Don't execute example code

Making the code executable would require adding a lot of duplicated
hidden lines, plus libc isn't available.

src/send-and-sync.md

@@ -82,7 +82,7 @@ implement Send and Sync. Let's call it a `Carton`.
 We start by writing code to take a value allocated on the stack and transfer it
 to the heap.
 
-```rust
+```rust,ignore
 use std::mem::size_of;
 use std::ptr::NonNull;
 
@@ -114,7 +114,7 @@ to access it. [`Box`][box-doc] implements [`Deref`][deref-doc] and
 [`DerefMut`][deref-mut-doc] so that you can access the inner value. Let's do
 that.
 
-```rust
+```rust,ignore
 use std::ops::{Deref, DerefMut};
 
 impl<T> Deref for Carton<T> {
@@ -153,7 +153,7 @@ safely be Send unless it shares mutable state with something else without
 enforcing exclusive access to it. Each `Carton` has a unique pointer, so
 we're good.
 
-```rust
+```rust,ignore
 // Safety: No one besides us has the raw pointer, so we can safely transfer the
 // Carton to another thread.
 unsafe impl<T> Send for Carton<T> {}
@@ -166,7 +166,7 @@ write to the pointer, and the borrow checker enforces that mutable
 references must be exclusive, there are no soundness issues making `Carton`
 sync either.
 
-```rust
+```rust,ignore
 // Safety: Our implementation of DerefMut requires writers to mutably borrow the
 // Carton, so the borrow checker will only let us have references to the Carton
 // on multiple threads if no one has a mutable reference to the Carton.