#Rust 编译器中一个令人惊喜的枚举大小优化

/// Foo is either a 32-bit integer or character.
enum Foo {
    Int(u32),
    Char(char),
}

assert_eq!(std::mem::size_of::<Foo>(), 8);

/// Print the memory representation of a value of a type T.
fn print_memory_representation<T: std::fmt::Debug>(t: T) {
    print!("type={} value={t:?}: ", std::any::type_name::<T>());
    let start = &t as *const _ as *const u8;
    for i in 0..std::mem::size_of::<T>() {
        print!("{:02x} ", unsafe {*start.offset(i as isize)});
    }
    println!();
}

print_memory_representation(Foo::Int(5));
// type=Foo value=Int(5): 00 00 00 00 05 00 00 00
//                        |-- tag --| |- value -|

print_memory_representation(Foo::Char('A'));
// type=Foo value=Char('A'): 01 00 00 00 41 00 00 00 
//                           |-- tag --| |- value -|

enum Option<char> {
    None,
    Some(char),
}

assert_eq!(std::mem::size_of::<Option<char>>(), 4);

let a: char = 'A'
print_memory_representation(a);
// type=char value='A': 41 00 00 00 

print_memory_representation(Some(a));
// type=Option<char> value=Some('A'): 41 00 00 00 

let none: Option<char> = None;
print_memory_representation(none);
// type=Option<char> value=None: 00 00 11 00 

enum Inner {
    A(u32),
    B(u32),
}

assert_eq!(std::mem::size_of::<Inner>(), 8);

print_memory_representation(Inner::A(2));
// type=Inner value=A(2): 00 00 00 00 02 00 00 00 
//                        |-- tag --| |- value -|

print_memory_representation(Inner::B(3));
// type=Inner value=B(3): 01 00 00 00 03 00 00 00 
//                        |-- tag --| |- value -|

enum Outer {
    C(u32),
    D(Inner),
}

assert_eq!(std::mem::size_of::<Outer>(), 8);

print_memory_representation(Outer::C(5));
// type=Outer value=C(5): 02 00 00 00 05 00 00 00
//                        |-- tag --| |- value -|

print_memory_representation(Outer::D(Inner::A(2)));
// type=Outer value=D(A(2)): 00 00 00 00 02 00 00 00 
//                           |-- tag --| |- value -|

print_memory_representation(Outer::D(Inner::B(3)));
// type=Outer value=D(B(3)): 01 00 00 00 03 00 00 00 
//                           |-- tag --| |- value -|