Auto merge of #12047 - ARandomDev99:12007-empty-enum-variants-with-brackets, r=Jarcho

New Lint: empty_enum_variants_with_brackets

This PR:
- adds a new early pass lint that checks for enum variants with no fields that were defined using brackets. **Category: Restriction**
- adds relevant UI tests for the new lint.

Closes #12007

```
changelog: New lint: [`empty_enum_variants_with_brackets`]
```

7 files changed, 148 insertions, 6 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e8eadd5db4d..e4e068f19d4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5105,6 +5105,7 @@ Released 2018-09-13
 [`else_if_without_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#else_if_without_else
 [`empty_drop`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_drop
 [`empty_enum`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_enum
+[`empty_enum_variants_with_brackets`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_enum_variants_with_brackets
 [`empty_line_after_doc_comments`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_doc_comments
 [`empty_line_after_outer_attr`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_outer_attr
 [`empty_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_loop
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index 5963c4c77d0..9ab38dab1e5 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/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,
diff --git a/clippy_lints/src/empty_structs_with_brackets.rs b/clippy_lints/src/empty_with_brackets.rs
index 3cf67b3ecbf..969df6d85b5 100644
--- a/clippy_lints/src/empty_structs_with_brackets.rs
+++ b/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::MaybeIncorrect,
+                    );
+                },
+            );
+        }
+    }
 }
 
 fn has_no_ident_token(braces_span_str: &str) -> bool {
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 712fe73f6a1..dc24243b574 100644
--- a/clippy_lints/src/lib.rs
+++ b/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));
diff --git a/tests/ui/empty_enum_variants_with_brackets.fixed b/tests/ui/empty_enum_variants_with_brackets.fixed
new file mode 100644
index 00000000000..1a5e78dd47f
--- /dev/null
+++ b/tests/ui/empty_enum_variants_with_brackets.fixed
@@ -0,0 +1,27 @@
+#![warn(clippy::empty_enum_variants_with_brackets)]
+#![allow(dead_code)]
+
+pub enum PublicTestEnum {
+    NonEmptyBraces { x: i32, y: i32 }, // No error
+    NonEmptyParentheses(i32, i32),     // No error
+    EmptyBraces,                    //~ ERROR: enum variant has empty brackets
+    EmptyParentheses,                //~ ERROR: enum variant has empty brackets
+}
+
+enum TestEnum {
+    NonEmptyBraces { x: i32, y: i32 }, // No error
+    NonEmptyParentheses(i32, i32),     // No error
+    EmptyBraces,                    //~ ERROR: enum variant has empty brackets
+    EmptyParentheses,                //~ ERROR: enum variant has empty brackets
+    AnotherEnum,                       // No error
+}
+
+enum TestEnumWithFeatures {
+    NonEmptyBraces {
+        #[cfg(feature = "thisisneverenabled")]
+        x: i32,
+    }, // No error
+    NonEmptyParentheses(#[cfg(feature = "thisisneverenabled")] i32), // No error
+}
+
+fn main() {}
diff --git a/tests/ui/empty_enum_variants_with_brackets.rs b/tests/ui/empty_enum_variants_with_brackets.rs
new file mode 100644
index 00000000000..ca20b969a24
--- /dev/null
+++ b/tests/ui/empty_enum_variants_with_brackets.rs
@@ -0,0 +1,27 @@
+#![warn(clippy::empty_enum_variants_with_brackets)]
+#![allow(dead_code)]
+
+pub enum PublicTestEnum {
+    NonEmptyBraces { x: i32, y: i32 }, // No error
+    NonEmptyParentheses(i32, i32),     // No error
+    EmptyBraces {},                    //~ ERROR: enum variant has empty brackets
+    EmptyParentheses(),                //~ ERROR: enum variant has empty brackets
+}
+
+enum TestEnum {
+    NonEmptyBraces { x: i32, y: i32 }, // No error
+    NonEmptyParentheses(i32, i32),     // No error
+    EmptyBraces {},                    //~ ERROR: enum variant has empty brackets
+    EmptyParentheses(),                //~ ERROR: enum variant has empty brackets
+    AnotherEnum,                       // No error
+}
+
+enum TestEnumWithFeatures {
+    NonEmptyBraces {
+        #[cfg(feature = "thisisneverenabled")]
+        x: i32,
+    }, // No error
+    NonEmptyParentheses(#[cfg(feature = "thisisneverenabled")] i32), // No error
+}
+
+fn main() {}
diff --git a/tests/ui/empty_enum_variants_with_brackets.stderr b/tests/ui/empty_enum_variants_with_brackets.stderr
new file mode 100644
index 00000000000..6b2f286147f
--- /dev/null
+++ b/tests/ui/empty_enum_variants_with_brackets.stderr
@@ -0,0 +1,36 @@
+error: enum variant has empty brackets
+  --> $DIR/empty_enum_variants_with_brackets.rs:7:16
+   |
+LL |     EmptyBraces {},
+   |                ^^^
+   |
+   = note: `-D clippy::empty-enum-variants-with-brackets` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::empty_enum_variants_with_brackets)]`
+   = help: remove the brackets
+
+error: enum variant has empty brackets
+  --> $DIR/empty_enum_variants_with_brackets.rs:8:21
+   |
+LL |     EmptyParentheses(),
+   |                     ^^
+   |
+   = help: remove the brackets
+
+error: enum variant has empty brackets
+  --> $DIR/empty_enum_variants_with_brackets.rs:14:16
+   |
+LL |     EmptyBraces {},
+   |                ^^^
+   |
+   = help: remove the brackets
+
+error: enum variant has empty brackets
+  --> $DIR/empty_enum_variants_with_brackets.rs:15:21
+   |
+LL |     EmptyParentheses(),
+   |                     ^^
+   |
+   = help: remove the brackets
+
+error: aborting due to 4 previous errors
+