Declare new structs in a custom derive macro?

Is is possible to use a custom derive macro to define new structures?

Something like:

struct MyStruct {
    foo: i32,
    bar: f32

where the macro would then in turn end up generating something like:

pub struct {
    first : MyStruct,
    second : MyStruct,
    type_from_name : #name,
    modified : Modified_ #name,

where the type_from_name and modified are just guesses at the syntax.
The idea is that type_from_name will have the type of the struct the macro was invoked on, and modified would set the type to a modification of that type name (assuming that is declared elsewhere).

The concept is that I want to write a crate that will "take" a user-defined struct as an input and define a whole hierarchy of structures around it, with those parent structs ultimately being used for serialization/deserialization.

And I realize that this construction may not exist because it's not meant to, and the real answer is: "Whatever you're trying to do should be a method or a trait". I'm still thinking through how to do ...

Yes; custom derive macros can produce arbitrary items (impl blocks, struct definitions, etc) at the same level as the annotated struct. According to the reference:

The input TokenStream is the token stream of the item that has the derive attribute on it. The output TokenStream must be a set of items that are then appended to the module or block that the item from the input TokenStream is in.

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