align strings when trailing backslash is used

[nikomatsakis]

Rust has this nifty feature that a trailing backslash ignores leading whitespace on the next line when found in a string constant. This means that if I have a Rust source file like:

fn main() {
    println!("foo\
bar");
}

I can instead indent it like:

fn main() {
    println!("foo\
              bar");
}

But rustfmt doesn't do this, from what I can tell. Perhaps we ought to?

[marcusklaas]

We used to do this by default, but we have disabled it because it often did not do the right thing.. I'm sure there's a configuration option for this.

[nikomatsakis]

@marcusklaas hmm, are there older issues yo can point me at? in what way could it not do the right thing? it seems like if people don't want the strings aligned, they should not use \...

[topecongiro]

Closed via #1724.