aserror: support Send + !Sync errors too

src/aserror.rs

@@ -18,6 +18,13 @@ impl AsDynError for dyn Error + 'static {
     }
 }
 
+impl AsDynError for dyn Error + Send + 'static {
+    #[inline]
+    fn as_dyn_error(&self) -> &(dyn Error + 'static) {
+        self
+    }
+}
+
 impl AsDynError for dyn Error + Send + Sync + 'static {
     #[inline]
     fn as_dyn_error(&self) -> &(dyn Error + 'static) {

tests/test_source.rs

@@ -1,4 +1,4 @@
-use std::error::Error as _;
+use std::error::Error;
 use std::io;
 use thiserror::Error;
 
@@ -16,6 +16,13 @@ pub struct ExplicitSource {
     io: io::Error,
 }
 
+#[derive(Error, Debug)]
+#[error("boxed source")]
+pub struct BoxedSource {
+    #[source]
+    source: Box<dyn Error + Send + 'static>,
+}
+
 #[test]
 fn test_implicit_source() {
     let io = io::Error::new(io::ErrorKind::Other, "oh no!");
@@ -32,3 +39,11 @@ fn test_explicit_source() {
     };
     error.source().unwrap().downcast_ref::<io::Error>().unwrap();
 }
+
+#[test]
+fn test_boxed_source() {
+    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
+    let _error = BoxedSource {
+        source: Box::new(io),
+    };
+}