Validating should be pretty trivial. Explicitly handling all cases would look something like this:

    #[inline]
    const fn is_valid_scalar(c: u32) -> bool {
        match c {
            0..=0xD7FF => true,
            0xD800..=0xDFFF => false, // surrogate pairs
            0xE000..=0x10FFFF => true,
            0x110000..=u32::MAX => false, // outside of Unicode range
        }
    }

But this could be reduced down to a simple if statement that tests for the invalid ranges.