Explain why we should avoid static mutable variable and introduce interior mutability #390
Conversation
| @@ -23,6 +23,8 @@ fn main() { | |||
|
|
|||
| But this has a few problems. It is a mutable global variable, and in Rust, these are always unsafe to interact with. These variables are also visible across your whole program, which means the borrow checker is unable to help you track references and ownership of these variables. | |||
|
|
|||
| Up until Rust 2024, the usage of static **mutable** variable has been discouraged. Since Rust 2024, static mutable variable results in an error. Of course, there are legitimate use case of having a shared mutable state, it's just that Rust want you to really think about the scope of the mutability. We'll talk about this later when we talk about interior mutability. | |||
There was a problem hiding this comment.
When read literally, "Since Rust 2024, static mutable variable results in an error." is wrong.
A static mutable variable in itself is not an issue at all, if you are comfortable with unsafe code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a9bb1d76aa1ef7cb07442d890438cd7d
What's discouraged is using references to static mutable variables:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=7b96c2c8023526d86d8f9b619eda9846
And even there, it's not a hard error, but a deny lint, that can be overridden (#[allow(static_mut_refs)])
Perhaps make that "Since Rust 2024, references to static mutable variables are denied by default."?
|
|
||
| Of course, none of the above are safe. In order to get a proper interior mutability that is safe, we should implement `Sync` ourself and put in our synchronization primitive specific to the hardware such as locking the resource with atomic swap and guarding the code with interrupt-free section (disable interrupt, run some code, enable interrupt), also known as critical section. | ||
|
|
||
| You could check how [rust-console gba](https://github.com/rust-console/gba/blob/6a3fcc1ee6493a499af507f8394ee542500721b7/src/gba_cell.rs#L63) implement it for reference. |
There was a problem hiding this comment.
Isn't this example a bit too specific and complicated?
I don't have a better example at hand though. Not a big deal, if someone has a better suggestion, it can easily be replaced later.
|
I very much like what you wrote, it explains the topic well and is easily understandable! What I did not yet check is how well this section fits into the book. I only read the diff in isolation, and the last time I read the book was a long time ago, so that should be judged by someone more familiar with the book. |
Closes #389