E147: A type has no default value because it reaches itself through & fields

T::default() gives every field its default: 0, false, “”, an empty collection. A &U field can never be null, so its default is a new U::default() of its own, made on the heap. That is what lets every collection share one header from the moment it is created.

A type that reaches itself through & fields cannot be defaulted this way, because each new value would need another new value inside it, forever:

struct Node {    v: int,    next: &Node}fn main(): int {    mut n: Node = Node::default();   // E147 -- 'Node' reaches itself through '&' fields    return n.v;}

The cycle can be longer (A has a &B, B has a &A), and it can pass through fields that hold a struct by value; the error names the type you defaulted and the type that repeats.

Two ways out:

1. Make one field on the cycle a nullable reference, &?Node. It defaults to null, which ends the chain; check it with if (n.next != null) before using it:

struct Node {    v: int,    next: &?Node}fn main(): int {    mut n: Node = Node::default();   // fine: next is null    return n.v;}

2. Build the value with a constructor, Node::new(...), and pass it the &Node it should point to.

A &U field that does not lead back to its own type is fine: struct Holder { h: &Counter } defaults to a fresh Counter::default().

Run turf --explain E147 in a terminal to read this again.