Generate error delegation when delegation is not resolved

compiler/rustc_ast_lowering/src/delegation.rs

@@ -152,10 +152,21 @@ pub(crate) fn lower_delegation(
     ) -> DelegationResults<'hir> {
         let span = self.lower_span(delegation.path.segments.last().unwrap().ident.span);
 
-        let ids = self.get_delegation_ids(
-            self.resolver.delegation_infos[&self.local_def_id(item_id)].resolution_node,
-            span,
-        );
+        // Delegation can be unresolved in illegal places such as function bodies in extern blocks (see #151356)
+        let ids = if let Some(delegation_info) =
+            self.resolver.delegation_infos.get(&self.local_def_id(item_id))
+        {
+            self.get_delegation_ids(delegation_info.resolution_node, span)
+        } else {
+            return self.generate_delegation_error(
+                self.dcx().span_delayed_bug(
+                    span,
+                    format!("LoweringContext: the delegation {:?} is unresolved", item_id),
+                ),
+                span,
+                delegation,
+            );
+        };
 
         match ids {
             Ok(ids) => {

tests/ui/delegation/unresolved-delegation-ice-151356.rs

@@ -0,0 +1,11 @@
+#![allow(incomplete_features)]
+#![feature(fn_delegation)]
+
+extern "C" {
+    fn a() {
+        //~^ ERROR incorrect function inside `extern` block
+        reuse foo {}
+    }
+}
+
+pub fn main() {}

tests/ui/delegation/unresolved-delegation-ice-151356.stderr

@@ -0,0 +1,19 @@
+error: incorrect function inside `extern` block
+  --> $DIR/unresolved-delegation-ice-151356.rs:5:8
+   |
+LL |   extern "C" {
+   |   ---------- `extern` blocks define existing foreign functions and functions inside of them cannot have a body
+LL |       fn a() {
+   |  ________^___-
+   | |        |
+   | |        cannot have a body
+LL | |
+LL | |         reuse foo {}
+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 1 previous error
+