Why does Rust enum with Box create cycle warning even with phantom?

Question

Im working with Rust and I have the following code: public enum Expr { Apply(Box, Box), } It seems like Im getting a cycle warning because Expr contains Box. However, Im 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?

CarlkawIy · Accepted Answer

The cycle warning occurs because your Expr enum definition creates a recursive type. In Rust, using Box inside the Expr enum type causes a recursive relationship because Box directly references Expr. This is why Rust gives a cycle warning. Even though you mention phantom types, it seems that youre still providing a concrete type Expr for the Box instead of a phantom generic type that wouldnt require a real instance. Consider if you wanted to use a phantom type, you would use a generic parameter in the enum that doesnt need to exist at runtime, something like Box> if T is a generic parameter not Expr itself.

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Why does Rust enum with Box create cycle warning even with phantom?

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