Tracking issue for RFC 3519: arbitrary_self_types

[arielb1]

This is the tracking issue for RFC 3519: Arbitrary self types v2.

The feature gate for this issue is #![feature(arbitrary_self_types)].

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Accept RFC.
  • Arbitrary self types v2 rfcs#3519
• Implement.
  • Arbitrary self types v2: pointers feature gate. #129664
  • Arbitrary self types: diagnostics for mismatched self types #129968
• Decide about generics in arbitrary self types
  • Decide about generics in arbitrary self types #129147
• Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • See the instructions.
• Add formatting for new syntax to the style guide.
  • See the nightly style procedure.

Unresolved Questions

None.

Related

• Changing the type of self in methods doesn't work #27941

Implementation history

TODO.

---

(Below follows content that predated the accepted Arbitrary Self Types v2 RFC.)

• figure out the object safety situation
• figure out the handling of inference variables behind raw pointers
• decide whether we want safe virtual raw pointer methods

Object Safety

See #27941 (comment)

Handling of inference variables

Calling a method on *const _ could now pick impls of the form

impl RandomType {
    fn foo(*const Self) {}
}

Because method dispatch wants to be "limited", this won't really work, and as with the existing situation on &_ we should be emitting an "the type of this value must be known in this context" error.

This feels like fairly standard inference breakage, but we need to check the impact of this before proceeding.

Safe virtual raw pointer methods

e.g. this is UB, so we might want to force the call <dyn Foo as Foo>::bar to be unsafe somehow - e.g. by not allowing dyn Foo to be object safe unless bar was an unsafe fn

trait Foo {
    fn bar(self: *const Self);
}

fn main() {
    // creates a raw pointer with a garbage vtable
    let foo: *const dyn Foo = unsafe { mem::transmute([0usize, 0x1000usize]) };
    // and call it
    foo.bar(); // this is UB
}

However, even today you could UB in safe code with mem::size_of_val(foo) on the above code, so this might not be actually a problem.

More information

There's no reason the self syntax has to be restricted to &T, &mut T and Box<T>, we should allow for more types there, e.g.

trait MyStuff {
    fn do_async_task(self: Rc<Self>);
}

impl MyStuff for () {
    fn do_async_task(self: Rc<Self>) {
        // ...
    }
}

Rc::new(()).do_async_stuff();

[porky11]

Why would you need this?
Why wouldn't you write an impl like this:

impl MyStuff for Rc<()> {
    fn do_async_task(self) {
        // ...
    }
}

I'd rather define the trait different. Maybe like this:

trait MyStuff: Rc {
    fn do_async_task(self);
}

In this case, Rc would be a trait type. If every generic type implemented a specific trait (this could be implemented automatically for generic types) this seems more understandable to me.

[cuviper]

This could only be allowed for trait methods, right?

For inherent methods, I can't impl Rc<MyType>, but if impl MyType can add methods with self: Rc<Self>, it seems like that would enable weird method shadowing.

[arielb1]

@cuviper

This is still pending lang team decisions (I hope there will be at least 1 RFC) but I think it will only be allowed for trait method impls.

[arielb1]

@porky11

You can't implement anything for Rc<YourType> from a crate that does not own the trait.

[arielb1]

So changes needed:

• remove the current error message for trait methods only, but still have a feature gate.
• make sure fn(self: Rc<Self>) doesn't accidentally become object-safe
• make sure method dispatch woks for Rc<Self> methods
• add tests

[SimonSapin]

I’ll look into this.

[arielb1]

Note that this is only supported to work with trait methods (and trait impl methods), aka

trait Foo {
    fn foo(self: Rc<Self>);
}
impl Foo for () {
    fn foo(self: Rc<Self>) {}
}

and is NOT supposed to work for inherent impl methods:

struct Foo;
impl Foo {
    fn foo(self: Rc<Self>) {}
}

[SimonSapin]

I got caught in some more Stylo work that's gonna take a while, so if someone else wants to work on this in the meantime feel free.

