refactor: simplify downcasting logic with std::mem::transmute

Simplified the downcasting implementations by replacing pointer casting logic with `std::mem::transmute`, ensuring type safety after matching. Added tests to validate various downcasting behaviors for both owned and trait-object error scenarios, improving overall reliability and test coverage.

Signed-off-by: Harald Hoyer <harald@hoyer.xyz>

src/lib.rs

@@ -338,11 +338,8 @@ impl<U: 'static + Display + Debug> ErrorDown for Error<U> {
     #[inline]
     fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&Error<T>> {
         if self.is_chain::<T>() {
-            #[allow(clippy::cast_ptr_alignment)]
-            unsafe {
-                #[allow(trivial_casts)]
-                Some(*(self as *const dyn StdError as *const &Error<T>))
-            }
+            // Use transmute when we've verified the types match
+            unsafe { Some(std::mem::transmute::<&Error<U>, &Error<T>>(self)) }
         } else {
             None
         }
@@ -351,11 +348,8 @@ impl<U: 'static + Display + Debug> ErrorDown for Error<U> {
     #[inline]
     fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut Error<T>> {
         if self.is_chain::<T>() {
-            #[allow(clippy::cast_ptr_alignment)]
-            unsafe {
-                #[allow(trivial_casts)]
-                Some(&mut *(self as *mut dyn StdError as *mut &mut Error<T>))
-            }
+            // Use transmute when we've verified the types match
+            unsafe { Some(std::mem::transmute::<&mut Error<U>, &mut Error<T>>(self)) }
         } else {
             None
         }
@@ -363,11 +357,8 @@ impl<U: 'static + Display + Debug> ErrorDown for Error<U> {
     #[inline]
     fn downcast_inner_ref<T: 'static + StdError>(&self) -> Option<&T> {
         if self.is_chain::<T>() {
-            #[allow(clippy::cast_ptr_alignment)]
-            unsafe {
-                #[allow(trivial_casts)]
-                Some(&(*(self as *const dyn StdError as *const &Error<T>)).kind)
-            }
+            // Use transmute when we've verified the types match
+            unsafe { Some(std::mem::transmute::<&U, &T>(&self.kind)) }
         } else {
             None
         }
@@ -376,11 +367,8 @@ impl<U: 'static + Display + Debug> ErrorDown for Error<U> {
     #[inline]
     fn downcast_inner_mut<T: 'static + StdError>(&mut self) -> Option<&mut T> {
         if self.is_chain::<T>() {
-            #[allow(clippy::cast_ptr_alignment)]
-            unsafe {
-                #[allow(trivial_casts)]
-                Some(&mut (*(self as *mut dyn StdError as *mut &mut Error<T>)).kind)
-            }
+            // Use transmute when we've verified the types match
+            unsafe { Some(std::mem::transmute::<&mut U, &mut T>(&mut self.kind)) }
         } else {
             None
         }
@@ -913,4 +901,125 @@ mod tests {
         assert!(err.source().is_some());
         assert!(err.source().unwrap().is_chain::<io::Error>());
     }
+
+    // Helper error types for testing
+    #[derive(Debug)]
+    struct TestError(String);
+
+    impl std::fmt::Display for TestError {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            write!(f, "{}", self.0)
+        }
+    }
+
+    impl std::error::Error for TestError {}
+
+    #[test]
+    fn test_downcast_chain_operations() {
+        // Create a test error chain
+        let original_error = Error::new(
+            TestError("test message".to_string()),
+            None,
+            Some("test location".to_string()),
+        );
+
+        // Test is_chain
+        assert!(original_error.is_chain::<TestError>());
+        assert!(!original_error.is_chain::<io::Error>());
+
+        // Test downcast_chain_ref
+        let downcast_ref = original_error.downcast_chain_ref::<TestError>();
+        assert!(downcast_ref.is_some());
+        let downcast_kind = downcast_ref.unwrap().kind();
+        assert_eq!(format!("{}", downcast_kind), "test message");
+        assert_eq!(
+            format!("{:?}", downcast_kind),
+            "TestError(\"test message\")"
+        );
+
+        // Test invalid downcast_chain_ref
+        let invalid_downcast = original_error.downcast_chain_ref::<io::Error>();
+        assert!(invalid_downcast.is_none());
+
+        // Test downcast_chain_mut
+        let mut mutable_error = original_error;
+        let downcast_mut = mutable_error.downcast_chain_mut::<TestError>();
+        assert!(downcast_mut.is_some());
+        assert_eq!(downcast_mut.unwrap().kind().0, "test message");
+
+        // Test invalid downcast_chain_mut
+        let invalid_downcast_mut = mutable_error.downcast_chain_mut::<io::Error>();
+        assert!(invalid_downcast_mut.is_none());
+    }
+
+    #[test]
+    fn test_downcast_inner_operations() {
+        // Create a test error
+        let mut error = Error::new(
+            TestError("inner test".to_string()),
+            None,
+            Some("test location".to_string()),
+        );
+
+        // Test downcast_inner_ref
+        let inner_ref = error.downcast_inner_ref::<TestError>();
+        assert!(inner_ref.is_some());
+        assert_eq!(inner_ref.unwrap().0, "inner test");
+        // Test invalid downcast_inner_ref
+        let invalid_inner = error.downcast_inner_ref::<io::Error>();
+        assert!(invalid_inner.is_none());
+
+        // Test downcast_inner_mut
+        let inner_mut = error.downcast_inner_mut::<TestError>();
+        assert!(inner_mut.is_some());
+        assert_eq!(inner_mut.unwrap().0, "inner test");
+
+        // Test invalid downcast_inner_mut
+        let invalid_inner_mut = error.downcast_inner_mut::<io::Error>();
+        assert!(invalid_inner_mut.is_none());
+    }
+
+    #[test]
+    fn test_error_down_for_dyn_error() {
+        // Create a boxed error
+        let error: Box<dyn std::error::Error + 'static> = Box::new(Error::new(
+            TestError("dyn test".to_string()),
+            None,
+            Some("test location".to_string()),
+        ));
+
+        // Test is_chain through trait object
+        assert!(error.is_chain::<TestError>());
+        assert!(!error.is_chain::<io::Error>());
+
+        // Test downcast_chain_ref through trait object
+        let chain_ref = error.downcast_chain_ref::<TestError>();
+        assert!(chain_ref.is_some());
+        assert_eq!(chain_ref.unwrap().kind().0, "dyn test");
+
+        // Test downcast_inner_ref through trait object
+        let inner_ref = error.downcast_inner_ref::<TestError>();
+        assert!(inner_ref.is_some());
+        assert_eq!(inner_ref.unwrap().0, "dyn test");
+    }
+
+    #[test]
+    fn test_error_down_with_sync_send() {
+        // Create a boxed error with Send + Sync
+        let error: Box<dyn std::error::Error + Send + Sync> = Box::new(Error::new(
+            TestError("sync test".to_string()),
+            None,
+            Some("test location".to_string()),
+        ));
+
+        // Test operations on Send + Sync error
+        assert!(error.is_chain::<TestError>());
+        assert!(error.downcast_chain_ref::<TestError>().is_some());
+        assert!(error.downcast_inner_ref::<TestError>().is_some());
+
+        // Test invalid downcasts
+        assert!(!error.is_chain::<io::Error>());
+        assert!(error.downcast_chain_ref::<io::Error>().is_none());
+        assert!(error.downcast_inner_ref::<io::Error>().is_none());
+    }
 }