Rollup merge of rust-lang#139113 - folkertdev:sanitizer-unstable-book-check-block, r=rcvalle

unstable book: in a sanitizer example, check the code

Use some `#` directives to make sure the code checks on x86_64, and does not produce errors on other platforms. This example still used an older version of `#[naked]`, and because the snippet was ignored that was missed before.

I'm not sure when this gets built on CI exactly, so it might be worthwhile to try and build it for a non-x86_64 architecture to make sure that works. I'm not sure how to verify locally that e.g. on aarch64 this code works without errors/warnings.

try-job: aarch64-apple

Author: GuillaumeGomez

src/doc/unstable-book/src/compiler-flags/sanitizer.md

@@ -244,7 +244,7 @@ See the [Clang ControlFlowIntegrity documentation][clang-cfi] for more details.
 
 ## Example 1: Redirecting control flow using an indirect branch/call to an invalid destination
 
-```rust,ignore (making doc tests pass cross-platform is hard)
+```rust
 use std::arch::naked_asm;
 use std::mem;
 
@@ -253,6 +253,7 @@ fn add_one(x: i32) -> i32 {
 }
 
 #[unsafe(naked)]
+# #[cfg(target_arch = "x86_64")]
 pub extern "C" fn add_two(x: i32) {
     // x + 2 preceded by a landing pad/nop block
     naked_asm!(
@@ -276,6 +277,7 @@ fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
     f(arg) + f(arg)
 }
 
+# #[cfg(target_arch = "x86_64")]
 fn main() {
     let answer = do_twice(add_one, 5);
 
@@ -292,6 +294,7 @@ fn main() {
 
     println!("The next answer is: {}", next_answer);
 }
+# #[cfg(not(target_arch = "x86_64"))] fn main() {}
 ```
 Fig. 1. Redirecting control flow using an indirect branch/call to an invalid
 destination (i.e., within the body of the function).