Add format_concat_args lint to optimize string formatting

examples/supplementary/format_concat_args/.cargo/config.toml

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+[target.'cfg(all())']
+rustflags = ["-C", "linker=dylint-link"]
+
+# For Rust versions 1.74.0 and onward, the following alternative can be used
+# (see https://github.com/rust-lang/cargo/pull/12535):
+# linker = "dylint-link"

examples/supplementary/format_concat_args/.gitignore

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+/target

examples/supplementary/format_concat_args/Cargo.toml

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+[package]
+name = "format_concat_args"
+version = "0.1.0"
+authors = ["Your Name <[email protected]>"]
+description = "A Dylint lint to suggest using `concat!(...)` instead of `format!(...)` for all-constant Display arguments."
+edition = "2021"
+publish = false
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+clippy_utils = { git = "https://github.com/rust-lang/rust-clippy", rev = "7bb54d91be1af212faaa078786c1d2271a67d4f9" }
+dylint_linting = { path = "../../../utils/linting" }
+
+[dev-dependencies]
+dylint_testing = { path = "../../../utils/testing" }
+
+[features]
+rlib = ["dylint_linting/constituent"]
+
+[lints]
+workspace = true
+
+[package.metadata.rust-analyzer]
+rustc_private = true

examples/supplementary/format_concat_args/README.md

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+# format_concat_args
+
+
+## Examples
+
+
+## Building and Testing
+
+## Limitations
+
+## References
+

examples/supplementary/format_concat_args/rust-toolchain

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+[toolchain]
+channel = "nightly-2025-02-20"
+components = ["llvm-tools-preview", "rustc-dev"]

examples/supplementary/format_concat_args/src/lib.rs

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+#![feature(rustc_private)]
+
+extern crate rustc_ast;
+extern crate rustc_errors;
+extern crate rustc_hir;
+extern crate rustc_lint;
+extern crate rustc_middle;
+extern crate rustc_session;
+extern crate rustc_span;
+
+#[cfg(not(feature = "rlib"))]
+dylint_linting::dylint_library!();
+
+use clippy_utils::{
+    diagnostics::span_lint_and_sugg,
+    is_expn_of,
+};
+use rustc_hir::Expr;
+use rustc_lint::{Lint, LateContext, LateLintPass, Level};
+use rustc_session::declare_lint_pass;
+use rustc_span::sym;
+
+// Declare the lint directly
+pub static FORMAT_CONCAT_ARGS: &Lint = &Lint {
+    name: "format_concat_args",
+    default_level: Level::Allow,
+    desc: "Checks for `format!(...)` invocations where `concat!(...)` could be used instead.",
+    edition_lint_opts: None,
+    report_in_external_macro: true,
+    future_incompatible: None,
+    is_externally_loaded: false,
+    eval_always: false,
+    feature_gate: None,
+    crate_level_only: false,
+};
+
+// Declare the lint pass
+declare_lint_pass!(FormatConcatArgs => [FORMAT_CONCAT_ARGS]);
+
+impl<'tcx> LateLintPass<'tcx> for FormatConcatArgs {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        // Check if the expression is a `format!` macro invocation
+        if is_expn_of(expr.span, sym::format_args.as_str()).is_some() {
+            let expn_data = expr.span.ctxt().outer_expn_data();
+
+            // Ensure this is from `std::format!` specifically
+            if expn_data.macro_def_id != cx.tcx.get_diagnostic_item(sym::format_macro) {
+                return; // Not a std::format! macro call
+            }
+
+            span_lint_and_sugg(
+                cx,
+                FORMAT_CONCAT_ARGS,
+                expr.span,
+                "this `format!(...)` invocation might be replaceable with `concat!(...)`",
+                "consider using concat! if all arguments are constant",
+                "concat!(...)".to_string(),
+                rustc_errors::Applicability::HasPlaceholders,
+            );
+        }
+    }
+}
+
+#[cfg(not(feature = "rlib"))]
+#[allow(unused_extern_crates)]
+#[allow(clippy::float_arithmetic, clippy::option_option, clippy::unreachable)]
+fn main() {
+   dylint_linting::test(env!("CARGO_PKG_NAME"), &[]);
+}

examples/supplementary/format_concat_args/test_lint.rs

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+// for testing,
+fn main() {
+    // Should trigger the lint
+    let _s1 = format!("simple string");
+    let _s2 = format!("hello {}", "world");
+
+    // Should not trigger (not format!)
+    let _s3 = "simple string".to_string();
+    println!("hello {}", "world");
+}