Don't visit foreign function bodies when lowering ast to hir

[ayazhafiz]

Previously the existence of bodies inside a foreign function block would
cause a panic in the hir NodeCollector during its collection of crate
bodies to compute a crate hash:

rust/src/librustc_middle/hir/map/collector.rs

Lines 154 to 158 in e59b08e

	// Insert bodies into the map
	for (id, body) in self.krate.bodies.iter() {
	let bodies = &mut self.map[id.hir_id.owner].with_bodies.as_mut().unwrap().bodies;
	assert!(bodies.insert(id.hir_id.local_id, body).is_none());
	}

The collector walks the hir tree and creates a map of hir nodes, then
attaching bodies in the crate to their owner in the map. For a code like

extern "C" {
    fn f() {
        fn g() {}
    }
}

The crate bodies include the body of the function g. But foreign
functions cannot have bodies, and while the parser AST permits a foreign
function to have a body, the hir doesn't. This means that the body of
f is not present in the hir, and so neither is g. So when the
NodeCollector finishes the walking the hir, it has no record of g,
cannot find an owner for the body of g it sees in the crate bodies,
and blows up.

Why do the crate bodies include the body of g? The AST walker has a
need a for walking function bodies, and FFIs share the same AST node as
functions in other contexts.

There are at least two options to fix this:

• Don't unwrap the map entry for an hir node in the NodeCollector
• Modifier the ast->hir lowering visitor to ignore foreign function
  blocks

I don't think the first is preferrable, since we want to know when we
can't find a body for an hir node that we thought had one (dropping this
information may lead to an invalid hash). So this commit implements the
second option.

Closes #74120

[rust-highfive]

r? @eddyb

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[ayazhafiz]

@eddyb friendly ping for this :)

[eddyb]

Ugh I should've mentioned that you should PM me on Zulip once you've addressed the comments.

[eddyb]

@bors r+

[bors]

📌 Commit d442bf7 has been approved by eddyb

[bors]

⌛ Testing commit d442bf7 with merge 57c460f246822f269a5c8c3df7f9787c37e3dd13...

[tmandry]

Rolled up,
@bors retry

[bors]

⌛ Testing commit d442bf7 with merge 7e6d6e5...