ACP: `#[derive(Debug)]` on unions to print `MyUnion { ... }`

Proposal

Problem statement

#[derive(Debug)] currently rejects unions, but there are times when a basic implementation would be convenient.

Motivating examples or use cases

It is not possible to derive Debug for aggregate types if they contain a union:

#[derive(Debug)] // `MyUnion` doesn't implement `Debug`
struct Foo {
    x: u32,
    u: MyUnion,
}

union MyUnion {
    i: i32,
    f: f32
}

This means that Debug must be handwritten for either Foo or MyUnion.

Solution sketch

When #[derive(Debug)] is applied to a union, represent it as an opaque struct.

#[derive(Debug)]
union MyUnion {
    i: i32,
    f: f32
}

The above would expand to:

impl fmt::Debug for MyUnion {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("MyUnion").finish_non_exhaustive()
    }
}

Which prints as:

MyUnion { .. }

This makes it easy to use in other types that #[derive(Debug)]. The result is similar to how UnsafeCell is formatted (UnsafeCell { .. })

Alternatives

Links and related work

Brief Zulip discussion: https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/.60derive.28Debug.29.60.20on.20.60union.60s

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

No implementation files or tests are named. Start by reading the proposal and its linked Zulip discussion, then determine whether the libs-api team approves the behavior and where Rust's derive implementation would need to change. Done means an agreed design and a reviewed implementation that supports #[derive(Debug)] on unions with the proposed opaque formatting.