Function replace_in_file 

pub fn replace_in_file<S: BuildHasher>(
    source: Option<&str>,
    destination: Option<&str>,
    settings: &Settings<S>,
) -> Result<()>

Replaces all occurrences of variables of the form ${KEY} in a input stream with their respective values.

Errors

If a variable key was found in the stream, but vars contains no entry for it, and fail_on_missing is true.

If reading from the source failed.

If writing to the destination failed.