Allow constructing non_exhaustive structs using ..Default::default()

[scottmcm]

You'll probably be interested in this thread, and upcoming RFC, from ekuber:

Pre-pre-RFC: syntactic sugar for `Default::default()`

Edit: the intent behind this thread was to flesh out enough of the scope to be able to write an RFC, and to that end it was wildly successful. I will be opening an RFC soon. This first post is now slightly divorced from my current thinking (the intent is the same, but the details have changed quite a bit), but I have left a comment further down-thread with a summary of my intentions for the RFC. I have interacted with some verbose types that are designed to leverage a Default implementation he…