skip updating when external binding is existed

[bvanjoi]

Fixes #128813

For following code:

extern crate core;

fn f() {
    use ::core;
}

macro_rules! m {
    () => {
        extern crate std as core;
    };
}

m!();

fn main() {}

• In the first loop, we define extern crate core and use ::core will be referred to core (yes, it does not consider if there are some macros that are not expanded. Ideally, this should be delayed if there are some unexpanded macros in the root, but I didn't change it like that because it seems like a huge change).
• Then m is expanded, which makes extern_prelude('core') return std rather than core, causing the inconsistency.

r? @petrochenkov

[petrochenkov]

Could you also add a test case for a crate that is not originally in the extern prelude due to command line options or built-ins, like core?

extern crate foo;

macro_rules! second_foo {
    () => {
        extern crate std as foo;
    };
}

second_foo!();

fn main() {
    use ::foo;
}

@rustbot author

[bvanjoi]

In the latest patch:

• rename issue-95992-extern-2.rs into empty.rs
• rename issue-95992-extern-1.rs into issue-95992-extern.rs
• skip updating the external binding if it exists.

[bvanjoi]

@rustbot ready

[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit df019a9 has been approved by petrochenkov

It is now in the queue for this repository.