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What is the overhead of Rust's Option type?

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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Problem

In Rust, references can never be null, so in case where you actually need null, such as a linked list, you use the Option type:

struct Element {
    value: i32,
    next: Option>,
}

How much overhead is involved in this in terms of memory allocation and steps to dereference compared to a simple pointer? Is there some "magic" in the compiler/runtime to make Option cost-free, or less costly than if one were to implement Option by oneself in a non-core library using the same enum construct, or by wrapping the pointer in a vector?

Solution

Yes, there is some compiler magic that optimises Option to a single pointer (most of the time).

use std::mem::size_of;

macro_rules! show_size {
    (header) => (
        println!("{:4}    {}", "Type", "T", "Option");
    );
    ($t:ty) => (
        println!("{:(), size_of::>())
    )
}

fn main() {
    show_size!(header);
    show_size!(i32);
    show_size!(&i32);
    show_size!(Box);
    show_size!(&[i32]);
    show_size!(Vec);
    show_size!(Result>);
}

The following sizes are printed (on a 64-bit machine, so pointers are 8 bytes):

// As of Rust 1.22.1
Type                      T    Option
i32                       4    8
&i32                      8    8
Box                  8    8
&[i32]                   16   16
Vec                 24   24
Result      8   16

Note that &i32, Box, &[i32], Vec all use the non-nullable pointer optimization inside an Option!

Code Snippets

use std::mem::size_of;

macro_rules! show_size {
    (header) => (
        println!("{:<22} {:>4}    {}", "Type", "T", "Option<T>");
    );
    ($t:ty) => (
        println!("{:<22} {:4} {:4}", stringify!($t), size_of::<$t>(), size_of::<Option<$t>>())
    )
}

fn main() {
    show_size!(header);
    show_size!(i32);
    show_size!(&i32);
    show_size!(Box<i32>);
    show_size!(&[i32]);
    show_size!(Vec<i32>);
    show_size!(Result<(), Box<i32>>);
}

Context

Stack Overflow Q#16504643, score: 121

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