clippy_lints/src/new_without_default.rs

   1 use clippy_utils::diagnostics::span_lint_hir_and_then;
   2 use clippy_utils::paths;
   3 use clippy_utils::sugg::DiagnosticBuilderExt;
   4 use clippy_utils::{get_trait_def_id, return_ty};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir as hir;
   8 use rustc_hir::HirIdSet;
   9 use rustc_lint::{LateContext, LateLintPass, LintContext};
  10 use rustc_middle::lint::in_external_macro;
  11 use rustc_middle::ty::{Ty, TyS};
  12 use rustc_session::{declare_tool_lint, impl_lint_pass};
  13 use rustc_span::sym;
  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     /// **Why is this bad?** The user might expect to be able to use
  21     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  22     /// type can be constructed without arguments.
  23     ///
  24     /// **Known problems:** Hopefully none.
  25     ///
  26     /// **Example:**
  27     ///
  28     /// ```ignore
  29     /// struct Foo(Bar);
  30     ///
  31     /// impl Foo {
  32     ///     fn new() -> Self {
  33     ///         Foo(Bar::new())
  34     ///     }
  35     /// }
  36     /// ```
  37     ///
  38     /// To fix the lint, add a `Default` implementation that delegates to `new`:
  39     ///
  40     /// ```ignore
  41     /// struct Foo(Bar);
  42     ///
  43     /// impl Default for Foo {
  44     ///     fn default() -> Self {
  45     ///         Foo::new()
  46     ///     }
  47     /// }
  48     /// ```
  49     pub NEW_WITHOUT_DEFAULT,
  50     style,
  51     "`fn new() -> Self` method without `Default` implementation"
  52 }
  53
  54 #[derive(Clone, Default)]
  55 pub struct NewWithoutDefault {
  56     impling_types: Option<HirIdSet>,
  57 }
  58
  59 impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);
  60
  61 impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
  62     #[allow(clippy::too_many_lines)]
  63     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
  64         if let hir::ItemKind::Impl(hir::Impl {
  65             of_trait: None, items, ..
  66         }) = item.kind
  67         {
  68             for assoc_item in items {
  69                 if let hir::AssocItemKind::Fn { has_self: false } = assoc_item.kind {
  70                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
  71                     if in_external_macro(cx.sess(), impl_item.span) {
  72                         return;
  73                     }
  74                     if let hir::ImplItemKind::Fn(ref sig, _) = impl_item.kind {
  75                         let name = impl_item.ident.name;
  76                         let id = impl_item.hir_id();
  77                         if sig.header.constness == hir::Constness::Const {
  78                             // can't be implemented by default
  79                             return;
  80                         }
  81                         if sig.header.unsafety == hir::Unsafety::Unsafe {
  82                             // can't be implemented for unsafe new
  83                             return;
  84                         }
  85                         if impl_item
  86                             .generics
  87                             .params
  88                             .iter()
  89                             .any(|gen| matches!(gen.kind, hir::GenericParamKind::Type { .. }))
  90                         {
  91                             // when the result of `new()` depends on a type parameter we should not require
  92                             // an
  93                             // impl of `Default`
  94                             return;
  95                         }
  96                         if sig.decl.inputs.is_empty() && name == sym::new && cx.access_levels.is_reachable(id) {
  97                             let self_def_id = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
  98                             let self_ty = cx.tcx.type_of(self_def_id);
  99                             if_chain! {
 100                                 if TyS::same_type(self_ty, return_ty(cx, id));
 101                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 102                                 then {
 103                                     if self.impling_types.is_none() {
 104                                         let mut impls = HirIdSet::default();
 105                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 106                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 107                                                 if let Some(local_def_id) = ty_def.did.as_local() {
 108                                                     impls.insert(cx.tcx.hir().local_def_id_to_hir_id(local_def_id));
 109                                                 }
 110                                             }
 111                                         });
 112                                         self.impling_types = Some(impls);
 113                                     }
 114
 115                                     // Check if a Default implementation exists for the Self type, regardless of
 116                                     // generics
 117                                     if_chain! {
 118                                         if let Some(ref impling_types) = self.impling_types;
 119                                         if let Some(self_def) = cx.tcx.type_of(self_def_id).ty_adt_def();
 120                                         if let Some(self_local_did) = self_def.did.as_local();
 121                                         then {
 122                                             let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_local_did);
 123                                             if impling_types.contains(&self_id) {
 124                                                 return;
 125                                             }
 126                                         }
 127                                     }
 128
 129                                     span_lint_hir_and_then(
 130                                         cx,
 131                                         NEW_WITHOUT_DEFAULT,
 132                                         id,
 133                                         impl_item.span,
 134                                         &format!(
 135                                             "you should consider adding a `Default` implementation for `{}`",
 136                                             self_ty
 137                                         ),
 138                                         |diag| {
 139                                             diag.suggest_prepend_item(
 140                                                 cx,
 141                                                 item.span,
 142                                                 "try this",
 143                                                 &create_new_without_default_suggest_msg(self_ty),
 144                                                 Applicability::MaybeIncorrect,
 145                                             );
 146                                         },
 147                                     );
 148                                 }
 149                             }
 150                         }
 151                     }
 152                 }
 153             }
 154         }
 155     }
 156 }
 157
 158 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 159     #[rustfmt::skip]
 160     format!(
 161 "impl Default for {} {{
 162     fn default() -> Self {{
 163         Self::new()
 164     }}
 165 }}", ty)
 166 }