Return mutable instance of entry of an object

Question

I want to understand if it is allowed to get a mutable reference to an entry of a vector inside a struct in Rust.

I have a vector in a struct and I want to return a reference to one of its indexed fields.

Sample code:

#[derive(Debug)]
pub struct Definition {
    v1: Vec<(String, i32)>,
}

impl Definition {
    fn new() -> Self {
        Self {
            v1: Vec::new(),
        }
    }

    fn insert_some_values(&mut self) {
        self.v1.push(("V1".to_string(), 0));
        self.v1.push(("V2".to_string(), 1));
        self.v1.push(("V3".to_string(), 2));
        self.v1.push(("V4".to_string(), 3));
    }

    fn search(&mut self, st: String) -> Result<&mut (String, i32), u32> {
        for i in &self.v1 {
            if st.eq(&i.0) {
                return Ok(I); // Is this supported in someway?
            }
        }
        return Err(0);
    }
}

fn main() {
    let mut def = Definition::new();
    def.insert_some_values();
    let res = def.search("V2".to_string());
    println!("{:?}", res);
}

Obviously, it errors in compiling only:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:23:27
   |
23 |                 return Ok(i);
   |                           ^ types differ in mutability
   |
   = note: expected mutable reference `&mut (String, i32)`
                      found reference `&(String, i32)`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

But I want to understand if there is some way to support it?

Most likely, it is not supported since in this way, it is essentially losing part ownership of the field to the caller and that is not allowed. Perhaps only partial ownership of the individual items in a struct is allowed but not within the object.

But if it is supported, can someone please help?

Thanks!

Playground