Add 2024 unsafe extern blocks.

src/SUMMARY.md

@@ -49,3 +49,4 @@
   - [Rustfmt: Combine all delimited exprs as last argument](rust-2024/rustfmt-overflow-delimited-expr.md)
   - [`gen` keyword](rust-2024/gen-keyword.md)
   - [Macro fragment specifiers](rust-2024/macro-fragment-specifiers.md)
+  - [`unsafe extern` blocks](rust-2024/unsafe-extern.md)

src/rust-2024/unsafe-extern.md

@@ -0,0 +1,59 @@
+# `unsafe extern` blocks
+
+🚧 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/123743>.
+
+## Summary
+
+- [`extern` blocks] must now be marked with the `unsafe` keyword.
+
+[`extern` blocks]: ../../reference/items/external-blocks.html
+
+## Details
+
+Rust 1.xx <!--TODO--> added the ability in all editions to mark [`extern` blocks] with the `unsafe` keyword.[^RFC3484] Adding the `unsafe` keyword helps to emphasize that it is the responsibility of the author of the `extern` block to ensure that the signatures are correct. If the signatures are not correct, then it may result in undefined behavior.
+
+The syntax for an unsafe `extern` block looks like this:
+
+```rust
+# #![feature(unsafe_extern_blocks)]
+
+unsafe extern "C" {
+    // sqrt (from libm) may be called with any `f64`
+    pub safe fn sqrt(x: f64) -> f64;
+
+    // strlen (from libc) requires a valid pointer,
+    // so we mark it as being an unsafe fn
+    pub unsafe fn strlen(p: *const std::ffi::c_char) -> usize;
+
+    // this function doesn't say safe or unsafe, so it defaults to unsafe
+    pub fn free(p: *mut core::ffi::c_void);
+
+    pub safe static IMPORTANT_BYTES: [u8; 256];
+}
+```
+
+In addition to being able to mark an `extern` block as `unsafe`, you can also specify if individual items in the `extern` block are `safe` or `unsafe`. Items marked as `safe` can be used without an `unsafe` block.
+
+Starting with the 2024 Edition, it is now required to include the `unsafe` keyword on an `extern` block. This is intended to make it very clear that there are safety requirements that must be upheld by the extern definitions.
+
+[^RFC3484]: See [RFC 3484](https://github.com/rust-lang/rfcs/blob/master/text/3484-unsafe-extern-blocks.md) for the original proposal.
+
+## Migration
+
+The [`missing_unsafe_on_extern`] lint can update `extern` blocks to add the `unsafe` keyword. 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 the signatures in the `extern` block are correct. It is still your responsibility to manually review their definition.
+
+Alternatively, you can manually enable the lint to find places where there are `unsafe` blocks that need to be updated.
+
+```rust
+// Add this to the root of your crate to do a manual migration.
+#![warn(missing_unsafe_on_extern)]
+```
+
+[`missing_unsafe_on_extern`]: ../../rustc/lints/listing/allowed-by-default.html#missing-unsafe-on-extern