Add new `useless_concat` lint (#13829)

Fixes #13793.

Interestingly enough, to actually check that the macro call has at least
two arguments, we need to use the rust lexer after getting the original
source code snippet.

changelog: Add new `useless_concat` lint

3 files changed, 107 insertions, 0 deletions

diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index bb825c7655f..5fcb851dfeb 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -764,6 +764,7 @@ 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,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 006145cc623..ca1a0b35710 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -393,6 +393,7 @@ 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;
@@ -937,6 +938,7 @@ 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)));
diff --git a/clippy_lints/src/useless_concat.rs b/clippy_lints/src/useless_concat.rs
new file mode 100644
index 00000000000..1ed1fbb3b9c
--- /dev/null
+++ b/clippy_lints/src/useless_concat.rs
@@ -0,0 +1,104 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::macros::macro_backtrace;
+use clippy_utils::paths::CONCAT;
+use clippy_utils::source::snippet_opt;
+use clippy_utils::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 = "1.89.0"]
+    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_s, _) = 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.
+            && CONCAT.matches(cx, macro_call.def_id)
+            // 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 => {
+                        if token_s == "true" || token_s == "false" {
+                            literal = Some(token_s);
+                        } else {
+                            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,
+                    _ => {},
+                }
+            }
+            // 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",
+                    format!("{lit_s:?}"),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+}