Add unsafe to extern build scripts in Cargo Book

src/doc/src/reference/build-script-examples.md

@@ -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(); }
@@ -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;
 }
 

src/doc/src/reference/semver.md

@@ -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;
 }
@@ -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
 }
@@ -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
 }