Function return does not always perform a move

[brson]

This is a problem for unique pointers

use std;
import std::unsafe;

fn f(addr: @mutable int) -> ~int {
    let i = ~100;
    let a: int = unsafe::reinterpret_cast(i);
    log a;
    *addr = a;
    // Since i is not a temporary it gets copied, which is not what you expect
    ret i;
}

fn main() {
    let addr = @mutable 0;
    let i <- f(addr);
    let expected = *addr;
    let actual: int = unsafe::reinterpret_cast(i);
    log expected;
    log actual;
    assert expected == actual;
}

[marijnh]

It should not always perform a move. Consider returning foo.x, moving out of a data structure is invalid. Whether it moves should definitely depend on the type. For uniques, returning something that can't be moved is (if I understand correctly) an error. For other types, it should continue to implicitly copy.

[brson]

Uniques don't have to be moved. They are allowed to be copied; they just don't like it. What approach can we take to make returning more likely to be a move? Is it true that it's always safe to do a move when returning a local?

[marijnh]

The optimization proposed in https://github.com/graydon/rust/issues/925 would help here.

[marijnh]

This has been fixed by #1177