Well you don't have to use it just because it is added, but it will allow library writers to provide more ergonomic interfaces. This specific type system featur

Well you don't have to use it just because it is added, but it will allow library writers to provide more ergonomic interfaces. This specific type system feature allows for stuff that weren't possible before but make sense and you'd expect to be valid code that compiles: the most common example is the lending iterator. Have a struct that owns a vector of data. Can you implement an iterator that can return a reference of each item when you call next()? Not without GATs, because each time you return from next() you're creating a new lifetime that doesn't exist in your trait definition. That's why current standard library iterators are basically structs that own a reference to what you iterate. See the GAT RFC which explains this concept much better than I can: https://github.com/rust-lang/rfcs/blob/master/text/1598-generic_associated_types.md https://github.com/rust-lang/rfcs/blob/master/text/1598-gene... TL;DR it's not a "new" feature per se but something that could have been present from the beginning without making the language different.