Dropping partially moved values after the moved values are dropped

Question

I expect the following code to compile and print Foo(6) as the value of b, owning the reference to a is dropped after the match block.

It seems related to this compiler error:

error[E0502]: cannot borrow `a` as immutable because it is also borrowed as mutable
  --> src/main.rs:26:22
   |
13 |     let b = get_foo(&mut a);
   |                          - mutable borrow occurs here
...
26 |     println!("{:?}", a);
   |                      ^ immutable borrow occurs here
27 | }
   | - mutable borrow ends here

Dropping the value of b doesn't work either, because it is partially moved:

error[E0382]: use of partially moved value: `b`
  --> src/main.rs:24:10
   |
18 |         Some(value) => *value = y,
   |              ----- value moved here
...
24 |     drop(b);
   |          ^ value used here after move
   |
   = note: move occurs because `(b:std::prelude::v1::Some).0` has type `&mut u32`, which does not implement the `Copy` trait

Is there a better way to fix this rather than putting lines let b and match b into an inner block? That just looks weird and ugly.

Shouldn't the compiler understand that the reference is dropped, and be able to compile that code?

#[derive(Debug)]
struct Foo(u32);

fn get_foo(bar: &mut Foo) -> Option<&mut u32> {
    Some(&mut bar.0)
}

pub fn test() {
    let mut x = 5;
    let mut y = 6;
    let mut a = Foo(x);

    // {
    
    let b = get_foo(&mut a);

    match b {
        Some(value) => *value = y,
        _ => (),
    }
    
    // }

    //    drop(b);

    println!("{:?}", a);
}

score 4 · Accepted Answer · answered Mar 26 '18 at 20:18

Is there a better way to fix this

Yes, but not in stable Rust. You need non-lexical lifetimes:

#![feature(nll)]

#[derive(Debug)]
struct Foo(u32);

fn get_foo(bar: &mut Foo) -> Option<&mut u32> {
    Some(&mut bar.0)
}

pub fn test() {
    let x = 5;
    let y = 6;
    let mut a = Foo(x);

    let b = get_foo(&mut a);

    if let Some(value) = b {
        *value = y;
    }

    println!("{:?}", a);
}

fn main() {}

Until then, just use the extra block.

Dropping the value of b doesn't work either

drop has nothing to do with borrows.

Dropping partially moved values after the moved values are dropped

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