Incorrect warning for unused field.

[rcls]

(Note entirely sure what the intended behaviour is here, but named fields versus tuple fields are treated differently, which seems wrong.)

Given the following code: (https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8c1b3d36eae323990f50c01d182c042d)

#[derive(Debug)]
pub struct Foo{x:i32}

fn main() {
    let foo = Foo{x:1};
    println!("{:?}", foo);
}

The current output is:

warning: field is never read: `x`

But the field is actually read due to the #[derive(Default)] and the use of that.

Arguable, a warning here is sensible in that as the usage is "debug" output only. But in that case it is hard to see why this doesn't warn also (https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a03f26d9f9c53bda612313913bfaf67e):

#[derive(Debug)]
pub struct Foo(i32);

fn main() {
    let foo = Foo(1);
    println!("{:?}", foo);
}

At least these two should be treated consistently?

[ehuss]

Thanks for the report! This is currently intended behavior, as otherwise adding #[derive(Debug)] would prevent the unused warning from working. I understand it can be problematic as actually using the debug formatting seems like it should count as a "use". There is a deeper explanation at #88900 (comment).

Closing as a duplicate of #88900, and the tuple issue is #92790.