rust - 为什么结构解构后会出现 drop call？

Question

我正在包装一个 C API。为了简化这个问题，我使用 NonNull::dangling 代替。

use std::ptr::NonNull;

struct Foo(NonNull<i32>);

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Foo::drop: {:?}", self.0);
    }
}

struct Moo(NonNull<i32>);
//impl Drop for Moo exists, but not important for question

fn f1() -> Result<Foo, String> {
    Ok(Foo(unsafe { NonNull::dangling() }))
}

fn f2() -> Result<Moo, String> {
    f1().map(|Foo(x)| Moo(x))//1
}

fn main() {
    f2();
}

在第 (1) 点，我解包/解构 Foo。我希望在那之后，不应该调用 Foo::drop，但是由于某种原因打印了 Foo::drop。

解构 (let Struct1 { field1, field2, .. } = struct1;) 应该阻止调用 Struct1::drop 我错了吗？

Answer 1

如果您将 NonNull 替换为未实现 Copy 的结构, 行为更清晰:

#[derive(Debug)]
struct NoCopy;

struct Foo(NoCopy);

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Foo::drop: {:?}", self.0);
    }
}

struct Moo(NoCopy);
//impl Drop for Moo exists, but not important for question

fn f1() -> Result<Foo, String> {
    Ok(Foo(NoCopy))
}

fn f2() -> Result<Moo, String> {
    f1().map(|Foo(x)| Moo(x))//1
}

fn main() {
    f2();
}

这会导致此错误:

error[E0509]: cannot move out of type `Foo`, which implements the `Drop` trait
  --> src/main.rs:20:15
   |
20 |     f1().map(|Foo(x)| Moo(x))//1
   |               ^^^^-^
   |               |   |
   |               |   data moved here
   |               cannot move out of here
   |
note: move occurs because `x` has type `NoCopy`, which does not implement the `Copy` trait
  --> src/main.rs:20:19
   |
20 |     f1().map(|Foo(x)| Moo(x))//1
   |            

因此，在 (1) 中，您从 Foo 中复制了 NonNull 并且 Foo 被丢弃。