Multi tool use

Why does this Rust binary tree overflow its stack in tests? [duplicate]

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I've written a binary tree-like structure, but I've been getting stack-overflow errors in some of my stress tests. I've reduced the error-causing code down to the following:
struct Node {
index: usize,
right: Option>,
}

struct BoxedTree {
root: Option<Box<Node>>,
}

fn build_degenerate() {
let mut t = BoxedTree {
root: Some(Box::new(Node {
index: 0,
right: None,
})),
};
let mut n = t.root.as_mut().unwrap();
for i in 1..50000 {
let cur = n;
let p = &mut cur.right;
*p = Some(Box::new(Node {
index: i,
right: None,
}));
n = p.as_mut().unwrap();
}
println!("{}", n.index);
}

fn main() {
build_degenerate();
}

#[cfg(test)]
mod tests {
use super::*;

#[test]
fn mytest() {
build_degenerate();
}
}


Running cargo test gives me:

cargo test

running 1 test

thread 'tests::mytest' has overflowed its stack
fatal runtime error: stack overflow
error: process didn't exit successfully: ...


I don't understand how this code could be causing a stack overflow, as all data seems to be allocated on the heap, and there is no recursion.

Even stranger, this code causes an overflow when I run cargo test, but not when I run cargo run, even though mytest and main call the same code.

cargo test

cargo run

mytest

main

What's going wrong here?

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