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mention `WithOptConstParam` (#1346)

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lcnr 2022-05-09 02:19:43 +02:00 committed by GitHub
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src/constants.md
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@ -4,6 +4,14 @@ Constants used in the type system are represented as [`ty::Const`].
The variants of their [`ty::ConstKind`] mostly mirror the variants of [`ty::TyKind`] The variants of their [`ty::ConstKind`] mostly mirror the variants of [`ty::TyKind`]
with the two *additional* variants being `ConstKind::Value` and `ConstKind::Unevaluated`. with the two *additional* variants being `ConstKind::Value` and `ConstKind::Unevaluated`.
## `WithOptConstParam` and dealing with the query system
To typecheck constants used in the type system, we have to know their expected type.
For const arguments in type dependent paths, e.g. `x.foo::<{ 3 + 4 }>()`, we don't know
the expected type for `{ 3 + 4 }` until we are typechecking the containing function.
As we may however have to evaluate that constant during this typecheck, we would get a cycle error.
For more details, you can look at [this document](https://hackmd.io/@rust-const-generics/Bk5GHW-Iq).
## Unevaluated constants ## Unevaluated constants