new lint: passthrough enumerate

What it does

check for calls. enumerate() where the index is only passed through deeper down the iter chain without actually being used

Advantage

simpler code

Drawbacks

No response

Example

pub fn foo(x: Vec<Option<String>>) {
    let is = x
        .clone()
        .into_iter()
        .enumerate() // move form here
        .map(|(i, o)| (i, o.unwrap_or_default())) // "i" not used, only passed through
        .filter(|(_i, o)| o.len() > 3)
        .collect::<Vec<_>>();

    let could_be = x
        .into_iter()
        .map(|o| o.unwrap_or_default())
        .enumerate() // to here
        .filter(|(_i, o)| o.len() > 3)
        .collect::<Vec<_>>();
}

The issue provides only Rust examples and names no implementation files or tests. Start by locating existing Clippy lints that inspect enumerate() and their corresponding UI tests, then trace how lint suggestions are recorded. Done means the new lint detects the shown passthrough pattern and has coverage for the relevant cases.