Verbose error message on boxroot_create failure (#158)

src/root.rs

@@ -24,10 +24,45 @@ impl core::fmt::Debug for Root {
 
 impl Eq for Root {}
 
+// Adaptation from C++: https://gitlab.com/ocaml-rust/ocaml-boxroot/-/blob/c119eb0c88f3f628e683ee90f1b694b51c60a0cb/boxroot/cpp/boxroot.cpp
+fn boxroot_raise_error() -> ! {
+    use ocaml_boxroot_sys::Status;
+    let status = unsafe { ocaml_boxroot_sys::boxroot_status() };
+    let error_message = match status {
+        Status::ToreDown => "boxroot_teardown has previously been called",
+        Status::Invalid => "With systhreads, boxroot_setup must be called after caml_thread_initialize but before any thread is created",
+        Status::Running | Status::NotSetup => {
+            #[cfg(not(feature = "no-std"))]
+            {
+                use std::io::{Error, ErrorKind};
+                match Error::last_os_error().kind() {
+                    ErrorKind::PermissionDenied => {
+                        "You tried calling boxroot_create or boxroot_modify without holding the domain lock"
+                    }
+                    ErrorKind::OutOfMemory => {
+                        "Allocation failure of the backing store"
+                    }
+                    _ => "Unknown Error::last_os_error().kind()",
+                }
+            }
+            #[cfg(feature = "no-std")]
+            {
+                "Unknown error (details unavailable in no-std environment)"
+            }
+        }
+        _ => "Unknown ocaml_boxroot_sys::Status",
+    };
+
+    panic!("Boxroot error: {}", error_message);
+}
+
 impl Root {
     /// Create a new root
     pub unsafe fn new(v: sys::Value) -> Root {
-        Root(ocaml_boxroot_sys::boxroot_create(v).expect("boxroot_create failed"))
+        match ocaml_boxroot_sys::boxroot_create(v) {
+            Some(root) => Root(root),
+            None => boxroot_raise_error(),
+        }
     }
 
     /// Get value from root