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

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:

e59b08e62e/src/librustc_middle/hir/map/collector.rs (L154-L158)

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

```rust
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

src/librustc_ast_lowering/item.rs

@@ -6,7 +6,8 @@ use rustc_ast::ast::*;
 use rustc_ast::attr;
 use rustc_ast::node_id::NodeMap;
 use rustc_ast::ptr::P;
-use rustc_ast::visit::{self, AssocCtxt, Visitor};
+use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
+use rustc_ast::walk_list;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
@@ -76,6 +77,43 @@ impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
         }
     }
 
+    // Forked from the original method because we don't want to descend into foreign function
+    // blocks. Such blocks are semantically invalid and the hir does not preserve them, so lowering
+    // items contained in them may be unexpected by later passes.
+    fn visit_foreign_item(&mut self, item: &'a ForeignItem) {
+        let Item { id: _, span: _, ident, ref vis, ref attrs, ref kind, tokens: _ } = *item;
+        self.visit_vis(vis);
+        self.visit_ident(ident);
+        walk_list!(self, visit_attribute, attrs);
+        match kind {
+            ForeignItemKind::Static(ty, _, expr) => {
+                self.visit_ty(ty);
+                walk_list!(self, visit_expr, expr);
+            }
+            ForeignItemKind::Fn(_, sig, generics, body) => {
+                self.visit_generics(generics);
+                let kind = FnKind::Fn(FnCtxt::Foreign, ident, sig, vis, body.as_deref());
+                match kind {
+                    FnKind::Fn(_, _, sig, _, _) => {
+                        self.visit_fn_header(&sig.header);
+                        visit::walk_fn_decl(self, &sig.decl);
+                    }
+                    FnKind::Closure(decl, _) => {
+                        visit::walk_fn_decl(self, decl);
+                    }
+                }
+            }
+            ForeignItemKind::TyAlias(_, generics, bounds, ty) => {
+                self.visit_generics(generics);
+                walk_list!(self, visit_param_bound, bounds);
+                walk_list!(self, visit_ty, ty);
+            }
+            ForeignItemKind::MacCall(mac) => {
+                self.visit_mac(mac);
+            }
+        }
+    }
+
     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
             AssocCtxt::Trait => {

src/test/ui/foreign/issue-74120-lowering-of-ffi-block-bodies.rs

@@ -0,0 +1,11 @@
+// Previously this ICE'd because `fn g()` would be lowered, but the block associated with `fn f()`
+// wasn't.
+
+// compile-flags: --crate-type=lib
+
+extern "C" {
+    fn f() {
+    //~^ incorrect function inside `extern` block
+        fn g() {}
+    }
+}

src/test/ui/foreign/issue-74120-lowering-of-ffi-block-bodies.stderr

@@ -0,0 +1,19 @@
+error: incorrect function inside `extern` block
+  --> $DIR/issue-74120-lowering-of-ffi-block-bodies.rs:7:8
+   |
+LL |   extern "C" {
+   |   ---------- `extern` blocks define existing foreign functions and functions inside of them cannot have a body
+LL |       fn f() {
+   |  ________^___-
+   | |        |
+   | |        cannot have a body
+LL | |
+LL | |         fn g() {}
+LL | |     }
+   | |_____- help: remove the invalid body: `;`
+   |
+   = help: you might have meant to write a function accessible through FFI, which can be done by writing `extern fn` outside of the `extern` block
+   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+error: aborting due to previous error
+