moveit
A library for safe, in-place construction of Rust (and C++!) objects.
How It Works
moveit
revolves around unsafe trait
s that impose additional guarantees
on !Unpin
types, such that they can be moved in the C++ sense. There are
two senses of "move" frequently used:
- The Rust sense, which is a blind memcpy and analogous-ish to the C++ "std::is_trivially_moveable` type-trait. Rust moves also render the moved-from object inaccessible.
- The C++ sense, where a move is really like a mutating
Clone
operation, which leave the moved-from value accessible to be destroyed at the end of the scope.
C++ also has constructors, which are special functions that produce a new
value in a particular location. In particular, C++ constructors may assume
that the address of *this
will not change; all C++ objects are effectively
pinned and new objects must be constructed using copy or move constructors.
The [New
], [CopyNew
], and [MoveNew
] traits bring these concepts
into Rust. A [New
] is like a nilary [FnOnce
], except that instead of
returning its result, it writes it to a Pin<&mut MaybeUninit<T>>
, which is
in the "memory may be repurposed" state described in the
Pin
documentation (i.e., either it is freshly allocated or the
destructor was recently run). This allows a [New
] to rely on the
pointer's address remaining stable, much like *this
in C++.
Types that implement [CopyNew
] may be copy-constructed: given any
pointer to T: CopyNew
, we can generate a constructor that constructs a
new, identical T
at a designated location. [MoveNew
] types may be
move-constructed: given an owning pointer (see [DerefMove
]) to T
,
we can generate a similar constructor, except that it also destroys the
T
and the owning pointer's storage.
None of this violates the existing Pin
guarantees: moving out of a
Pin<P>
does not perform a move in the Rust sense, but rather in the C++
sense: it mutates through the pinned pointer in a safe manner to construct
a new P::Target
, and then destroys the pointer and its contents.
In general, move-constructible types are going to want to be !Unpin
so
that they can be self-referential. Self-referential types are one of the
primary motivations for move constructors.
Constructors
A constructor is any type that implements [New
]. Constructors are like
closures that have guaranteed RVO, which can be used to construct a
self-referential type in-place. To use the example from the Pin<T>
docs:
use PhantomPinned;
use MaybeUninit;
use Pin;
use ptr;
use NonNull;
use new;
use New;
use moveit;
// This is a self-referential struct because the slice field points to the
// data field. We cannot inform the compiler about that with a normal
// reference, as this pattern cannot be described with the usual borrowing
// rules. Instead we use a raw pointer, though one which is known not to be
// null, as we know it's pointing at the string.
// The constructor can't be used directly, and needs to be emplaced.
moveit!
// The pointer should point to the correct location,
// so long as the struct hasn't moved.
// Meanwhile, we are free to move the pointer around.
let mut still_unmoved = unmoved;
assert_eq!;
// Since our type doesn't implement Unpin, this will fail to compile:
// let mut new_unmoved = Unmovable::new("world".to_string());
// std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
// However, we can implement `MoveNew` to allow it to be "moved" again.
The [new
] module provides various helpers for making constructors. As a
rule, functions which, in Rust, would normally construct and return a value
should return impl New
instead. This is analogous to have async fn
s and
.iter()
functions work.
Emplacement
The example above makes use of the [moveit!()
] macro, one of many ways to
turn a constructor into a value. moveit
gives you two choices for running
a constructor:
- On the stack, using the [
MoveRef
] type (this is what [moveit!()
] generates). - On the heap, using the extension methods from the [
Emplace
] trait.
For example, we could have placed the above in a Box
by writing
Box::emplace(Unmovable::new())
.
License: Apache-2.0
This is not an officially supported Google product.