55 /// can be derived by `#[derive(Default)]`:
  56 ///
  57 /// ```rust
  58 /// struct Foo;
  59 ///
  60 /// impl Foo {
  61 ///     fn new() -> Self {
  62 ///         Foo
  63 ///     }
  64 /// }
  65 /// ```
  66 ///
  67 /// Instead, use:
  68 ///
  69 /// ```rust
  70 /// #[derive(Default)]
  71 /// struct Foo;
  72 ///
  73 /// impl Foo {
  74 ///     fn new() -> Self {
  75 ///         Foo
  76 ///     }
  77 /// }
  78 /// ```
  79 ///
  80 /// You can also have `new()` call `Default::default()`.
  81 declare_clippy_lint! {
  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<NodeSet>,
  90 }
  91
  92 impl LintPass for NewWithoutDefault {
  93     fn get_lints(&self) -> LintArray {
  94         lint_array!(NEW_WITHOUT_DEFAULT)
  95     }
  96
  97     fn name(&self) -> &'static str {
  98         "NewWithoutDefault"
  99     }
 100 }
 101
 102 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
 103     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item) {
 104         if let hir::ItemKind::Impl(_, _, _, _, None, _, ref items) = item.node {
 105             for assoc_item in items {
 106                 if let hir::AssociatedItemKind::Method { has_self: false } = assoc_item.kind {
 107                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
 108                     if in_external_macro(cx.sess(), impl_item.span) {
 109                         return;
 110                     }
 111                     if let hir::ImplItemKind::Method(ref sig, _) = impl_item.node {
 112                         let name = impl_item.ident.name;
 113                         let id = impl_item.hir_id;
 114                         let node_id = cx.tcx.hir().hir_to_node_id(id);
 115                         if sig.header.constness == hir::Constness::Const {
 116                             // can't be implemented by default
 117                             return;
 118                         }
 119                         if sig.header.unsafety == hir::Unsafety::Unsafe {
 120                             // can't be implemented for unsafe new
 121                             return;
 122                         }
 123                         if impl_item.generics.params.iter().any(|gen| match gen.kind {
 124                             hir::GenericParamKind::Type { .. } => true,
 125                             _ => false,
 126                         }) {
 127                             // when the result of `new()` depends on a type parameter we should not require
 128                             // an
 129                             // impl of `Default`
 130                             return;
 131                         }
 132                         if sig.decl.inputs.is_empty() && name == "new" && cx.access_levels.is_reachable(node_id) {
 133                             let self_did = cx.tcx.hir().local_def_id_from_hir_id(cx.tcx.hir().get_parent_item(id));
 134                             let self_ty = cx.tcx.type_of(self_did);
 135                             if_chain! {
 136                                 if same_tys(cx, self_ty, return_ty(cx, node_id));
 137                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 138                                 then {
 139                                     if self.impling_types.is_none() {
 140                                         let mut impls = NodeSet::default();
 141                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 142                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 143                                                 if let Some(node_id) = cx.tcx.hir().as_local_node_id(ty_def.did) {
 144                                                     impls.insert(node_id);
 145                                                 }
 146                                             }
 147                                         });
 148                                         self.impling_types = Some(impls);
 149                                     }
 150
 151                                     // Check if a Default implementation exists for the Self type, regardless of
 152                                     // generics
 153                                     if_chain! {
 154                                         if let Some(ref impling_types) = self.impling_types;
 155                                         if let Some(self_def) = cx.tcx.type_of(self_did).ty_adt_def();
 156                                         if self_def.did.is_local();
 157                                         then {
 158                                             let self_id = cx.tcx.hir().local_def_id_to_node_id(self_def.did.to_local());
 159                                             if impling_types.contains(&self_id) {
 160                                                 return;
 161                                             }
 162                                         }
 163                                     }
 164
 165                                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 166                                         span_lint_node_and_then(
 167                                             cx,
 168                                             NEW_WITHOUT_DEFAULT,
 169                                             id,
 170                                             impl_item.span,
 171                                             &format!(
 172                                                 "you should consider deriving a `Default` implementation for `{}`",
 173                                                 self_ty
 174                                             ),
 175                                             |db| {
 176                                                 db.suggest_item_with_attr(
 177                                                     cx,
 178                                                     sp,
 179                                                     "try this",
 180                                                     "#[derive(Default)]",
 181                                                     Applicability::MaybeIncorrect,
 182                                                 );
 183                                             });
 184                                     } else {
 185                                         span_lint_node_and_then(
 186                                             cx,
 187                                             NEW_WITHOUT_DEFAULT,
 188                                             id,
 189                                             impl_item.span,
 190                                             &format!(
 191                                                 "you should consider adding a `Default` implementation for `{}`",
 192                                                 self_ty
 193                                             ),
 194                                             |db| {
 195                                                 db.suggest_prepend_item(
 196                                                     cx,
 197                                                     item.span,
 198                                                     "try this",
 199                                                     &create_new_without_default_suggest_msg(self_ty),
 200                                                     Applicability::MaybeIncorrect,
 201                                                 );
 202                                             },
 203                                         );
 204                                     }
 205                                 }
 206                             }
 207                         }
 208                     }
 209                 }
 210             }
 211         }
 212     }
 213 }
 214
 215 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 216     #[rustfmt::skip]
 217     format!(
 218 "impl Default for {} {{
 219     fn default() -> Self {{
 220         Self::new()
 221     }}
 222 }}", ty)
 223 }
 224
 225 fn can_derive_default<'t, 'c>(ty: Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 226     match ty.sty {
 227         ty::Adt(adt_def, substs) if adt_def.is_struct() => {
 228             for field in adt_def.all_fields() {
 229                 let f_ty = field.ty(cx.tcx, substs);
 230                 if !implements_trait(cx, f_ty, default_trait_id, &[]) {
 231                     return None;
 232                 }
 233             }
 234             Some(cx.tcx.def_span(adt_def.did))
 235         },
 236         _ => None,
 237     }
 238 }