[kennytm]

Is this supposed to allow any type as long as it involves Self? Or must it impl Deref<Target=Self>?

trait MyStuff {
    fn a(self: Option<Self>);
    fn b(self: Result<Self, Self>);
    fn c(self: (Self, Self, Self));
    fn d(self: Box<Box<Self>>);
}

impl MyStuff for i32 {
   ...
}

Some(1).a();  // ok?
Ok(2).b();  // ok?
(3, 4, 5).c(); // ok?
(box box 6).d(); // ok?

[mikeyhew]

I've started working on this issue. You can see my progress on this branch

[mikeyhew]

@arielb1 You seem adamant that this should only be allowed for traits and not structs. Aside from method shadowing, are there other concerns?

[arielb1]

@mikeyhew

inherent impl methods are loaded based on the type. You shouldn't be able to add a method to Rc<YourType> that is usable without any use statement.

[arielb1]

That's it, if you write something like

trait Foo {
    fn bar(self: Rc<Self>);
}

Then it can only be used if the trait Foo is in-scope. Even if you do something like fn baz(self: u32); that only works for modules that use the trait.

If you write an inherent impl, then it can be called without having the trait in-scope, which means we have to be more careful to not allow these sorts of things.

[mikeyhew]

@arielb1 Can you give an example of what we want to avoid? I'm afraid I don't really see what the issue is. A method you define to take &self will still be callable on Rc<Self>, the same as if you define it to take self: Rc<Self>. And the latter only affectsRc<MyStruct>, not Rc<T> in general.

[mikeyhew]

I've been trying to figure out how we can support dynamic dispatch with arbitrary self types. Basically we need a way to take a CustomPointer<Trait>, and do two things: (1) extract the vtable, so we can call the method, and (2) turn it into a CustomPointer<T> without knowing T.

(1) is pretty straightforward: call Deref::deref and extract the vtable from that. For (2), we'll effectively need to do the opposite of how unsized coercions are implemented for ADTs. We don't know T, but we can can coerce to CustomPointer<()>, assuming CustomPointer<()> has the same layout as CustomPointer<T> for all T: Sized. (Is that true?)

The tough question is, how do we get the type CustomPointer<()>? It looks simple in this case, but what if CustomPointer had multiple type parameters and we had a CustomPointer<Trait, Trait>? Which type parameter do we switch with ()? In the case of unsized coercions, it's easy, because the type to coerce to is given to us. Here, though, we're on our own.

@arielb1 @nikomatsakis any thoughts?

[nikomatsakis]

@arielb1

and is NOT supposed to work for inherent impl methods:

Wait, why do you not want it work for inherent impl methods? Because of scoping? I'm confused. =)

[nikomatsakis]

@mikeyhew

I've been trying to figure out how we can support dynamic dispatch with arbitrary self types.

I do want to support that, but I expected it to be out of scope for this first cut. That is, I expected that if a trait uses anything other than self, &self, &mut self, or self: Box<Self> it would be considered no longer object safe.

[mikeyhew]

@nikomatsakis

I do want to support that, but I expected it to be out of scope for this first cut.

I know, but I couldn't help looking into it, it's all very interesting to me :)

[arielb1]

Wait, why do you not want it work for inherent impl methods? Because of scoping? I'm confused. =)

We need some sort of "orphan rule" to at least prevent people from doing things like this:

struct Foo;
impl Foo {
    fn frobnicate<T>(self: Vec<T>, x: Self) { /* ... */ }
}

Because then every crate in the world can call my_vec.frobnicate(...); without importing anything, so if 2 crates do this there's a conflict when we link them together.

Maybe the best way to solve this would be to require self to be a "thin pointer to Self" in some way (we can't use Deref alone because it doesn't allow for raw pointers - but Deref + deref of raw pointers, or eventually an UnsafeDeref trait that reifies that - would be fine).

I think that if we have the deref-back requirement, there's no problem with allowing inherent methods - we just need to change inherent method search a bit to also look at defids of derefs. So that's probably a better idea than restricting to trait methods only.

Note that the CoerceSized restriction for object safety is orthogonal if we want allocators:

struct Foo;
impl Tr for Foo {
    fn frobnicate<A: Allocator+?Sized>(self: RcWithAllocator<Self, A>) { /* ... */ }
}

Where an RcWithAllocator<Self, A> can be converted to a doubly-fat RcWithAllocator<Tr, Allocator>.

