Add `Error::downcast_ref()` method

src/error.rs

@@ -336,10 +336,22 @@ impl StdError for Error {
 }
 
 impl Error {
+    /// Wraps an external error object.
     pub fn external<T: Into<Box<dyn StdError + Send + Sync>>>(err: T) -> Self {
         Error::ExternalError(err.into().into())
     }
 
+    /// Attempts to downcast the external error object to a concrete type by reference.
+    pub fn downcast_ref<T>(&self) -> Option<&T>
+    where
+        T: StdError + 'static,
+    {
+        match self {
+            Error::ExternalError(err) => err.downcast_ref(),
+            _ => None,
+        }
+    }
+
     pub(crate) fn bad_self_argument(to: &str, cause: Error) -> Self {
         Error::BadArgument {
             to: Some(to.to_string()),
@@ -423,19 +435,19 @@ impl<T> ErrorContext for StdResult<T, Error> {
     }
 }
 
-impl std::convert::From<AddrParseError> for Error {
+impl From<AddrParseError> for Error {
     fn from(err: AddrParseError) -> Self {
         Error::external(err)
     }
 }
 
-impl std::convert::From<IoError> for Error {
+impl From<IoError> for Error {
     fn from(err: IoError) -> Self {
         Error::external(err)
     }
 }
 
-impl std::convert::From<Utf8Error> for Error {
+impl From<Utf8Error> for Error {
     fn from(err: Utf8Error) -> Self {
         Error::external(err)
     }

tests/chunk.rs

@@ -1,7 +1,7 @@
 use std::fs;
 use std::io;
 
-use mlua::{Error, Lua, Result};
+use mlua::{Lua, Result};
 
 #[test]
 fn test_chunk_path() -> Result<()> {
@@ -18,8 +18,7 @@ fn test_chunk_path() -> Result<()> {
     assert_eq!(i, 321);
 
     match lua.load(&*temp_dir.path().join("module2.lua")).exec() {
-        Err(Error::ExternalError(err))
-            if err.downcast_ref::<io::Error>().unwrap().kind() == io::ErrorKind::NotFound => {}
+        Err(err) if err.downcast_ref::<io::Error>().unwrap().kind() == io::ErrorKind::NotFound => {}
         res => panic!("expected io::Error, got {:?}", res),
     };