core: add Value impl for Box<T> where T: Value (#2071)

This commit adds a `Value` implementation for `Box<T> where T: Value`.
This is *primarily* intended to make `Box<dyn Error + ...>` implement
`Value`, building on the `Value` impls for `dyn Error + ...` added in
#2066, but it may be useful for other boxed values as well.

Refs: #1308

tracing-core/src/field.rs

@@ -588,6 +588,21 @@ impl<'a> Value for fmt::Arguments<'a> {
     }
 }
 
+#[cfg(feature = "alloc")]
+impl<T: ?Sized> crate::sealed::Sealed for alloc::boxed::Box<T> where T: Value {}
+
+#[cfg(feature = "alloc")]
+#[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
+impl<T: ?Sized> Value for alloc::boxed::Box<T>
+where
+    T: Value,
+{
+    #[inline]
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        self.as_ref().record(key, visitor)
+    }
+}
+
 impl fmt::Debug for dyn Value {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // We are only going to be recording the field value, so we don't
@@ -1155,4 +1170,24 @@ mod test {
         });
         assert_eq!(result, "123".to_owned());
     }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn record_error() {
+        let fields = TEST_META_1.fields();
+        let err: Box<dyn std::error::Error + Send + Sync + 'static> =
+            std::io::Error::new(std::io::ErrorKind::Other, "lol").into();
+        let values = &[
+            (&fields.field("foo").unwrap(), Some(&err as &dyn Value)),
+            (&fields.field("bar").unwrap(), Some(&Empty as &dyn Value)),
+            (&fields.field("baz").unwrap(), Some(&Empty as &dyn Value)),
+        ];
+        let valueset = fields.value_set(values);
+        let mut result = String::new();
+        valueset.record(&mut |_: &Field, value: &dyn fmt::Debug| {
+            use core::fmt::Write;
+            write!(&mut result, "{:?}", value).unwrap();
+        });
+        assert_eq!(result, format!("{}", err));
+    }
 }