From #537 (comment):

error: Invalid rust syntax, expected literal, got Some(Punct { ch: '-', spacing: Alone, span: #0 bytes(20762..20763) })

#[derive(Encode, Decode)]
pub enum TypeOfFile {
    Unknown = -1,
}

It appears that enum discriminators are encoded as u32, and changing this to i32 will change the byte encoding of positive discriminators as well.

I've done some work on a patch for this if this is a desirable change.