Add a few From, Deref, and AsRef impls for wasmtime_internal_error::Error (#12201)

This matches `anyhow::Error` and makes migrating existing code easier. Found
while migrating existing code.

Author: fitzgen

crates/error/src/context.rs

@@ -1,7 +1,9 @@
 use crate::{
-    Error, ErrorExt, Result,
+    Error, ErrorExt, OutOfMemory, Result,
+    boxed::try_new_uninit_box,
     error::{OomOrDynError, OomOrDynErrorMut, OomOrDynErrorRef},
 };
+use alloc::boxed::Box;
 use core::any::TypeId;
 use core::fmt;
 use core::ptr::NonNull;
@@ -252,6 +254,17 @@ unsafe impl<C> ErrorExt for ContextError<C>
 where
     C: fmt::Display + Send + Sync + 'static,
 {
+    fn ext_as_dyn_core_error(&self) -> &(dyn core::error::Error + Send + Sync + 'static) {
+        self
+    }
+
+    fn ext_into_boxed_dyn_core_error(
+        self,
+    ) -> Result<Box<dyn core::error::Error + Send + Sync + 'static>, OutOfMemory> {
+        let boxed = try_new_uninit_box()?;
+        Ok(Box::write(boxed, self) as _)
+    }
+
     fn ext_source(&self) -> Option<OomOrDynErrorRef<'_>> {
         let error = self.error.as_ref()?;
         Some(error.inner.unpack())

crates/error/src/error.rs

@@ -22,7 +22,16 @@ use std::backtrace::{Backtrace, BacktraceStatus};
 /// not lie about whether they are or are not an instance of the given type id's
 /// associated type (or a newtype wrapper around that type). Violations will
 /// lead to unsafety.
-pub(crate) unsafe trait ErrorExt: core::error::Error + Send + Sync + 'static {
+pub(crate) unsafe trait ErrorExt: Send + Sync + 'static {
+    /// Get this error as a shared reference to a `dyn core::error::Error` trait
+    /// object.
+    fn ext_as_dyn_core_error(&self) -> &(dyn core::error::Error + Send + Sync + 'static);
+
+    /// Get this error as a boxed `dyn core::error::Error` trait object.
+    fn ext_into_boxed_dyn_core_error(
+        self,
+    ) -> Result<Box<dyn core::error::Error + Send + Sync + 'static>, OutOfMemory>;
+
     /// Get a shared borrow of the next error in the chain.
     fn ext_source(&self) -> Option<OomOrDynErrorRef<'_>>;
 
@@ -488,6 +497,20 @@ impl From<Error> for Box<dyn core::error::Error + Send + Sync + 'static> {
     }
 }
 
+impl From<Error> for Box<dyn core::error::Error + Send + 'static> {
+    #[inline]
+    fn from(error: Error) -> Self {
+        error.into_boxed_dyn_error()
+    }
+}
+
+impl From<Error> for Box<dyn core::error::Error + 'static> {
+    #[inline]
+    fn from(error: Error) -> Self {
+        error.into_boxed_dyn_error()
+    }
+}
+
 /// Convert a [`Error`] into an [`anyhow::Error`].
 ///
 /// # Example
@@ -517,6 +540,28 @@ impl From<Error> for anyhow::Error {
     }
 }
 
+impl core::ops::Deref for Error {
+    type Target = dyn core::error::Error + Send + Sync + 'static;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_ref()
+    }
+}
+
+impl AsRef<dyn core::error::Error> for Error {
+    #[inline]
+    fn as_ref(&self) -> &(dyn core::error::Error + 'static) {
+        self.inner.unpack().as_dyn_core_error()
+    }
+}
+
+impl AsRef<dyn core::error::Error + Send + Sync> for Error {
+    #[inline]
+    fn as_ref(&self) -> &(dyn core::error::Error + Send + Sync + 'static) {
+        self.inner.unpack().as_dyn_core_error()
+    }
+}
+
 impl Error {
     /// Construct a new `Error` from a type that implements
     /// `core::error::Error`.
@@ -1107,38 +1152,22 @@ impl Error {
 #[repr(transparent)]
 struct ForeignError<E>(E);
 
