Update Error::downcast_ref to follow Error::chain

`Error::downcast_ref` now descends through `BadArgument` and `CallbackError`,
matching `chain` traversal logic

Author: khvzak

src/error.rs

@@ -359,7 +359,9 @@ impl Error {
     {
         match self {
             Error::ExternalError(err) => err.downcast_ref(),
-            Error::WithContext { cause, .. } => Self::downcast_ref(cause),
+            Error::BadArgument { cause, .. }
+            | Error::CallbackError { cause, .. }
+            | Error::WithContext { cause, .. } => Self::downcast_ref(cause),
             _ => None,
         }
     }

tests/error.rs

@@ -1,4 +1,5 @@
 use std::error::Error as _;
+use std::sync::Arc;
 use std::{fmt, io};
 
 use mlua::{Error, ErrorContext, Lua, Result};
@@ -77,6 +78,31 @@ fn test_error_chain() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_error_downcast() -> Result<()> {
+    let lua = Lua::new();
+
+    let func = lua.create_function(|_, ()| Err::<(), _>(Error::external(io::Error::other("boom"))))?;
+    let err = func.call::<()>(()).unwrap_err();
+    assert!(matches!(err, Error::CallbackError { .. }));
+    assert!(err.downcast_ref::<io::Error>().is_some());
+    assert!(err.downcast_ref::<fmt::Error>().is_none());
+
+    let bad_arg = Error::BadArgument {
+        to: Some("func".to_string()),
+        pos: 1,
+        name: Some("arg".to_string()),
+        cause: Arc::new(Error::external(io::Error::new(
+            io::ErrorKind::InvalidInput,
+            "bad",
+        ))),
+    };
+    assert!(bad_arg.downcast_ref::<io::Error>().is_some());
+    assert!(bad_arg.downcast_ref::<fmt::Error>().is_none());
+
+    Ok(())
+}
+
 #[test]
 fn test_external_error() {
     // `Error::external` should preserve `mlua::Error`