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Merge pull request #21 from lcnr/uwu-new-book
uwu a list of all the issues
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‎SUMMARY.md

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- [📅 Roadmap for 2021](./vision/roadmap.md)
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- [❓How to vision doc](./vision/how_to_vision_doc.md)
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- [🔍 Meetings](./meetings/README.md)
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- [📚 Design Documents](./design-docs/README.md)
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- [Anon const substs](./design-docs/anon-const-substs.md)
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- [Const eval requirements](./design-docs/const-eval-requirements.md)
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- [Generic const param types](./design-docs/generic-const-param-types.md)
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- [📚 Design](./design/README.md)
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- [❗ Constraining generic parameters](./design/constraining-generic-parameters.md)
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- [❗⚖️🔄 Generic const parameter types](./design/generic-const-param-types.md)
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- [❗🔙 Restrictions on const evaluation](./design/const-eval-requirements.md)
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- [❗🔙 ⚖️ Structural equality](./design/structural-equality.md)
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- [❗⚖️ Valid const parameter types](./design/valid-const-parameter-types.md)
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- [❗ Valtrees](./design/valtrees.md)
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- [❔ Exhaustiveness](./design/exhaustiveness.md)
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- [❔🔙 Functions as const parameters](./design/functions-as-const-parameters.md)
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- [✏️ Draft RFCs](./draft-rfcs/README.md)
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‎design/README.md

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# 📚 Design
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For an explanation of how const generics currently works in the compiler,
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also check out the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/constants.html).
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Const generics has quite a few deep rooted issues and design challenges.
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In this document we try to highlight some specific topics which are fairly
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pervasive while working on this feature.
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### 🔙 Backwards compatability
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Rust was not design with const generics in mind, and we've also made
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some unfortunate decisions even after work on const generics started.
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Future extensions of const generics must not cause substantial breakage
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to existing code, or if they do, this breakage has to be contained somehow.
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A possible solution to many such changes are editions.
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### ⚖️ No perfect solution
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There isn't just one *perfect* version of const generics we are working towards.
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Instead there are a lot of major and minor tradeoffs between learnability, expressiveness,
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implementation complexity and many other factors. As we can't perfectly tell how
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const generics will be used in the future, making these decisions can be especially
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hard. For any such issues we should try to reach out to the wider community before
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coming to a final conclusion.
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### 🔄 The query system
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Const generics mixes evaluation with typechecking which very quickly reaches the edge
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of what is possible with the query system. It might make sense to look at these issues in
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unison and consider whether there are some more fundamental changes to the compiler which
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provide a better solution to them instead of [adding more hacks](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.WithOptConstParam.html).
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### ❔ Optional extensions
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There are a lot of things which are nice to have but not strictly necessary or maybe not even
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desirable. This means that decisions which prevents them may still be the correct ones,
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i.e. these are not strictly a blocking concern.
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We should still try to find alternatives before blocking these though.
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## Const parameter types ([`adt_const_params`](https://github.com/rust-lang/rust/issues/95174))
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On stable, only integers, `bool`, and `char` is allowed as the type of const parameters.
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We want to extend this to more types in the future.
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```rust
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struct NamedType<const NAME: &'static str> { ... }
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```
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Additionally, we want to support generic const parameter types and must not stabilize anything
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which prevents the addition of that in the future.
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```rust
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struct WithGenericArray<T, const N: usize, const ARRAY: [T; N]> { ... }
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```
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This feature interacts with the following topics:
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- [❗ Constraining generic parameters](./design/constraining-generic-parameters.html)
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- [❗🔙 ⚖️ Structural equality](./design/structural-equality.html)
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- [❗⚖️ Valid const parameter types](./design/valid-const-parameter-types.html)
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- [❗ Valtrees](./design/valtrees.html)
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- [❗⚖️🔄 Generic const parameter types](./design/generic-const-param-types.html)
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- [❔ Exhaustiveness](./design/exhaustiveness.html)
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- [❔🔙 Functions as const parameters](./design/functions-as-const-parameters.html)
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## Generic constants in the type system ([`generic_const_exprs`](https://github.com/rust-lang/rust/issues/76560))
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- ❗🔙 🔄 Unused substs
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- ❗🔄 Self referential where clauses
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- ❗🔄 Evaluation without first checking where-clauses
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### Unifying generic constants
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- [❗🔙 Restrictions on const evaluation](./design/const-eval-requirements.html)
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- ❗ Do not leak implementation details
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- ❗ Splitting constants during unification
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- [❗ Constraining generic parameters](./design/constraining-generic-parameters.html)
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- ❔⚖️ Extending unification logic
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### Const evaluatable bounds
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ALL OF THIS
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## Repeat length backcompatability lint ([`const_evaluatable_unchecked`](https://github.com/rust-lang/rust/issues/76200))
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close this or wait until it's possible on stable.

