add new lint, empty_enum_variants_with_brackets

- Add a new early pass lint.
- Add relevant UI tests.

clippy_lints/src/declared_lints.rs

@@ -150,7 +150,8 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::else_if_without_else::ELSE_IF_WITHOUT_ELSE_INFO,
     crate::empty_drop::EMPTY_DROP_INFO,
     crate::empty_enum::EMPTY_ENUM_INFO,
-    crate::empty_structs_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS_INFO,
+    crate::empty_with_brackets::EMPTY_ENUM_VARIANTS_WITH_BRACKETS_INFO,
+    crate::empty_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS_INFO,
     crate::endian_bytes::BIG_ENDIAN_BYTES_INFO,
     crate::endian_bytes::HOST_ENDIAN_BYTES_INFO,
     crate::endian_bytes::LITTLE_ENDIAN_BYTES_INFO,

clippy_lints/src/empty_with_brackets.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::snippet_opt;
-use rustc_ast::ast::{Item, ItemKind, VariantData};
+use rustc_ast::ast::{Item, ItemKind, Variant, VariantData};
 use rustc_errors::Applicability;
 use rustc_lexer::TokenKind;
 use rustc_lint::{EarlyContext, EarlyLintPass};
@@ -27,9 +27,38 @@ declare_clippy_lint! {
     restriction,
     "finds struct declarations with empty brackets"
 }
-declare_lint_pass!(EmptyStructsWithBrackets => [EMPTY_STRUCTS_WITH_BRACKETS]);
 
-impl EarlyLintPass for EmptyStructsWithBrackets {
+declare_clippy_lint! {
+    /// ### What it does
+    /// Finds enum variants without fields that are declared with empty brackets.
+    ///
+    /// ### Why is this bad?
+    /// Empty brackets while defining enum variants are redundant and can be omitted.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// enum MyEnum {
+    ///     HasData(u8),
+    ///     HasNoData(), // redundant parentheses
+    /// }
+    /// ```
+    ///
+    /// Use instead:
+    /// ```no_run
+    /// enum MyEnum {
+    ///     HasData(u8),
+    ///     HasNoData,
+    /// }
+    /// ```
+    #[clippy::version = "1.77.0"]
+    pub EMPTY_ENUM_VARIANTS_WITH_BRACKETS,
+    restriction,
+    "finds enum variants with empty brackets"
+}
+
+declare_lint_pass!(EmptyWithBrackets => [EMPTY_STRUCTS_WITH_BRACKETS, EMPTY_ENUM_VARIANTS_WITH_BRACKETS]);
+
+impl EarlyLintPass for EmptyWithBrackets {
     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
         let span_after_ident = item.span.with_lo(item.ident.span.hi());
 
@@ -53,6 +82,27 @@ impl EarlyLintPass for EmptyStructsWithBrackets {
             );
         }
     }
+
+    fn check_variant(&mut self, cx: &EarlyContext<'_>, variant: &Variant) {
+        let span_after_ident = variant.span.with_lo(variant.ident.span.hi());
+
+        if has_brackets(&variant.data) && has_no_fields(cx, &variant.data, span_after_ident) {
+            span_lint_and_then(
+                cx,
+                EMPTY_ENUM_VARIANTS_WITH_BRACKETS,
+                span_after_ident,
+                "enum variant has empty brackets",
+                |diagnostic| {
+                    diagnostic.span_suggestion_hidden(
+                        span_after_ident,
+                        "remove the brackets",
+                        "",
+                        Applicability::MachineApplicable,
+                    );
+                },
+            );
+        }
+    }
 }
 
 fn has_no_ident_token(braces_span_str: &str) -> bool {

clippy_lints/src/lib.rs

@@ -115,7 +115,7 @@ mod duplicate_mod;
 mod else_if_without_else;
 mod empty_drop;
 mod empty_enum;
-mod empty_structs_with_brackets;
+mod empty_with_brackets;
 mod endian_bytes;
 mod entry;
 mod enum_clike;
@@ -949,7 +949,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
         })
     });
     store.register_early_pass(|| Box::new(crate_in_macro_def::CrateInMacroDef));
-    store.register_early_pass(|| Box::new(empty_structs_with_brackets::EmptyStructsWithBrackets));
+    store.register_early_pass(|| Box::new(empty_with_brackets::EmptyWithBrackets));
     store.register_late_pass(|_| Box::new(unnecessary_owned_empty_strings::UnnecessaryOwnedEmptyStrings));
     store.register_early_pass(|| Box::new(pub_use::PubUse));
     store.register_late_pass(|_| Box::new(format_push_string::FormatPushString));