multiple layers of pointers work!

crates/memory/src/error.rs

@@ -0,0 +1,12 @@
+use crate::Region;
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum GuestError {
+    #[error("Invalid enum value {0}")]
+    InvalidEnumValue(&'static str),
+    #[error("Out of bounds: {0:?}")]
+    PtrOutOfBounds(Region),
+    #[error("Borrowed: {0:?}")]
+    PtrBorrowed(Region),
+}