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_session::{declare_tool_lint, impl_lint_pass};
  12 use rustc_span::sym;
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// 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?
  21     /// 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     /// ### Example
  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     #[clippy::version = "pre 1.29.0"]
  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,
  65             ref generics,
  66             self_ty: impl_self_ty,
  67             items,
  68             ..
  69         }) = item.kind
  70         {
  71             for assoc_item in items {
  72                 if assoc_item.kind == (hir::AssocItemKind::Fn { has_self: false }) {
  73                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
  74                     if in_external_macro(cx.sess(), impl_item.span) {
  75                         return;
  76                     }
  77                     if let hir::ImplItemKind::Fn(ref sig, _) = impl_item.kind {
  78                         let name = impl_item.ident.name;
  79                         let id = impl_item.hir_id();
  80                         if sig.header.constness == hir::Constness::Const {
  81                             // can't be implemented by default
  82                             return;
  83                         }
  84                         if sig.header.unsafety == hir::Unsafety::Unsafe {
  85                             // can't be implemented for unsafe new
  86                             return;
  87                         }
  88                         if impl_item
  89                             .generics
  90                             .params
  91                             .iter()
  92                             .any(|gen| matches!(gen.kind, hir::GenericParamKind::Type { .. }))
  93                         {
  94                             // when the result of `new()` depends on a type parameter we should not require
  95                             // an
  96                             // impl of `Default`
  97                             return;
  98                         }
  99                         if_chain! {
 100                             if sig.decl.inputs.is_empty();
 101                             if name == sym::new;
 102                             if cx.access_levels.is_reachable(impl_item.def_id);
 103                             let self_def_id = cx.tcx.hir().get_parent_item(id);
 104                             let self_ty = cx.tcx.type_of(self_def_id);
 105                             if self_ty == return_ty(cx, id);
 106                             if let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default);
 107                             then {
 108                                 if self.impling_types.is_none() {
 109                                     let mut impls = HirIdSet::default();
 110                                     cx.tcx.for_each_impl(default_trait_id, |d| {
 111                                         if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 112                                             if let Some(local_def_id) = ty_def.did.as_local() {
 113                                                 impls.insert(cx.tcx.hir().local_def_id_to_hir_id(local_def_id));
 114                                             }
 115                                         }
 116                                     });
 117                                     self.impling_types = Some(impls);
 118                                 }
 119
 120                                 // Check if a Default implementation exists for the Self type, regardless of
 121                                 // generics
 122                                 if_chain! {
 123                                     if let Some(ref impling_types) = self.impling_types;
 124                                     if let Some(self_def) = cx.tcx.type_of(self_def_id).ty_adt_def();
 125                                     if let Some(self_local_did) = self_def.did.as_local();
 126                                     let self_id = cx.tcx.hir().local_def_id_to_hir_id(self_local_did);
 127                                     if impling_types.contains(&self_id);
 128                                     then {
 129                                         return;
 130                                     }
 131                                 }
 132
 133                                 let generics_sugg = snippet(cx, generics.span, "");
 134                                 let self_ty_fmt = self_ty.to_string();
 135                                 let self_type_snip = snippet(cx, impl_self_ty.span, &self_ty_fmt);
 136                                 span_lint_hir_and_then(
 137                                     cx,
 138                                     NEW_WITHOUT_DEFAULT,
 139                                     id,
 140                                     impl_item.span,
 141                                     &format!(
 142                                         "you should consider adding a `Default` implementation for `{}`",
 143                                         self_type_snip
 144                                     ),
 145                                     |diag| {
 146                                         diag.suggest_prepend_item(
 147                                             cx,
 148                                             item.span,
 149                                             "try adding this",
 150                                             &create_new_without_default_suggest_msg(&self_type_snip, &generics_sugg),
 151                                             Applicability::MaybeIncorrect,
 152                                         );
 153                                     },
 154                                 );
 155                             }
 156                         }
 157                     }
 158                 }
 159             }
 160         }
 161     }
 162 }
 163
 164 fn create_new_without_default_suggest_msg(self_type_snip: &str, generics_sugg: &str) -> String {
 165     #[rustfmt::skip]
 166     format!(
 167 "impl{} Default for {} {{
 168     fn default() -> Self {{
 169         Self::new()
 170     }}
 171 }}", generics_sugg, self_type_snip)
 172 }