rust index

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use std::collections::HashMap; use std::ops::Index; #[derive(Debug,Clone)] struct Cell{     name:String } type Col = HashMap<String, Vec<Cell>>; #[derive(Debug,Clone)] struct DF{     columns: Vec<Col> }   impl Index<&str> for DF{     type Output = i32;     fn index(&self, c:&str)->&i32 {         println!("索引字符是: {}", &c);           &2_i32     } }   fn main() {     let mut a = HashMap::new();     a.insert(String::from("A"), vec![Cell{name:String::from("A1")}, Cell{name:String::from("A2")}]);     let mut b = HashMap::new();     b.insert(String::from("B"), vec![Cell{name:String::from("B1")}, Cell{name:String::from("B2")}]);     let df = DF{ columns:vec![a, b]};     println!("{:?}", df["C"]); }