How to displaydoc an Option?

[sffc]

How can I write a displaydoc impl for the following struct?

#[displaydoc("{kind}: {str_context}")]
pub struct DataError {
    /// Broad category of the error.
    pub kind: DataErrorKind,

    /// Additional context, if available.
    pub str_context: Option<&'static str>,
}

This errors out with:

`std::option::Option<&str>` cannot be formatted with the default formatter

What I would like is to make an error that contains ": {str_context}" when Some and nothing when None.

Do I need to manually implement Display, or can I use displaydoc?

[yaahc]

Right now we don't have any support displaying options like this. We do have something similar on Path/PathBuf, where we will call display() transparently in the background, so I wouldn't be against adding some support for convenient formatting for Option.

I do have questions about the specific case you mentioned. It looks like you'd want to print "Some DataErrorKind Display: " with a trailing : and whitespace even when the str_context is None. Is this the correct behavior you'd like to see? Without knowing your specific usecase in detail my intuition is you'd want to show "Some DataErrorKind Display: some str_context" for the Some case and "SomeDataErrorKind Display" in the None case. I imagine we'd need something a bit more complicated to support the later case.

[sffc]

Without knowing your specific usecase in detail my intuition is you'd want to show "Some DataErrorKind Display: some str_context" for the Some case and "SomeDataErrorKind Display" in the None case.

Yes, that is what I meant. I would like to format with "{}: {}" when the context is Some and "{}" when the context is None. I was thinking a way to go about this would be to format the struct as a whole with "{}{}", where the second placeholder is ": {}" if Some or "" if None.

[20jasper]

A workaround I use is a newtype, eg TOption that has its own Display impl to passthrough or display a default message. I also impl From<Option<T>> for Option<T> for ease of use