Support any source() that derefs to std::error::Error

impl/src/expand.rs

@@ -35,10 +35,11 @@ fn struct_error(input: &DeriveInput, data: &DataStruct) -> Result<TokenStream> {
     };
 
     let source_method = source.map(|source| {
-        let member = quote_spanned!(source.span()=> &self.#source);
+        let member = quote_spanned!(source.span()=> self.#source);
         quote! {
             fn source(&self) -> std::option::Option<&(dyn std::error::Error + 'static)> {
-                std::option::Option::Some(#member)
+                use thiserror::private::AsDynError;
+                std::option::Option::Some(#member.as_dyn_error())
             }
         }
     });

src/aserror.rs

@@ -0,0 +1,23 @@
+use std::error::Error;
+
+pub trait AsDynError {
+    fn as_dyn_error(&self) -> &(dyn Error + 'static);
+}
+
+impl<T: Error + 'static> AsDynError for T {
+    fn as_dyn_error(&self) -> &(dyn Error + 'static) {
+        self
+    }
+}
+
+impl AsDynError for dyn Error + 'static {
+    fn as_dyn_error(&self) -> &(dyn Error + 'static) {
+        self
+    }
+}
+
+impl AsDynError for dyn Error + Send + Sync + 'static {
+    fn as_dyn_error(&self) -> &(dyn Error + 'static) {
+        self
+    }
+}

src/lib.rs

@@ -1 +1,9 @@
+mod aserror;
+
 pub use thiserror_impl::*;
+
+// Not public API.
+#[doc(hidden)]
+pub mod private {
+    pub use crate::aserror::AsDynError;
+}