Object safe Format

[EdBuilds]

Hello,
Would it be possible to make the Format trait object safe? I would like to pass messages around by using

&dyn defmt::Format 

But the static function in the trait prevents that.
Cheers!

[jonathanpallant]

I looked into this for you.

The serialiser needs to report what kind of thing is being serialised so the deserialiser can unpack it. This is done using an interned string:

pub fn fmt<T: Format + ?Sized>(f: &T) {
    istr(&T::_format_tag());
    f._format_data();
}

pub fn fmt_slice<T: Format>(values: &[T]) {
    usize(&values.len());
    istr(&T::_format_tag());
    for value in values {
        value._format_data();
    }
}

If _format_tag() takes &self, then you can't call T::_format_tag() when serialising a slice, and we can't guarantee that the slice has non-zero length, so we can't call values[0]._format_tag() either. So the trait has to include a static method, and that means we can't have dynamic trait objects.

So with the current design, no, it's not possible to have a &dyn defmt::Format. Can you elaborate on why you would like to make &dyn references to things that can be formatted with defmt? What would you wish to do with those reference?