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//! lint when there is an enum with no variants
use clippy_utils::diagnostics::span_lint_and_help;
use rustc_hir::{Item, ItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
declare_clippy_lint! {
/// ### What it does
/// Checks for `enum`s with no variants.
///
/// As of this writing, the `never_type` is still a
/// nightly-only experimental API. Therefore, this lint is only triggered
/// if the `never_type` is enabled.
///
/// ### Why is this bad?
/// If you want to introduce a type which
/// can't be instantiated, you should use `!` (the primitive type "never"),
/// or a wrapper around it, because `!` has more extensive
/// compiler support (type inference, etc...) and wrappers
/// around it are the conventional way to define an uninhabited type.
/// For further information visit [never type documentation](https://doc.rust-lang.org/std/primitive.never.html)
///
///
/// ### Example
/// ```rust
/// enum Test {}
/// ```
///
/// Use instead:
/// ```rust
/// #![feature(never_type)]
///
/// struct Test(!);
/// ```
#[clippy::version = "pre 1.29.0"]
pub EMPTY_ENUM,
pedantic,
"enum with no variants"
}
declare_lint_pass!(EmptyEnum => [EMPTY_ENUM]);
impl<'tcx> LateLintPass<'tcx> for EmptyEnum {
fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
// Only suggest the `never_type` if the feature is enabled
if !cx.tcx.features().never_type {
return;
}
if let ItemKind::Enum(..) = item.kind {
let ty = cx.tcx.type_of(item.owner_id).subst_identity();
let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
if adt.variants().is_empty() {
span_lint_and_help(
cx,
EMPTY_ENUM,
item.span,
"enum with no variants",
None,
"consider using the uninhabited type `!` (never type) or a wrapper \
around it to introduce a type which can't be instantiated",
);
}
}
}
}
|
