Function preprocess 

pub fn preprocess(
    opts: &Options,
    name: &str,
    fs: &dyn FileSystem,
) -> Preprocessed

Preprocesses one file and renders the result.

name is the path as the user wrote it, which is the name the output and every diagnostic about the file use. It is not canonicalised, because a message naming a path nobody typed is a message that is harder to act on.