Tracking issue for RFC 2514, "Union initialization and Drop" · Issue #55149 · rust-lang/rust (original) (raw)

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This is a tracking issue for the RFC "Union initialization and Drop" (rust-lang/rfcs#2514).

Successor of #32836.

Steps:

Unresolved questions:

There should be more tests in particular for all move-related behaviors (1, 2, 3, 4). Done in unions: test move behavior of non-Copy fields #75559.
Should we try to avoid the DerefMut-related pitfall? And if yes, should we maybe try harder, e.g. lint against using * below a union type when describing a place? That would make people write let v = &mut u.f; *v = Vec::new();. It is not clear that this helps in terms of pointing out that an automatic drop may be happening. (Implementation at do not apply DerefMut on union field #75584.)
We could allow moving out of a union field even if the union implements Drop. That would have the effect of making the union considered uninitialized, i.e., it would not be dropped implicitly when it goes out of scope. However, it might be useful to not let people do this accidentally. The same effect can always be achieved by having a dropless union wrapped in a newtype struct with the desired Drop.
Should we allow impl Copy for Union even when the union has non-Copy fields? (Proposed here.)

Implementation history