src/tools/clippy/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, return_ty, span_lint_hir_and_then};
   4 use if_chain::if_chain;
   5 use rustc_errors::Applicability;
   6 use rustc_hir as hir;
   7 use rustc_hir::HirIdSet;
   8 use rustc_lint::{LateContext, LateLintPass, LintContext};
   9 use rustc_middle::lint::in_external_macro;
  10 use rustc_middle::ty::{Ty, TyS};
  11 use rustc_session::{declare_tool_lint, impl_lint_pass};
  12
  13 declare_clippy_lint! {
  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     /// **Why is this bad?** The user might expect to be able to use
  19     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  20     /// type can be constructed without arguments.
  21     ///
  22     /// **Known problems:** Hopefully none.
  23     ///
  24     /// **Example:**
  25     ///
  26     /// ```ignore
  27     /// struct Foo(Bar);
  28     ///
  29     /// impl Foo {
  30     ///     fn new() -> Self {
  31     ///         Foo(Bar::new())
  32     ///     }
  33     /// }
  34     /// ```
  35     ///
  36     /// To fix the lint, add a `Default` implementation that delegates to `new`:
  37     ///
  38     /// ```ignore
  39     /// struct Foo(Bar);
  40     ///
  41     /// impl Default for Foo {
  42     ///     fn default() -> Self {
  43     ///         Foo::new()
  44     ///     }
  45     /// }
  46     /// ```
  47     pub NEW_WITHOUT_DEFAULT,
  48     style,
  49     "`fn new() -> Self` method without `Default` implementation"
  50 }
  51
  52 #[derive(Clone, Default)]
  53 pub struct NewWithoutDefault {
  54     impling_types: Option<HirIdSet>,
  55 }
  56
  57 impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);
  58
  59 impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
  60     #[allow(clippy::too_many_lines)]
  61     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
  62         if let hir::ItemKind::Impl {
  63             of_trait: None, items, ..
  64         } = item.kind
  65         {
  66             for assoc_item in items {
  67                 if let hir::AssocItemKind::Fn { has_self: false } = assoc_item.kind {
  68                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
  69                     if in_external_macro(cx.sess(), impl_item.span) {
  70                         return;
  71                     }
  72                     if let hir::ImplItemKind::Fn(ref sig, _) = impl_item.kind {
  73                         let name = impl_item.ident.name;
  74                         let id = impl_item.hir_id;
  75                         if sig.header.constness == hir::Constness::Const {
  76                             // can't be implemented by default
  77                             return;
  78                         }
  79                         if sig.header.unsafety == hir::Unsafety::Unsafe {
  80                             // can't be implemented for unsafe new
  81                             return;
  82                         }
  83                         if impl_item
  84                             .generics
  85                             .params
  86                             .iter()
  87                             .any(|gen| matches!(gen.kind, hir::GenericParamKind::Type { .. }))
  88                         {
  89                             // when the result of `new()` depends on a type parameter we should not require
  90                             // an
  91                             // impl of `Default`
  92                             return;
  93                         }
  94                         if sig.decl.inputs.is_empty() && name == sym!(new) && cx.access_levels.is_reachable(id) {
  95                             let self_def_id = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
  96                             let self_ty = cx.tcx.type_of(self_def_id);
  97                             if_chain! {
  98                                 if TyS::same_type(self_ty, return_ty(cx, id));
  99                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 100                                 then {
 101                                     if self.impling_types.is_none() {
 102                                         let mut impls = HirIdSet::default();
 103                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 104                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 105                                                 if let Some(local_def_id) = ty_def.did.as_local() {
 106                                                     impls.insert(cx.tcx.hir().local_def_id_to_hir_id(local_def_id));
 107                                                 }
 108                                             }
 109                                         });
 110                                         self.impling_types = Some(impls);
 111                                     }
 112
 113                                     // Check if a Default implementation exists for the Self type, regardless of
 114                                     // generics
 115                                     if_chain! {
 116                                         if let Some(ref impling_types) = self.impling_types;
 117                                         if let Some(self_def) = cx.tcx.type_of(self_def_id).ty_adt_def();
 118                                         if let Some(self_local_did) = self_def.did.as_local();
 119                                         then {
 120                                             let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_local_did);
 121                                             if impling_types.contains(&self_id) {
 122                                                 return;
 123                                             }
 124                                         }
 125                                     }
 126
 127                                     span_lint_hir_and_then(
 128                                         cx,
 129                                         NEW_WITHOUT_DEFAULT,
 130                                         id,
 131                                         impl_item.span,
 132                                         &format!(
 133                                             "you should consider adding a `Default` implementation for `{}`",
 134                                             self_ty
 135                                         ),
 136                                         |diag| {
 137                                             diag.suggest_prepend_item(
 138                                                 cx,
 139                                                 item.span,
 140                                                 "try this",
 141                                                 &create_new_without_default_suggest_msg(self_ty),
 142                                                 Applicability::MaybeIncorrect,
 143                                             );
 144                                         },
 145                                     );
 146                                 }
 147                             }
 148                         }
 149                     }
 150                 }
 151             }
 152         }
 153     }
 154 }
 155
 156 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 157     #[rustfmt::skip]
 158     format!(
 159 "impl Default for {} {{
 160     fn default() -> Self {{
 161         Self::new()
 162     }}
 163 }}", ty)
 164 }