Feature: unchecked access to enum interior

I'll use Option as an example, but this applies to any enum:

Sometimes you may have a *mut Option<T>, and want to get a *mut T from that, without dereferencing the pointer, and without checking whether it is actually a Some(T). AFAICT, this is currently impossible to do in rust, unless you have some way to construct a valid T (which would allow calculating an offset before-hand and then using pointer arithmetic).

Using mem::uninitialized(), or mem::zeroed() to construct a T before-hand to calculate the pointer offset doesn't work, because of enum layout optimization, which means it could be UB.

This came up when writing some concurrency code: in this case de-referencing the pointer would be invalid because the memory may be being concurrently written to. The unsafe code is able to be correct because it checks at run-time that it has exclusive access, and that it was indeed a Some(T) before it ever tries to dereference the pointer.

No files or tests are named; start by reviewing the RFC process and Rust’s enum-layout and raw-pointer rules. A complete contribution would need a concrete, reviewed proposal for unchecked enum-interior access that addresses the stated concurrency and undefined-behavior constraints.