Add format_concat_args lint to optimize string formatting

[ginzahatemi]

This PR addresses #1601 by implementing a new lint that detects cases where format!() could be
replaced with concat!() for improved performance.

Problem

When all arguments to a format!() macro are compile-time constants and use
the standard Display formatting trait, the formatting can be done at compile time
with concat!() instead of being deferred to runtime.

Solution

This lint analyzes format string placeholders and their arguments to detect:

1. When all placeholders use the Display trait (default {} format)
2. When all arguments are const-evaluatable (compile-time constants)

When both conditions are met, it suggests replacing with an equivalent concat!()
expression.

Examples

// Will trigger the lint:
format!("hello {}", "world")  // suggests: concat!("hello ", "world")
format!("{}/file.txt", env!("CARGO_MANIFEST_DIR"))  // suggests: concat!(env!("CARGO_MANIFEST_DIR"), "/file.txt")

// Will not trigger:
format!("debug: {:?}", some_struct)  // Uses Debug trait
format!("value: {}", runtime_variable)  // Not const-evaluatable

Fixes #1601

[ginzahatemi]

Hello @smoelius, sorry to bother you. Could you please help me with the CI failures I'm encountering?

I tried to fix it myself, but it looks like I am repeatedly getting the clippy error. I tried adding the following to my Cargo.toml file:

clippy_utils = { workspace = true }

But I'm still seeing the same errors. Any idea what might be going wrong?

Thank You!

[smoelius]

@ginzahatemi Sorry for being slow to respond.

Here are a few things I noticed.

I think the compilation errors you are seeing are from a mismatch between the rust-toolchain file and the clippy_utils version.

Specifically, for the clippy_utils version you are using, I think the toolchain should be nightly-2025-04-03.

[smoelius]

Suggested change

	[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"
	[build]
	target-dir = "../../target/examples"
	[target.'cfg(all())']
	linker = "dylint-link"

To match the other examples' configs.

[smoelius]

This file can be removed. A .gitignore in a parent directory accomplishes this.

[smoelius]

Suggested change

	edition = "2021"
	edition = "2024"

[smoelius]

Suggested change

	channel = "nightly-2025-02-20"
	channel = "nightly-2025-04-03"

There's a test to ensure that all examples use the same toolchain, and this is the toolchain they currently use.

Also, I think this is the toolchain that goes with the version of clippy_utils you are currently using.

[smoelius]

Suggested change

	version = "0.1.0"
	version = "4.1.0"

To match the versions used elsewhere in this repository.

[smoelius]

@ginzahatemi Thank you very much for your work on this! I will pick up this PR, if you don't mind.