Mark fields used by debug format as used

[long-long-float]

Fix #123068

In the following code, a field value ProcessError::Utf8.0 is used by "{:?}".
However a "never read" warning is shown.

#[derive(Debug)]
enum ProcessError {
    Utf8(i32),
}

fn main() {
    println!("{:?}", ProcessError::Utf8(42));
}

warning: field `0` is never read
 --> test.rs:3:10
  |
3 |     Utf8(i32),
  |     ---- ^^^
  |     |
  |     field in this variant
  |
  = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
  |
3 |     Utf8(()),
  |          ~~

warning: 1 warning emitted

In this PR, I fixed to mark all enum (for now) fields used by "{:?}".

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[asquared31415]

I was under the impression that this was intentional, and that there was meant to be a note along the lines of "Debug is ignored for dead code analysis"

[Nilstrieb]

Yes, if we mark this as used then the lint Iis basically useless.
If you only use a field for its Debug output, apply #[allow(dead_code)] (or in the future, #[expect()]) with a comment about this.

[asquared31415]

There is however a bug here in that there's not a note explaining it

[long-long-float]

Oh, I misunderstood. I will make new PR to explain a note about that derived traits by Debug are intentionally ignored.

[Nilstrieb]

cool, thanks!