Fix using downcast with anyhow::Error

crates/core/src/error/context.rs

@@ -303,4 +303,12 @@ where
             }
         }
     }
+
+    #[cfg(feature = "anyhow")]
+    fn ext_into_anyhow(mut self) -> anyhow::Error {
+        match self.error.take() {
+            Some(error) => anyhow::Error::from(error).context(self.context),
+            None => anyhow::Error::msg(self),
+        }
+    }
 }

crates/core/src/error/error.rs

@@ -64,6 +64,10 @@ pub(crate) unsafe trait ErrorExt: Send + Sync + 'static {
     /// Take the backtrace from this error, if any.
     #[cfg(feature = "backtrace")]
     fn take_backtrace(&mut self) -> Option<Backtrace>;
+
+    /// Conversion into an `anyhow::Error`.
+    #[cfg(feature = "anyhow")]
+    fn ext_into_anyhow(self) -> anyhow::Error;
 }
 
 /// Morally a `dyn ErrorExt` trait object that holds its own vtable.
@@ -542,7 +546,7 @@ impl From<Error> for Box<dyn core::error::Error + 'static> {
 impl From<Error> for anyhow::Error {
     #[inline]
     fn from(e: Error) -> Self {
-        anyhow::Error::from_boxed(e.into_boxed_dyn_error())
+        e.inner.into_anyhow()
     }
 }
 
@@ -1241,6 +1245,11 @@ where
     fn take_backtrace(&mut self) -> Option<Backtrace> {
         None
     }
+
+    #[cfg(feature = "anyhow")]
+    fn ext_into_anyhow(self) -> anyhow::Error {
+        anyhow::Error::new(self.0)
+    }
 }
 
 /// `ErrorExt` wrapper for types given to `Error::msg`.
@@ -1319,6 +1328,11 @@ where
     fn take_backtrace(&mut self) -> Option<Backtrace> {
         None
     }
+
+    #[cfg(feature = "anyhow")]
+    fn ext_into_anyhow(self) -> anyhow::Error {
+        anyhow::Error::msg(self.0)
+    }
 }
 
 /// `ErrorExt` wrapper for `Box<dyn core::error::Error>`.
@@ -1374,6 +1388,11 @@ unsafe impl ErrorExt for BoxedError {
     fn take_backtrace(&mut self) -> Option<Backtrace> {
         None
     }
+
+    #[cfg(feature = "anyhow")]
+    fn ext_into_anyhow(self) -> anyhow::Error {
+        anyhow::Error::from_boxed(self.0)
+    }
 }
 
 /// `ErrorExt` wrapper for `anyhow::Error`.
@@ -1430,6 +1449,11 @@ unsafe impl ErrorExt for AnyhowError {
     fn take_backtrace(&mut self) -> Option<Backtrace> {
         None
     }
+
+    #[cfg(feature = "anyhow")]
+    fn ext_into_anyhow(self) -> anyhow::Error {
+        self.0
+    }
 }
 
 pub(crate) enum OomOrDynErrorRef<'a> {
@@ -1754,6 +1778,23 @@ impl OomOrDynError {
         }
     }
 
+    #[cfg(feature = "anyhow")]
+    pub(crate) fn into_anyhow(self) -> anyhow::Error {
+        if self.is_oom() {
+            // Safety: `self.is_oom()` is true.
+            anyhow::Error::from(unsafe { *self.unchecked_oom() })
+        } else {
+            debug_assert!(self.is_boxed_dyn_error());
+            // Safety: this is a boxed dyn error.
+            let ptr = unsafe { self.unchecked_into_dyn_error() };
+            // Safety: invariant of the type that the pointer is valid.
+            let vtable = unsafe { ptr.as_ref().vtable };
+            // Safety: the pointer is valid and the vtable is associated with
+            // this pointer's concrete error type.
+            unsafe { (vtable.into_anyhow)(ptr) }
+        }
+    }
+
     /// Given that this is known to be an instance of the type associated with
     /// the given `TypeId`, do an owning-downcast to that type, writing the
     /// result through the given `ret_ptr`, and deallocating `self` along the

crates/core/src/error/vtable.rs

@@ -45,6 +45,10 @@ pub(crate) struct Vtable {
     /// Upon successful return, a `T` will have been written to that memory
     /// block.
     pub(crate) downcast: unsafe fn(OwnedPtr<DynError>, TypeId, NonNull<u8>),
+
+    /// Conversion into `anyhow::Error` from `Box<Self>`.
+    #[cfg(feature = "anyhow")]
+    pub(crate) into_anyhow: unsafe fn(OwnedPtr<DynError>) -> anyhow::Error,
 }
 
 impl Vtable {
@@ -63,6 +67,8 @@ impl Vtable {
             into_boxed_dyn_core_error: into_boxed_dyn_core_error::<E>,
             drop_and_deallocate: drop_and_deallocate::<E>,
             downcast: downcast::<E>,
+            #[cfg(feature = "anyhow")]
+            into_anyhow: into_anyhow::<E>,
         }
     }
 }
@@ -174,3 +180,14 @@ where
         }
     }
 }
+
+#[cfg(feature = "anyhow")]
+unsafe fn into_anyhow<E>(error: OwnedPtr<DynError>) -> anyhow::Error
+where
+    E: ErrorExt,
+{
+    let error = error.cast::<ConcreteError<E>>();
+    // Safety: implied by all vtable functions' safety contract.
+    let error = unsafe { error.into_box() };
+    error.error.ext_into_anyhow()
+}

crates/core/tests/tests.rs

@@ -905,3 +905,40 @@ fn oom_requested_allocation_size() {
     let oom = OutOfMemory::new(usize::MAX);
     assert!(oom.requested_allocation_size() <= usize::try_from(isize::MAX).unwrap());
 }
+
+#[test]
+#[cfg(feature = "anyhow")]
+fn anyhow_preserves_downcast() -> Result<()> {
+    {
+        let e: Error = TestError(1).into();
+        assert!(e.downcast_ref::<TestError>().is_some());
+        let e: anyhow::Error = e.into();
+        assert!(e.downcast_ref::<TestError>().is_some());
+    }
+    {
+        let e = Error::from(TestError(1)).context("hi");
+        assert!(e.downcast_ref::<TestError>().is_some());
+        assert!(e.downcast_ref::<&str>().is_some());
+        let e: anyhow::Error = e.into();
+        assert!(e.downcast_ref::<TestError>().is_some());
+        assert!(e.downcast_ref::<&str>().is_some());
+    }
+    Ok(())
+}
+
+#[test]
+#[cfg(feature = "anyhow")]
+fn anyhow_source_loses_downcast() -> Result<()> {
+    let e: anyhow::Error = TestError(1).into();
+    assert!(e.downcast_ref::<TestError>().is_some());
+
+    let e: Error = Error::from_anyhow(e);
+    // FIXME: this should actually test for `is_some`
+    assert!(e.downcast_ref::<TestError>().is_none());
+
+    // Even while the above is broken, when going back to `anyhow` we should
+    // preserve the original error.
+    let e: anyhow::Error = e.into();
+    assert!(e.downcast_ref::<TestError>().is_some());
+    Ok(())
+}