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"

Original description for historic preservation

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.)

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

Unresolved Questions

None

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

[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.