Modules need to be inserted in the type namespace

[CohenArthur]

mod test {
    mod Toto {

    }

    struct Toto;
}

https://godbolt.org/z/r55x4jrGh

Thanks to @P-E-P for the testcase and godbolt link

This is important as the prelude import in core fails since it only contains a mod declaration for v1:

// core/src/lib.rs
use prelude::v1::*;

// core/src/prelude/mod.rs
pub mod v1;

// core/src/prelude/v1.rs
pub use crate::option::Option::{None, Some};