Add new useless_concat lint

CHANGELOG.md

@@ -6170,6 +6170,7 @@ 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

@@ -770,6 +770,7 @@ 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

@@ -388,6 +388,7 @@ 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;
@@ -967,5 +968,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(manual_ignore_case_cmp::ManualIgnoreCaseCmp));
     store.register_late_pass(|_| Box::new(unnecessary_literal_bound::UnnecessaryLiteralBound));
     store.register_late_pass(move |_| Box::new(arbitrary_source_item_ordering::ArbitrarySourceItemOrdering::new(conf)));
+    store.register_late_pass(|_| Box::new(useless_concat::UselessConcat));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/useless_concat.rs

@@ -0,0 +1,101 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::macros::macro_backtrace;
+use clippy_utils::match_def_path;
+use clippy_utils::source::snippet_opt;
+use rustc_ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lexer::{Cursor, 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,
+    suspicious,
+    "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 current_pos = 0;
+            let mut cursor = Cursor::new(&original_code);
+            let mut nb_commas = 0;
+            let mut nb_idents = 0;
+            loop {
+                let token = cursor.advance_token();
+                match token.kind {
+                    TokenKind::Eof => break,
+                    TokenKind::Literal { .. } => {
+                        if literal.is_some() {
+                            return;
+                        }
+                        literal = Some(&original_code[current_pos..current_pos + token.len as usize]);
+                    },
+                    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,
+                    _ => {},
+                }
+                current_pos += token.len as usize;
+            }
+            // 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.unwrap_or("\"\"").into(),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+}

tests/ui/useless_concat.fixed

@@ -0,0 +1,19 @@
+#![warn(clippy::useless_concat)]
+#![allow(clippy::print_literal)]
+
+macro_rules! my_concat {
+    ($fmt:literal $(, $e:expr)*) => {
+        println!(concat!("ERROR: ", $fmt), $($e,)*);
+    }
+}
+
+fn main() {
+    let x = ""; //~ useless_concat
+    let x = "a"; //~ useless_concat
+    println!("b: {}", "a"); //~ useless_concat
+    // Should not lint.
+    let x = concat!("a", "b");
+    let local_i32 = 1;
+    my_concat!("{}", local_i32);
+    let x = concat!(file!(), "#L", line!());
+}

tests/ui/useless_concat.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::useless_concat)]
+#![allow(clippy::print_literal)]
+
+macro_rules! my_concat {
+    ($fmt:literal $(, $e:expr)*) => {
+        println!(concat!("ERROR: ", $fmt), $($e,)*);
+    }
+}
+
+fn main() {
+    let x = concat!(); //~ useless_concat
+    let x = concat!("a"); //~ useless_concat
+    println!("b: {}", concat!("a")); //~ useless_concat
+    // Should not lint.
+    let x = concat!("a", "b");
+    let local_i32 = 1;
+    my_concat!("{}", local_i32);
+    let x = concat!(file!(), "#L", line!());
+}

tests/ui/useless_concat.stderr

@@ -0,0 +1,23 @@
+error: unneeded use of `concat!` macro
+  --> tests/ui/useless_concat.rs:11:13
+   |
+LL |     let x = concat!();
+   |             ^^^^^^^^^ help: replace with: `""`
+   |
+   = note: `-D clippy::useless-concat` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::useless_concat)]`
+
+error: unneeded use of `concat!` macro
+  --> tests/ui/useless_concat.rs:12:13
+   |
+LL |     let x = concat!("a");
+   |             ^^^^^^^^^^^^ help: replace with: `"a"`
+
+error: unneeded use of `concat!` macro
+  --> tests/ui/useless_concat.rs:13:23
+   |
+LL |     println!("b: {}", concat!("a"));
+   |                       ^^^^^^^^^^^^ help: replace with: `"a"`
+
+error: aborting due to 3 previous errors
+