Tracking Issue for function delegation (`fn_delegation`, RFC 3530)

[petrochenkov]

This is a tracking issue for the RFC "Implement function delegation in rustc" (https://github.com/rust-lang/rfcs/pull/3530). The feature gate for the issue is #![feature(fn_delegation)].

The feature was accepted for experimentation in https://github.com/rust-lang/rust/pull/117978#issuecomment-1823066970.

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

• [ ] Implement the RFC (roughly in the order of priority)
  • [x] https://github.com/rust-lang/rust/pull/118947 Parse syntax and implement basic desugaring
  • [x] https://github.com/rust-lang/rust/pull/122500 Support renaming
  • [x] https://github.com/rust-lang/rust/pull/122500 Support modifiers like const, extern, async
  • [x] https://github.com/rust-lang/rust/pull/123413 Support list delegation (design)
  • [x] https://github.com/rust-lang/rust/pull/124135 Support glob delegation and overriding it with explicit definitions (design)
  • [ ] https://github.com/rust-lang/rust/pull/126699 Support autoref/deref and coercions for the first argument (design)
  • [ ] Support inheriting generic parameters (design)
  • [ ] https://github.com/rust-lang/rust/pull/125929
  • [ ] https://github.com/rust-lang/rust/pull/126161
  • [ ] Support providing generic arguments (design)
  • [ ] https://github.com/rust-lang/rust/pull/123958
  • [ ] Add implicit #[inline] unless specified otherwise
  • [ ] Consider whether some selected attributes need to be inherited
  • [ ] Self type mapping (in RFC)
  • [ ] Conversions for arguments and return value in the generated body (in RFC 1, in RFC 2)
  • [ ] Delegation for associated constants and types
  • [ ] Support impl Trait in traits
  • [ ] One line sugar for glob-delegating a whole trait impl
  • [ ] Support recursive delegation (delegation items referring to other local delegation items)
  • [ ] Support type-relative paths (old prototype)
  • [ ] Infer delegation target from the target expr instead of using an explicit path (in RFC)
• [ ] Adjust documentation (see instructions on rustc-dev-guide)
• [ ] Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• [ ] Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Many of them in the RFC draft.

Implementation history

https://github.com/rust-lang/rust/pulls?q=is%3Apr+label%3AF-fn_delegation

[petrochenkov]

cc @Bryanskiy

[jdonszelmann]

I was looking through the code of the experimental implementation today (#117978), and I found something that surprised me. The experimental implementation of delegation has an id in it:

However, a Delegation is used as an ItemKind, and any ItemKind is associated with an Item. Now every Item already has an ID associated with it:

I looked through all the code, and I think for delegation the Item id is never used, and the Delegation specific one is always used but seems redundant.

Let me know if I'm misunderstanding something here, maybe I am, but I just wanted to let you know in case it's redundant.

[petrochenkov]

@jdonszelmann In

trait Trait { fn method(); }

impl Trait for Struct {
  reuse some::method;
}

the same Resolver::partial_res_map table keeps resolutions for both some::method and Trait::method (pointing from the impl to the trait). The former uses the Delegation's id as a key, and the latter uses Item's id as a key, so they need different ids.

The Delegation's id can be removed if a separate table is added for associated item resolutions in impls, instead of reusing partial_res_map.

[petrochenkov]

I'll update the list of subtasks in the issue a bit later, but for now I'll just post one of them for @Bryanskiy .

Suppose we want to delegate implementation of a whole trait with 10 methods, 5 of them are by ref and 5 are by mutable ref.

struct Wrapper(Inner);

impl Trait for Wrapper {
  fn ref1() { Trait::ref1(&self.0) }
  fn ref2() { Trait::ref2(&self.0) }
  fn ref3() { Trait::ref3(&self.0) }
  fn ref4() { Trait::ref4(&self.0) }
  fn ref5() { Trait::ref5(&self.0) }

  fn mut1() { Trait::mut1(&mut self.0) }
  fn mut2() { Trait::mut2(&mut self.0) }
  fn mut3() { Trait::mut3(&mut self.0) }
  fn mut4() { Trait::mut4(&mut self.0) }
  fn mut5() { Trait::mut5(&mut self.0) }
}

Right now we cannot do it using glob delegation and an explicit path, due to the difference between &self.0 and &mut self.0. We have to write

impl Trait for Wrapper {
  reuse Trait::* { &self.0 }
  reuse Trait::{mut1, mut2, mut3, mut4, mut5} { &mut self.0 }
}

Ideally we would use just

impl Trait for Wrapper {
  reuse Trait::* { self.0 }
}

and self.0 would autoref/autoderef/coerce automatically.

For that in the generated body Trait::method(self.0) we should treat self.0 as if it was a method receiver in a method call, rather than a regular argument. The difference with existing method calls is that in self.0.method() the method is resolved as a part of applying conversions to self.0, but in our case it is already pre-resolved to Trait::method. The types team should know how to do it correctly.

(This was mentioned in the RFC in application to a different case - https://github.com/petrochenkov/rfcs/blob/delegation/text/0000-fn-delegation.md#the-proposed-algorithm.)

[jdonszelmann]

@petrochenkov that makes sense. Would you like it if I gave it a comment or a slightly more descriptive name?

[petrochenkov]

The separate table approach may actually be a better way.