generics - expected type parameter `T`, found `&T`

Question

I have a generic struct, but I have a problem.

#[derive(Debug,Clone,Serialize,Deserialize)]
    pub struct Item<T> {
      pub edges: Option<Vec<T>>
    }
    impl<T> Item<T> {
      pub fn to_result(self) -> Option<T>{
        match self.edges {
          Some(edges) =>{
            if edges.is_empty() { return None; }
            return edges.first();
          },
          None => None
        }
      }
    }

I get this error:

    expected type parameter `T`, found `&T`

    note: expected enum `std::option::Option<T>`
             found enum `std::option::Option<&T>`
    help: type parameters must be constrained to match other types
    note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters

The edges.first() gets a reference, which causes an error.

please help me.

标签： genericsrust

Answer 1

You are consuming the vector and attempting to send a reference to the first element.

pub fn to_result(self) -> Option<T>{
    self.edges?.into_iter().next()
}

There is no need to check if contained vector has any elements or not. You can use into_iter() to consume the vector and send the first element if available. Playground

To make your code work, you can do the following.

impl<T> Item<T> {

    pub fn to_result(self) -> Option<T>{
        match self.edges {
            Some(edges) =>{
                if edges.is_empty() { return None; }
                return edges.into_iter().next();
            },
            None => None
        }
    }
}