Rust: Can't destructure a boxed struct without an intermediate let binding

Created on 12 Jan 2018 · 3Comments · Source: rust-lang/rust

This code fails to compile:

struct Foo;

struct Bar {
    a: Foo,
    b: Foo,
}

fn main() {
    let bar = Box::new(Bar { a: Foo, b: Foo });

    let Bar { a, b } = *bar;
}

error[E0382]: use of moved value: `bar`
  --> x.rs:11:18
   |
11 |     let Bar { a, b } = *bar;
   |               -  ^ value used here after move
   |               |
   |               value moved here
   |
   = note: move occurs because `bar.a` has type `Foo`, which does not implement the `Copy` trait

However inserting an intermediate let binding in between the box deref and the destructure fixes it:

fn main() {
    let bar = Box::new(Bar { a: Foo, b: Foo });

    let Bar { a, b } = { let intermediate = *bar; intermediate };
}

This feels like a bug.

Affected versions:

rustc 1.23.0 (766bd11c8 2018-01-01)
rustc 1.25.0-nightly (73ac5d6a8 2018-01-11)

C-bug T-compiler

Source

charliesome

👍2

Most helpful comment

Also happens with box patterns.

#![feature(box_patterns)]

struct Foo;

struct Bar {
    a: Foo,
    b: Foo,
}

fn main() {
    let bar = Box::new(Bar { a: Foo, b: Foo });

    let box Bar { a, b } = bar;
}

nagisa on 12 Jan 2018

😄4

All 3 comments

Also happens with box patterns.

#![feature(box_patterns)]

struct Foo;

struct Bar {
    a: Foo,
    b: Foo,
}

fn main() {
    let bar = Box::new(Bar { a: Foo, b: Foo });

    let box Bar { a, b } = bar;
}

nagisa on 12 Jan 2018

😄4

This is a dupe of https://github.com/rust-lang/rust/issues/16223.