Add 2024 unsafe attributes.

src/SUMMARY.md

@@ -50,3 +50,4 @@
   - [`gen` keyword](rust-2024/gen-keyword.md)
   - [Macro fragment specifiers](rust-2024/macro-fragment-specifiers.md)
   - [`unsafe extern` blocks](rust-2024/unsafe-extern.md)
+  - [Unsafe attributes](rust-2024/unsafe-attributes.md)

src/rust-2024/unsafe-attributes.md

@@ -0,0 +1,81 @@
+# Unsafe attributes
+
+🚧 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 <https://github.com/rust-lang/rust/issues/123757>.
+
+## Summary
+
+- The following attributes must now be marked as `unsafe`:
+    - [`export_name`]
+    - [`link_section`]
+    - [`no_mangle`]
+
+[`export_name`]: ../../reference/abi.html#the-export_name-attribute
+[`link_section`]: ../../reference/abi.html#the-link_section-attribute
+[`no_mangle`]: ../../reference/abi.html#the-no_mangle-attribute
+
+## Details
+
+Rust 1.xx <!--TODO--> added the ability in all editions to mark certain attributes as `unsafe` to indicate that they have soundness requirements that must be upheld.[^RFC3325] The syntax for an unsafe attribute looks like this:
+
+```rust
+# #![feature(unsafe_attributes)]
+// SAFETY: there is no other global function of this name
+#[unsafe(no_mangle)]
+pub fn example() {}
+```
+
+Marking the attribute with `unsafe` highlights that there are safety requirements that must be upheld that the compiler cannot verify on its own.
+
+Starting with the 2024 Edition, it is now required to mark these attributes as `unsafe`. The following section describes the safety requirements for these attributes.
+
+[^RFC3325]: See [RFC 3325](https://rust-lang.github.io/rfcs/3325-unsafe-attributes.html) for the original proposal.
+
+### Safety requirements
+
+The [`no_mangle`], [`export_name`], and [`link_section`] attributes influence the symbol names and linking behavior of items. Care must be taken to ensure that these attributes are used correctly.
+
+Because the set of symbols across all linked libraries is a global namespace, there can be issues if there is a symbol name collision between libraries. Typically this isn't an issue for normally defined functions because [symbol mangling] helps ensure that the symbol name is unique. However, attributes like `export_name` can upset that assumption of uniqueness.
+
+For example, in previous editions the following crashes on most Unix-like platforms despite containing only safe code:
+
+```rust,no_run,edition2021
+fn main() {
+    println!("Hello, world!");
+}
+
+#[export_name = "malloc"]
+fn foo() -> usize { 1 }
+```
+
+In the 2024 Edition, it is now required to mark these attributes as unsafe to emphasize that it is required to ensure that the symbol is defined correctly:
+
+<!-- TODO: edition2024 -->
+```rust
+# #![feature(unsafe_attributes)]
+// SAFETY: There should only be a single definition of the loop symbol.
+#[unsafe(export_name="loop")]
+fn arduino_loop() {
+    // ...
+}
+```
+
+[symbol mangling]: ../../rustc/symbol-mangling/index.html
+[`unsafe_attr_outside_unsafe`]: ../../rustc/lints/listing/allowed-by-default.html#unsafe-attr-outside-unsafe
+
+## Migration
+
+The [`unsafe_attr_outside_unsafe`] lint can update these attributes to use the `unsafe(...)` format. The lint is part of the `rust-2024-compatibility` lint group which is included in the automatic edition migration. In order to migrate your code to be Rust 2024 Edition compatible, run:
+
+```sh
+cargo fix --edition
+```
+
+Just beware that this automatic migration will not be able to verify that these attributes are being used correctly. It is still your responsibility to manually review their usage.
+
+Alternatively, you can manually enable the lint to find places where these attributes need to be updated.
+
+```rust
+// Add this to the root of your crate to do a manual migration.
+#![warn(unsafe_attr_outside_unsafe)]
+```