specifying default value makes field required

[jgarvin]

How to reproduce

With this declaration passing the argument is not required:

#[clap(short, long)]
cores: Vec<u32>,

with this declaration it is:

#[clap(short, long, default_value = "")]
cores: Vec<u32>,

I would expect specifying a default value to imply optional, so this doesn't seem like it can be intentional.

Version

• Rust: rustc 1.46.0-nightly (16957bd4d 2020-06-30)
• Clap: "3.0.0-beta.1"

[pksunkara]

I think you are trying to assign your own semantics to clap here. Having a default value makes the arg optional but the value of the arg will not be undefined but will instead be the default value.

[CreepySkeleton]

default_value doesn't work with Vec. I don't think it should because it implies "only one value" while Vec is all about multiple values. Ideally, this should be a compile time error.

But default_valueS must work as described - its presence should not affect requiredness. This is a bug in derive.

[CreepySkeleton]

@jgarvin The temporary workaround is #[clap(required = false)].

[pksunkara]

@CreepySkeleton can you explain what exactly the problem is and what the solution should be. I am afraid that I don't understand.

[CreepySkeleton]

@pksunkara

#[derive(Clap)]
struct Opts {
    // this option is required because it's not Option (please excuse the pun)
    a: u32,

    // this option is NOT required, despite the fact it's not Option
    // if absent, the default value will be used
    #[clap(default_value = "1")]
    b: u32,

    // this should be a compile time error because default_value (singular)
    // is kinda meaningless with `Vec`
    //
    // This case is what OP complained about: it turns out the field somehow becomes required (WTF???).
    #[clap(default_value = "1")]
    c: Vec<u32>,

    // this should be a legit - and preferable! - usage
    // the field is not required
    #[clap(default_values = &["1"])]
    d: Vec<u32>
}

[epage]

With master, I can't reproduce this issue

use clap::Clap;

#[derive(Clap, Debug)]
struct Opts {
    // this option is required because it's not Option (please excuse the pun)
    //a: u32,

    // this option is NOT required, despite the fact it's not Option
    // if absent, the default value will be used
    //#[clap(default_value = "1")]
    //b: u32,

    // this should be a compile time error because default_value (singular)
    // is kinda meaningless with `Vec`
    //
    // This case is what OP complained about: it turns out the field somehow becomes required (WTF???).
    #[clap(default_value = "1")]
    c: Vec<u32>,
    // this should be a legit - and preferable! - usage
    // the field is not required
    //#[clap(default_values = &["1"])]
    //d: Vec<u32>,
}

fn main() {
    let opts = Opts::parse(); // panics
    dbg!(opts);
}

❯ cargo run
   Compiling test-clap v0.1.0 (/home/epage/src/personal/test-clap)
    Finished dev [unoptimized + debuginfo] target(s) in 0.60s
     Running `target/debug/test-clap`
[src/main.rs:27] opts = Opts {
    c: [
        1,
    ],
}

❯ cargo run -- --help
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/test-clap --help`
test-clap 

USAGE:
    test-clap [C]...

ARGS:
    <C>...    [default: 1]

FLAGS:
    -h, --help       Print help information
    -V, --version    Print version information

Am I doing something wrong?