Function encoding_mem_is_ascii 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn encoding_mem_is_ascii(
    buffer: *const u8,
    len: usize,
) -> bool

Checks whether the buffer is all-ASCII.

May read the entire buffer even if it isn’t all-ASCII. (I.e. the function is not guaranteed to fail fast.)

Undefined behavior

UB ensues if buffer and buffer_len don’t designate a valid memory block or if buffer is NULL. (If buffer_len is 0, buffer may be bogus but still has to be non-NULL.)