$ cat /tmp/a.c 
#1 "a/file/with/incredibly/long/path/name/that/exceeds/80/characters/and/i/need/to/type/some/more"
int main() { return 0; }

$ clang /tmp/a.c

$ clang-format /tmp/a.c -style=LLVM -i

$ cat /tmp/a.c 
#1 "a/file/with/incredibly/long/path/name/that/exceeds/80/characters/and/i/"   \
   "need/to/type/some/more"
int main() { return 0; }

$ clang /tmp/a.c
/tmp/a.c:2:4: error: invalid flag line marker directive
   "need/to/type/some/more"
   ^
1 error generated.


Not a very common use case, I admit, but I have found it useful to be able to reformat preprocessed files. E.g., when I am hunting for compiler bugs, and I want to reduce the source manually, I would like to reprocess it first, because normal preprocessed output looks quite ugly.