Summary

Unions do not allow fields of types that require drop glue (the code that is automatically run when a variables goes out of scope: recursively dropping the variable and all its fields), but they may still impl Drop themselves. We specify when one may move out of a union field and when the union’s drop is called. To avoid undesired implicit calls of drop, we also restrict the use of DerefMut when unions are involved.

Motivation

Currently, it is unstable to have a non-Copy field in the union. The main reason for this is that having fields which need drop glue raises some hard questions about whether to call that drop glue when assigning a union field, and how to make programming with such unions less of a time bomb (triggered by accidentally dropping data one meant to just overwrite). Not much progress has been made on stabilizing the unstable union features. This RFC proposes a route forwards that side-steps the time bomb: Do not allow fields with drop glue.

Guide-level explanation

Union Definition

When defining a union, it is a hard error to use a field type that requires drop glue. Examples:

// Accepted
union Example1<T> {
    // `ManuallyDrop<T>` never has drop glue, even if `T` does.
    f1: ManuallyDrop<T>,
    // `RefCell<i32>` is a fully known type, and does not have drop glue.
    f2: RefCell<i32>,
}
union Example2<T: Copy> {
    // `Copy` types never have drop glue.
    f1: T,
}
trait Trait3 { type Assoc: Copy; }
union Example3<T: Trait3> {
    // `T::Assoc` is `Copy` and hence cannot have drop glue.
    f1: T::Assoc,
}

// Rejected
union Example4<T> {
    // `T` might have drop glue, and then `RefCell<T>` would as well.
    f1: RefCell<T>,
}
trait Trait5 { type Assoc; }
union Example5<T: Trait5> {
    // `T::Assoc` might have drop glue.
    f1: T::Assoc,
}

Ruling out possibly dropping types may seem restrictive, but thanks to ManuallyDrop it in fact is not: If the compiler rejects a union definition, you can always wrap field types in ManuallyDrop to obtain a working definition. This means you have to manually take care of when to drop the data, but that is already something to be concerned with when working on unions.

As a consequence, it is quite obvious that writing to a union field will never implicitly call drop. Such a write is hence always a safe operation. This removes a whole class of pitfalls related to drop being called in tricky unsafe code when you might not expect that to happen. (However, see below for some pitfalls that remain.)

Reading from a union field and creating a reference remain unsafe: We cannot guarantee that the field contains valid data.

Union initialization and Drop

In two cases, the compiler cares about whether a (field of a) variable is initialized: When deciding whether a move from the field/variable is allowed (for cases where the type is not Copy), and when deciding whether or not the variable has to be dropped when it goes out of scope.

A union just does very simple initialization tracking: There is a single boolean state for the entire union and all of its fields. Nested inner fields are tracked just like they are for structs; however, when the union becomes (un)initialized, then all nested inner fields of all union fields are (un)initialized at once. So, (un)initializing a union field also (un)initializes its siblings. For example:

// This code creates bad references and transmutes to `Vec` in incorrect ways.
// This is just to demonstrate what the compiler would accept in terms of
// tracking initialization.

struct S(i32); // not `Copy`, no drop glue
union U { f1: ManuallyDrop<Vec<i32>>, f2: (S, S), f3: i32 }

let mut u: U;
// Now `u` is not initialized: `&u`, `&u.f2` and `&u.f2.0` are all rejected.

// We can write into uninitialized inner fields:
u.f2.1 = S(42);
{ let _x = &u.f2.1; } // This field is initialized now.
// But this does not change the initialization state of the union itself,
// or any other (inner) field.

// We can initialize by assigning an entire field:
u.f1 = ManuallyDrop::new(Vec::new());
// Now *all (nested) fields* of `u` are initialized, including the siblings of `f1`:
{ let _x = &u.f2; }
{ let _x = &u.f2.0; }

// Equivalently, we can assign the entire union:
u = U { f2: (S(42), S(23) };
// Now `u` is still initialized.

// Copying does not change anything:
let _x = u.f3;
// Now `u` is still initialized.

// We can move out of an initialized union:
let v = u.f1;
// Now `f1` *and its siblings* are no longer initialized (they got "moved out of"):
// `let _x = u.f2;` would hence get rejected, as would `&u.f1` and `foo(u)`.
u.f1 = v;
// Now `u` and all of its fields are initialized again ("moving back in").

