Error text for derived numeric ranges is unclear

[epage]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Rust Version

rustc 1.55.0 (c8dfcfe04 2021-09-06)

Clap Version

master

Minimal reproducible code

use clap::Parser;

/// Simple program to greet a person
#[derive(Parser, Debug)]
#[clap(about, version, author)]
struct Args {
    /// Name of the person to greet
    #[clap(short, long)]
    name: String,

    /// Number of times to greet
    #[clap(short, long, default_value_t = 1)]
    count: u8,
}

fn main() {
    let args = Args::parse();

    for _ in 0..args.count {
        println!("Hello {}!", args.name)
    }
}

Steps to reproduce the bug with the above code

In the clap repo

$ cargo run --example demo --features derive -- --name foo --count 20000000

Actual Behaviour

error: Invalid value for '--count <COUNT>': number too large to fit in target type

For more information try --help

Expected Behaviour

error: Invalid value for '--count <COUNT>': 20000000 too large, must be <255

For more information try --help

Additional Context

From reddit post https://www.reddit.com/r/rust/comments/rhxkau/clap_300rc7/hp9drzd/

Debug Output

No response

[epage]

The challenge with fixing this is that the error is coming from FromStr. We don't inspect the actual type information to be able to provide a more user-meaningful error.

A workaround is for the user to provide their own parse function.

[epage]

From the core of clap's perspective, this is a wont-fix. The user is supplying the type and relying on the default parser.

Two thoughts on where we can go with this

• A crate of types designed to handle cases like this (ideally using const-generics to allow restricting the range further)
• Provide data in the application domain rather than the CLI domain #2683 might lead to us doing custom trait implementations and we could instead build this into clap.

[epage]

Huh, this was supposed to be closed by #3743 3734