src/tools/clippy/clippy_lints/src/empty_enum.rs

   1 //! lint when there is an enum with no variants
   2
   3 use clippy_utils::diagnostics::span_lint_and_help;
   4 use rustc_hir::{Item, ItemKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7
   8 declare_clippy_lint! {
   9     /// ### What it does
  10     /// Checks for `enum`s with no variants.
  11     ///
  12     /// As of this writing, the `never_type` is still a
  13     /// nightly-only experimental API. Therefore, this lint is only triggered
  14     /// if the `never_type` is enabled.
  15     ///
  16     /// ### Why is this bad?
  17     /// If you want to introduce a type which
  18     /// can't be instantiated, you should use `!` (the primitive type "never"),
  19     /// or a wrapper around it, because `!` has more extensive
  20     /// compiler support (type inference, etc...) and wrappers
  21     /// around it are the conventional way to define an uninhabited type.
  22     /// For further information visit [never type documentation](https://doc.rust-lang.org/std/primitive.never.html)
  23     ///
  24     ///
  25     /// ### Example
  26     /// ```rust
  27     /// enum Test {}
  28     /// ```
  29     ///
  30     /// Use instead:
  31     /// ```rust
  32     /// #![feature(never_type)]
  33     ///
  34     /// struct Test(!);
  35     /// ```
  36     #[clippy::version = "pre 1.29.0"]
  37     pub EMPTY_ENUM,
  38     pedantic,
  39     "enum with no variants"
  40 }
  41
  42 declare_lint_pass!(EmptyEnum => [EMPTY_ENUM]);
  43
  44 impl<'tcx> LateLintPass<'tcx> for EmptyEnum {
  45     fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
  46         // Only suggest the `never_type` if the feature is enabled
  47         if !cx.tcx.features().never_type {
  48             return;
  49         }
  50
  51         if let ItemKind::Enum(..) = item.kind {
  52             let ty = cx.tcx.type_of(item.owner_id).subst_identity();
  53             let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
  54             if adt.variants().is_empty() {
  55                 span_lint_and_help(
  56                     cx,
  57                     EMPTY_ENUM,
  58                     item.span,
  59                     "enum with no variants",
  60                     None,
  61                     "consider using the uninhabited type `!` (never type) or a wrapper \
  62                     around it to introduce a type which can't be instantiated",
  63                 );
  64             }
  65         }
  66     }
  67 }