Mara Bos 2967e58be3
Rollup merge of #78728 - a1phyr:const_cell_into_inner, r=dtolnay
Constantify `UnsafeCell::into_inner` and related

Tracking issue: #78729

This PR constantifies:
- `UnsafeCell::into_inner`
- `Cell::into_inner`
- `RefCell::into_inner`
- `Atomic*::into_inner`

r? `````@dtolnay`````
2020-11-08 13:36:14 +01:00
..
2020-11-05 12:57:18 +09:00
2020-10-06 00:48:34 +02:00