Fix debug ICE when previous arg span during extra comma removal comes from macro context

[jieyouxu]

Manually calculate prev.shrink_to_hi().to(span) to avoid empty span when prev comes from a macro context, such as in the code below:

#![feature(never_type)]
fn f(a: !) {}

fn main() {
    f(panic!(), 1);
}

Fixes #112437.

[rustbot]

r? @oli-obk

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[lukas-code]

This fix looks incorrect if the argument comes from a macro variable, for example for the following the span will now start after 1 and go all the way to 2:

fn foo(_: i32) {}

macro_rules! m {
    ($e:expr) => {
        foo(1, $e)
    }
}

fn main() {
    m!(2)
}

I think you can swap the arguments to get a better span: span.to(prev.shrink_to_hi()).

[jieyouxu]

Unfortunately that isn't correct either: Span::to will also return incomplete span when the "current" argument span (span) is from a macro context...

e.g.

f(1, panic!());

I will have to think about how to correctly fix this span if either comes from macro contexts...

[jieyouxu]

Unfortunately I do not know how to properly fix this issue, closing for now.