// When we move out of an inner field, the other union fields become uninitialized
// even if they are `Copy`.
let s = u.f2.1;
// Now `u.f1` and `u.f3` are no longer initialized.  But `u.f2.0` is:
let s = u.f2.0;

If the union implements Drop, the same restrictions as for structs apply: It is not possible to initialize a field before initializing the entire variable, and it is not possible to move out of a field. For example:

// This code creates bad references and transmutes to `Vec` in incorrect ways.
// This is just to demonstrate what the compiler would accept in terms of
// tracking initialization.

struct S(i32); // not `Copy`, no drop glue

union U { f1: ManuallyDrop<Vec<i32>>, f2: (S, S), f3: u32 }
impl Drop for U {
    fn drop(&mut self) {
        println!("Goodbye!");
    }
}

let mut u: U;
// `u.f1 = ...;` gets rejected: Cannot initialize a union with `Drop` by assigning a field.
u = U { f2: (S(42), S(1)) };
// Now `u` is initialized.

// `let v = u.f1;` gets rejected: Cannot move out of union that implements `Drop`.
let v_ref = &mut u.f1; // creating a reference is allowed
let _x = u.f3; // copying out is allowed

When a union implementing Drop goes out of scope, its destructor gets called if and only if the union is currently considered initialized: (Continuing the example from above.)

{
    let u = U { f2: (S(0), S(1)) };
    // drop gets called
}
{
    let u = U { f1: ManuallyDrop::new(Vec::new()) };
    foo(u);
    // drop does NOT get called
}

Potential pitfalls around DerefMut

There is still a potential pitfall left around assigning to union fields: If the assignment implicitly happens through a DerefMut, it may call drop glue. For example:

#![feature(untagged_unions)]

use std::mem::ManuallyDrop;

union U<T> { x:(), f: ManuallyDrop<T> }

fn main() {
    let mut u : U<(Vec<i32>,)> = U { x: () };
    unsafe { u.f.0 = Vec::new() }; // uninitialized `Vec` being dropped
}

This requires unsafe because it desugars to ManuallyDrop::deref_mut(&mut u.f).0, and while writing to a union field is safe, taking a reference is not.

For this reason, DerefMut auto-deref is not applied when working on a union or its fields. However, note that manually dereferencing is still possible, so (*u.f).0 = Vec::new() is still a way to drop an uninitialized field! But this can never happen when no * is involved, and hopefully dereferencing an element of a union is a clear enough signal that the union better be initialized properly for this to make sense.

Reference-level explanation

Union definition

When defining a union, it is a hard error to use a field type that requires drop glue. This is checked as follows:

  • Proceed recursively down the given type, insofar as the type involved is known at compile-time. For example, u32, &mut T and ManuallyDrop<T> are known to not have drop glue no matter the choice of T.
  • When hitting a type variable where no progress can be made, check that T: Copy as a proxy for T not requiring drop glue.

Note: Currently, union fields with drop glue are allowed on nightly with an unstable feature. This RFC proposes to remove support for that entirely; code using nightly might have to be changed.

Writing to union fields

Writing to union fields is currently unsafe when the field has drop glue. This check is no longer needed, because union fields will never have drop glue. Moreover, writing to a nested field (e.g., u.f1.x = 0;) is currently unsafe as well, this should also become a safe operation as long as the path (expanded, i.e., after auto-derefs are inserted) consists only of field projections, not deref’s. Note that this is sound only because ManuallyDrop’s only field is private (so, in fact, this is not sound inside the module that defines ManuallyDrop).

Union initialization tracking

A “fragment” is a place of the form local_var.field.field.field, without any implicit derefs. A fragment can be either initialized or uninitialized. This state is approximated statically: The type system will only allow accesses to definitely initialized fragments. Drop elaboration needs to know the precise state of a fragment, for which purpose it adds run-time drop flags as needed.

If a fragment has some uninitialized nested fragments then it is still uninitialized and accesses to this fragment as a whole are prevented. This applies even if it also has a nested initialized fragment (in which case we speak of a partially initialized fragment). If a fragment has only initialized nested fragments then it is initialized as a whole and can be accessed.

A fragment becomes initialized when it is assigned to, or created using an initializer, or it is a union field and a sibling becomes initialized, or all its nested fragments become initialized. A fragment becomes uninitialized when it doesn’t implement Copy and is moved out from, or it is a union field (possibly Copy) and its sibling becomes uninitialized, or some of its nested fragments becomes uninitialized.