‎design/const-eval-requirements.md

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# ❗🔙 Restrictions on const evaluation
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For generic constants in the type system to be sound, const evaluation must
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not be able to differentiate between values considered equal by the type system.
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Const evaluation must also be fully deterministic and must not depend on any external state
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when used in the type system.
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## Floats
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Using floats during CTFE is fully determinstic. So using
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them inside of the type system is fine. CTFE can however
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produce different results than what would happen on real hardware,
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but this is not a concern for const generics.
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## Dealing with references and allocations
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Other sources of non-determinism are allocations. This non-determinism
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must however not be observed during const-evaluation (TODO: link to const-eval).
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Any references used in a constant are considered equal if their targets are equal, which is also determistic.
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This is needed by [valtrees](./design/valtrees.html). The specific design of valtrees also adds some other
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additional constraints to const evaluation which are listed on its page.

‎design/constraining-generic-parameters.md

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# ❗ Constraining generic parameters
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Given an impl, the compiler has to be able to decide the generic arguments used by that impl.
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Consider the following snippet:
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```rust
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struct Weird<const N: usize>;
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impl<const A: usize, const B: usize> Weird<{ A + B }> {
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fn returns_a() -> usize {
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A
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}
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}
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```
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When calling `Weird::<3>::returns_a()`, there is no way to restrict the generic parameters `A` or `B` so this has to error.
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If a generic parameter is used by an injective expression, then we should allow this. The most relevant case here are
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constructors:
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```rust
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struct UsesOption<const N: Option<usize>>;
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impl<const N: usize> UsesOption<{ Some(N) }> {}
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```
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Here it is very clear which `N` we should use given `UsesOption::<{ Some(3) }>`.
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## Current status
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**Blocked**: Before making any decisions here we should first figure out [structural equality](./design/structural-equality.md)

‎design/exhaustiveness.md

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# ❔ Exhaustiveness
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Whether we want to be able to unify different partial impls to consider them exhaustive.
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```rust
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trait ForBool<const B: bool> {}
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impl ForBool<true> for u8 {}
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impl ForBool<false> for u8 {}
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// Does `for<const B: bool> u8: ForBool<B>` hold?
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```
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## Status
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**Idle**: While not blocked, it may make sense to wait until we are able to [constrain generic parameters](./design/constraining-generic-parameters.html).

‎design/functions-as-const-parameters.md

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# ❔🔙 Functions as const parameters
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**[project-const-generics#7](https://github.com/rust-lang/project-const-generics/issues/7)**
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It would be nice to allow function pointers as const parameter types.
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```rust
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fn foo<const N: fn() -> usize>() {}
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```
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While they do have a sensible definition of structural equality at compile time,
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comparing them is not deterministic at runtime. This behavior might be fairly surprising,
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so it might be better to not allow this.
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## Current status
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**Planning/Blocked**: needs `generic_const_exprs` to be closer to stable before it makes sense to start working on this.

‎design-docs/generic-const-param-types.md renamed to ‎design/generic-const-param-types.md

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# Generic const param types
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# ❗⚖️🔄 Generic const parameter types
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We want to support the types of const parameters
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Potential dangers include partially resolved types and projections.
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[WithOptConstParam]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.WithOptConstParam.html
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[WithOptConstParam]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.WithOptConstParam.html
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## Status
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**Blocked**: We should first stabilize `feature(adt_const_params)`.

‎design/structural-equality.md

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# ❗🔙 ⚖️ Structural equality
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We have to figure out which values and types can be sensibly compared at compile time.
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## Status
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**In progress**: [lcnr](https://github.com/lcnr/) is working on this.

‎design/valid-const-parameter-types.md

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# ❗⚖️ Valid const parameter types
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**[project-const-generics#6](https://github.com/rust-lang/project-const-generics/issues/6)**
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We have to decide which types may be used as a const parameter and whether there needs to be explicit opt-in.
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## Status
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**Blocked**: Waiting on [structural equality](./design/structural-equality.html).

‎design/valtrees.md

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# ❗ Valtrees
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A new representation for `ty::Const`.
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## ❗ Figure out how they interact with references wrt padding
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**[project-const-generics#20](https://github.com/rust-lang/project-const-generics/issues/20)**
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## Status
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**In Progress**: [b-naber](https://github.com/rust-lang/project-const-generics/issues/20) is working on this.

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