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 use rustc_span::sym;
  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     /// **Why is this bad?** The user might expect to be able to use
  20     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  21     /// type can be constructed without arguments.
  22     ///
  23     /// **Known problems:** Hopefully none.
  24     ///
  25     /// **Example:**
  26     ///
  27     /// ```ignore
  28     /// struct Foo(Bar);
  29     ///
  30     /// impl Foo {
  31     ///     fn new() -> Self {
  32     ///         Foo(Bar::new())
  33     ///     }
  34     /// }
  35     /// ```
  36     ///
  37     /// To fix the lint, add a `Default` implementation that delegates to `new`:
  38     ///
  39     /// ```ignore
  40     /// struct Foo(Bar);
  41     ///
  42     /// impl Default for Foo {
  43     ///     fn default() -> Self {
  44     ///         Foo::new()
  45     ///     }
  46     /// }
  47     /// ```
  48     pub NEW_WITHOUT_DEFAULT,
  49     style,
  50     "`fn new() -> Self` method without `Default` implementation"
  51 }
  52
  53 #[derive(Clone, Default)]
  54 pub struct NewWithoutDefault {
  55     impling_types: Option<HirIdSet>,
  56 }
  57
  58 impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);
  59
  60 impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
  61     #[allow(clippy::too_many_lines)]
  62     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
  63         if let hir::ItemKind::Impl(hir::Impl {
  64             of_trait: None, items, ..
  65         }) = item.kind
  66         {
  67             for assoc_item in items {
  68                 if let hir::AssocItemKind::Fn { has_self: false } = assoc_item.kind {
  69                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
  70                     if in_external_macro(cx.sess(), impl_item.span) {
  71                         return;
  72                     }
  73                     if let hir::ImplItemKind::Fn(ref sig, _) = impl_item.kind {
  74                         let name = impl_item.ident.name;
  75                         let id = impl_item.hir_id();
  76                         if sig.header.constness == hir::Constness::Const {
  77                             // can't be implemented by default
  78                             return;
  79                         }
  80                         if sig.header.unsafety == hir::Unsafety::Unsafe {
  81                             // can't be implemented for unsafe new
  82                             return;
  83                         }
  84                         if impl_item
  85                             .generics
  86                             .params
  87                             .iter()
  88                             .any(|gen| matches!(gen.kind, hir::GenericParamKind::Type { .. }))
  89                         {
  90                             // when the result of `new()` depends on a type parameter we should not require
  91                             // an
  92                             // impl of `Default`
  93                             return;
  94                         }
  95                         if sig.decl.inputs.is_empty() && name == sym::new && cx.access_levels.is_reachable(id) {
  96                             let self_def_id = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
  97                             let self_ty = cx.tcx.type_of(self_def_id);
  98                             if_chain! {
  99                                 if TyS::same_type(self_ty, return_ty(cx, id));
 100                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 101                                 then {
 102                                     if self.impling_types.is_none() {
 103                                         let mut impls = HirIdSet::default();
 104                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 105                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 106                                                 if let Some(local_def_id) = ty_def.did.as_local() {
 107                                                     impls.insert(cx.tcx.hir().local_def_id_to_hir_id(local_def_id));
 108                                                 }
 109                                             }
 110                                         });
 111                                         self.impling_types = Some(impls);
 112                                     }
 113
 114                                     // Check if a Default implementation exists for the Self type, regardless of
 115                                     // generics
 116                                     if_chain! {
 117                                         if let Some(ref impling_types) = self.impling_types;
 118                                         if let Some(self_def) = cx.tcx.type_of(self_def_id).ty_adt_def();
 119                                         if let Some(self_local_did) = self_def.did.as_local();
 120                                         then {
 121                                             let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_local_did);
 122                                             if impling_types.contains(&self_id) {
 123                                                 return;
 124                                             }
 125                                         }
 126                                     }
 127
 128                                     span_lint_hir_and_then(
 129                                         cx,
 130                                         NEW_WITHOUT_DEFAULT,
 131                                         id,
 132                                         impl_item.span,
 133                                         &format!(
 134                                             "you should consider adding a `Default` implementation for `{}`",
 135                                             self_ty
 136                                         ),
 137                                         |diag| {
 138                                             diag.suggest_prepend_item(
 139                                                 cx,
 140                                                 item.span,
 141                                                 "try this",
 142                                                 &create_new_without_default_suggest_msg(self_ty),
 143                                                 Applicability::MaybeIncorrect,
 144                                             );
 145                                         },
 146                                     );
 147                                 }
 148                             }
 149                         }
 150                     }
 151                 }
 152             }
 153         }
 154     }
 155 }
 156
 157 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 158     #[rustfmt::skip]
 159     format!(
 160 "impl Default for {} {{
 161     fn default() -> Self {{
 162         Self::new()
 163     }}
 164 }}", ty)
 165 }