Function cast_from_source 

pub fn cast_from_source<S: DeserializeOwned, V: ClipVariant>(
    file: &str,
    source: &str,
    can_speak: impl Fn(&str) -> bool,
) -> Result<Cast<S>, Vec<ContentError>>

Parses and validates cast source, attributing errors to file at each entry’s line: no voice is empty, every key is a speaker that is NARRATION_SPEAKER or one that can_speak, or such a speaker followed by the suffix of one of V’s variants. A key that is a speaker is the speaker’s, whatever it ends with. Settings S rejects are a parse error.