Obtaining a Vec<X> from a &HashSet<X>

Question

Assume I have some function do_stuff that takes a &HashSet<X> and wants to collect the items into a Vec<X> by copying all the elements. What is the canonical way to do that? I have basically found the following ways to do this:

1. Using _.clone().into_iter().collect() (GodBolt Link):

   use std::collections::HashSet;
   
   #[derive(Copy, Clone, Eq, Hash, PartialEq)]
   struct X {}
   
   struct Y {
       xs: HashSet<X>,
   }
   
   fn do_stuff(y: &Y) -> Vec<X> {
       y.xs.clone().into_iter().collect()
   }
   

   This is recommended here:

   • How can I collect Vec from &HashSet? - Answer by at54321

2. Using _.iter().cloned().collect() (GodBolt Link):

   use std::collections::HashSet;
   
   #[derive(Copy, Clone, Eq, Hash, PartialEq)]
   struct X {}
   
   struct Y {
       xs: HashSet<X>,
   }
   
   fn do_stuff(y: &Y) -> Vec<X> {
       y.xs.iter().cloned().collect()
   }
   

   This is recommended here:

   • Build HashSet from a vector in Rust - Answer by Fuji
   • How can I collect Vec from &HashSet? - Comment by Masklinn
   • Is there a built in function to compute the difference of two sets? - Answer by Paul
   • HashSet from Vec while maintaining the order of elements - Answer by connicpu
   • What is the idiomatic way of converting a Vec of references to a Vec of values? - Answer by hellow

3. Using Vec::from_iter(_.iter().cloned()) (GodBolt Link):

   use std::collections::HashSet;
   
   #[derive(Copy, Clone, Eq, Hash, PartialEq)]
   struct X {}
   
   struct Y {
       xs: HashSet<X>,
   }
   
   fn do_stuff(y: &Y) -> Vec<X> {
       Vec::from_iter(y.xs.iter().cloned())
   }
   

   This is recommended here:

   • Build HashSet from a vector in Rust - Answer by bluss
   • Collecting from HashSet to Vector - Answer by notriddle
   • HashSet from Vec while maintaining the order of elements - Question by StackLeak

I have two questions:

1. What is the essential difference between the above methods?

2. What is the canonical way to collect the items from a &HashSet<X> into a Vec<X>?

Related:

• How to determine whether to use .clone() and .cloned() with iterators - Answer by Chayim Friedman

Answer 1

_.clone().into_iter().collect()

This one creates an intermediate copy of HashSet. Which is not something you want, its an unneeded allocation and processing. The rule of thumb is: prefer .cloned() over .clone().

These two:

_.iter().cloned().collect()

Vec::from_iter(_.iter().cloned())

are equivalent. The first one is more readable IMO.