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Document that "extern blocks must be unsafe" in Rust 2024 #140216

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Apr 26, 2025
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Document that "extern blocks must be unsafe" in Rust 2024 #140216

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8 changes: 6 additions & 2 deletions library/std/src/keyword_docs.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -381,11 +381,15 @@ mod enum_keyword {}
/// lazy_static;`. The other use is in foreign function interfaces (FFI).
///
/// `extern` is used in two different contexts within FFI. The first is in the form of external
/// blocks, for declaring function interfaces that Rust code can call foreign code by.
/// blocks, for declaring function interfaces that Rust code can call foreign code by. This use
/// of `extern` is unsafe, since we are asserting to the compiler that all function declarations
/// are correct. If they are not, using these items may lead to undefined behavior.
///
/// ```rust ignore
/// // SAFETY: The function declarations given below are in
/// // line with the header files of `my_c_library`.
/// #[link(name = "my_c_library")]
/// extern "C" {
/// unsafe extern "C" {
/// fn my_c_function(x: i32) -> bool;
/// }
/// ```
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