Merge pull request #1857 from aochagavia/drop

Stabilize drop order

text/1857-stabilize-drop-order.md

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+- Feature Name: stable_drop_order
+- Start Date: 2017-01-19
+- RFC PR: https://github.com/rust-lang/rfcs/pull/1857
+- Rust Issue: https://github.com/rust-lang/rust/issues/43034
+
+# Summary
+[summary]: #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
+[motivation]: #motivation
+
+After lots of discussion on [issue 744](https://github.com/rust-lang/rfcs/issues/744),
+there seems to be consensus about the need for a stable drop order. See, for instance,
+[this](https://github.com/rust-lang/rfcs/issues/744#issuecomment-231215181) and
+[this](https://github.com/rust-lang/rfcs/issues/744#issuecomment-231237499) 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](https://internals.rust-lang.org/t/rolling-out-or-unrolling-struct-field-reorderings/4485)
+about struct field reorderings). It is highly probable that similar breakage
+would result from changes to the drop order. See for instance, the
+[comment](https://github.com/rust-lang/rfcs/issues/744#issuecomment-225918642)
+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
+[design]: #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:
+
+```rust
+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:
+
+```rust
+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.
+
+```rust
+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:
+
+```rust
+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](https://aochagavia.github.io/blog/exploring-rusts-unspecified-drop-order/)
+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
+[how-we-teach-this]: #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
+[drawbacks]: #drawbacks
+
+* The counter-intuitive drop order is here to stay.
+
+# Alternatives
+[alternatives]: #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
+[unresolved]: #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]`?