Support custom string for None/Some(T)

[Nokel81]

It would be very useful to be able to support specialized display strings for when an argument of an enum is of type Option<T> and is the variant None. This would also remove the Some(...) from the display on the Some variant.

[dtolnay]

Could you show an example of what you would like to be able to write?

Nothing in thiserror automatically puts "None" or "Some" into the display string, that must be from using {:?} on the field in your format string.

[Nokel81]

I guess I meant that for the type Option<T: Display> you would provided two strings, one for Some(t) which would be the same as {} for the T and the other not having one at all.

Example:

#[derive(Error)]
pub enum MyError {
    #[error(
        1.Some:"a variant error occurred with {1}: {0}"; 
        1.None:"there was an empty variant error: {0}"
    )]
    Variant(String, Option<usize>)
}

[dtolnay]

I would prefer not to build fancier support for this into thiserror. For now you can handwrite the Display impl, or make this work using the existing features depending on your preference.

#[derive(Error, Debug)]
pub enum MyError {
    Variant(String, Option<usize>)
}

impl Display for MyError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            MyError::Variant(a, Some(b)) => {
                write!(f, "a variant error occurred with {}: {}", b, a)
            }
            MyError::Variant(a, None) => {
                write!(f, "there was an empty variant error: {}", a)
            } 
        }
    }
}

#[derive(Error, Debug)]
pub enum MyError {
    #[error("{}: {0}", match .1 {
        Some(n) => format!("a variant error occurred with {}", n),
        None => format!("there was an empty variant error"),
    })]
    Variant(String, Option<usize>),
}

[Nokel81]

Would it be possible to get the second one into the documentation? I didn't even know that existed.

[dtolnay]

I believe this is largely covered by the third bullet of https://github.com/dtolnay/thiserror/tree/ff9524e1fd65fc49cfe905e9456c255e470fb239#details which mentions that format arguments may be arbitrary expressions, with multiple examples.

[Nokel81]

Excellent, thanks.