What are the differences between the multiple ways to create zero-sized structs?

Question

I found four different ways to create a struct with no data:

• struct A{} // empty struct / empty braced struct
  

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Answer 1

struct D; // unit struct

This is the usual way for people to write a zero-sized struct.

struct A{} // empty struct / empty braced struct
struct B(); // empty tuple struct

These are just special cases of basic struct and tuple struct which happen to have no parameters. RFC 1506 explains the rational to allow those (they didn't used to):

Permit tuple structs and tuple variants with 0 fields. This restriction is artificial and can be lifted trivially. Macro writers dealing with tuple structs/variants will be happy to get rid of this one special case.

As such, they could easily be generated by macros, but people will rarely write those on their own.

struct C(()); // unit-valued tuple struct

This is another special case of tuple struct. In Rust, () is a type just like any other type, so struct C(()); isn't much different from struct E(u32);. While the type itself isn't very useful, forbidding it would make yet another special case that would need to be handled in macros or generics (struct F(T) can of course be instantiated as F<()>).

Note that there are many other ways to have empty types in Rust. Eg. it is possible to have a function return Result<(), !> to indicate that it doesn't produce a value, and cannot fail. While you might think that returning () in that case would be better, you might have to do that if you implement a trait that dictates you to return Result but lets you choose T = () and E = !.