Add mutable to data field

book/src/how_it_works.md

@@ -14,9 +14,12 @@ only need the pointer to be valid (which basically means `ptr..ptr+size` should
 aligned. So Zngur can use the pointer to the below `data` in those functions:
 
 ```C++
-alignas(align_value) ::std::array<uint8_t, size_value> data;
+alignas(align_value) mutable ::std::array<uint8_t, size_value> data;
 ```
 
+The `mutable` keyword is equivalent to the `UnsafeCell` in Rust. It allows modifying `const` objects (which happens with
+Rust interior mutable types) without triggering UB.
+
 To support running destructors of Rust types in C++, Zngur uses `std::ptr::drop_in_place` which has similar constraints to `read` and
 `write`. But to prevent double free, Zngur needs to track if a Rust type is moved out. It does this using a boolean field called
 `drop_flag`, which is `false` if the value doesn't need drop (it is uninitialized or moved out from) and otherwise `true`. So a C++ wrapper
@@ -25,7 +28,7 @@ for a typical Rust type will look like this:
 ```C++
 struct MultiBuf {
 private:
-  alignas(8)::std::array<uint8_t, 32> data;
+  alignas(8) mutable ::std::array<uint8_t, 32> data;
   bool drop_flag;
 
 public:

zngur-generator/src/cpp.rs

@@ -748,7 +748,7 @@ public:
                                 r#"
 {{
 private:
-    alignas({align}) ::std::array<uint8_t, {size}> data;
+    alignas({align}) mutable ::std::array<uint8_t, {size}> data;
             "#,
                             )?;
                         }