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Mark fields used by debug format as used #124371

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@long-long-float
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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 "{:?}".

@rustbot
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rustbot commented Apr 25, 2024

r? @fee1-dead

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 25, 2024
@asquared31415
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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"

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

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

@Noratrieb
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cool, thanks!

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field "never read" warning for field that is used by derive(Debug)

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@long-long-float @rustbot @asquared31415 @Noratrieb @fee1-dead