Detect `println!("{x}", x)`

[estebank]

When encountering println!("{x}", x) the user either meant println!("{x}") or println!("{x}", x=x). The later is only likely in <=2018 editions, while the former is what they actually likely wanted. We should suggest it.

[atamakahere-git]

Hello, I would like to work on this issue with some guidance.
It would be great if someone can give me a lead to start.
@rustbot claim

[estebank]

You'll want to start looking at

rust/compiler/rustc_builtin_macros/src/format.rs

Line 660 in 71db0dd

diag.span_label(fmt_span, "formatting specifier missing");

and work your way backwards from there.

You should look at the unused list

rust/compiler/rustc_builtin_macros/src/format.rs

Line 457 in 71db0dd

let unused = used

and check if the name corresponds to one of the pieces. If it does, then point at it while mentioning that it is being captured by the macro already.

[atamakahere-git]

Hello @estebank ,
I was trying to do the same as you told but I'm not able to check if the unused arg is present in the piece,
I can check the currently unused arg in the closure here

rust/compiler/rustc_builtin_macros/src/format.rs

Line 461 in 71db0dd

.map(|(i, _)| {

Current unused arg is:-

args.explicit_args()[i].expr.to_ty().unwrap().kind.is_simple_path().unwrap()

Gives out "x" for println!("{x}",x);
But I'm not able to compare it to pieces as it contains either String() literal or NextArgument() and NextArgument() doesn't contain the literal it refers to.

Please correct me If I misunderstood something.

[Solo-steven]

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