[Discussion] Super borrow, for super let, placement new, and hybrid borrow

language design
3

I have several concerns about all the previous RFCs:

  1. If 2 lifetimes are provided, is the short lifetime really necessary? (My proposal: no)
  2. Is it possible store downgradable borrows into something like Vec? (My proposal: no)
  3. Is it possible that we have to downgrade and upgrade the borrow several times? (My proposal: in the function body, super 'a value could be regarded as value itself, thus it could be downgraded many times, or even re-borrow as another super 'b value. At the end of function body, the return value might contains a lifetime which is shorter than super borrow's lifetime.)
  4. How to tell users what could be downgraded and what could not? (My proposal: imagining all the borrow/reborrow/downgrade happens in the same caller. After a mut borrow ends, immutable borrows of-coursely could be created.)

Actually I've made something almost same to your RFC in July 2024. And finally I found that, the borrow could be combined with super-let, since it could be regarded as something occurs on a higher stack.

What's more, I strongly suspect that the short lifetime will cause some corner cases, since it cannot be either enlarged or shorten. For a most simple case, since &'short mut lives at least 'short, and we cannot define lifetime shorter than the whole function scope, thus &'short mut lives for the whole function scope, which makes &'long cannot be obtained in the function.

fn poc<T>(&<'short mut, 'long> item: T) -> &'long T {
    &*item // Can we actually do this?
    // firstly, item is borrowed for at least 'short and 'short at least lives here
    // which makes &'long impossible to be borrowed.
    // thus *item here is *(item as &'short mut), and thus only `&'short T` is generated.
}