What is the null pointer optimization in Rust?

Question

In Learning Rust With Entirely Too Many Linked Lists, the author mentions:

However, if we have a special kind of enum:

enum Foo {
             


        

Answer 1

enum is a tagged union. Without optimization it looks like

Foo::A;    // tag 0x00 data 0xXX
Foo::B(2); // tag 0x01 data 0x02

The null pointer optimization removes the separate tag field.

Foo::A;    // tag+data 0x00
Foo::B(2); // tag+data 0x02