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`]

Author: bors

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 {