-impl<E> fmt::Debug for ForeignError<E>
-where
-    E: core::error::Error + Send + Sync + 'static,
-{
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.0, f)
-    }
-}
-
-impl<E> fmt::Display for ForeignError<E>
+// Safety: `ext_is` is correct, `ext_move` always writes to `dest`.
+unsafe impl<E> ErrorExt for ForeignError<E>
 where
     E: core::error::Error + Send + Sync + 'static,
 {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(&self.0, f)
+    fn ext_as_dyn_core_error(&self) -> &(dyn core::error::Error + Send + Sync + 'static) {
+        &self.0
     }
-}
 
-impl<E> core::error::Error for ForeignError<E>
-where
-    E: core::error::Error + Send + Sync + 'static,
-{
-    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
-        self.0.source()
+    fn ext_into_boxed_dyn_core_error(
+        self,
+    ) -> Result<Box<dyn core::error::Error + Send + Sync + 'static>, OutOfMemory> {
+        let boxed = try_new_uninit_box()?;
+        Ok(Box::write(boxed, self.0) as _)
     }
-}
 
-// Safety: `ext_is` is correct, `ext_move` always writes to `dest`.
-unsafe impl<E> ErrorExt for ForeignError<E>
-where
-    E: core::error::Error + Send + Sync + 'static,
-{
     fn ext_source(&self) -> Option<OomOrDynErrorRef<'_>> {
         None
     }
@@ -1206,6 +1235,17 @@ unsafe impl<M> ErrorExt for MessageError<M>
 where
     M: fmt::Debug + fmt::Display + Send + Sync + 'static,
 {
+    fn ext_as_dyn_core_error(&self) -> &(dyn core::error::Error + Send + Sync + 'static) {
+        self
+    }
+
+    fn ext_into_boxed_dyn_core_error(
+        self,
+    ) -> Result<Box<dyn core::error::Error + Send + Sync + 'static>, OutOfMemory> {
+        let boxed = try_new_uninit_box()?;
+        Ok(Box::write(boxed, self) as _)
+    }
+
     fn ext_source(&self) -> Option<OomOrDynErrorRef<'_>> {
         None
     }
@@ -1247,27 +1287,19 @@ where
 #[repr(transparent)]
 struct BoxedError(Box<dyn core::error::Error + Send + Sync + 'static>);
 
-impl fmt::Debug for BoxedError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.0, f)
+// Safety: `ext_is` is implemented correctly and `ext_move` always
+// writes to its pointer.
+unsafe impl ErrorExt for BoxedError {
+    fn ext_as_dyn_core_error(&self) -> &(dyn core::error::Error + Send + Sync + 'static) {
+        &*self.0
     }
-}
 
-impl fmt::Display for BoxedError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(&self.0, f)
+    fn ext_into_boxed_dyn_core_error(
+        self,
+    ) -> Result<Box<dyn core::error::Error + Send + Sync + 'static>, OutOfMemory> {
+        Ok(self.0)
     }
-}
 
-impl core::error::Error for BoxedError {
-    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
-        self.0.source()
-    }
-}
-
-// Safety: `ext_is` is implemented correctly and `ext_move` always
-// writes to its pointer.
-unsafe impl ErrorExt for BoxedError {
     fn ext_source(&self) -> Option<OomOrDynErrorRef<'_>> {
         None
     }
@@ -1311,31 +1343,20 @@ unsafe impl ErrorExt for BoxedError {
 #[cfg(feature = "anyhow")]
 struct AnyhowError(anyhow::Error);
 
