Re: [PATCH v2 02/25] rust: types: add `ForLt` trait for higher-ranked lifetime support

[Danilo Krummrich]

On Wed May 6, 2026 at 11:50 PM CEST, Danilo Krummrich wrote:
> From: Gary Guo <gary@xxxxxxxxxxx>
>
> There are a few cases, e.g. when dealing with data referencing each other,
> one might want to write code that are generic over lifetimes. For example,
> if you want take a function that takes `&'a Foo` and gives `Bar<'a>`, you
> can write:
>
>     f: impl for<'a> FnOnce(&'a Foo) -> Bar<'a>,
>
> However, it becomes tricky when you want that function to not have a fixed
> `Bar`, but have it be generic again. In this case, one needs something that
> is generic over types that are themselves generic over lifetimes.
>
> `ForLt` provides such support. It provides a trait `ForLt` which describes
> a type generic over lifetime. One may use `ForLt::Of<'a>` to get an
> instance of a type for a specific lifetime.
>
> For the case of cross referencing, one would almost always want the
> lifetime to be covariant. Therefore this is also made a requirement for the
> `ForLt` trait, so functions with `ForLt` trait bound can assume covariance.
>
> A macro `ForLt!()` is provided to be able to obtain a type that implements
> `ForLt`. For example, `ForLt!(for<'a> Bar<'a>)` would yield a type that
> `<TheType as ForLt>::Of<'a>` is `Bar<'a>`. This also works with lifetime
> elision, e.g. `ForLt!(Bar<'_>)` or for types without lifetime at all, e.g.
> `ForLt!(u32)`.
>
> The API design draws inspiration from the higher-kinded-types [1] crate,
> however different design decision has been taken (e.g. covariance
> requirement) and the implementation is independent.
>
> License headers use "Apache-2.0 OR MIT" because I anticipate this to be
> used in pin-init crate too which is licensed as such.
>
> Link: https://docs.rs/higher-kinded-types/ [1]
>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>

Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>