Auto merge of #8594 - FoseFx:unit_like_struct_brackets, r=giraffate

add `empty_structs_with_brackets`

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Closes #8591

I'm already sorry for the massive diff 😅

changelog: New lint [`empty_structs_with_brackets`]

CHANGELOG.md

@@ -3263,6 +3263,7 @@ Released 2018-09-13
 [`empty_enum`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_enum
 [`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
+[`empty_structs_with_brackets`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_structs_with_brackets
 [`enum_clike_unportable_variant`]: https://rust-lang.github.io/rust-clippy/master/index.html#enum_clike_unportable_variant
 [`enum_glob_use`]: https://rust-lang.github.io/rust-clippy/master/index.html#enum_glob_use
 [`enum_variant_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#enum_variant_names

clippy_lints/src/empty_structs_with_brackets.rs

@@ -0,0 +1,99 @@
+use clippy_utils::{diagnostics::span_lint_and_then, source::snippet_opt};
+use rustc_ast::ast::{Item, ItemKind, VariantData};
+use rustc_errors::Applicability;
+use rustc_lexer::TokenKind;
+use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Finds structs without fields (a so-called "empty struct") that are declared with brackets.
+    ///
+    /// ### Why is this bad?
+    /// Empty brackets after a struct declaration can be omitted.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct Cookie {}
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// struct Cookie;
+    /// ```
+    #[clippy::version = "1.62.0"]
+    pub EMPTY_STRUCTS_WITH_BRACKETS,
+    restriction,
+    "finds struct declarations with empty brackets"
+}
+declare_lint_pass!(EmptyStructsWithBrackets => [EMPTY_STRUCTS_WITH_BRACKETS]);
+
+impl EarlyLintPass for EmptyStructsWithBrackets {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
+        let span_after_ident = item.span.with_lo(item.ident.span.hi());
+
+        if let ItemKind::Struct(var_data, _) = &item.kind
+            && has_brackets(var_data)
+            && has_no_fields(cx, var_data, span_after_ident) {
+            span_lint_and_then(
+                cx,
+                EMPTY_STRUCTS_WITH_BRACKETS,
+                span_after_ident,
+                "found empty brackets on struct declaration",
+                |diagnostic| {
+                    diagnostic.span_suggestion_hidden(
+                        span_after_ident,
+                        "remove the brackets",
+                        ";".to_string(),
+                        Applicability::MachineApplicable);
+                    },
+            );
+        }
+    }
+}
+
+fn has_no_ident_token(braces_span_str: &str) -> bool {
+    !rustc_lexer::tokenize(braces_span_str).any(|t| t.kind == TokenKind::Ident)
+}
+
+fn has_brackets(var_data: &VariantData) -> bool {
+    !matches!(var_data, VariantData::Unit(_))
+}
+
+fn has_no_fields(cx: &EarlyContext<'_>, var_data: &VariantData, braces_span: Span) -> bool {
+    if !var_data.fields().is_empty() {
+        return false;
+    }
+
+    // there might still be field declarations hidden from the AST
+    // (conditionaly compiled code using #[cfg(..)])
+
+    let Some(braces_span_str) = snippet_opt(cx, braces_span) else {
+        return false;
+    };
+
+    has_no_ident_token(braces_span_str.as_ref())
+}
+
+#[cfg(test)]
+mod unit_test {
+    use super::*;
+
+    #[test]
+    fn test_has_no_ident_token() {
+        let input = "{ field: u8 }";
+        assert!(!has_no_ident_token(input));
+
+        let input = "(u8, String);";
+        assert!(!has_no_ident_token(input));
+
+        let input = " {
+                // test = 5
+        }
+        ";
+        assert!(has_no_ident_token(input));
+
+        let input = " ();";
+        assert!(has_no_ident_token(input));
+    }
+}

clippy_lints/src/lib.register_lints.rs

@@ -129,6 +129,7 @@ store.register_lints(&[
     duration_subsec::DURATION_SUBSEC,
     else_if_without_else::ELSE_IF_WITHOUT_ELSE,
     empty_enum::EMPTY_ENUM,
+    empty_structs_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS,
     entry::MAP_ENTRY,
     enum_clike::ENUM_CLIKE_UNPORTABLE_VARIANT,
     enum_variants::ENUM_VARIANT_NAMES,

clippy_lints/src/lib.register_restriction.rs

@@ -16,6 +16,7 @@ store.register_group(true, "clippy::restriction", Some("clippy_restriction"), ve
     LintId::of(default_union_representation::DEFAULT_UNION_REPRESENTATION),
     LintId::of(disallowed_script_idents::DISALLOWED_SCRIPT_IDENTS),
     LintId::of(else_if_without_else::ELSE_IF_WITHOUT_ELSE),
+    LintId::of(empty_structs_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS),
     LintId::of(exhaustive_items::EXHAUSTIVE_ENUMS),
     LintId::of(exhaustive_items::EXHAUSTIVE_STRUCTS),
     LintId::of(exit::EXIT),

clippy_lints/src/lib.rs

@@ -209,6 +209,7 @@ mod drop_forget_ref;
 mod duration_subsec;
 mod else_if_without_else;
 mod empty_enum;
+mod empty_structs_with_brackets;
 mod entry;
 mod enum_clike;
 mod enum_variants;
@@ -869,6 +870,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         })
     });
     store.register_early_pass(|| Box::new(crate_in_macro_def::CrateInMacroDef));
+    store.register_early_pass(|| Box::new(empty_structs_with_brackets::EmptyStructsWithBrackets));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/use_self.rs

