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::NodeSet;
  10 use rustc::{declare_tool_lint, lint_array};
  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<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                         if sig.header.constness == hir::Constness::Const {
 115                             // can't be implemented by default
 116                             return;
 117                         }
 118                         if sig.header.unsafety == hir::Unsafety::Unsafe {
 119                             // can't be implemented for unsafe new
 120                             return;
 121                         }
 122                         if impl_item.generics.params.iter().any(|gen| match gen.kind {
 123                             hir::GenericParamKind::Type { .. } => true,
 124                             _ => false,
 125                         }) {
 126                             // when the result of `new()` depends on a type parameter we should not require
 127                             // an
 128                             // impl of `Default`
 129                             return;
 130                         }
 131                         if sig.decl.inputs.is_empty() && name == "new" && cx.access_levels.is_reachable(id) {
 132                             let self_did = cx.tcx.hir().local_def_id_from_hir_id(cx.tcx.hir().get_parent_item(id));
 133                             let self_ty = cx.tcx.type_of(self_did);
 134                             if_chain! {
 135                                 if same_tys(cx, self_ty, return_ty(cx, id));
 136                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 137                                 then {
 138                                     if self.impling_types.is_none() {
 139                                         let mut impls = NodeSet::default();
 140                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 141                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 142                                                 if let Some(node_id) = cx.tcx.hir().as_local_node_id(ty_def.did) {
 143                                                     impls.insert(node_id);
 144                                                 }
 145                                             }
 146                                         });
 147                                         self.impling_types = Some(impls);
 148                                     }
 149
 150                                     // Check if a Default implementation exists for the Self type, regardless of
 151                                     // generics
 152                                     if_chain! {
 153                                         if let Some(ref impling_types) = self.impling_types;
 154                                         if let Some(self_def) = cx.tcx.type_of(self_did).ty_adt_def();
 155                                         if self_def.did.is_local();
 156                                         then {
 157                                             let self_id = cx.tcx.hir().local_def_id_to_node_id(self_def.did.to_local());
 158                                             if impling_types.contains(&self_id) {
 159                                                 return;
 160                                             }
 161                                         }
 162                                     }
 163
 164                                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 165                                         span_lint_hir_and_then(
 166                                             cx,
 167                                             NEW_WITHOUT_DEFAULT,
 168                                             id,
 169                                             impl_item.span,
 170                                             &format!(
 171                                                 "you should consider deriving a `Default` implementation for `{}`",
 172                                                 self_ty
 173                                             ),
 174                                             |db| {
 175                                                 db.suggest_item_with_attr(
 176                                                     cx,
 177                                                     sp,
 178                                                     "try this",
 179                                                     "#[derive(Default)]",
 180                                                     Applicability::MaybeIncorrect,
 181                                                 );
 182                                             });
 183                                     } else {
 184                                         span_lint_hir_and_then(
 185                                             cx,
 186                                             NEW_WITHOUT_DEFAULT,
 187                                             id,
 188                                             impl_item.span,
 189                                             &format!(
 190                                                 "you should consider adding a `Default` implementation for `{}`",
 191                                                 self_ty
 192                                             ),
 193                                             |db| {
 194                                                 db.suggest_prepend_item(
 195                                                     cx,
 196                                                     item.span,
 197                                                     "try this",
 198                                                     &create_new_without_default_suggest_msg(self_ty),
 199                                                     Applicability::MaybeIncorrect,
 200                                                 );
 201                                             },
 202                                         );
 203                                     }
 204                                 }
 205                             }
 206                         }
 207                     }
 208                 }
 209             }
 210         }
 211     }
 212 }
 213
 214 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 215     #[rustfmt::skip]
 216     format!(
 217 "impl Default for {} {{
 218     fn default() -> Self {{
 219         Self::new()
 220     }}
 221 }}", ty)
 222 }
 223
 224 fn can_derive_default<'t, 'c>(ty: Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 225     match ty.sty {
 226         ty::Adt(adt_def, substs) if adt_def.is_struct() => {
 227             for field in adt_def.all_fields() {
 228                 let f_ty = field.ty(cx.tcx, substs);
 229                 if !implements_trait(cx, f_ty, default_trait_id, &[]) {
 230                     return None;
 231                 }
 232             }
 233             Some(cx.tcx.def_span(adt_def.did))
 234         },
 235         _ => None,
 236     }
 237 }