You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: project.rs
    path of file: ./repos/risingwave/src/batch/src/executor
    
    the code of the file until where you have to start completion: // Copyright 2024 RisingWave Labs
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use std::sync::Arc;

use futures::{Stream, StreamExt};
use itertools::Itertools;
use risingwave_common::array::DataChunk;
use risingwave_common::catalog::{Field, Schema};
use risingwave_expr::expr::{build_from_prost, BoxedExpression, Expression};
use risingwave_pb::batch_plan::plan_node::NodeBody;

use crate::error::{BatchError, Result};
use crate::executor::{
    BoxedDataChunkStream, BoxedExecutor, BoxedExecutorBuilder, Executor, ExecutorBuilder,
};
use crate::task::BatchTaskContext;

pub struct ProjectExecutor {
    expr: Vec<BoxedExpression>,
    child: BoxedExecutor,
    schema: Schema,
    identity: String,
}

impl Executor for ProjectExecutor {
    fn schema(&self) -> &Schema {
    
    
    async fn test_project_executor() -> Result<()> {
        let chunk = DataChunk::from_pretty(
            "
            i     i
            1     7
            2     8
            33333 66666
            4     4
            5     3
        ",
        );

        let expr1 = InputRefExpression::new(DataType::Int32, 0);
        let expr_vec = vec![Box::new(expr1) as BoxedExpression];

        let schema = schema_unnamed! { DataType::Int32, DataType::Int32 };
        let mut mock_executor = MockExecutor::new(schema);
        mock_executor.add(chunk);

        let fields = expr_vec
            .iter()
            .map(|expr| Field::unnamed(expr.return_type()))
            .collect::<Vec<Field>>();

        let proj_executor = Box::new(ProjectExecutor {
            expr: expr_vec,
            child: Box::new(mock_executor),
            schema: Schema { fields },
            identity: "ProjectExecutor".to_string(),
        });

        let fields = &proj_executor.schema().fields;
        assert_eq!(fields[0].data_type, DataType::Int32);

        let mut stream = proj_executor.execute();
        let result_chunk = stream.next().await.unwrap().unwrap();
        assert_eq!(result_chunk.dimension(), 1);
        assert_eq!(
            result_chunk
                .column_at(0)
                .as_int32()
                .iter()
                .collect::<Vec<_>>(),
            vec![Some(1), Some(2), Some(33333), Some(4), Some(5)]
        );
        Ok(())
    }