@@ -34,7 +34,7 @@ declare_clippy_lint! {
     ///
     /// ### Example
     /// ```rust
-    /// struct Foo {}
+    /// struct Foo;
     /// impl Foo {
     ///     fn new() -> Foo {
     ///         Foo {}
@@ -43,7 +43,7 @@ declare_clippy_lint! {
     /// ```
     /// could be
     /// ```rust
-    /// struct Foo {}
+    /// struct Foo;
     /// impl Foo {
     ///     fn new() -> Self {
     ///         Self {}

tests/ui-toml/struct_excessive_bools/test.rs

@@ -4,6 +4,6 @@ struct S {
     a: bool,
 }
 
-struct Foo {}
+struct Foo;
 
 fn main() {}

tests/ui/case_sensitive_file_extension_comparisons.rs

@@ -2,7 +2,7 @@
 
 use std::string::String;
 
-struct TestStruct {}
+struct TestStruct;
 
 impl TestStruct {
     fn ends_with(self, arg: &str) {}

tests/ui/crashes/ice-2774.rs

@@ -8,7 +8,7 @@ pub struct Bar {
 }
 
 #[derive(Eq, PartialEq, Debug, Hash)]
-pub struct Foo {}
+pub struct Foo;
 
 #[allow(clippy::implicit_hasher)]
 // This should not cause a "cannot relate bound region" ICE.

tests/ui/crashes/ice-6179.rs

@@ -4,7 +4,7 @@
 #![warn(clippy::use_self)]
 #![allow(dead_code)]
 
-struct Foo {}
+struct Foo;
 
 impl Foo {
     fn new() -> Self {

tests/ui/crashes/ice-6792.rs

@@ -7,7 +7,7 @@ trait Trait {
     fn broken() -> Self::Ty;
 }
 
-struct Foo {}
+struct Foo;
 
 impl Trait for Foo {
     type Ty = Foo;

tests/ui/crashes/needless_lifetimes_impl_trait.rs

@@ -3,7 +3,7 @@
 
 trait Foo {}
 
-struct Bar {}
+struct Bar;
 
 struct Baz<'a> {
     bar: &'a Bar,

tests/ui/crashes/regressions.rs

@@ -6,6 +6,6 @@ pub fn foo(bar: *const u8) {
 
 // Regression test for https://github.com/rust-lang/rust-clippy/issues/4917
 /// <foo
-struct A {}
+struct A;
 
 fn main() {}

tests/ui/default_numeric_fallback_f64.fixed

@@ -134,7 +134,7 @@ mod enum_ctor {
 }
 
 mod method_calls {
-    struct StructForMethodCallTest {}
+    struct StructForMethodCallTest;
 
     impl StructForMethodCallTest {
         fn concrete_arg(&self, f: f64) {}

tests/ui/default_numeric_fallback_f64.rs

@@ -134,7 +134,7 @@ mod enum_ctor {
 }
 
 mod method_calls {
-    struct StructForMethodCallTest {}
+    struct StructForMethodCallTest;
 
     impl StructForMethodCallTest {
         fn concrete_arg(&self, f: f64) {}

tests/ui/default_numeric_fallback_i32.fixed

@@ -133,7 +133,7 @@ mod enum_ctor {
 }
 
 mod method_calls {
-    struct StructForMethodCallTest {}
+    struct StructForMethodCallTest;
 
     impl StructForMethodCallTest {
         fn concrete_arg(&self, x: i32) {}

tests/ui/default_numeric_fallback_i32.rs

@@ -133,7 +133,7 @@ mod enum_ctor {
 }
 
 mod method_calls {
-    struct StructForMethodCallTest {}
+    struct StructForMethodCallTest;
 
     impl StructForMethodCallTest {
         fn concrete_arg(&self, x: i32) {}

tests/ui/drop_forget_copy.rs

@@ -5,7 +5,7 @@ use std::mem::{drop, forget};
 use std::vec::Vec;
 