[mikeyhew]

@arielb1

Because then every crate in the world can call my_vec.frobnicate(...); without importing anything, so if 2 crates do this there's a conflict when we link them together.

Are saying is that there would be a "conflicting symbols for architechture x86_64..." linker error?

Maybe the best way to solve this would be to require self to be a "thin pointer to Self" in some way (we can't use Deref alone because it doesn't allow for raw pointers - but Deref + deref of raw pointers, or eventually an UnsafeDeref trait that reifies that - would be fine).

I'm confused, are you still talking about frobnicate here, or have you moved on to the vtable stuff?

[arielb1]

I'm confused, are you still talking about frobnicate here, or have you moved on to the vtable stuff?

The deref-back requirement is supposed to be for everything, not only object-safety. It prevents the problem when one person does

struct MyType;
impl MyType {
    fn foo<T>(self: Vec<(MyType, T)>) { /* ... */ }
}   

While another person does

struct OurType;
impl OurType {
    fn foo<T>(self: Vec<(T, OurType)>) {/* ... */ }
}   

And now you have a conflict on Vec<(MyType, OurType)>. If you include the deref-back requirement, there is no problem with allowing inherent impls.

[adetaylor]

Hello, friends of Arbitrary Self Types.

Update on general progress

A further update on progress towards the plan above.

I consider step 2 "done". This PR shows, I believe, that it's possible to detect shadowing situations without significant performance impact. The devil might be in the detail (after all, this doesn't actually yet emit any shadowing errors) but for now it's good enough for me to move forward.

Step 3: avoiding "generic" arbitrary self types

During the preparation of the RFC it was broadly felt that we should ban "generic" self types but we didn't really define what "generic" meant, and I didn't pin it down enough.

Some of the commentary:

• Arbitrary self types v2 rfcs#3519 (comment)
• Arbitrary self types v2 rfcs#3519 (comment)
• Arbitrary self types v2 rfcs#3519 (comment)
• A zulip comment
• I've a feeling there's more which I've been unable to find, including the initial impetus to ban generics from arbitrary self types.

It seems to be widely felt that:

impl SomeType {
 fn m<R: Deref<Target=Self>>(self: R) {}
}

would be confusing, but (per those comments) it's actually pretty hard to distinguish that from various legitimate cases for arbitrary self types with generic receivers:

impl SomeType {
  fn method1(self: MyBox<Self, impl Allocator>) {}
  fn method2<const ID: u64>(self: ListArc<Self, ID>) {}
}

I played around with different tests here on the self type in wfcheck.rs but none of them yield the right sort of filter of good/bad cases (which is unsurprising since we haven't quite defined what that means, but I thought I might hit inspiration).

From those comment threads, the most concrete proposal (from @joshtriplett) is:

just disallow the case where the top-level receiver type itself is not concrete.

I plan to have a crack at that, unless folks have other thoughts.
cc @Nadrieril who had opinions here.

If we do this, I might need to revisit the diagnostics mentioned in the bits of RFC removed by this commit.

[adetaylor]

Update: (again relative to the plan in this comment further up)

• Step 3 will hopefully be discussed here, I'm pausing work on that bit to see if a consensus emerges
• Step 4 in progress in this PR

[adetaylor]

Regular status update, relative to the plan outlined before:

• Step 3: still under discussion at Decide about generics in arbitrary self types #129147: options include:
  • do nothing (which seems relatively likely)
  • filter out simple generics using the pull request at Reject generic self types. #130098, which is just about ready to merge if we choose that option
  • do something much more complex along the lines of the unfinished Reject generic self types using impl defid: do not merge. #130120.
• Step 4 landed in Arbitrary self types v2: pointers feature gate. #129664 and seems to have stuck
• Step 5: PR in progress at Rename Receiver -> LegacyReceiver #130225

There was some discussion at the Rust for Linux conference about whether we should do steps 5, 6 and 7 as a single atomic PR, but nobody felt strongly so I'll stick with the plan.