Rust

For example maybe with ML in your background Generic Associated Types are obvious, or maybe not, with GATs Rust can express the idea that some trait can have associated types which are generic. So e.g. today an Iterator has an associated type saying which Item type it iterates over, but that associated type has to be specific, like this is an Iterator over Strings, there are some traits people would like to write where you'd want to express that the associated type has some generic properties but not tie down the specifics. Evidently the Rust for Linux have some use for this feature.

But equally maybe you don't know what Rust's traits are. Traits are similar to the "interfaces" feature in many object oriented languages, in that they express some capability or property common to multiple types. A trait must be explicitly implemented for any particular type, either with the definition of the type itself, or with the definition of the trait, and each such implementation stands alone. So you can be sure that if SomeTrait is implemented for ThisThing, either the author of ThisThing intended that, or the author of SomeTrait or perhaps both, as a result traits have Semantics - there is no risk of their being a mere accident of syntax as with say C++ Concepts.

Maybe you know about Rust stability, or maybe not, in Rust there's a concept of "unstable" features. These features exist, and they work in whatever build of Rust you have, but tomorrow there might be a new Rust version and they're altered, or renamed, or gone. In contrast all the stable features of Rust are promised to still work into the indefinite future, none have been removed since 1.0 in 2015. You have to specifically opt in to having unstable features, and to each specific unstable feature you want. Today Rust for Linux needs several such features. Internally Rust uses some unstable features, but without opting in you can't and probably most people shouldn't, thus it is desirable for Rust for Linux to rely as much as possible on stable features only.