Handle partial union matches

src/util/serialize.zig

@@ -278,37 +278,42 @@ pub fn DeserializerContext(comptime Result: type, comptime From: type, comptime
             util.deepFree(allocator, val);
         }
 
+        const DeserializeError = error{ ParseFailure, MissingField, DuplicateUnionMember, SparseSlice, OutOfMemory };
+
         fn deserialize(
             self: *@This(),
             allocator: std.mem.Allocator,
             comptime T: type,
             intermediary: anytype,
             comptime field_ref: FieldRef,
-        ) !?T {
+        ) DeserializeError!?T {
             if (comptime Context.options.isScalar(T)) {
                 const val = @field(intermediary.static, util.comptimeJoin(".", field_ref));
-                return try self.context.deserializeScalar(allocator, T, val orelse return null);
+                return self.context.deserializeScalar(allocator, T, val orelse return null) catch return error.ParseFailure;
             }
 
             switch (@typeInfo(T)) {
-                // At most one of any union field can be active at a time, and it is embedded
+                // At most one of any union field can be active at a time
-                // in its parent container
                 .Union => |info| {
                     var result: ?T = null;
                     errdefer if (result) |v| self.deserializeFree(allocator, v);
-                    // TODO: errdefer cleanup
+                    var partial_match_found: bool = false;
-                    const union_ref: FieldRef = if (Context.options.embed_unions) field_ref[0 .. field_ref.len - 1] else field_ref;
                     inline for (info.fields) |field| {
                         const F = field.field_type;
-                        const new_field_ref = union_ref ++ &[_][]const u8{field.name};
+                        const maybe_value = self.deserialize(allocator, F, intermediary, field_ref) catch |err| switch (err) {
-                        const maybe_value = try self.deserialize(allocator, F, intermediary, new_field_ref);
+                            error.MissingField => blk: {
+                                partial_match_found = true;
+                                break :blk @as(?F, null);
+                            },
+                            else => |e| return e,
+                        };
                         if (maybe_value) |value| {
-                            // TODO: errdefer cleanup
                             errdefer self.deserializeFree(allocator, value);
                             if (result != null) return error.DuplicateUnionMember;
                             result = @unionInit(T, field.name, value);
                         }
                     }
+                    if (partial_match_found and result == null) return error.MissingField;
                     return result;
                 },