refactor: simplify downcasting logic with std::mem::transmute (#20)

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.

.github/workflows/coverage.yml

@@ -13,31 +13,26 @@ on:
     types:
       - created
 
+
 jobs:
-  test:
-    name: coverage
+  coverage:
     runs-on: ubuntu-latest
+    env:
+      CARGO_TERM_COLOR: always
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
           target: x86_64-unknown-linux-gnu
           toolchain: nightly
           components: llvm-tools-preview
-
       - name: Install cargo-llvm-cov
-        run: >
-          curl -LsSf 'https://github.com/taiki-e/cargo-llvm-cov/releases/download/v0.5.23/cargo-llvm-cov-x86_64-unknown-linux-musl.tar.gz'
-          | tar xzf -
-          && mv cargo-llvm-cov $HOME/.cargo/bin
-
-      - name: Run cargo-llvm-cov
-        run: cargo llvm-cov --doctests --all --all-features --lcov --output-path lcov.info
-
+        uses: taiki-e/install-action@cargo-llvm-cov
+      - name: Generate code coverage
+        run: cargo +nightly llvm-cov --all-features --workspace --codecov --doctests --output-path codecov.json
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v3
         with:
-          directory: ./
-          fail_ci_if_error: false
-          files: ./lcov.info
-          verbose: true
+          token: ${{ secrets.CODECOV_TOKEN }} # not required for public repos
+          files: codecov.json
+          fail_ci_if_error: true

