Union initialization and Drop

text/0000-union-initialization-and-drop.md

@@ -0,0 +1,365 @@
+- Feature Name: union_initialization_and_drop
+- Start Date: 2018-08-03
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Unions do not allow fields of types that require drop glue, 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
+[motivation]: #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
+[guide-level-explanation]: #guide-level-explanation
+
+## Union Definition
+
+When defining a union, it is a hard error to use a field type that requires drop glue.
+Examples:
+```rust
+// 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 `Cell<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:
+
+```rust
+// 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:
+
+```rust
+// 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.)
+
+```rust
+{
+    let u = U { f2: (S(0), S(1)) };
+    // drop gets called
+}
+{
+    let u = U { f1: ManuallyDrop::new(Vec::new()) };
+    foo(u.f2);
+    // 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:
+
+```rust
+#![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 droped
+}
+```
+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
+[reference-level-explanation]: #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](https://github.com/petrochenkov/rfcs/blob/e5266bd105f592f7408b8592c5c3deaccba7f1ec/text/1444-union.md#initialization-state).)
+
+## 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
+[drawbacks]: #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
+[rationale-and-alternatives]: #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](https://github.com/petrochenkov/rfcs/blob/e5266bd105f592f7408b8592c5c3deaccba7f1ec/text/1444-union.md#initialization-state).
+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.  @petrochenkov hence
+proposes 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
+[prior-art]: #prior-art
+
+I do not know of any language combining initialization tracking and destructors
+with unions: C++ does not have destructors for unions, and it does not track
+whether fields of a data structures are initialized to (dis)allow references or
+moves.
+
+# Unresolved questions
+[unresolved-questions]: #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.

text/1444-union.md

@@ -10,6 +10,8 @@ Provide native support for C-compatible unions, defined via a new "contextual
 keyword" `union`, without breaking any existing code that uses `union` as an
 identifier.
 
+**Note:** This RFC has been partially superseded by `unions-and-drop`.
+
 # Motivation
 [motivation]: #motivation