Sorry... I open the issue with reference in new issue, and am unable to add the bug label to this issue. 😳

I added self.inject_prelude(); in

This resolves the issue, but I don't understand what "so that blocks can properly shadow them" means.

The shadowing is about stuff like

(assuming a core entry in the extern prelude)

fn f() {
   mod core {
       pub fn foo () {}
   }
   core::foo();
}

This should look into the local module, not the prelude.

It seems that it can be resolved correctly with the modifications above:

The problem with that approach might be that it fails for nsted block def maps

fn foo() {
    mod core { mod mem { fn drop<T>(_: T) {} } }
    {
        use self as _; // forces a block def map here
        core::mem::drop(0); // should resolve to the local item, not the prelude one
    }
}