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+#### Note: this error code is no longer emitted by the compiler.
+
+You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
+in order to make a new `Foo` value. This is because there would be no way a
+first instance of `Foo` could be made to initialize another instance!
+
+Here's an example of a struct that has this problem:
+
+```
+struct Foo { x: Box<Foo> } // error
+```
+
+One fix is to use `Option`, like so:
+
+```
+struct Foo { x: Option<Box<Foo>> }
+```
+
+Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.