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rustsize

Why is the size of Option<bool> equal to 1?

On Rust Playground, the size of an Option<bool> is 1. Is rust packing the option information into the bool itself? So the Option represents the 3 possible states: None, Some(false), Some(true)?

use std::mem::size_of;

fn main() {
    println!("{}", size_of::<Option<bool>>()); // 1
}
Solution

  • Yes. This is called niche optimization. bool has a niche of 7 bits (7 unused bits), and we pack Option::None into those.

    You can inspect the bytes (playground):

    fn main() {
    fn print_bytes(v: Option<bool>) {
        println!("{:08b}", unsafe { std::mem::transmute::<Option<bool>, u8>(v) });
    }
    print_bytes(Some(false));
    print_bytes(Some(true));
    print_bytes(None);
}

    00000000
00000001
00000010

    You can see we use the second bit to indicate None.

    Note that as said in the comment by @Cerberus, the memory layout is not guaranteed for #[repr(rust)] types and one should not rely on it.