Rust project goals: Immobile types and guaranteed destructors

None. The question is more what happens when you put a `!Move` type into, say, `Vec`, because that's a collection type that regularly moves its elements to a new allocation when it grows or shrinks.

Should it be possible to construct a `Vec` whose size can never change? Is there a subset of Vec's API that can be annotated with `where T: ?Move`? These are all important design questions, with the potential to break 99% of existing Rust code.

Ok, I guess somebody has to provide the youngsters/uninitiated with some context. What is going on here can be viewed as part of a process of Rust (potentially) incrementally adopting the C++ model, because the Rust model is limited in important ways.

Specifically, Rust's "necessarily-trivial-destructive" moves make it possible for a memory location previously holding a valid object to become invalid without a destructor (or any other handler) being called. Accommodating this possibility resulted in unforeseen (by many) limitations, particularly in the safe subset. (See "the leakpocalypse".) This was partially addressed by the introduction of "pinning" into the Rust language. The posted github page suggests that this sort of pinning is not the ideal approach, and that it is more effective to make the "unmovability" of an object a property of the object's type, rather than a property of the reference to the object, as is the case with the pinning approach.

To be clear, we're talking about Rust-style "necessarily-trivial-destructive" movability here. Traditionally, C++ doesn't really support this sort of movability. That is, even if an object's contents are ("conceptually") moved to a different location, the original source object remains (at its original location) until it is otherwise destroyed (and its destructor called). So in C++, all types are "immovable" in the sense of the posted github page.

The github page notes how these "immovable" types can support self-references completely in the safe subset in a way that pinning can't.

> This unblocks patterns that are currently impossible in safe Rust.

For an idea of some other unblocked patterns, you can consider so-called "norad" pointers [1] (and proxy pointers [2]) in the SaferCPlusPlus library. Analogous to how `RefCell` references can be used to express references that cannot be statically verified to conform to Rust's "aliasing-xor-mutability" restrictions, "norad" pointers can be used to express references that cannot be statically verified to be lifetime safe. This would include, for example, all manner of cyclic references beyond just "self-references".

I think you could implement a version of these norad pointers in Rust that can safely target these immovable types (whose destructor is guaranteed to be called while the object is still in its original location). But note that the C++ implementation uses static inheritance (which Rust does not support) to avoid the noise having to access the target object as "interior" content (like with `RefCell`s).

With the availability of these flexible references, one could imagine immovable types becoming popular in things like games / entity component systems, GUI frameworks, browser engines, and any place where "back pointers" would be convenient. One might even imagine that at some point, types being "immovable" could become the popular default for object types in Rust (among biological and/or non-biological Rust programmers). At which point, people may decide that actually they do want (the contents of) some of their immovable types to be "movable", but they don't necessarily need the object to be destructively movable. So you could imagine the introduction of standard `nondestructive_move()` (and `nondestructive_move_from()`) methods that would be companions of the existing `clone()` (and `clone_from()`) methods. At which point Rust would have counterparts for C++ copy and move constructors (and assignment operators).

In my view, this adoption of the C++ model (potentially) addresses Rust's main limitation. With one consequence being to potentially make automated translation of C and C++ code to (reasonable code in) the safe subset of Rust much more feasible than seems to be currently.

[1] https://github.com/duneroadrunner/SaferCPlusPlus/blob/master...

[2] https://github.com/duneroadrunner/SaferCPlusPlus/blob/master...

> and is a major source why people dislike Rust's async

It's worth mentioning that there is, in fact, no language out there other than Rust that can even do this in the first place.

Some languages give the illusion that they support it by boxing the stack frame of async functions and letting a garbage collector deal with the consequences, but that comes with significant drawbacks too (additional GC pressure, heap allocation overhead, requiring a GC in the first place).

You can do it with C++ coroutines, but it's much harder to do correctly than in Rust if you want to maintain any sense of conviction that the system is correct.

The main reason that structured async concurrency would be so awesome to have is that it feels like Rust has the right set of features that could enable it with a set of constraints that are so much more attractive than any other language out there can provide - no overhead, "just works" with no drawbacks.

(For the record, you can actually get pretty far today using primitives like `FuturesUnordered` instead of `tokio::spawn` and similar, but this sidesteps the runtime's scheduler, so YMMV. This basically creates a task-local mini-scheduler for your futures, which may or may not be sufficient.)

I guess `!Move` is largely equivalent to `Unpin` for the purposes of `Pin`, so for example Pin's safe constructor `Pin::new()` can be re-expressed in terms of `!Move` instead of `Unpin`. Today you need unsafe code to pin a `!Unpin` (i.e. "movable") type.

But I also suspect there are important differences between `!Move` and `Unpin` that I'm not sure about.

I agree with most of that, but there's one important detail (that I'm sure the designers of this proposal are thinking about): The current contract of Pin in Rust (the only standard support for expressing immobility) doesn't just cover the object's own location, but also any references derived from the object, commonly called "pin projection".

For example, if you have a `Pin>>`, it's safe to turn that into a `Pin`.

Any system that replaces `Pin` will probably have to maintain that same property, which wouldn't naïvely happen using C++-like move semantics, right? Or maybe I'm overassuming?