On Wed May 6, 2026 at 11:50 PM CEST, Danilo Krummrich wrote: > From: Gary Guo <[email protected]> > > 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 <[email protected]>
Signed-off-by: Danilo Krummrich <[email protected]>
