Using an enum for the default_value or the possible_values

[Vagelis-Prokopiou]

Please complete the following tasks

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

Rust Version

rustc 1.57.0 (f1edd0429 2021-11-29)

Clap Version

3.0.6

Minimal reproducible code

use std::str::FromStr;
use clap::{Parser};


#[derive(Debug)]
pub enum TranslationProvider { DeepL, Amazon, Google }

impl std::fmt::Display for TranslationProvider {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        match self {
            TranslationProvider::DeepL => { write!(f, "deepl") }
            TranslationProvider::Amazon => { write!(f, "amazon") }
            TranslationProvider::Google => { write!(f, "google") }
        }
    }
}

impl FromStr for TranslationProvider {
    type Err = String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        if s == "deepl" { return Ok(Self::DeepL); }
        if s == "amazon" { return Ok(Self::Amazon); }
        if s == "google" { return Ok(Self::Google); }
        return Err("This translation provider is not valid".to_owned());
    }
}

/// File translator using DeepL
#[derive(Parser, Debug)]
#[clap(about, version, author)]
struct Args {
    /// The translation provider to use
    #[clap(
    long,
    default_value(&TranslationProvider::DeepL.to_string().as_str()),
    possible_values([
    clap::PossibleValue::new(TranslationProvider::DeepL.to_string().as_str()),
    ]))]
    translation_provider: TranslationProvider,
}

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

Steps to reproduce the bug with the above code

cargo run -- --help

Actual Behaviour

I get a build error:

error[E0716]: temporary value dropped while borrowed
  --> src/main.rs:38:20
   |
32 | #[derive(Parser, Debug)]
   |          ------
   |          |
   |          lifetime `'b` defined here
   |          argument requires that borrow lasts for `'b`
...
35 |     /// The translation provider to use
   |                                       - temporary value is freed at the end of this statement
...
38 |     default_value(&TranslationProvider::DeepL.to_string().as_str()),
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use

error[E0716]: temporary value dropped while borrowed
  --> src/main.rs:40:30
   |
32 | #[derive(Parser, Debug)]
   |          ------
   |          |
   |          lifetime `'b` defined here
   |          argument requires that borrow lasts for `'b`
...
35 |     /// The translation provider to use
   |                                       - temporary value is freed at the end of this statement
...
40 |     clap::PossibleValue::new(TranslationProvider::DeepL.to_string().as_str()),
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use

Expected Behaviour

I think that we should be able to use the string representation of enums some way. Either as String or as String.as_str() or as &String.

Additional Context

No response

Debug Output

No response

[epage]

In #1041, we plan to evaluate removing the lifetime parameter, most likely with a type like Cow<'static, str> so we can have &'static str and String together. This would help in your case.

We already provide default_value_t (see also the tutorial) which resolves the first problem you had.

Personally, when an enum maps to string values, I do something like

impl TranslationProvider {
    fn as_str(self) -> &'static str {
        match self {
            ...
        }
    }
}


impl std::fmt::Display for TranslationProvider {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        self.as_str().fmt(f)
    }
}

This is an alternative that resolves your second lifetime issue because you can just call as_str() rather than to_string().as_str().

Alternatively, you could derive or impl the ArgEnum trait (see also the tutorial).

[Vagelis-Prokopiou]

Thanks for the feedback @epage.
Removing the lifetime parameter is the best solution to my mind too.
For now, I will try your suggestions out, or will use any other workaround.

If you need anything else from me, let me know.

This issue can be linked to the issue you mentioned, and can be closed, I suppose.