Trailing whitespace followed by long unbreakable line causes error[internal]: left behind trailing whitespace

Input code

fn f() {
    match x {
        0 => S {
            
        },
        _ => AAAAAAAAA,
    }
}

(Note, the 4th line contains 12 spaces, which you can see if you select the text.)

Actual result

$ rustfmt file.rs --config max_width=20
error[internal]: left behind trailing whitespace
 --> /usr/local/google/home/mkember/a.rs:4:4:1
  |
4 |
  | ^^^^^^^^^^^^
  |

warning: rustfmt has failed to format. See previous 1 errors. 
$ echo $?
1

Expected result

Here are some possibilities, from best to worst (IMO):

1. Successfully delete the trailing whitespace. I'm not sure why the long line that follows prevents it from doing so.
2. Print a warning that it wasn't able to remove trailing whitespace, but still succeed (exit status 0).
3. Provide an option that makes such errors non-fatal (I though error_on_unformatted=false would, but it does not).

If you're running rustfmt manually on your own code this isn't such a big deal. But in my case I am running it automatically on Rust bindings code generated by a program. I want to fail the whole program is rustfmt fails on a real problem, like invalid Rust syntax. I don't want it to fail just because it wasn't able to format optimally. One solution (at least for this example) is to make max_width very large, but that makes everything else less readable. The whole point of running rustfmt is so I can generate ugly code without worrying about formatting.

Reproduce the issue using the Rust snippet in file.rs and run rustfmt with max_width=20. Trace the trailing-whitespace handling that produces the internal error, then verify that this input removes the whitespace or otherwise completes without an inappropriate formatting failure.