padding in format args doesn't work with debug impls

[nikomatsakis]

This:

fn main() {
    println!("Hello {0:5}!", "x");
}

prints what I expect:

Hello x    !

But this:

fn main() {
    println!("Hello {0:5?}!", "x");
}

prints Hello "x"! (play), not:

Hello "x"  !

Is this intentional? If so, the format docs are pretty misleading. Otherwise, a regression perhaps? I'm not sure what is...T-libs?

cc @alexcrichton

[sfackler]

Each implementation of a fmt::* trait needs to individually decide if and how it's going to handle formatting flags. Debug impls tend to not handle them in general.

[alexcrichton]

Yeah this is "intentional" in the sense that it's not expected to work. We likely want to be more clear about this in the documentation though

[nikomatsakis]

Oh, interesting. I thought padding was imposed externally. OK, that's a shame. It'd be useful I think to have in std::fmt some wrappers that "impose" formatting from the outside -- e.g., I often wish that instead of using a flag like {:22$} I could do this:

use std::fmt;
...
format!("{}", fmt::padded(&foo, 22))

where fmt::padded takes the output of foo.fmt() and pads it to be of the given width. These could then be readily composed, so {:22$?} would be:

use std::fmt;
....
format!("{}", fmt::padded(fmt::debug(&foo), 22))

I should just make some things like this in the compiler I guess. Anyway. I'll close then!

[paul-callahan]

easy work around:

#[derive(Debug)]
pub enum Unprintable {
    X
}
#[test]
fn test_x() {
    println!("Hello {0:5}!", format!("{:?}", Unprintable::X));
}

[sourcefrog]

#59933 mentions this in the docs.