"overflow while adding drop-check rules" while implementing a fingertree - data-structures

Question

I'm trying to define a finger tree structure and implement its basic operations as an exercise in Rust. I've come up with the following, which is basically what's described in this paper.
use self::FingerTree::{Empty, Single, Deep};
use self::Digit::{One, Two, Three, Four};
enum Digit<A> {
One(A),
Two(A, A),
Three(A, A, A),
Four(A, A, A, A),
}
enum Node<V, A> {
Node2(V, A, A),
Node3(V, A, A, A),
}
enum FingerTree<V, A> {
Empty,
Single(A),
Deep {
size: V,
prefix: Digit<A>,
tree: Box<FingerTree<V, Node<V, A>>>,
suffix: Digit<A>,
},
}
fn main() {
let e: FingerTree<i32, String> = Empty;
}
Compilation gives me an error that I don't understand:
error[E0320]: overflow while adding drop-check rules for FingerTree<i32, std::string::String>
--> fingertree.rs:28:9
|
28 | let e: FingerTree<i32, String> = Empty;
| ^
|
note: overflowed on enum Node variant Node2 field 0 type: i32
--> fingertree.rs:28:9
|
28 | let e: FingerTree<i32, String> = Empty;
| ^
error[E0320]: overflow while adding drop-check rules for FingerTree<i32, std::string::String>
--> fingertree.rs:28:38
|
28 | let e: FingerTree<i32, String> = Empty;
| ^^^^^
|
note: overflowed on enum Node variant Node2 field 0 type: i32
--> fingertree.rs:28:38
|
28 | let e: FingerTree<i32, String> = Empty;
| ^^^^^
Why is this not working? How do I make it work?

Answer 1

You have created an infinite type.
Instantiating FingerTree<V, A> instantiates FingerTree<V, Node<V, A>> which instantiates FingerTree<V, Node<V, Node<V, A>>> which instantiates, ... and there's no end in sight.
The compiler cannot tell that the type will not actually be used at run-time, so prepares itself for the worst. And the worst is infinite.
Simply replacing the type of tree by Box<FingerTree<V, A>> solves the issue, though it may not be correct for the situation at hand.