Borrowing and consuming pattern matching for noncopyable types

[ellie20]

@John_McCall

It's almost inherent in the keywords that let is an easy thing to reach for, while borrowing is very explicit and intentional. That's a dichotomy we should lean in to. So I think it's unfortunate that getting a switch to borrow the original value under this proposal requires rewriting all of the cases to use borrowing, and it's even more unfortunate that it's possible to mismatch borrowing with the overall effect of the switch.

Personally, I think it could be a good idea to go even further and make it so let, if let, and switch as a whole are ownership-neutral and just infer it from context. I mentioned this idea before, but basically, I think it would be ergonomically better if users could just use let when they're in the middle of writing code, without committing to either a consuming or borrowing access at the moment.