Investigate size hints for formatting

[alexcrichton]

There is a closed RFC on this topic with some lingering questions at the end, but one of the deficiencies of format! is that it does not have size hints (and hence cannot preallocate a String). It would be good to look into this!

[jD91mZM2]

Wow... this has been here since 2015 😮
I personally very much like this, as format! is currently slower than pushing to a pre-allocated string, which makes concatenation a pain.

If you do add some size_hint function, I'd very much appreciate some macro to generate both write and size hint from a single format. Such as

derive_display! {
    for Greeting,

    "Hello {}!",
    self.who length self.who.len()
}

would become

impl Display for Greeting {
    fn write(&self, fmt: &mut Formatter) -> Result {
        write!(fmt, "Hello {}", self.who)
    }
    fn size_hint(&self) -> usize {
        6 + self.who.len()
    }
}

I have no idea how you would implement such a macro, but it would be epic...

[GuillaumeGomez]

Anything new on this? :)