It's... not the most compelling feature in the world. The main use case I think I see is slightly easing the burden of writing external function signatures for

It's... not the most compelling feature in the world. The main use case I think I see is slightly easing the burden of writing external function signatures for a function that passes a user-definable type to a function callback parameter like qsort does. But there's relatively few of those functions in existence, and often times, you want a more ergonomic function definition anyways (see e.g. https://news.ycombinator.com/item?id=39267277 https://news.ycombinator.com/item?id=39267277 for qsort). The other major use case is if you've got something like a Vec<T> where several of the helper methods can be erasable. But it's already possible to do this with helper functions--the standard library takes advantage of it--so it's not entirely clear that you need a language-level feature for this use case.