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Flesh out detail behind disallowing refs to static mut
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eggyal committed Apr 11, 2024
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Expand Up @@ -25,13 +25,17 @@ unsafe {
}
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Shared or mutable references of mutable static are almost always a mistake and can lead to undefined behavior and various other problems in your code.
For example, another thread writing to the `static mut` will cause an aliasing violation and incur [Undefined Behavior].
Merely taking such a reference in violation of Rust's mutability XOR aliasing requirement has always been *instantaneous* [undefined behavior], **even if the reference is never read from or written to**. Furthermore, upholding mutability XOR aliasing for a `static mut` requires *reasoning about your code globally*, which can be particularly difficult in the face of reentrancy and/or multithreading.

<!-- TODO: Discuss possible alternatives. -->
## Alternatives

Wherever possible, it is **strongly recommended** to use instead an *immutable* `static` of a type that provides *interior mutability* behind some *locally-reasoned abstraction* (which greatly reduces the complexity of ensuring that Rust's mutability XOR aliasing requirement is upheld).

In situations where no locally-reasoned abstraction is possible and you are therefore compelled still to reason globally about accesses to your `static` variable, you must now use raw pointers such as can be obtained via the [`addr_of_mut!`] macro. By first obtaining a raw pointer rather than directly taking a reference, (the safety requirements of) accesses through that pointer will be more familiar to `unsafe` developers and can be deferred until/limited to smaller regions of code.

[Undefined Behavior]: ../../reference/behavior-considered-undefined.html
[`static mut`]: ../../reference/items/static-items.html#mutable-statics
[`addr_of_mut!`]: https://docs.rust-lang.org/core/ptr/macro.addr_of_mut.html

## Migration

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