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Why does Rust enum with Box create cycle warning even with phantom?
I'm working with Rust and I have the following code:
public enum Expr {
Apply(Box<Expr>, Box<Expr>),
}
It seems like I'm getting a cycle warning because Expr contains Box<Expr>. However, I'm using phantom types, and I thought that would make it safe to compile. Can someone explain why this cycle warning is happening despite using phantom types?
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Answers
1The cycle warning occurs because your Expr enum definition creates a recursive type. In Rust, using Box<Expr> inside the Expr enum type causes a recursive relationship because Box<Expr> directly references Expr. This is why Rust gives a cycle warning.
Even though you mention phantom types, it seems that you're still providing a concrete type Expr for the Box instead of a phantom generic type that wouldn't require a real instance. Consider if you wanted to use a phantom type, you would use a generic parameter in the enum that doesn't need to exist at runtime, something like Box<PhantomData<T>> if T is a generic parameter not Expr itself.
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