Add must-use-type to the book.

Author: KarelPeeters

book/src/SUMMARY.md

@@ -19,6 +19,7 @@
     - [Preventing the Derivation of `Copy` and `Clone`](./nocopy.md)
     - [Preventing the Derivation of `Debug`](./nodebug.md)
     - [Preventing the Derivation of `Default`](./nodefault.md)
+    - [Annotating types with `#[must-use]`](./must-use-types.md)
 - [Generating Bindings to C++](./cpp.md)
 - [Generating Bindings to Objective-c](./objc.md)
 - [Using Unions](./using-unions.md)

book/src/must-use-types.md

@@ -0,0 +1,27 @@
+# Annotating types with `#[must-use]`
+
+`bindgen` can be instructed to annotate certain types with 
+[`#[must_use]`](https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute).
+
+Some libraries have a common error type, returned by lots of their functions, 
+which needs to be checked after every call. In these cases it's useful to add `#[must_use]` to this type, so the Rust
+compiler emits a warning when the check is missing.
+### Library
+
+* [`bindgen::Builder::must_use_type`](https://docs.rs/bindgen/latest/bindgen/struct.Builder.html#method.must_use_type)
+
+### Command Line
+
+* `--must-use-type <regex>`
+
+### Annotations
+
+```c
+/** <div rustbindgen mustusetype></div> */
+struct ErrorType {
+    // ...
+};
+
+...
+```
+