improve doc comment for dynamic_struct::Reader::has(), and increase t…

…est coverage

capnp/src/dynamic_struct.rs

@@ -194,7 +194,9 @@ impl<'a> Reader<'a> {
         }
     }
 
-    /// Returns `false` if the field is a pointer and the pointer is null.
+    /// On a field that is part of a union, returns `true` if the field
+    /// is active in the union and is not a null pointer. On non-union fields,
+    /// returns `true` if the field is not a null pointer.
     pub fn has(&self, field: Field) -> Result<bool> {
         assert_eq!(self.schema.raw, field.parent.raw);
         let proto = field.get_proto();

capnpc/test/dynamic.rs

@@ -67,6 +67,8 @@ fn test_unions() {
     let dynamic: dynamic_struct::Reader<'_> = dynamic.downcast();
     {
         let u: dynamic_struct::Reader<'_> = dynamic.get_named("union0").unwrap().downcast();
+        assert!(u.has_named("u0f1s32").unwrap());
+        assert!(!u.has_named("u0f1s16").unwrap());
         assert_eq!(
             "u0f1s32",
             u.which().unwrap().unwrap().get_proto().get_name().unwrap()