Those are used for generic in that case. Foo<T> supports more types, often limited with certain traits they must support. Here it's passing the actual type on

Those are used for generic in that case. Foo<T> supports more types, often limited with certain traits they must support. Here it's passing the actual type on instantiation, in that case there are two types: an u8 slice and a number of inputs, just guestimating the latter as I did not check the original definition in the source. See https://doc.rust-lang.org/book/ch10-01-syntax.html https://doc.rust-lang.org/book/ch10-01-syntax.html for a better explanation.