Why can a trait not construct itself?

Question

This code give me a compilation error:

trait IBoo {
    fn new() -> Box<IBoo>;
}

while this code compiles without any error:

trait IBoo {
    //fn new() -> Box<IBoo>;
}

trait IFoo {
    fn new() -> Box<IBoo>;
}

1. Why does the first one not compile? rustc --explain E0038 does not give me a direct hint why it is not possible.
2. Is it possible to combine construction and methods in one interface (trait)?

Answer 1

This is from the description of E0038:

Method has no receiver

Methods that do not take a self parameter can't be called since there won't be a way to get a pointer to the method table for them.

trait Foo {
    fn foo() -> u8;
}

This could be called as <Foo as Foo>::foo(), which would not be able to pick an implementation.

Adding a Self: Sized bound to these methods will generally make this compile.

trait Foo {
    fn foo() -> u8 where Self: Sized;
}

You can do this:

trait IBoo {
    fn new() -> Box<IBoo>
    where
        Self: Sized;
}

In other cases, you can place the restriction on the entire impl: