Fix ICE #138415 for invalid extern function body

compiler/rustc_ast_lowering/src/item.rs

@@ -1724,8 +1724,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
             return;
         };
         let define_opaque = define_opaque.iter().filter_map(|(id, path)| {
-            let res = self.resolver.get_partial_res(*id).unwrap();
-            let Some(did) = res.expect_full_res().opt_def_id() else {
+            let res = self.resolver.get_partial_res(*id);
+            let Some(did) = res.and_then(|res| res.expect_full_res().opt_def_id()) else {
                 self.dcx().span_delayed_bug(path.span, "should have errored in resolve");
                 return None;
             };

library/core/src/macros/mod.rs

@@ -1744,7 +1744,7 @@ pub(crate) mod builtin {
     }
 
     /// Provide a list of type aliases and other opaque-type-containing type definitions.
-    /// This list will be used in the body of the item it is applied to to define opaque
+    /// This list will be used in the body of the item it is applied to define opaque
     /// types' hidden types.
     /// Can only be applied to things that have bodies.
     #[unstable(

tests/ui/type-alias-impl-trait/define_opaques_attr/invalid-extern-fn-body.rs

@@ -0,0 +1,11 @@
+#![feature(type_alias_impl_trait)]
+
+extern "C" {
+    fn a() {
+        //~^ ERROR incorrect function inside `extern` block
+        #[define_opaque(String)]
+        fn c() {}
+    }
+}
+
+pub fn main() {}

tests/ui/type-alias-impl-trait/define_opaques_attr/invalid-extern-fn-body.stderr

@@ -0,0 +1,20 @@
+error: incorrect function inside `extern` block
+  --> $DIR/invalid-extern-fn-body.rs:4:8
+   |
+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 | |         #[define_opaque(String)]
+LL | |         fn c() {}
+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
+