Middle-ground for wrapping function/macro arguments

[ssokolow]

A lot of the assert! calls in my code are just too long to fit on a single line, but that "tiny first and last argument, big middle one" structure produces a lot of wasted space for tall layouts.

assert!(1 == 1,
        "12345678901234567890123456789012345678901234567890123456789012345678{}",
        901);

In my Python programs, I resolve this sort of issue by using a word-wrap-esque style like this:

assert!(1 == 1,
        "12345678901234567890123456789012345678901234567890123456789012345678{}", 901);

...or, in rare situations when the above isn't any better, like this...

assert!(1 == 1, "1234567890123456789012345678901234567890123456789012\
        3456789012345678{}", 901);

For asserts of the form "Failed to foo. Got {}", it also has the benefit that the variable name is right next to the substitution placeholder, rather than requiring a full-width visual CRLF.

In Rust, it's annoying to have to throw out rustfmt's attempts to change these every time I run it and it'd be nice if there were some kind of layout option which left them intact.

[nrc]

cc rust-lang/style-team#86