The types are there for garbage collection, which is there for integration with the Web APIs which are all defined in terms of garbage collected objects.
The thing is that anyone can show up and spend time discussing ideas in Rust project spaces. From the outside it is easy to confuse that with actual movement toward landing changes in the language.
(The communication aspect of this is something that has bothered me many times in the past- even people who are lang team members often phrase things in a way that makes it sound like something is on its way in, when it's still just in the stage of "we're kinda noodling with ideas.")
Yoshua is part of the "loud contingent" being described. He's not on the lang team, and he's been "working on" things like keyword generics for years without any indication that they are going to make it into the language.
Or put another way, a hypothetical feature that you made up in your head is the thing that requires source access. Editions do not let you change the semantics of types.
To be fair, Rust tooling does tend toward build-from-source. But this is for completely different reasons than the edition system: if you had a way to build a crate and then feed the binary into builds by future compilers, it would require zero additional work to link it into a crate using a different edition.
Not at all. It's much more efficient to implement a GC on x86 or ARM than it is on Wasm 1.0/2.0, because you control the stack layout, and you don't have an impenetrable security boundary with the JS runtime that your GC needs to interop with.
Not to mention the issue that bundling a GC implementation as part of your web page can be prohibitive in terms of download size.
The reason rust-analyzer doesn't update diagnostics until you save is historical. Originally, people tried to build IDE support by reusing rustc itself, but this proved too slow and cumbersome at the time.
Rust-analyzer reimplemented the frontend in a more IDE-friendly architecture, but focused more on name resolution than on type checking. So it delegated diagnostics to literally just running `cargo check`.
As parts of rustc get rewritten over time (the trait solver, borrow checker) they have also been made more IDE-friendly and reusable, so rust-analyzer is slowly gaining the ability to surface more type checking diagnostics as you edit, without delegating to `cargo check`.
TCO is less of an optimization (which are typically best-effort on the part of the compiler) and more of an actual semantic change that expands the set of valid programs. It's like a new control flow construct that lives alongside `while` loops.
Giving special treatment to code that "explicitly wants" to handle unwinding means two things:
* You have to know when an API can unwind, and you have to make it an error to unwind when the caller isn't expecting it. If this is done statically, you are getting into effect annotation territory. If this is done dynamically, are essentially just injecting drop bombs into code that doesn't expect unwinding. Either way, you are multiplying complexity for generic code. (Not to mention you have to invent a whole new set of idioms for panic-free code.)
* You still have to be able to clean up the resources held by a caller that does expect unwinding. So all your vocabulary/glue/library code (the stuff that can't just assume panic=abort) still needs these "scoped panic hooks" in all the same places it has any level of panic awareness in Drop today.
So for anyone to actually benefit from this, they have to be writing panic-free code with whatever new static or dynamic tools come with this, and they have to be narrowly scoped and purpose-specific enough that they could essentially already today afford panic=abort. Who is this even for?
> Suppose, instead, we had a mechanism that allowed registering arbitrary panic hooks, and unregistering them when no longer needed, in any order. Then, we could do RAII-style resource handling: you could have a `CursesTerminal` type, which is responsible for cleaning up the terminal, and it cleans up the terminal on `Drop` and on panic. To do the latter, it would register a panic hook, and deregister that hook on `Drop`.
This doesn't get rid of unwinding at all- it's an inefficient reimplementation of it. There's a reason language implementations have switched away from having the main execution path register and unregister destructors and finally blocks, to storing them in a side table and recovering them at the time of the throw.
There are also currently the unstable rustc_layout_scalar_valid_range_start and rustc_layout_scalar_valid_range_end attributes (which are used in the definition of NonNull, etc.) which could be used for some bit patterns.
Hypothetically Rust could make `Mutex<InnerBlah>` work with just two bits in the same way it makes `Option<&T>` the same size as `&T`. Annotate `InnerBlah` with the information about which bits are available and let `Mutex` use them.
You don't need any of that, and you can keep cancellation too.
The core of an eager cooperative multitasking system does not even need the concept of an executor. You can spawn a new task by giving it some stack space and running its body to its first suspension point, right there on the current thread. When it suspends, the leaf API (e.g. `lock`) grabs the current top of the stack and stashes it somewhere, and when it's time to resume it again just runs the next part of the task right there on the current thread.
You can build different kinds of schedulers on top of this first-class ability to resume a particular leaf call in a task. For example, a `lock` integrated with a particular scheduler might queue up the resume somewhere instead of invoking it immediately. Or, a generic `lock` might be wrapped with an adapter that re-suspends and queues that up. None of this requires that the language know anything about the scheduler at all.
This is all typical of how higher level languages implement both stackful and stackless coroutines. The difference is that we want control over the "give it some stack space" part- we want the compiler to compute a maximum size and have us specify where to store it, whether that's on the heap (e.g. tokio::spawn) or nested in some other task's stack (e.g. join, select) or some statically-allocated storage (e.g. on a microcontroller).
(Of course the question then becomes, how do you ensure `lock` can't resume the task after it's been freed, either due to normal resumption or cancellation? Rust answers this with `Waker`, but this conflates the unit of stack ownership with the unit of scheduling, and in the process enables intermediate futures to route a given wakeup incorrectly. These must be decoupled so that `lock` can hold onto both the overall stack and the exact leaf suspension point it will eventually resume.)
Cancellation doesn't change much here. Given a task held from the "caller end" (as opposed to the leaf callee resume handles above), the language needs to provide a way to destruct the stack and let the decoupled `Waker` mechanism respond. This still propagates naturally to nested tasks like join/select arms, though there is now an additional wrinkle that a nested task may be actively running (and may even be the thing that indirectly provoked the cancellation).
> in principle the exact same optimization could be done for stackful coroutines.
Yes, I totally agree, and this is sort of what I imagine a better design would look like.
> One of the reasons Rust does it the way it currently does is because the implementation avoids requiring support from, e.g., LLVM
This I would argue is simply a failure of imagination. All you need from the LLVM layer is tail calls, and then you can manage the stack layout yourself in essentially the same way Rust manages Future layout.
You don't even need arbitrary tail calls. The compiler can limit itself to the sorts of things LLVM asks for- specific calling convention, matching function signatures, etc. when transferring control between tasks, because it can store most of the state in the stack that it laid out itself.
"Not inert" does not at all imply "a single runtime within std+compiler." You've jumped way too far in the opposite direction there.
The problem is that the particular interface Rust chose for controlling dispatch is not granular enough. When you are doing your own dispatch, you only get access to separate tasks, but for individual futures you are at the mercy of combinators like `select!` or `FuturesUnordered` that only have a narrow view of the system.
A better design would continue to avoid heap allocations and allow you to do your own dispatch, but operate in terms of individual suspended leaf futures. Combinators like `join!`/`select!`/etc. would be implemented more like they are in thread-based systems, waiting for sub-tasks to complete, rather than being responsible for driving them.
This one is relevant because it avoids heap allocation while running the iterator and for loop body concurrently. Which is exactly the kind of thing that `async` does.
The requirement is that the futures are not separate heap allocations, not that they are inert.
It's not at all obvious that Rust's is the only possible design that would work here. I strongly suspect it is not.
In fact, early Rust did some experimentation with exactly the sort of stack layout tricks you would need to approach this differently. For example, see Graydon's post here about the original implementation of iterators, as lightweight coroutines: https://old.reddit.com/r/ProgrammingLanguages/comments/141qm...