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1857-stabilize-drop-order.md

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Summary

I propose we specify and stabilize drop order in Rust, instead of treating it as an implementation detail. The stable drop order should be based on the current implementation. This results in avoiding breakage and still allows alternative, opt-in, drop orders to be introduced in the future.

Motivation

After lots of discussion on issue 744, there seems to be consensus about the need for a stable drop order. See, for instance, this and this comment.

The current drop order seems counter-intuitive (fields are dropped in FIFO order instead of LIFO), but changing it would inevitably result in breakage. There have been cases in the recent past when code broke because of people relying on unspecified behavior (see for instance the post about struct field reorderings). It is highly probable that similar breakage would result from changes to the drop order. See for instance, the comment from @sfackler, which reflects the problems that would arise:

Real code in the wild does rely on the current drop order, including rust-openssl, and there is no upgrade path if we reverse it. Old versions of the libraries will be subtly broken when compiled with new rustc, and new versions of the libraries will be broken when compiled with old rustc.

Introducing a new drop order without breaking things would require figuring out how to:

  • Forbid an old compiler (with the old drop order) from compiling recent Rust code (which could rely on the new drop order).
  • Let the new compiler (with the new drop order) recognize old Rust code (which could rely on the old drop order). This way it could choose to either: (a) fail to compile; or (b) compile using the old drop order.

Both requirements seem quite difficult, if not impossible, to meet. Even in case we figured out how to meet those requirements, the complexity of the approach would probably outweight the current complexity of having a non-intuitive drop order.

Finally, in case people really dislike the current drop order, it may still be possible to introduce alternative, opt-in, drop orders in a backwards compatible way. However, that is not covered in this RFC.

Detailed design

The design is the same as currently implemented in rustc and is described below. This behavior will be enforced by run-pass tests.

Tuples, structs and enum variants

Struct fields are dropped in the same order as they are declared. Consider, for instance, the struct below:

struct Foo {
    bar: String,
    baz: String,
}

In this case, bar will be the first field to be destroyed, followed by baz.

Tuples and tuple structs show the same behavior, as well as enum variants of both kinds (struct and tuple variants).

Note that a panic during construction of one of previous data structures causes destruction in a different order. Since the object has not yet been constructed, its fields are treated as local variables (which are destroyed in LIFO order). See the example below:

let x = MyStruct {
    field1: String::new(),
    field2: String::new(),
    field3: panic!()
};

In this case, field2 is destructed first and field1 second, which may seem counterintuitive at first but makes sense when you consider that the initialized fields are actually temporary variables. Note that the drop order depends on the order of the fields in the initializer and not in the struct declaration.

Slices and Vec

Slices and vectors show the same behavior as structs and enums. This behavior can be illustrated by the code below, where the first elements are dropped first.

for x in xs { drop(x) }

If there is a panic during construction of the slice or the Vec, the drop order is reversed (that is, when using [] literals or the vec![] macro). Consider the following example:

let xs = [X, Y, panic!()];

Here, Y will be dropped first and X second.

Allowed unspecified behavior

Besides the previous constructs, there are other ones that do not need a stable drop order (at least, there is not yet evidence that it would be useful). It is the case of vec![expr; n] and closure captures.

Vectors initialized with vec![expr; n] syntax clone the value of expr in order to fill the vector. In case clone panics, the values produced so far are dropped in unspecified order. The order is closely tied to an implementation detail and the benefits of stabilizing it seem small. It is difficult to come up with a real-world scenario where the drop order of cloned objects is relevant to ensure some kind of invariant. Furthermore, we may want to modify the implementation in the future.

Closure captures are also dropped in unspecified order. At this moment, it seems like the drop order is similar to the order in which the captures are consumed within the closure (see this blog post for more details). Again, this order is closely tied to an implementation that we may want to change in the future, and the benefits of stabilizing it seem small. Furthermore, enforcing invariants through closure captures seems like a terrible footgun at best (the same effect can be achieved with much less obscure methods, like passing a struct as an argument).

Note: we ignore slices initialized with [expr; n] syntax, since they may only contain Copy types, which in turn cannot implement Drop.

How We Teach This

When mentioning destructors in the Rust book, Reference and other documentation, we should also mention the overall picture for a type that implements Drop. In particular, if a struct/enum implements Drop, then when it is dropped we will first execute the user's code and then drop all the fields (in the given order). Thus any code in Drop must leave the fields in an initialized state such that they can be dropped. If you wish to interleave the fields being dropped and user code being executed, you can make the fields into Option and have a custom drop that calls take() (or else wrap your type in a union with a single member and implement Drop such that it invokes ptr::read() or something similar).

It is also important to mention that union types never drop their contents.

Drawbacks

  • The counter-intuitive drop order is here to stay.

Alternatives

  • Figure out how to let rustc know the language version targeted by a given program. This way we could introduce a new drop order without breaking code.
  • Introduce a new drop order anyway, try to minimize breakage by running crater and hope for the best.

Unresolved questions

  • Where do we draw the line between the constructs where drop order should be stabilized and the rest? Should the drop order of closure captures be specified? And the drop order of vec![expr; n]?