bugfix: make subset use correct index when constructing Type

Author: dzfranklin

capnp/src/schema.rs

@@ -204,8 +204,9 @@ impl FieldSubset {
     }
 
     pub fn get(self, index: u16) -> Field {
+        let index = self.indices[index as usize];
         Field {
-            proto: self.fields.get(self.indices[index as usize] as u32),
+            proto: self.fields.get(index as u32),
             index,
             parent: self.parent,
         }

capnpc/test/test.capnp

@@ -24,6 +24,15 @@
 
 using Rust = import "rust.capnp";
 
+struct FieldSubsetIndexesCorrectly {
+  common @2 :Text;
+
+  union {
+    unset @0 :Void;
+    variant @1 :UInt8;
+  }
+}
+
 struct TestPrimList {
     uint8List  @0 : List(UInt8);
     int8List   @1 : List(Int8);

capnpc/test/test.rs

@@ -106,6 +106,22 @@ mod tests {
         };
     }
 
+    #[test]
+    fn field_subset_indexes_corrently() {
+        use crate::test_capnp::field_subset_indexes_correctly;
+        use capnp::{
+            introspect::{Introspect, TypeVariant},
+            schema::StructSchema,
+        };
+
+        let TypeVariant::Struct(schema) = field_subset_indexes_correctly::Owned::introspect().which() else { unreachable!() };
+        let schema = StructSchema::new(schema);
+
+        let subset = schema.get_non_union_fields().unwrap();
+        let field = subset.get(0);
+        assert_matches!(field.get_type().which(), TypeVariant::Text);
+    }
+
     #[test]
     fn test_prim_list() {
         use crate::test_capnp::test_prim_list;