clippy::used_underscore_binding fires on an unused function parameter

[Kriskras99]

Summary

clippy::used_underscore_binding fires on an unused function parameter.

To reproduce:

git clone https://github.com/Kriskras99/ferris_dancing.git
cd ferris_dancing
git checkout acd683a
cargo +nightly clippy

You don't need the submodule to reproduce, and you can ignore the dead_code warning from another crate in the workspace

Lint Name

used_underscore_binding

Reproducer

I tried this code:

impl BinaryDeserialize<'_> for Xtx {
    type Ctx = ();
    type Output = Self;

    #[tracing::instrument(skip(reader))]
    fn deserialize_at_with_ctx(
        reader: &(impl ReadAtExt + ?Sized),
        position: &mut u64,
        _ctx: (),
    ) -> Result<Self, ReadError> {
        let start = *position;
        let magic = reader.read_at::<u32le>(position)?;
        test_eq(&magic, &0x4E76_4644)?;

        let size = reader.read_at::<u32le>(position)?;
        test_eq(&size, &0x10)?;

        let major_version = reader.read_at::<u32le>(position)?;
        test_eq(&major_version, &0x1)?;

        let minor_version = reader.read_at::<u32le>(position)?;

        let mut blocks = Vec::new();

        loop {
            match reader.read_at::<u32le>(position) {
                Ok(magic) => {
                    *position -= 4;
                    if magic != 0x4E76_4248 {
                        break;
                    }
                }
                Err(ReadError::IoError {
                    error: _,
                    backtrace: _,
                }) => break,
                Err(error) => return Err(error),
            }
            tracing::trace!("Block start: {}", *position - start);
            let block = reader.read_at::<Block>(position)?;
            blocks.push(block);
        }

        let mut images = Vec::new();

        let mut index = 0;
        while index < blocks.len() {
            let block = blocks.get(index).unwrap_or_else(|| unreachable!());
            match &block.data {
                BlockData::TextureHeader(hdr) => {
                    let second_block = blocks.get(index + 1);
                    let data = match second_block {
                        Some(block) => match &block.data {
                            BlockData::Data(data) => Ok(data),
                            _ => Err(ReadError::custom("Found header without data".to_string())),
                        },
                        None => Err(ReadError::custom("Found header without data".to_string())),
                    }?;

                    images.push(parse_data_block_to_image(hdr, data)?);

                    index += 2;

                    Ok(())
                }
                BlockData::Data(_) => {
                    Err(ReadError::custom("Found data without a header".to_string()))
                }
                BlockData::Three(_) => {
                    index += 1;
                    Ok(())
                }
            }?;
        }

        Ok(Self {
            major_version,
            minor_version,
            images,
        })
    }
}

I saw this happen:

warning: used binding `_ctx` which is prefixed with an underscore. A leading underscore signals that a binding will not be used
  --> ubiart_toolkit/src/cooked/xtx/parser.rs:30:9
   |
30 |         _ctx: (),
   |         ^^^^
   |
note: `_ctx` is defined here
  --> ubiart_toolkit/src/cooked/xtx/parser.rs:30:9
   |
30 |         _ctx: (),
   |         ^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_binding
   = note: `-W clippy::used-underscore-binding` implied by `-W clippy::pedantic`
   = help: to override `-W clippy::pedantic` add `#[allow(clippy::used_underscore_binding)]`

I expected to see this happen:
No output as _ctx is not used

Version

rustc 1.80.0-nightly (1871252fc 2024-05-15)
binary: rustc
commit-hash: 1871252fc8bb672d40787e67404e6eaae7059369
commit-date: 2024-05-15
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4

Additional Labels

No response

[AjithPanneerselvam]

I'd like to work on this issue if it is accepted :)

@rustbot claim