Here's the minimal reproduction I managed:

fn foo() {
    option.or_else(|| {
some_macro!(
"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
);
})
}

Rustfmt refuses to format this.

Some notes:

• The indentation of the "aaaaa" line doesn't matter, but its length does. Removing a single a is enough for rustfmt to stop choking.
• Replacing option.or_else with option_or_else is enough for rustfmt to stop choking.
• The long line doesn't need to be a string. Literals and variable names work too, as do operators. But different types of content trigger the bug at different lenghts.

Anyway, I guess you know rustfmt internals better than I do, so I'll stop there.

gl;hf