Can't use Linux wc on one-line file

Question

I have a one-line file with no newline character at the end of the line. When i run the following:

diff oneline-file.txt any-file.txt | wc -c

I get:

Warning: missing newline character in oneline-file.txt

Why is this an error? How can i fix it? I could do this first:

echo "\n" >> oneline-file.txt

I'd rather do something that does not change the file. Thx.

Answer 1

Thanks to Barmar. This steered me in the right direction. Here's what I used:

diff <(sed 's/$/\n/g' oneline-file.txt) <(cat any-file.txt) | wc -c