Handle captured identifiers in format strings

[nyurik]

Since 1.58 Rust allows println!("Hello, {person}!"); style strings (documentations). I would like to propose cargo fmt to automatically convert println!("Hello, {}!", person); and other format!-like calls to the inlined versions, either by default, or, at least initially, as an opt-in.

Original code	After cargo fmt	Notes
println!("{}", a)	println!("{a}")	positional in-lining
println!("{a}", a=b)	println!("{b}")	named param
println!("{a:0$}", width)	println!("{a:width$}")	width param
println!("{a:.0$}", prec)	println!("{a:.prec$}")	precision param

[calebcartwright]

I wonder if rustfix might be a better venue for such transformations

[nyurik]

I wonder if rustfix might be a better venue for such transformations

I just looked at rustfix readme: The magic of rustfix is entirely dependent on the diagnostics implemented in the Rust compiler (and external lints, like clippy).

My understanding is that rustfix won't actually decide anything, but rather use whatever suggestion is made by rustc or clippy. I am not sure this type of fix would belong in the rustc itself. Do you think clippy would be a better target?

[calebcartwright]

I think clippy would probably be the best place to start, just to see if they happen to already have a lint or if they have one planned. If they do, then that's something rustfix could pick up.

[nyurik]

Thanks @calebcartwright , turns out clippy already has a similar (smaller-scoped) request, so closing in favor of rust-lang/rust-clippy#8368

[calebcartwright]

Perfect, thanks for sharing!