value_parser possibly returning multiple tuples (instead of just one)

[ribeaud]

Hi. Following the example given in the documentation, I created a parse_key_value method like this:

/// Parse a single key-value pair
fn parse_key_val<T: FromStr, U: FromStr>(input: &str) -> Result<Vec<(T, U)>>
where
    <T as FromStr>::Err: std::error::Error + Send + Sync + 'static,
    <U as FromStr>::Err: std::error::Error + Send + Sync + 'static,
{
    input
        .trim()
        .split(',')
        .map(|item| {
            let idx = item
                .find('=')
                .with_context(|| format!("invalid KEY=value: no `=` found in `{}`", input))?;
            Ok((input[..idx].parse()?, input[idx + 1..].parse()?))
        })
        .collect::<Result<Vec<(T, U)>>>()
}

And I'm using it as folllowing:

    #[arg(short, long, env="SETT_METADATA_CUSTOM", value_name="NAME=VALUE", value_parser = parse_key_val::<String, String>)]
    metadata: Option<Vec<(String, String)>>,

I do not get any error during compile time. However this code panicks on runtime:

Mismatch between definition and access of `metadata`. Could not downcast to (alloc::string::String, alloc::string::String), need to downcast to alloc::vec::Vec<(alloc::string::String, alloc::string::String)>

Any clue how to solve this? Thanks!

[epage]

Option<Vec<_>> has special meaning in clap's derive, see https://docs.rs/clap/latest/clap/_derive/index.html#arg-types, which also mentions a workaround and where further discussion is

To force any inferred type (like Vec) to be treated as T, you can refer to the type by another means, like using std::vec::Vec instead of Vec. For improving this, see #4626.

[epage]

To make this more explicit, I'm referring to

    #[arg(short, long, env="SETT_METADATA_CUSTOM", value_name="NAME=VALUE", value_parser = parse_key_val::<String, String>)]
    metadata: Option<std::vec::Vec<(String, String)>>,