Strange compilation error "error: expected ," in arg derive

[apbr]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.71.0 (8ede3aae2 2023-07-12)

Clap Version

4.4.2

Minimal reproducible code

use clap::Parser;

#[derive(Parser, Debug)]
#[command(author, version, about)]
pub struct Args {
    #[arg(long, short, default_value_t = "foo".to_string())]
    pub some_value: String,
}

fn main() {
    let args = Args::parse();
    println!("Hello, world!");
}

Steps to reproduce the bug with the above code

cargo run

Actual Behaviour

I do get this error:

error: expected `,`
 --> src/main.rs:6:47
  |
6 |     #[arg(long, short, default_value_t = "foo".to_string())]
  |                                               ^

This message is not really helpful.

Expected Behaviour

I don't see why this should not just compile. At least there should be a more conclusive error message.

Additional Context

I know I can work around this by using #[arg(long, short, default_value_t = String::from("foo"))] or #[arg(long, short, default_value_t = ("foo".to_string()))].
Though the latter option gives the following warning when compiling:

warning: unnecessary parentheses around assigned value
 --> src/main.rs:6:42
  |
6 |     #[arg(long, short, default_value_t = ("foo".to_string()))]
  |                                          ^                 ^
  |
  = note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
  |
6 -     #[arg(long, short, default_value_t = ("foo".to_string()))]
6 +     #[arg(long, short, default_value_t = "foo".to_string())]
  |

Debug Output

No response

[epage]

Not quite sure what is going on. If I comment out the code that generates code for default_value_t, it still happens.

[Sinfaen]

also seeing this

[epage]

For anyone blocked on this, there are two workarounds for this, depending on your situation

• Use default_value
• Move the call out to a function

See

#!/usr/bin/env nargo
---
[dependencies]
clap = { version = "4", features = ["derive"] }
---

use clap::Parser;

#[derive(Parser, Debug)]
#[command(author, version, about)]
pub struct Args {
    //#[arg(long, short, default_value = "foo")]
    #[arg(long, short, default_value_t = "foo".to_string())]
    //#[arg(long, short, default_value_t = default())]
    pub some_value: String,
}

fn default() -> String {
    "foo".to_string()
}

fn main() {
    let args = Args::parse();
    println!("Hello, world!");
}

[lolbinarycat]

there's another workaround i've found: surround it in parenthesies: default_value_t = ("foo".to_string()).

unfortunately rustc deems these "unnecessary" and will output a warning about it.