stdout / stderr policy

[gnzlbg]

Currently rustfmt emits a lot of output, including warnings, Warnings, internal errors, etc.

Most of it seems to be information relevant for debugging rustfmt, but not that relevant to users which cannot do much about it in most cases.

It would be great to clean some of this up such that:

• the warnings emitted are things that users can do something about (e.g. a configuration option that doesn't exist)
• the errors emitted are errors that make rustfmt fail such that no formatting is applied (e.g. parsing errors, configuration option with incorrect value)

For all other things rustfmt could have a --verbose option (or cargo fmt -vv) that prints what are currently internal errors, exceeded max line limit, and other things that while useful for debugging, aren't really actionable for users.

[nrc]

We've changed up the output quite a bit in Rustfmt and I think it is much better. The two invariants you provide sound good and we should check that we abide by them. We have --quiet and --verbose modes too.

[ModProg]

I would really like when rustfmt could print a warning if it did not format, printing on what line it failed.

I often times start to bisecting my file with comments to find the offending line, just to notice that it is a too long string literal that I can easily break up my self as soon as I finally find it, e.g. I had this recently prohibiting format:

fn main() {
    Some("").map(|value| {
                    assert_eq!(value, "invalid key `{value}`, https://www.w3.org/TR/2011/WD-html5-20110525/syntax.html#attributes-0") 
        });
}