+// Safety: `ext_is` is implemented correctly and `ext_move` always
+// writes to its pointer.
 #[cfg(feature = "anyhow")]
-impl fmt::Debug for AnyhowError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.0, f)
-    }
-}
-
-#[cfg(feature = "anyhow")]
-impl fmt::Display for AnyhowError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(&self.0, f)
+unsafe impl ErrorExt for AnyhowError {
+    fn ext_as_dyn_core_error(&self) -> &(dyn core::error::Error + Send + Sync + 'static) {
+        self.0.as_ref()
     }
-}
 
-#[cfg(feature = "anyhow")]
-impl core::error::Error for AnyhowError {
-    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
-        self.0.source()
+    fn ext_into_boxed_dyn_core_error(
+        self,
+    ) -> Result<Box<dyn core::error::Error + Send + Sync + 'static>, OutOfMemory> {
+        Ok(self.0.into_boxed_dyn_error())
     }
-}
 
-// Safety: `ext_is` is implemented correctly and `ext_move` always
-// writes to its pointer.
-#[cfg(feature = "anyhow")]
-unsafe impl ErrorExt for AnyhowError {
     fn ext_source(&self) -> Option<OomOrDynErrorRef<'_>> {
         None
     }

crates/error/src/vtable.rs

@@ -1,5 +1,4 @@
 use super::{ConcreteError, DynError, ErrorExt, OomOrDynErrorMut, OomOrDynErrorRef, OutOfMemory};
-use crate::boxed::try_new_uninit_box;
 use crate::ptr::{MutPtr, OwnedPtr, SharedPtr};
 use alloc::boxed::Box;
 use core::{any::TypeId, fmt, ptr::NonNull};
@@ -73,7 +72,7 @@ where
     let error = error.cast::<ConcreteError<E>>();
     // Safety: implied by all vtable functions' safety contract.
     let error = unsafe { error.as_ref() };
-    fmt::Display::fmt(&error.error, f)
+    fmt::Display::fmt(error.error.ext_as_dyn_core_error(), f)
 }
 
 unsafe fn debug<E>(error: SharedPtr<'_, DynError>, f: &mut fmt::Formatter<'_>) -> fmt::Result
@@ -83,7 +82,7 @@ where
     let error = error.cast::<ConcreteError<E>>();
     // Safety: implied by all vtable functions' safety contract.
     let error = unsafe { error.as_ref() };
-    fmt::Debug::fmt(&error.error, f)
+    fmt::Debug::fmt(error.error.ext_as_dyn_core_error(), f)
 }
 
 unsafe fn source<E>(error: SharedPtr<'_, DynError>) -> Option<OomOrDynErrorRef<'_>>
@@ -125,7 +124,7 @@ where
     let error = error.cast::<ConcreteError<E>>();
     // Safety: implied by all vtable functions' safety contract.
     let error = unsafe { error.as_ref() };
-    &error.error as _
+    error.error.ext_as_dyn_core_error()
 }
 
 unsafe fn into_boxed_dyn_core_error<E>(
@@ -137,11 +136,7 @@ where
     let error = error.cast::<ConcreteError<E>>();
     // Safety: implied by all vtable functions' safety contract.
     let error = unsafe { error.into_box() };
-
-    let boxed = try_new_uninit_box()?;
-    let error = Box::write(boxed, error.error);
-
-    Ok(error as _)
+    error.error.ext_into_boxed_dyn_core_error()
 }
 
 unsafe fn drop_and_deallocate<E>(error: OwnedPtr<DynError>)

crates/error/tests/tests.rs

@@ -200,6 +200,14 @@ fn is() {
     assert!(!e.is::<&str>());
 }
 
+#[test]
+fn is_for_root_cause_with_initial_error_source() {
+    let error = std::fmt::Error;
+    let error = ChainError::new("whoops", Some(Box::new(error)));
+    let error = Error::new(error);
+    assert!(error.root_cause().is::<std::fmt::Error>());
+}
+
 #[test]
 #[cfg(feature = "backtrace")]
 fn backtrace() {