commit dbd366bde69315f83cfceccbdafda3f9e084c622
author Devin Jeanpierre <jeanpierreda@google.com> Fri Mar 18 16:46:49 2022 +0000
committer Marcel Hlopko <hlopko@google.com> Fri Mar 25 22:26:41 2022 +0000
tree 04dc16f9d012adb28e6cd2f146fbe27b5f2f9dfb
parent 2c1e85c13ebf5c445772641ce40082ad5684f645

Delete blanket move constructor impls for `Unpin` types.

We have basically four options:

0. Do not allow calling inherent move constructors at all for `Unpin` types.
1. write a blanket impl, and only if the type isn't covered by the blanket impl do we allow calling the inherent move constructor. So we could keep the blanket impl, and only allow the C++ move constructor to be called on Unpin types if they are not default-constructible.
2. don't write a blanket impl.
3. write a blanket impl and *specialize*, even though specialization is unstable.

Option #0 is, I think, a bit of a non-option. For example, it prohibits moving from a `Box<SomeCxxType>` where `SomeCxxType` is trivially relocatable and all the rest, but doesn't implement `Default`.

With regards to option #1, well, the blanket impl was a bit gross! (e.g. it zeroes integers.) And, to add to that, the idea that a C++ type's move constructor would only sometimes be called, depending on the presence of a default-constructor, is, I think, a bit outrageous.

So option #1 is also out, leaving #2 and #3. We can always do #3 later, so I figure we should do #2 for now.

(We can also always do #1 later, but perhaps it should be a different function.)

PiperOrigin-RevId: 435663437