generic const items: support effects#117530
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fmease
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fmease
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| #[derive(Clone, Copy)] | ||
| enum EffectContext { | ||
| Parametrized { host_param_id: LocalDefId }, | ||
| Invariant { value: bool }, |
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I struggle a bit with the modelling of this variant. I know that Invariant { value: true } is super useless and that I should probably make it into a unit variant. However, I don't know what I should name it. I don't know if EffectContext should mention const-specific stuff, I opted against it. Otherwise, AlwaysConst would be the most obvious choice.
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I don't know if we should even allow |
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…fee1-dead
Introduce `const Trait` (always-const trait bounds)
Feature `const_trait_impl` currently lacks a way to express “always const” trait bounds. This makes it impossible to define generic items like fns or structs which contain types that depend on const method calls (\*). While the final design and esp. the syntax of effects / keyword generics isn't set in stone, some version of “always const” trait bounds will very likely form a part of it. Further, their implementation is trivial thanks to the `effects` backbone.
Not sure if this needs t-lang sign-off though.
(\*):
```rs
#![feature(const_trait_impl, effects, generic_const_exprs)]
fn compute<T: const Trait>() -> Type<{ T::generate() }> { /*…*/ }
struct Store<T: const Trait>
where
Type<{ T::generate() }>:,
{
field: Type<{ T::generate() }>,
}
```
Lastly, “always const” trait bounds are a perfect fit for `generic_const_items`.
```rs
#![feature(const_trait_impl, effects, generic_const_items)]
const DEFAULT<T: const Default>: T = T::default();
```
Previously, we (oli, fee1-dead and I) wanted to reinterpret `~const Trait` as `const Trait` in generic const items which would've been quite surprising and not very generalizable.
Supersedes rust-lang#117530.
---
cc `@oli-obk`
As discussed
r? fee1-dead (or compiler)
bors
added a commit
to rust-lang-ci/rust
that referenced
this pull request
Dec 27, 2023
…fee1-dead
Introduce `const Trait` (always-const trait bounds)
Feature `const_trait_impl` currently lacks a way to express “always const” trait bounds. This makes it impossible to define generic items like fns or structs which contain types that depend on const method calls (\*). While the final design and esp. the syntax of effects / keyword generics isn't set in stone, some version of “always const” trait bounds will very likely form a part of it. Further, their implementation is trivial thanks to the `effects` backbone.
Not sure if this needs t-lang sign-off though.
(\*):
```rs
#![feature(const_trait_impl, effects, generic_const_exprs)]
fn compute<T: const Trait>() -> Type<{ T::generate() }> { /*…*/ }
struct Store<T: const Trait>
where
Type<{ T::generate() }>:,
{
field: Type<{ T::generate() }>,
}
```
Lastly, “always const” trait bounds are a perfect fit for `generic_const_items`.
```rs
#![feature(const_trait_impl, effects, generic_const_items)]
const DEFAULT<T: const Default>: T = T::default();
```
Previously, we (oli, fee1-dead and I) wanted to reinterpret `~const Trait` as `const Trait` in generic const items which would've been quite surprising and not very generalizable.
Supersedes rust-lang#117530.
---
cc `@oli-obk`
As discussed
r? fee1-dead (or compiler)
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Treat
~constbounds on generic const items as “constbounds” (always const)1.cc @oli-obk
r? @fee1-dead
Footnotes
Obviously, it would be of advantage to at some point introduce const trait bounds into the surface language to enable users to abstract over generic const items with const fns. ↩