Commits on Oct 28, 2021

1. Use type based qualification for unions …

   Union field access is currently qualified based on the qualification of
   a value previously assigned to the union. At the same time, every union
   access transmutes the content of the union, which might result in a
   different qualification.
   
   For example, consider constants A and B as defined below, under the
   current rules neither contains interior mutability, since a value used
   in the initial assignment did not contain `UnsafeCell` constructor.
   
   ```rust
   #![feature(untagged_unions)]
   
   union U { i: u32, c: std::cell::Cell<u32> }
   const A: U = U { i: 0 };
   const B: std::cell::Cell<u32> = unsafe { U { i: 0 }.c };
   ```
   
   To avoid the issue, the changes here propose to consider the content of
   a union as opaque and use type based qualification for union types.

   

   tmiasko committed Oct 28, 2021
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