In other words, union fields behave a lot like struct fields except that if one field changes initialization state, the others follow suit. In particular, if one union field becomes partially initialized (because one of its nested fragments got uninitialized), all its siblings become entirely uninitialized, including their nested fragments.

If a fragment is of a type which has an impl Drop, then its nested fragments cannot be separately (un)initialized. Only the entire fragment can be initialized by assignment, and the entire fragment can be uninitialized by moving out.

NOTE: To my knowledge, this already mostly matches the current implementation. The only exception is that “fragment becomes initialized when all its nested fragments become initialized” rule is not currently implemented for neither structs nor unions, so the compiler accepts less code than it should. However, impl Drop for Union and non-Copy union fields are behind a feature gate, so the effects of this on unions cannot currently be observed on stable compilers.

(This closely follows a previously proposed RFC by @petrochenkov.)

Potential pitfalls around DerefMut

When adding auto-derefs on the left-hand side of an assignment, as we traverse the path, once we hit a union, we stop adding further auto-derefs. So with s: Struct and u: Union, when encountering s.u.f.x, auto-deref does happen on s, but not on s.u or any of the later components.

Notice that this relies crucially on the only field of ManuallyDrop being private! If we could project directly through that field, no DerefMut would be needed to reproduce the problematic example from the “guide” section.

Drawbacks

This makes working with unions involving types that may have drop glue slightly more verbose than today: One has to write ManuallyDrop more often than one may want to.

The restriction placed on DerefMut is not fully backwards compatible: A type could implement Copy + DerefMut and actually rely on the deref coercion inside a union. That seems very unlikely, but should be tested with a crater run.

The initialization tracking rules are somewhat surprising, and one might prefer the compiler to just not track anything when it comes to unions. After all, the compiler fundamentally cannot know what part of the union is properly initialized. Unfortunately, not having any initialization tracking is not an option when non-Copy fields are involved: We have to decide if moving out of a union field is allowed.

Rationale and alternatives

Ruling out fields with drop glue does not, in fact, reduce the expressiveness of unions because one can use ManuallyDrop<T> to obtain a drop-glue-free version of T. If anything, having the ManuallyDrop in the union definition should help to drive home the point that no automatic dropping is happening, ever. (Before this RFC, automatic dropping is happening when assigning to a union field but not when the union goes out of scope. That seems to be the result of necessity, not of a coherent design.)

An alternative approach to proceed with unions has been previously proposed by @petrochenkov. That proposal replaces RFC 1444 and goes into a lot more points than this much more limited proposal. In particular, it allows fields with drop glue. However, it can be pretty hard for the programmer to predict when drop glue will be automatically invoked on assignment or not, because the initialization tracking (which this RFC adapts from @petrochenkov’s proposal) can sometimes be a little surprising when looking at individual fields: Whether u.f2 = ...; drops depends on whether u.f1 has been previously initialized. We hence have a lint to warn people that unions with drop-glue fields are not always very well-behaved. This RFC, on the other hand, side-steps the entire question by not allowing fields with drop glue. Initialization tracking thus has no effect on the code executed during an assignment of a union field. For unions that impl Drop, it still has an effect on what happens when the union goes out of scope, but in that case initialization is so restricted that I cannot think of any surprises. Together with the DerefMut restriction, that should make it very unlikely to accidentally call drop when it was not intended.

We could significantly simplify the initialization tracking by always applying the rules that are currently only applied to unions that impl Drop. However, that does not actually help with the pitfall described above. The more complex rules allow more code that many will reasonably expect to work, and do not seem to introduce any additional pitfalls.

We could reduce the relevance of state tracking further by not to allowing impl Drop for Union. It is still possible to add a wrapper struct around the union which has drop glue, so this does not restrict expressiveness. However, this seems unnecessarily cumbersome, and it does not seem to help avoid any surprises. State tracking around unions that impl Drop is pretty much as simple as it gets.

Prior art

I do not know of any language combining initialization tracking and destructors with unions: C++ never runs destructors for fields of unions, and it does not track whether fields of a data structures are initialized to (dis)allow references or moves.

Unresolved questions

Should we even 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.

We could allow moving out of a union field even if it 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.