'Why Vector iterator has type unknown in rust [Goland Rust-plugin]? [closed]

I am trying to learn rust, and I am following one tutorial on YouTube. In that video, he did this:

let v1 = vec![1,2,3];
let iter = v1.iter();
let sum = iter.fold(..othercode...)

I was inspecting this code to figure out what's going on and found out variable iter has a type of unknown.

Can anyone explain to me why v1.iter() has type unknown?

rust

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