Solutions to performance issues caused by large variant size of enum

Question

I am trying to build a tree-like data structure using the Node representation:

use std::cmp::Ordering;
use std::fmt::Debug;
use std::sync::Arc;
         


        

Answer 1

You don't need trait objects here because you don't need the unbounded polymorphism they provide.

Clippy tells you what to do:

warning: large size difference between variants
  --> src/main.rs:13:5
   |
13 | /     RelaxedBranch {
14 | |         children: [Option<Arc<Node<T>>>; BRANCH_FACTOR],
15 | |         sizes: [Option<usize>; BRANCH_FACTOR],
16 | |         len: usize,
17 | |     },
   | |_____^
   |
   = note: #[warn(large_enum_variant)] on by default
help: consider boxing the large fields to reduce the total size of the enum
  --> src/main.rs:13:5
   |
13 | /     RelaxedBranch {
14 | |         children: [Option<Arc<Node<T>>>; BRANCH_FACTOR],
15 | |         sizes: [Option<usize>; BRANCH_FACTOR],
16 | |         len: usize,
17 | |     },
   | |_____^
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.211/index.html#large_enum_variant

Switching to

RelaxedBranch {
    children: Box<[Option<Arc<Node<T>>>; BRANCH_FACTOR]>,
    sizes: Box<[Option<usize>; BRANCH_FACTOR]>,
    len: usize,
},

Reduces the size of Node<()> from 784 to 272. You could go further by combining all the fields into a new struct and boxing that.