Tracking issue for inherent associated types

[pnkfelix]

This is a tracking issue for the "inherent associate type" part of "inherent associated items" part of the RFC 195 "associated items"

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.

Original description

(This code is written in ancient version of Rust, and is not meant to be used directly.)

When developing a type-parametric impl, I found myself writing code somewhat like this:

    struct Grammar<T, NT> { ... }

    impl<T:Clone,NT:Eq+Hash+Clone+Primable> Grammar<T,NT> {
        fn to_nt_map(&self) -> HashMap<NT, ~[Prod<T,NT>]>
        {
            type Rules = ~[Prod<T,NT>];
            type NTMap = HashMap<NT, Rules>;        
          ...
        }

        fn from_nt_map(start: &NT, rules: &HashMap<NT, ~[Prod<T,NT>]>)
            -> Grammar<T,NT>
        {
            type Rules = ~[Prod<T,NT>];
            type NTMap = HashMap<NT, Rules>;
          ...
        }

        pub fn eliminate_left_recursion(&self) -> Grammar<T,NT>
        {
          ...
        }

        ...

    }

I cannot make a type definition for HashMap<NT, ~[Prod<T,NT>]> outside of the impl because it then those free references to NT and T would be unbound.

And I cannot put a type definition within the impl, even an impl for a struct such as this (it simply does not parse with the current grammar).

(Being able to put type definitions within impls will probably arise naturally from #5033, when we get around to that. But that is nonetheless a separate issue from this: Associated Types (and Associated Items) are about enabling certain patterns for the user of a trait, while this ticket describes a convenience for the implementor of a struct.)

Steps

• Implement the RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Implementation history

• Add incomplete feature gate for inherent associate types. #82516
• Correctly resolve Inherent Associated Types #103621
• Respect visibility & stability of inherent associated types #104348
• Properly substitute inherent associated types. #105224
• Normalize inherent associated types after substitution #105315
• Detect inherent associated types not having CamelCase #105768
• Type-directed probing for inherent associated types #105961
• Introduce AliasKind::Inherent for inherent associated types #109410
• rustdoc: render visibility on associated types #110945
• Pretty-print inherent projections correctly #111486
• Structurally normalize weak and inherent in new solver #114594
• Do not erase late bound regions when selecting inherent associated types #118118
• Relate Inherent Associated Types using eq #118262

[emberian]

Nominating; is backwards compatible. I can try and take this on if we decide we want it.

[pnkfelix]

assigning P-low priority (is backwards compatible going foward, as cmr said).

[reem]

Triage: How does this interact with Associated Types? At least a new syntax would probably have to be chosen.

[pnkfelix]

I think that this feature request is actually satisfied by the Associated Items RFC.

In particular, the section "inherent associated items" of RFC 59 implies (to me) that you can write the exact example that I had asked for.

(I did write in the issue description that this feature request is distinct from associated types, but nonetheless, it seems that the author of RFC 59 decided that it was a natural feature to include.)

[pnkfelix]

So, this now seems to be a subissue of #17307.

I will change the title to "implement inherent associated items."

[rohel01]

Hello, I hope this the right place to ask for my request.

Associated types currently only work with traits.
I think they are also useful for structs. Is there a reason for the current limitation ?

pub struct GenericStruct<A, B> {
    fa : A,
    fb : B,
}

struct GenericResult<A, B> {
    fa : A,
    fb : B,
}

impl <A, B> GenericStruct<A, B> {


    // I d like to write
    // type Result = Option<GenericResult<A, B>>;
    // and use it in the code

    fn new(a: A, b : B) -> Option<GenericResult<A, B>> {
        Some(GenericResult {fa: a, fb : b})

    }
}

[steveklabnik]

I'm going to tag this with the tags from #8995 since it was closed without this being implemented.

[yodaldevoid]

@steveklabnik I think you meant to reference #17307 rather than #8995, which is this issue.

[tinaun]

note that inherent associated consts work perfectly fine today

#[derive(Debug)]
struct Vector {
    x: f32,
    y: f32,
    z: f32,
}

impl Vector {
    const ZERO: Self = Vector { x: 0.0, y: 0.0, z: 0.0 };
    
    fn new(x: f32, y: f32, z: f32) -> Self {
        Vector {
            x, y, z
        }
    }
}

[eddyb]

@nikomatsakis Is it right to assume that this can't be implemented without lazy normalization?

[peku33]

Were there any changes made to this in one of last releases?

struct Foo {
    bar: Self::Bar,
}
impl Foo {
    pub type Bar = usize;
}

used to work a couple of nightly releases ago.

Now it fails to compile with error:

error[E0391]: cycle detected when computing predicates of `Foo`
  --> src\main.rs:5:1
   |
5  | struct Foo {
   | ^^^^^^^^^^
   |
note: ...which requires computing predicates of `Foo`...
  --> src\main.rs:5:1
   |
5  | struct Foo {
   | ^^^^^^^^^^
note: ...which requires computing inferred outlives predicates of `Foo`...
  --> src\main.rs:5:1
   |
5  | struct Foo {
   | ^^^^^^^^^^
   = note: ...which requires computing the inferred outlives predicates for items in this crate...
note: ...which requires computing type of `Foo::bar`...
  --> src\main.rs:6:5
   |
6  |     bar: Self::Bar,
   |     ^^^^^^^^^^^^^^
note: ...which requires computing normalized predicates of `Foo`...
  --> src\main.rs:5:1
   |
5  | struct Foo {
   | ^^^^^^^^^^
   = note: ...which again requires computing predicates of `Foo`, completing the cycle
note: cycle used when collecting item types in top-level module

[flouet-company]

I am just curious about the status of this issue. Is someone working on this or has someone made any progress in this space?

[fmease]

There's been a recent series of PRs that work towards fully implementing inherent associated types by @fee1-dead, @cjgillot and me.

• Correctly resolve Inherent Associated Types #103621
• Respect visibility & stability of inherent associated types #104348
• Properly substitute inherent associated types. #105224
• Normalize inherent associated types after substitution #105315
• Detect inherent associated types not having CamelCase #105768
• Type-directed probing for inherent associated types #105961

I highly suspect that the last one on that list (#105961) broke the code posted above. @peku33, could you please open an issue for the cycle error and tag me? Thanks. Note however that inherent_associated_types is still an incomplete feature and thus it's likely to break every so often.

Is someone working on this or has someone made any progress in this space?

@flouet-company, I'm assigned to several issues related to IATs and I'm in close contact with some members of T-types in order to properly implement inherent associated types.

[jhpratt]

I'd just like to say thank you to you three for working on this feature! I know it's not simple, but it's something a lot of people will appreciate when it's eventually completed.

[Cypher1]

This seems particularly important for resolving 'clippy::type_complexity' when using large types inside parameterized types with multiple arguments.

Normally we'd use a type Foo = Bar<A, B, C>; but in a generic type, those types may not be accessible except via more type parameters, leading to new complex types.

Keep to hear more about blockers :)

[fmease]

Keep to hear more about blockers

The most recent major blocker is fixing #108491 which is not easy it seems. Basically, you cannot use inherent associated types inside of structs, enums, unions or where clauses (as opposed to function bodies, constants and type aliases) and some function signatures without encountering a cycle error first. According to my current understanding this is due to architectural limitations in the compiler (see this semi-related Zulip topic, cc #22519 the equivalent for trait assoc tys).

Since #109410 got merged, I'm investigating approaches to solve this in a proper way without too much rework.