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`~const` trait and projection bounds do not imply their non-const counterparts This PR removes the hack where we install a non-const trait and projection bound for every `const_trait` and `~const` projection bound we have in the AST. It ends up messing up more things than it fixes, see words below. Fixes #119718 cc `@fmease` `@fee1-dead` `@oli-obk` r? fee1-dead or one of y'all i don't care --- My understanding is that this hack was added to support the following code: ```rust pub trait Owo<X = <Self as Uwu>::T> {} #[const_trait] pub trait Uwu: Owo {} ``` Which is concretely lifted from in the `FromResidual` and `Try` traits. Since within the param-env of `trait Uwu`, we only know that `Self: ~const Uwu` and not `Self: Uwu`, the projection `<Self as Uwu>::T` is not satsifyable. This causes problems such as #119718, since instantiations of `FnDef` types coming from `const fn` really do **only** implement one of `FnOnce` or `const FnOnce`! --- In the long-term, I believe that such code should really look something more like: ```rust #[const_trait] pub trait Owo<X = <Self as ~const Uwu>::T> {} #[const_trait] pub trait Uwu: Owo {} ``` ... and that we should introduce some sort of `<T as ~const Foo>::Bar` bound syntax, since due to the fact that `~const` bounds can be present in item bounds, e.g. ```rust #[const_trait] trait Foo { type Bar: ~const Destruct; } ``` It's easy to see that `<T as Foo>::Bar` and `<T as ~const Foo>::Bar` (or `<T as const Foo>::Bar`) can be distinct types with distinct item bounds! **Admission**: I know I've said before that I don't like `~const` projection syntax, I do at this point believe they're necessary to fully express bounds and types in a maybe-const world. |
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For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.