Confusing and/or incorrect error with non-copyable types and FRU

[brson]

struct NonCopyable {
    x: (),
    f: NonCopyable2
}

struct NonCopyable2 {
    g: ()
}

impl NonCopyable2: Drop {
    fn finalize(&self) { }
}

fn main() {
    let a = NonCopyable { x: (), f: NonCopyable2 { g: () } };
    let b = NonCopyable {
        x: (),
        .. a
    };
}

/home/brian/dev/rust/src/test/run-pass/test.rs:16:8: 16:11 warning: unused variable: `b`
/home/brian/dev/rust/src/test/run-pass/test.rs:16     let b = NonCopyable {
                                                          ^~~
/home/brian/dev/rust/src/test/run-pass/test.rs:16:12: 16:23 error: copying a noncopyable value
/home/brian/dev/rust/src/test/run-pass/test.rs:16     let b = NonCopyable {
                                                              ^~~~~~~~~~~
error: aborting due to previous error

One of the fields in a is noncopyable and doesn't get moved into b. I think this should work as written, but if not then the error message should be better.

[nikomatsakis]

Hmm, you know, a couple things:

• The message is wrong but not for the reason you suggest. You are copying the field f from a which is non-copyable.
• However, in fact I believe that a should be being moved here, which means that you are actually moving the field f from a which should be ok.

When doing my recent patch, I removed most of these "copyability" checks from kind.rs for this reason (that is, those copies would not actually be moves if the type were non-copyable) but I left this one. However, I think there is no need at all to check whether the base is non-copyable.

[nikomatsakis]

So to be clear: I agree there should be no error reported here (clearly, though, if we were to report an error, we are using the wrong span).

[nikomatsakis]

Fixed.