Function c4repl_isValidRemote 

pub unsafe extern "C" fn c4repl_isValidRemote(
    remoteAddress: C4Address,
    remoteDatabaseName: C4String,
    outError: *mut C4Error,
) -> bool

Checks whether the destination of a replication is valid. (c4repl_new makes the same checks; this function is exposed so CBL can fail sooner.)