Address review comments

compiler/rustc_monomorphize/src/partitioning/mod.rs

@@ -205,17 +205,33 @@ pub fn partition<'tcx>(
         tcx.sess.instrument_coverage() && !tcx.sess.instrument_coverage_except_unused_functions();
 
     if instrument_dead_code {
+        assert!(
+            post_inlining.codegen_units.len() > 0,
+            "There must be at least one CGU that code coverage data can be generated in."
+        );
+
         // Find the smallest CGU that has exported symbols and put the dead
         // function stubs in that CGU. We look for exported symbols to increase
-        // the likelyhood the linker won't throw away the dead functions.
-        let mut cgus_with_exported_symbols: Vec<_> = post_inlining
-            .codegen_units
-            .iter_mut()
-            .filter(|cgu| cgu.items().iter().any(|(_, (linkage, _))| *linkage == Linkage::External))
-            .collect();
-        cgus_with_exported_symbols.sort_by_key(|cgu| cgu.size_estimate());
+        // the likelihood the linker won't throw away the dead functions.
+        // FIXME(#92165): In order to truly resolve this, we need to make sure
+        // the object file (CGU) containing the dead function stubs is included
+        // in the final binary. This will probably require forcing these
+        // function symbols to be included via `-u` or `/include` linker args.
+        let mut cgus: Vec<_> = post_inlining.codegen_units.iter_mut().collect();
+        cgus.sort_by_key(|cgu| cgu.size_estimate());
 
-        let dead_code_cgu = cgus_with_exported_symbols.last_mut().unwrap();
+        let dead_code_cgu = if let Some(cgu) = cgus
+            .into_iter()
+            .rev()
+            .filter(|cgu| cgu.items().iter().any(|(_, (linkage, _))| *linkage == Linkage::External))
+            .next()
+        {
+            cgu
+        } else {
+            // If there are no CGUs that have externally linked items,
+            // then we just pick the first CGU as a fallback.
+            &mut post_inlining.codegen_units[0]
+        };
         dead_code_cgu.make_code_coverage_dead_code_cgu();
     }