add as_dyn trait

Signed-off-by: Bugen Zhao <[email protected]>

src/as_dyn.rs

@@ -0,0 +1,26 @@
+/// Extension trait for [`std::error::Error`] that casts the error to
+/// a trait object.
+pub trait AsDyn: crate::error_sealed::Sealed {
+    /// Casts the error to a trait object.
+    fn as_dyn(&self) -> &(dyn std::error::Error + '_);
+}
+
+impl<T: std::error::Error> AsDyn for T {
+    fn as_dyn(&self) -> &(dyn std::error::Error + '_) {
+        self
+    }
+}
+
+macro_rules! impl_as_dyn {
+    ($({$ty:ty},)*) => {
+        $(
+            impl AsDyn for $ty {
+                fn as_dyn(&self) -> &(dyn std::error::Error + '_) {
+                    self
+                }
+            }
+        )*
+    };
+}
+
+crate::for_dyn_error_types! { impl_as_dyn }

src/lib.rs

@@ -1,9 +1,11 @@
 #![feature(error_generic_member_access)] // TODO: it's nightly-only
 
+mod as_dyn;
 mod backtrace;
 mod error_box;
 mod report;
 
+pub use as_dyn::AsDyn;
 pub use report::{AsReport, Report};
 pub use thiserror_ext_derive::{Box, Construct, ContextInto};