New lint: suggest using "implicit named arguments"

[stanislav-tkach]

What it does

This lint suggests using the feature added in the 1.58 version of Rust: implicit named arguments for formatting macros.

Lint Name

implicit_named_arguments

Category

style

Advantage

• Slightly less verbose code.
• (Arguably) less error prone code.

Drawbacks

The only drawback that I can see is that not everyone would probably be happy with this lint, so perhaps it should be opt-in (pedantic).

Example

println!("1 = {}, 2 = {}, 3 = {}", first, second, third);

Could be written as:

println!("1 = {first}, 2 = {second}, 3 = {third}");

[stanislav-tkach]

Perhaps this issue duplicated #633.

[edmorley]

added in the 1.18 version of Rust

I think this should say 1.58, rather than 1.18:
https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1580-2022-01-13

[nyurik]

Initially I made a suggestion for this in rust-lang/rustfmt#5450 , but it seems clippy would be a better target. I do think that this issue should be expanded to cover all cases listed below. My adjusted original post:

Since 1.58 Rust allows captured identifiers in format calls, i.e. println!("Hello, {person}!"); style strings (documentations). I would like to propose cargo clippy to automatically convert println!("Hello, {}!", person); and other format!-like calls to the inlined versions.

Original code	After cargo clippy	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

[nyurik]

I began av implementation of this lint based on the PR #8518 by @Alexendoo. See first pass at nyurik@99f3352. So far I got it to identify most cases, but I am still struggling with the internal API -- for some reason the suggestions do not modify the source code. I am also not certain how to parse the structs in case the same value is used multiple times, i.e. ("{0} {0}", var) case. Any help and feedback is greatly appreciated :)

[Alexendoo]

You could do the same as in check_literal, using a map to only consider arguments that are used once

rust-clippy/clippy_lints/src/write.rs

Lines 423 to 430 in 818a91e

	fn check_literal(cx: &LateContext<'_>, format_args: &FormatArgsExpn<'_>, args: &[FormatArgsArg<'_>], name: &str) {
	let mut counts = HirIdMap::<usize>::default();
	for arg in args {
	*counts.entry(arg.value.hir_id).or_default() += 1;
	}
	for arg in args {
	if counts[&arg.value.hir_id] == 1

[nyurik]

I added some more implementation code, and some of it even works (kinda). Any feedback is welcome - probably best to do it on the #9233 PR. Thanks!