diff --git a/src/rust-2024/static-mut-references.md b/src/rust-2024/static-mut-references.md index 2b9e9d1d..01b61b46 100644 --- a/src/rust-2024/static-mut-references.md +++ b/src/rust-2024/static-mut-references.md @@ -1,44 +1,56 @@ # Disallow references to static mut 🚧 The 2024 Edition has not yet been released and hence this section is still "under construction". +More information may be found in the tracking issue at . ## Summary -- The [`static_mut_refs`] lint is now a hard error that cannot be disabled. - This prevents taking a shared or mutable reference to a `static mut`. +- The [`static_mut_refs`] lint level is now `deny` by default. + This checks for taking a shared or mutable reference to a `static mut`. [`static_mut_refs`]: ../../rustc/lints/listing/warn-by-default.html#static-mut-refs ## Details -Taking a reference to a [`static mut`] is no longer allowed: +The [`static_mut_refs`] lint detects taking a reference to a [`static mut`]. In the 2024 Edition, this lint is now `deny` by default to emphasize that you should avoid making these references. - + ```rust static mut X: i32 = 23; static mut Y: i32 = 24; unsafe { - let y = &X; // ERROR: reference of mutable static - let ref x = X; // ERROR: reference of mutable static - let (x, y) = (&X, &Y); // ERROR: reference of mutable static + let y = &X; // ERROR: shared reference to mutable static + let ref x = X; // ERROR: shared reference to mutable static + let (x, y) = (&X, &Y); // ERROR: shared reference to mutable static } ``` 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. +Note that there are some cases where implicit references are automatically created without a visible `&` operator. For example, these situations will also trigger the lint: + + +```rust +static mut NUMS: &[u8; 3] = &[0, 1, 2]; + +unsafe { + println!("{NUMS:?}"); // ERROR: shared reference to mutable static + let n = NUMS.len(); // ERROR: shared reference to mutable static +} +``` + ## 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. +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 [`&raw const` or `&raw mut` operators][raw]. 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 +[raw]: ../../reference/expressions/operator-expr.html#raw-borrow-operators ## Migration -🚧 The automatic migration for this has not yet been implemented. - - +There is no automatic migration to fix these references to `static mut`. To avoid undefined behavior you must rewrite your code to use a different approach as recommended in the [Alternatives](#alternatives) section.