What does Rust's unary || (parallel pipe) mean?

Question

In Non-Lexical Lifetimes: Introduction, Niko includes the following snippet:

fn get_default3<'m,K,V:Default>(map: &'m mut HashMap<K,V>,                                 key: K)                                 -> &'m mut V {     map.entry(key)        .or_insert_with(|| V::default()) } 

What does the || V::default() mean here?

Answer 1

It is a closure with zero arguments. This is a simplified example to show the basic syntax and usage (play):

fn main() {     let c = || println!("c called");     c();     c(); } 

This prints:

c called c called 

Another example from the documentation:

let plus_one = |x: i32| x + 1;  assert_eq!(2, plus_one(1)); 

Answer 2

It's a zero-argument lambda function.