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
         }
@@ -607,6 +595,7 @@ macro_rules! str_context {
         #[derive(Clone)]
         pub struct $e(pub String);
         impl $e {
+            #[allow(dead_code)]
             pub fn new<S: Into<String>>(s: S) -> Self {
                 $e(s.into())
             }
@@ -688,6 +677,12 @@ macro_rules! str_context {
 ///     do_some_io(filename).map_context(|e| ErrorKind::from(e))?;
 ///     Ok(())
 /// }
+///
+/// # fn main() {
+/// #   if let Err(e) = func1() {
+/// #       eprintln!("Error:\n{:?}", e);
+/// #   }
+/// # }
 /// ```
 #[macro_export]
 macro_rules! err_kind {
@@ -740,3 +735,291 @@ macro_rules! err_kind {
         }
     };
 }
+
+#[cfg(test)]
+mod tests {
+    use super::Context as _;
+    use super::*;
+    use std::io;
+
+    #[test]
+    fn test_error_chain_with_multiple_causes() {
+        // Create a chain of errors
+        let io_error = io::Error::new(io::ErrorKind::NotFound, "file not found");
+
+        str_context!(Level3Error);
+        str_context!(Level2Error);
+        str_context!(Level1Error);
+
+        let err = Result::<(), _>::Err(io_error.into())
+            .context(Level3Error("level 3".into()))
+            .context(Level2Error("level 2".into()))
+            .context(Level1Error("level 1".into()))
+            .unwrap_err();
+
+        // Test the error chain
+        assert!(err.is_chain::<Level1Error>());
+        assert!(err.find_chain_cause::<Level2Error>().is_some());
+        assert!(err.find_chain_cause::<Level3Error>().is_some());
+        assert!(err.find_chain_cause::<io::Error>().is_some());
+    }
+
+    #[test]
+    fn test_error_root_cause() {
+        let io_error = io::Error::new(io::ErrorKind::NotFound, "file not found");
+
+        str_context!(WrapperError);
+        let err = Result::<(), _>::Err(io_error.into())
+            .context(WrapperError("wrapper".into()))
+            .unwrap_err();
+
+        let root = err.root_cause().unwrap();
+        assert!(root.is_chain::<io::Error>());
+    }
+
+    #[test]
+    fn test_error_display_and_debug() {
+        str_context!(CustomError);
+        let err = Error::new(
+            CustomError("test error".into()),
+            None,
+            Some("src/lib.rs:100".into()),
+        );
+
+        // Test Display formatting
+        assert_eq!(format!("{}", err), "test error");
+
+        // Test alternate Display formatting
+        assert_eq!(format!("{:#}", err), "test error");
+
+        // Test Debug formatting
+        let debug_output = format!("{:?}", err);
+        assert!(debug_output.contains("test error"));
+        assert!(debug_output.contains("src/lib.rs:100"));
+    }
+
+    #[test]
+    fn test_error_annotation() {
+        let io_error = io::Error::new(io::ErrorKind::NotFound, "file not found");
+        let err = Result::<(), _>::Err(io_error.into())
+            .annotate()
+            .unwrap_err();
+
+        assert!(err.source().is_some());
+        err.source()
+            .unwrap()
+            .downcast_inner_ref::<io::Error>()
+            .unwrap();
+    }
+
+    #[test]
+    fn test_map_context() {
+        let io_error = io::Error::new(io::ErrorKind::NotFound, "file not found");
+
+        str_context!(MappedError);
+        let err = Result::<(), _>::Err(io_error.into())
+            .map_context(|e| MappedError(format!("Mapped: {}", e)))
+            .unwrap_err();
+
+        assert!(err.is_chain::<MappedError>());
+        assert!(err.find_chain_cause::<io::Error>().is_some());
+    }
+
+    #[test]
+    fn test_error_downcasting() {
+        str_context!(OriginalError);
+        let original = Error::new(OriginalError("test".into()), None, None);
+
+        let error: Box<dyn StdError + Send + Sync> = Box::new(original);
+
+        // Test downcast_chain_ref
+        assert!(error.is_chain::<OriginalError>());
+        assert!(error.downcast_chain_ref::<OriginalError>().is_some());
+
+        // Test downcast_inner_ref
+        let inner = error.downcast_inner_ref::<OriginalError>();
+        assert!(inner.is_some());
+    }
+
+    #[derive(Debug, Clone)]
+    enum TestErrorKind {
+        Basic(String),
+        Complex { message: String },
+    }
+
+    impl Display for TestErrorKind {
+        fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+            match self {
+                TestErrorKind::Basic(msg) => write!(f, "Basic error: {}", msg),
+                TestErrorKind::Complex { message } => write!(f, "Complex error: {}", message),
+            }
+        }
+    }
+
+    #[test]
+    fn test_err_kind_macro() {
+        err_kind!(TestError, TestErrorKind);
+
+        let err = TestError::from(TestErrorKind::Basic("test".into()));
+        assert!(matches!(err.kind(), TestErrorKind::Basic(_)));
+        // The annotated error should display "(passed error)" even in a chain
+        assert_eq!(format!("{}", err), "Basic error: test");
+        assert_eq!(format!("{:?}", err), "Basic(\"test\")");
+
+        let complex_err = TestError::from(TestErrorKind::Complex {
+            message: "test".into(),
+        });
+        assert!(matches!(complex_err.kind(), TestErrorKind::Complex { .. }));
+        // The annotated error should display "(passed error)" even in a chain
+        assert_eq!(format!("{}", complex_err), "Complex error: test");
+        assert_eq!(
+            format!("{:?}", complex_err),
+            "Complex { message: \"test\" }"
+        );
+    }
+    #[test]
+    fn test_annotated_error_display_and_debug() {
+        let annotated = AnnotatedError(());
+
+        // Test Display formatting
+        assert_eq!(format!("{}", annotated), "(passed error)");
+
+        // Test Debug formatting
+        assert_eq!(format!("{:?}", annotated), "(passed error)");
+
+        // Test with error chain
+        let io_error = io::Error::new(io::ErrorKind::NotFound, "file not found");
+        let err = Result::<(), _>::Err(io_error.into())
+            .annotate()
+            .unwrap_err();
+
+        // The annotated error should display "(passed error)" even in a chain
+        assert_eq!(format!("{}", err), "(passed error)");
+        assert!(format!("{:?}", err).contains("(passed error)"));
+
+        // Verify the error chain is preserved
+        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());
+    }
+}