Fix for ICE: eii: fn / macro rules None in find_attr()
Closesrust-lang/rust#149981
This used to ICE:
```rust
macro_rules! foo_impl {}
#[eii]
fn foo_impl() {}
```
`#[eii]` generates a macro (called `foo_impl`) and a default impl. So the partial expansion used to roughly look like the following:
```rust
macro_rules! foo_impl {} // actually resolves here
extern "Rust" {
fn foo_impl();
}
#[eii_extern_target(foo_impl)]
macro foo_impl {
() => {};
}
const _: () = {
#[implements_eii(foo_impl)] // assumed to name resolve to the macro v2 above
fn foo_impl() {}
};
```
Now, shadowing rules for macrov2 and macrov1 are super weird! Take a look at this: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=23f21421921360478b0ec0276711ad36
So instead of resolving to the macrov2, we resolve the macrov1 named the same thing.
A regression test was added to this, and some span_delayed_bugs were added to make sure we catch this in the right places. But that didn't fix the root cause.
To make sure this simply cannot happen again, I made it so that we don't even need to do a name resolution for the default. In other words, the new partial expansion looks more like:
```rust
macro_rules! foo_impl {}
extern "Rust" {
fn foo_impl(); // resolves to here now!!!
}
#[eii_extern_target(foo_impl)]
macro foo_impl {
() => {};
}
const _: () = {
#[implements_eii(known_extern_target=foo_impl)] // still name resolved, but directly to the foreign function.
fn foo_impl() {}
};
```
The reason this helps is that name resolution for non-macros is much more predictable. It's not possible to have two functions like that with the same name in scope.
We used to key externally implementable items off of the defid of the macro, but now the unique identifier is the foreign function's defid which seems much more sane.
Finally, I lied a tiny bit because the above partial expansion doesn't actually work.
```rust
extern "Rust" {
fn foo_impl(); // not to here
}
const _: () = {
#[implements_eii(known_extern_target=foo_impl)] // actually resolves to this function itself
fn foo_impl() {} // <--- so to here
};
```
So the last few commits change the expansion to actually be this:
```rust
macro_rules! foo_impl {}
extern "Rust" {
fn foo_impl(); // resolves to here now!!!
}
#[eii_extern_target(foo_impl)]
macro foo_impl {
() => {};
}
const _: () = {
mod dflt { // necessary, otherwise `super` doesn't work
use super::*;
#[implements_eii(known_extern_target=super::foo_impl)] // now resolves to outside the `dflt` module, so the foreign item.
fn foo_impl() {}
}
};
```
I apologize to whoever needs to review this, this is very subtle and I hope this makes it clear enough 😭.
a8028abedb