Reborrow of borrowed value

[sander2]

I think the code below should not be allowed by the compiler, but it is. Here, even though arg is mutably borrowed, we are allowed to make a copy (reborrow) of it. Why is this the case? As the comment shows, a read of arg is invalid, so why isn't a reborrow? I had a discussion about this on the IRC, but I would like a second opinion, as I'm still not convinced the behavior is consistent. The only difference I see between a reborrow let b = &*arg and a copy let c = arg is the lifetimes of the resulting borrows. But I don't see why that would make the first valid but not the second..

fn f(mut arg: & bool) {
  let a = & mut arg; 
  let b = &*arg; // allowed 
 // this would not be allowed: let c = arg;
}

Tested on 1.29.2.

[matthewjasper]

Duplicate of #38899, it's fixed by NLL (although there has to be a later use of a to get an error)

[sander2]

Ah alright, glad to know this is known bug. Sorry for the duplicate.