e189bdb687a56bcf389798f1d3a2f261fff2ef54 - rust: types: add `ForLt` trait for higher-ranked lifetime support

Mon, 25 May 2026 22:21:09 +0200

rust: types: add `ForLt` trait for higher-ranked lifetime supportThere are a few cases, e.g. when dealing with data referencing eachother, one might want to write code that is generic over lifetimes. Forexample, if you want to 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 afixed `Bar`, but have it be generic again. In this case, one needssomething that is generic over types that are themselves generic overlifetimes.`ForLt` provides such support. It provides a trait `ForLt` whichdescribes a type generic over a lifetime. One may use `ForLt::Of<'a>` toget an instance of a type for a specific lifetime.For the case of cross referencing, one would almost always want thelifetime to be covariant. Therefore this is also made a requirement forthe `ForLt` trait, so functions with `ForLt` trait bound can assumecovariance.A macro `ForLt!()` is provided to be able to obtain a type thatimplements `ForLt`. For example, `ForLt!(for<'a> Bar<'a>)` would yield atype that `::Of<'a>` is `Bar<'a>`. This also workswith lifetime elision, e.g. `ForLt!(Bar<'_>)` or for types withoutlifetime at all, e.g. `ForLt!(u32)`.The API design draws inspiration from the higher-kinded-types [1] crate,however a different design decision has been taken (e.g. covariancerequirement) and the implementation is independent.License headers use "Apache-2.0 OR MIT" because I anticipate this to beused in pin-init crate too which is licensed as such.Link: https://docs.rs/higher-kinded-types/ [1]Reviewed-by: Greg Kroah-Hartman Reviewed-by: Eliot Courtney Signed-off-by: Gary Guo Acked-by: Miguel Ojeda Reviewed-by: Alexandre Courbot Link: https://patch.msgid.link/20260525202921.124698-23-dakr@kernel.org[ Handle macro_rules! invocations in the ForLt! proc macro's covariance and WF checks. Since proc macros cannot expand macro_rules!, add a visit_macro() implementation to conservatively assume macro invocations may contain lifetimes, forcing them through the compiler-assisted covariance proof. Fix a few typos in the documentation and in the commit message, add empty lines before samples, add missing periods and consistently use markdown. - Danilo ]Signed-off-by: Danilo Krummrich List of files: /linux/rust/macros/for_lt.rs

Changes in for_lt.rs

e189bdb687a56bcf389798f1d3a2f261fff2ef54 - rust: types: add `ForLt` trait for higher-ranked lifetime support