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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. Fixes #90268. `@rust-lang/wg-const-eval` |
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