We've previously bumped into the need for Rust to know exactly how many bytes a particular data value can occupy. Most of the time, Rust can figure that out, and most of the time, it's not a problem, but there are a few cases where it's impossible to define a fixed size for a data value.

One fundamental example is a data structure, such as the following one, where an instance contains other instances of itself:

pub struct TreeNode {
    pub value: i32,
    pub left: TreeNode,
    pub right: TreeNode,
}

That looks reasonable at first glance, but Rust quite rightly points out that the calculated size is infinite (because the size of a TreeNode is the size of two TreeNodes plus 32 bits):

Just as the compiler suggests, we can fix this with a Box:

pub struct TreeNode {
    pub value: i32,
    pub left: Box<TreeNode>,
    pub right: Box<TreeNode>,
}

Now, the size of a TreeNode is the size of two Boxes plus 32 bits, which is entirely reasonable.