Add panic docs

book/src/SUMMARY.md

@@ -8,6 +8,7 @@
   - [Wellknown traits](./call_rust_from_cpp/wellknown_traits.md)
   - [Layout policy](./call_rust_from_cpp/layout_policy.md)
   - [Types with special support](./call_rust_from_cpp/special_types.md)
+  - [Panic and exceptions](./call_rust_from_cpp/panic_and_exceptions.md)
 - [Calling C++ from Rust](./call_cpp_from_rust/index.md)
   - [Calling C++ free functions](./call_cpp_from_rust/function.md)
   - [Adding methods to Rust types](./call_cpp_from_rust/rust_impl.md)

book/src/call_rust_from_cpp/panic_and_exceptions.md

@@ -0,0 +1,37 @@
+# Panic and exceptions
+
+By default, Zngur aborts the process if an unwinding (Rust panic or C++ exception) reaches the cross language boundary. Handling
+unwinding adds a non zero performance cost to every function call, and complicates things as catching an unwinding can result
+in a corrupt state for some objects. (See [unwind safety](https://doc.rust-lang.org/std/panic/trait.UnwindSafe.html))
+
+But Zngur has support for converting Rust panics into a C++ exception. To enable that, add
+
+```
+#convert_panic_to_exception
+```
+
+to your `main.zng` file. Now you can catch exceptions of kind `rust::Panic` for handling Rust panics:
+
+```C++
+try {
+    std::cout << "s[2] = " << *s.get(2).unwrap() << std::endl;
+    std::cout << "s[4] = " << *s.get(4).unwrap() << std::endl;
+} catch (rust::Panic e) {
+    std::cout << "Rust panic happened" << std::endl;
+}
+```
+
+assuming `s` contains `[2, 5, 7, 3]`, the above code prints:
+
+```
+s[2] = 7
+thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', examples/simple/src/generated.rs:184:39
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+s[4] = Rust panic happened
+```
+
+To disable the the log, you need to register a panic hook in the Rust side (See [this stack overflow question](https://stackoverflow.com/questions/35559267/suppress-panic-output-in-rust-when-using-paniccatch-unwind)). Note that
+the `rust::Panic` object contains nothing, and you will lose the panic message.
+
+For proper error handling, consider returning `Result` from your Rust functions, and throw native C++ exceptions with proper details in case of `Err` variant. Use
+this panic to exception mechanism only in places which you need `catch_unwind` in Rust (e.g. for increasing fault tolerance).