Rust error E0073: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

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 }.

From rustc --explain E0073 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.