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docs(refs): Add unsafe to extern while using build scripts in Cargo Book #15294

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Mar 11, 2025
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docs(refs): Add unsafe to extern while using build scripts in Cargo Book #15294

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4 changes: 2 additions & 2 deletions src/doc/src/reference/build-script-examples.md
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Original file line number Diff line number Diff line change
Expand Up @@ -252,7 +252,7 @@ void hello() {
// Note the lack of the `#[link]` attribute. We’re delegating the responsibility
// of selecting what to link over to the build script rather than hard-coding
// it in the source file.
extern { fn hello(); }
unsafe extern { fn hello(); }

fn main() {
unsafe { hello(); }
Expand Down Expand Up @@ -327,7 +327,7 @@ Let's round out the example with a basic FFI binding:

use std::os::raw::{c_uint, c_ulong};

extern "C" {
unsafe extern "C" {
pub fn crc32(crc: c_ulong, buf: *const u8, len: c_uint) -> c_ulong;
}

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6 changes: 3 additions & 3 deletions src/doc/src/reference/semver.md
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Expand Up @@ -766,7 +766,7 @@ pub struct SpecificLayout {
// Example usage that will break.
use updated_crate::SpecificLayout;

extern "C" {
unsafe extern "C" {
// This C function is assuming a specific layout defined in a C header.
fn c_fn_get_b(x: &SpecificLayout) -> u32;
}
Expand Down Expand Up @@ -820,7 +820,7 @@ pub struct SpecificLayout {
// Example usage that will break.
use updated_crate::SpecificLayout;

extern "C" {
unsafe extern "C" {
// This C function is assuming a specific layout defined in a C header.
fn c_fn_get_b(x: &SpecificLayout) -> u32; // Error: is not FFI-safe
}
Expand Down Expand Up @@ -941,7 +941,7 @@ pub struct Transparent<T>(T);
#![deny(improper_ctypes)]
use updated_crate::Transparent;

extern "C" {
unsafe extern "C" {
fn c_fn() -> Transparent<f64>; // Error: is not FFI-safe
}

Expand Down