src/tools/clippy/clippy_lints/src/new_without_default.rs

   1 use clippy_utils::diagnostics::span_lint_hir_and_then;
   2 use clippy_utils::return_ty;
   3 use clippy_utils::source::snippet;
   4 use clippy_utils::sugg::DiagnosticBuilderExt;
   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::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
  17     /// 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?
  22     /// The user might expect to be able to use
  23     /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  24     /// type can be constructed without arguments.
  25     ///
  26     /// ### Example
  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     #[clippy::version = "pre 1.29.0"]
  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,
  66             ref generics,
  67             self_ty: impl_self_ty,
  68             items,
  69             ..
  70         }) = item.kind
  71         {
  72             for assoc_item in items {
  73                 if assoc_item.kind == (hir::AssocItemKind::Fn { has_self: false }) {
  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_chain! {
 101                             if sig.decl.inputs.is_empty();
 102                             if name == sym::new;
 103                             if cx.access_levels.is_reachable(impl_item.def_id);
 104                             let self_def_id = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
 105                             let self_ty = cx.tcx.type_of(self_def_id);
 106                             if TyS::same_type(self_ty, return_ty(cx, id));
 107                             if let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default);
 108                             then {
 109                                 if self.impling_types.is_none() {
 110                                     let mut impls = HirIdSet::default();
 111                                     cx.tcx.for_each_impl(default_trait_id, |d| {
 112                                         if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 113                                             if let Some(local_def_id) = ty_def.did.as_local() {
 114                                                 impls.insert(cx.tcx.hir().local_def_id_to_hir_id(local_def_id));
 115                                             }
 116                                         }
 117                                     });
 118                                     self.impling_types = Some(impls);
 119                                 }
 120
 121                                 // Check if a Default implementation exists for the Self type, regardless of
 122                                 // generics
 123                                 if_chain! {
 124                                     if let Some(ref impling_types) = self.impling_types;
 125                                     if let Some(self_def) = cx.tcx.type_of(self_def_id).ty_adt_def();
 126                                     if let Some(self_local_did) = self_def.did.as_local();
 127                                     let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_local_did);
 128                                     if impling_types.contains(&self_id);
 129                                     then {
 130                                         return;
 131                                     }
 132                                 }
 133
 134                                 let generics_sugg = snippet(cx, generics.span, "");
 135                                 let self_ty_fmt = self_ty.to_string();
 136                                 let self_type_snip = snippet(cx, impl_self_ty.span, &self_ty_fmt);
 137                                 span_lint_hir_and_then(
 138                                     cx,
 139                                     NEW_WITHOUT_DEFAULT,
 140                                     id,
 141                                     impl_item.span,
 142                                     &format!(
 143                                         "you should consider adding a `Default` implementation for `{}`",
 144                                         self_type_snip
 145                                     ),
 146                                     |diag| {
 147                                         diag.suggest_prepend_item(
 148                                             cx,
 149                                             item.span,
 150                                             "try adding this",
 151                                             &create_new_without_default_suggest_msg(&self_type_snip, &generics_sugg),
 152                                             Applicability::MaybeIncorrect,
 153                                         );
 154                                     },
 155                                 );
 156                             }
 157                         }
 158                     }
 159                 }
 160             }
 161         }
 162     }
 163 }
 164
 165 fn create_new_without_default_suggest_msg(self_type_snip: &str, generics_sugg: &str) -> String {
 166     #[rustfmt::skip]
 167     format!(
 168 "impl{} Default for {} {{
 169     fn default() -> Self {{
 170         Self::new()
 171     }}
 172 }}", generics_sugg, self_type_snip)
 173 }