Fix doctest

Author: oli-obk

src/doc/unstable-book/src/language-features/intrinsics.md

@@ -16,14 +16,35 @@ some features that only exist on some backends. Backends can simply not implemen
 intrinsics without causing any code miscompilations or failures to compile.
 
 ```rust
-#![feature(core_intrinsics)]
+#![feature(rustc_attrs, effects)]
+#![allow(internal_features)]
 
 #[rustc_intrinsic]
-fn some_intrinsic_name() -> u32 {
-    42
+const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
+```
+
+Since these are just regular functions, it is perfectly ok to create the intrinsic twice:
+
+```rust
+#![feature(rustc_attrs, effects)]
+#![allow(internal_features)]
+
+#[rustc_intrinsic]
+const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
+
+mod foo {
+    #[rustc_intrinsic]
+    const unsafe fn const_deallocate(ptr: *mut u8, size: usize, align: usize) {
+        eprintln!("noisy const dealloc: {ptr:p}, {size}, {align}")
+    }
 }
+
 ```
 
+The behaviour on backends that override the intrinsic is exactly the same. On other
+backends, the intrinsic behaviour depends on which implementation is called, just like
+with any regular function.
+
 ## Intrinsics lowered to MIR instructions
 
 Various intrinsics have native MIR operations that they correspond to. Instead of requiring
@@ -53,4 +74,5 @@ extern "rust-intrinsic" {
 }
 ```
 
-As with any other FFI functions, these are always `unsafe` to call.
+As with any other FFI functions, these are by default always `unsafe` to call.
+You can add `#[rustc_safe_intrinsic]` to the intrinsic to make it safe to call.