src/tools/clippy/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::source::snippet;
   4 use clippy_utils::sugg::DiagnosticBuilderExt;
   5 use clippy_utils::{get_trait_def_id, return_ty};
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir as hir;
   9 use rustc_hir::HirIdSet;
  10 use rustc_lint::{LateContext, LateLintPass, LintContext};
  11 use rustc_middle::lint::in_external_macro;
  12 use rustc_middle::ty::{Ty, TyS};
  13 use rustc_session::{declare_tool_lint, impl_lint_pass};
  14 use rustc_span::sym;
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  18     /// implementation of
  19     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  20     ///
  21     /// **Why is this bad?** The user might expect to be able to use
  22     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  23     /// type can be constructed without arguments.
  24     ///
  25     /// **Known problems:** Hopefully none.
  26     ///
  27     /// **Example:**
  28     ///
  29     /// ```ignore
  30     /// struct Foo(Bar);
  31     ///
  32     /// impl Foo {
  33     ///     fn new() -> Self {
  34     ///         Foo(Bar::new())
  35     ///     }
  36     /// }
  37     /// ```
  38     ///
  39     /// To fix the lint, add a `Default` implementation that delegates to `new`:
  40     ///
  41     /// ```ignore
  42     /// struct Foo(Bar);
  43     ///
  44     /// impl Default for Foo {
  45     ///     fn default() -> Self {
  46     ///         Foo::new()
  47     ///     }
  48     /// }
  49     /// ```
  50     pub NEW_WITHOUT_DEFAULT,
  51     style,
  52     "`fn new() -> Self` method without `Default` implementation"
  53 }
  54
  55 #[derive(Clone, Default)]
  56 pub struct NewWithoutDefault {
  57     impling_types: Option<HirIdSet>,
  58 }
  59
  60 impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);
  61
  62 impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
  63     #[allow(clippy::too_many_lines)]
  64     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
  65         if let hir::ItemKind::Impl(hir::Impl {
  66             of_trait: None,
  67             ref generics,
  68             items,
  69             ..
  70         }) = item.kind
  71         {
  72             for assoc_item in items {
  73                 if let hir::AssocItemKind::Fn { has_self: false } = assoc_item.kind {
  74                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
  75                     if in_external_macro(cx.sess(), impl_item.span) {
  76                         return;
  77                     }
  78                     if let hir::ImplItemKind::Fn(ref sig, _) = impl_item.kind {
  79                         let name = impl_item.ident.name;
  80                         let id = impl_item.hir_id();
  81                         if sig.header.constness == hir::Constness::Const {
  82                             // can't be implemented by default
  83                             return;
  84                         }
  85                         if sig.header.unsafety == hir::Unsafety::Unsafe {
  86                             // can't be implemented for unsafe new
  87                             return;
  88                         }
  89                         if impl_item
  90                             .generics
  91                             .params
  92                             .iter()
  93                             .any(|gen| matches!(gen.kind, hir::GenericParamKind::Type { .. }))
  94                         {
  95                             // when the result of `new()` depends on a type parameter we should not require
  96                             // an
  97                             // impl of `Default`
  98                             return;
  99                         }
 100                         if sig.decl.inputs.is_empty() && name == sym::new && cx.access_levels.is_reachable(id) {
 101                             let self_def_id = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
 102                             let self_ty = cx.tcx.type_of(self_def_id);
 103                             if_chain! {
 104                                 if TyS::same_type(self_ty, return_ty(cx, id));
 105                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 106                                 then {
 107                                     if self.impling_types.is_none() {
 108                                         let mut impls = HirIdSet::default();
 109                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 110                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 111                                                 if let Some(local_def_id) = ty_def.did.as_local() {
 112                                                     impls.insert(cx.tcx.hir().local_def_id_to_hir_id(local_def_id));
 113                                                 }
 114                                             }
 115                                         });
 116                                         self.impling_types = Some(impls);
 117                                     }
 118
 119                                     // Check if a Default implementation exists for the Self type, regardless of
 120                                     // generics
 121                                     if_chain! {
 122                                         if let Some(ref impling_types) = self.impling_types;
 123                                         if let Some(self_def) = cx.tcx.type_of(self_def_id).ty_adt_def();
 124                                         if let Some(self_local_did) = self_def.did.as_local();
 125                                         then {
 126                                             let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_local_did);
 127                                             if impling_types.contains(&self_id) {
 128                                                 return;
 129                                             }
 130                                         }
 131                                     }
 132
 133                                     let generics_sugg = snippet(cx, generics.span, "");
 134                                     span_lint_hir_and_then(
 135                                         cx,
 136                                         NEW_WITHOUT_DEFAULT,
 137                                         id,
 138                                         impl_item.span,
 139                                         &format!(
 140                                             "you should consider adding a `Default` implementation for `{}`",
 141                                             self_ty
 142                                         ),
 143                                         |diag| {
 144                                             diag.suggest_prepend_item(
 145                                                 cx,
 146                                                 item.span,
 147                                                 "try this",
 148                                                 &create_new_without_default_suggest_msg(self_ty, &generics_sugg),
 149                                                 Applicability::MaybeIncorrect,
 150                                             );
 151                                         },
 152                                     );
 153                                 }
 154                             }
 155                         }
 156                     }
 157                 }
 158             }
 159         }
 160     }
 161 }
 162
 163 fn create_new_without_default_suggest_msg(ty: Ty<'_>, generics_sugg: &str) -> String {
 164     #[rustfmt::skip]
 165     format!(
 166 "impl{} Default for {} {{
 167     fn default() -> Self {{
 168         Self::new()
 169     }}
 170 }}", generics_sugg, ty)
 171 }