 #[derive(Copy, Clone)]
-struct SomeStruct {}
+struct SomeStruct;
 
 struct AnotherStruct {
     x: u8,

tests/ui/empty_structs_with_brackets.fixed

@@ -0,0 +1,25 @@
+// run-rustfix
+#![warn(clippy::empty_structs_with_brackets)]
+#![allow(dead_code)]
+
+pub struct MyEmptyStruct; // should trigger lint
+struct MyEmptyTupleStruct; // should trigger lint
+
+// should not trigger lint
+struct MyCfgStruct {
+    #[cfg(feature = "thisisneverenabled")]
+    field: u8,
+}
+
+// should not trigger lint
+struct MyCfgTupleStruct(#[cfg(feature = "thisisneverenabled")] u8);
+
+// should not trigger lint
+struct MyStruct {
+    field: u8,
+}
+struct MyTupleStruct(usize, String); // should not trigger lint
+struct MySingleTupleStruct(usize); // should not trigger lint
+struct MyUnitLikeStruct; // should not trigger lint
+
+fn main() {}

tests/ui/empty_structs_with_brackets.rs

@@ -0,0 +1,25 @@
+// run-rustfix
+#![warn(clippy::empty_structs_with_brackets)]
+#![allow(dead_code)]
+
+pub struct MyEmptyStruct {} // should trigger lint
+struct MyEmptyTupleStruct(); // should trigger lint
+
+// should not trigger lint
+struct MyCfgStruct {
+    #[cfg(feature = "thisisneverenabled")]
+    field: u8,
+}
+
+// should not trigger lint
+struct MyCfgTupleStruct(#[cfg(feature = "thisisneverenabled")] u8);
+
+// should not trigger lint
+struct MyStruct {
+    field: u8,
+}
+struct MyTupleStruct(usize, String); // should not trigger lint
+struct MySingleTupleStruct(usize); // should not trigger lint
+struct MyUnitLikeStruct; // should not trigger lint
+
+fn main() {}

tests/ui/empty_structs_with_brackets.stderr

@@ -0,0 +1,19 @@
+error: found empty brackets on struct declaration
+  --> $DIR/empty_structs_with_brackets.rs:5:25
+   |
+LL | pub struct MyEmptyStruct {} // should trigger lint
+   |                         ^^^
+   |
+   = note: `-D clippy::empty-structs-with-brackets` implied by `-D warnings`
+   = help: remove the brackets
+
+error: found empty brackets on struct declaration
+  --> $DIR/empty_structs_with_brackets.rs:6:26
+   |
+LL | struct MyEmptyTupleStruct(); // should trigger lint
+   |                          ^^^
+   |
+   = help: remove the brackets
+
+error: aborting due to 2 previous errors
+

tests/ui/fn_params_excessive_bools.rs

@@ -20,7 +20,7 @@ fn h(_: bool, _: bool, _: bool) {}
 fn e(_: S, _: S, _: Box<S>, _: Vec<u32>) {}
 fn t(_: S, _: S, _: Box<S>, _: Vec<u32>, _: bool, _: bool, _: bool, _: bool) {}
 
-struct S {}
+struct S;
 trait Trait {
     fn f(_: bool, _: bool, _: bool, _: bool);
     fn g(_: bool, _: bool, _: bool, _: Vec<u32>);

tests/ui/implicit_clone.rs

@@ -30,7 +30,7 @@ where
 }
 
 #[derive(Copy, Clone)]
-struct Kitten {}
+struct Kitten;
 impl Kitten {
     // badly named method
     fn to_vec(self) -> Kitten {
@@ -44,7 +44,7 @@ impl Borrow<BorrowedKitten> for Kitten {
     }
 }
 
-struct BorrowedKitten {}
+struct BorrowedKitten;
 impl ToOwned for BorrowedKitten {
     type Owned = Kitten;
     fn to_owned(&self) -> Kitten {

tests/ui/iter_nth_zero.fixed

@@ -3,7 +3,7 @@
 #![warn(clippy::iter_nth_zero)]
 use std::collections::HashSet;
 
-struct Foo {}
+struct Foo;
 
 impl Foo {
     fn nth(&self, index: usize) -> usize {

tests/ui/iter_nth_zero.rs

@@ -3,7 +3,7 @@
 #![warn(clippy::iter_nth_zero)]
 use std::collections::HashSet;
 
-struct Foo {}
+struct Foo;
 
 impl Foo {
     fn nth(&self, index: usize) -> usize {

tests/ui/large_types_passed_by_value.rs

@@ -37,7 +37,7 @@ pub trait PubLargeTypeDevourer {
     fn devoure_array_in_public(&self, array: [u8; 6666]);
 }
 
-struct S {}
+struct S;
 impl LargeTypeDevourer for S {
     fn devoure_array(&self, array: [u8; 6666]) {
         todo!();

tests/ui/let_and_return.rs

@@ -88,7 +88,7 @@ mod no_lint_if_stmt_borrows {
             ret
         }
 
-        struct Bar {}
+        struct Bar;
 
         impl Bar {
             fn new() -> Self {