Item inner preludes

[crlf0710]

TL;DR : This allows this code with the new prelude keyword:

mod bar {
     #[derive(Default)]
     pub(crate) struct Bar(pub(crate) usize);
     #[derive(Default)]
     pub(crate) struct Baz(pub(crate) usize);
}
struct Foo {
    bar: Bar,
    prelude {
          use bar::*;
    },
    baz: Baz,
}

impl Foo {
    fn new() -> Self {
        Foo {
             bar: Default::default(),
             baz: Default::default(),
        }
    }
   fn into_baz(self) -> Baz {
        self.baz
    }
    prelude {
          use bar::*;
    }
}

Although the similar names, the motivation is quite different from RFC 890, in that this tries to address these issues:

• Allowing use items in more cases, bringing more ergonomics

Possible Future extensions:

• Allowing nested ADT definitions, in a way not affecting the typesystem.
• Generalize this to the mod levels, to actually cover the original custom prelude use case

[SOF3]

Is this already possible to implement with an attribute macro?

impl Xxx {
    #![prelude(
        bar::*,
    )]

    // ...
}

expands to

mod _random {
    use super::*;
    use bar::*;

    impl Xxx {
        // ...
    }
}

use _random::*;

[Noratrieb]

this seems like a solution without a problem