rust - Rust 中 ZST/无人居住/单一表示类型的 MayUninit 和初始化

Question

问题很简单，但我一直找不到答案。以下代码在 Rust 中是否有效/安全:

use core::mem::MaybeUninit;

// This is a ZST
#[derive(Debug)]
struct MyStruct;

// This is an uninhabited type
#[derive(Debug)]
enum MyEnum {}

#[derive(Debug)]
enum OneVariantEnum {
    Variant1,
}

fn main() {
    let s: MaybeUninit<MyStruct> = MaybeUninit::uninit();
    println!("s: {:?}", unsafe { s.assume_init() });

    let e: MaybeUninit<MyEnum> = MaybeUninit::uninit();
    println!("e: {:?}", unsafe { e.assume_init() });

    // and what about this?
    let o: MaybeUninit<OneVariantEnum> = MaybeUninit::uninit();
    println!("o: {:?}", unsafe { o.assume_init() });
}

Answer 1

以下似乎是正确的，因为 MyStruct是一个 ZST:

let s: MaybeUninit<MyStruct> = MaybeUninit::uninit();
println!("s: {:?}", unsafe { s.assume_init() });

然而，构造一个无人居住类型的值总是 UB，所以以下是不正确的:

let e: MaybeUninit<MyEnum> = MaybeUninit::uninit();
println!("e: {:?}", unsafe { e.assume_init() });

在 Debug模式下，Rust 可能会捕捉到这个并 panic

thread 'main' panicked at 'attempted to instantiate uninhabited type MyEnum'

至于 OneVariantEnum ，是ZST，所以类似 MyStruct以下不涉及UB:

let o: MaybeUninit<OneVariantEnum> = MaybeUninit::uninit();
println!("o: {:?}", unsafe { o.assume_init() });