Add new useless_concat lint

CHANGELOG.md

@@ -6438,6 +6438,7 @@ Released 2018-09-13
 [`used_underscore_items`]: https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items
 [`useless_asref`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_asref
 [`useless_attribute`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_attribute
+[`useless_concat`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_concat
 [`useless_conversion`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
 [`useless_format`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_format
 [`useless_let_if_seq`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_let_if_seq

clippy_lints/src/declared_lints.rs

@@ -762,6 +762,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::unwrap_in_result::UNWRAP_IN_RESULT_INFO,
     crate::upper_case_acronyms::UPPER_CASE_ACRONYMS_INFO,
     crate::use_self::USE_SELF_INFO,
+    crate::useless_concat::USELESS_CONCAT_INFO,
     crate::useless_conversion::USELESS_CONVERSION_INFO,
     crate::vec::USELESS_VEC_INFO,
     crate::vec_init_then_push::VEC_INIT_THEN_PUSH_INFO,

clippy_lints/src/lib.rs

@@ -392,6 +392,7 @@ mod unwrap;
 mod unwrap_in_result;
 mod upper_case_acronyms;
 mod use_self;
+mod useless_concat;
 mod useless_conversion;
 mod vec;
 mod vec_init_then_push;
@@ -936,6 +937,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(unnecessary_literal_bound::UnnecessaryLiteralBound));
     store.register_early_pass(|| Box::new(empty_line_after::EmptyLineAfter::new()));
     store.register_late_pass(move |_| Box::new(arbitrary_source_item_ordering::ArbitrarySourceItemOrdering::new(conf)));
+    store.register_late_pass(|_| Box::new(useless_concat::UselessConcat));
     store.register_late_pass(|_| Box::new(unneeded_struct_pattern::UnneededStructPattern));
     store.register_late_pass(|_| Box::<unnecessary_semicolon::UnnecessarySemicolon>::default());
     store.register_late_pass(move |_| Box::new(non_std_lazy_statics::NonStdLazyStatic::new(conf)));

clippy_lints/src/useless_concat.rs

@@ -0,0 +1,108 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::macros::macro_backtrace;
+use clippy_utils::source::snippet_opt;
+use clippy_utils::{match_def_path, tokenize_with_text};
+use rustc_ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lexer::TokenKind;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks that the `concat!` macro has at least two arguments.
+    ///
+    /// ### Why is this bad?
+    /// If there are less than 2 arguments, then calling the macro is doing nothing.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let x = concat!("a");
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let x = "a";
+    /// ```
+    #[clippy::version = "1.85.0"]
+    pub USELESS_CONCAT,
+    complexity,
+    "checks that the `concat` macro has at least two arguments"
+}
+
+declare_lint_pass!(UselessConcat => [USELESS_CONCAT]);
+
+impl LateLintPass<'_> for UselessConcat {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        // Check that the expression is generated by a macro.
+        if expr.span.from_expansion()
+            // Check that it's a string literal.
+            && let ExprKind::Lit(lit) = expr.kind
+            && let LitKind::Str(_, _) = lit.node
+            // Get the direct parent of the expression.
+            && let Some(macro_call) = macro_backtrace(expr.span).next()
+            // Check if the `concat` macro from the `core` library.
+            && match_def_path(cx, macro_call.def_id, &["core", "macros", "builtin", "concat"])
+            // We get the original code to parse it.
+            && let Some(original_code) = snippet_opt(cx, macro_call.span)
+            // This check allows us to ensure that the code snippet:
+            // 1. Doesn't come from proc-macro expansion.
+            // 2. Doesn't come from foreign macro expansion.
+            //
+            // It works as follows: if the snippet we get doesn't contain `concat!(`, then it
+            // means it's not code written in the current crate so we shouldn't lint.
+            && let mut parts = original_code.split('!')
+            && parts.next().is_some_and(|p| p.trim() == "concat")
+            && parts.next().is_some_and(|p| p.trim().starts_with('('))
+        {
+            let mut literal = None;
+            let mut nb_commas = 0;
+            let mut nb_idents = 0;
+            for (token_kind, token_s, _) in tokenize_with_text(&original_code) {
+                match token_kind {
+                    TokenKind::Eof => break,
+                    TokenKind::Literal { .. } => {
+                        if literal.is_some() {
+                            return;
+                        }
+                        literal = Some(token_s);
+                    },
+                    TokenKind::Ident => nb_idents += 1,
+                    TokenKind::Comma => {
+                        nb_commas += 1;
+                        if nb_commas > 1 {
+                            return;
+                        }
+                    },
+                    // We're inside a macro definition and we are manipulating something we likely
+                    // shouldn't, so aborting.
+                    TokenKind::Dollar => return,
+                    _ => {},
+                }
+            }
+            let literal = match literal {
+                Some(lit) => {
+                    // Literals can also be number, so we need to check this case too.
+                    if lit.starts_with('"') {
+                        lit.to_string()
+                    } else {
+                        format!("\"{lit}\"")
+                    }
+                },
+                None => "\"\"".to_string(),
+            };
+            // There should always be the ident of the `concat` macro.
+            if nb_idents == 1 {
+                span_lint_and_sugg(
+                    cx,
+                    USELESS_CONCAT,
+                    macro_call.span,
+                    "unneeded use of `concat!` macro",
+                    "replace with",
+                    literal,
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+}