Move dead CGU marking code out of partition.

The other major steps in `partition` have their own function, so it's
nice for this one to be likewise.

Author: nnethercote

compiler/rustc_monomorphize/src/partitioning.rs

@@ -197,34 +197,8 @@ where
 
     let instrument_dead_code =
         tcx.sess.instrument_coverage() && !tcx.sess.instrument_coverage_except_unused_functions();
-
     if instrument_dead_code {
-        assert!(
-            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 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<_> = codegen_units.iter_mut().collect();
-        cgus.sort_by_key(|cgu| cgu.size_estimate());
-
-        let dead_code_cgu =
-            if let Some(cgu) = cgus.into_iter().rev().find(|cgu| {
-                cgu.items().iter().any(|(_, (linkage, _))| *linkage == Linkage::External)
-            }) {
-                cgu
-            } else {
-                // If there are no CGUs that have externally linked items,
-                // then we just pick the first CGU as a fallback.
-                &mut codegen_units[0]
-            };
-        dead_code_cgu.make_code_coverage_dead_code_cgu();
+        mark_code_coverage_dead_code_cgu(&mut codegen_units);
     }
 
     // Ensure CGUs are sorted by name, so that we get deterministic results.
@@ -545,6 +519,33 @@ fn internalize_symbols<'tcx>(
     }
 }
 
+fn mark_code_coverage_dead_code_cgu<'tcx>(codegen_units: &mut [CodegenUnit<'tcx>]) {
+    assert!(!codegen_units.is_empty());
+
+    // 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 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<&mut CodegenUnit<'tcx>> = codegen_units.iter_mut().collect();
+    cgus.sort_by_key(|cgu| cgu.size_estimate());
+
+    let dead_code_cgu = if let Some(cgu) = cgus
+        .into_iter()
+        .rev()
+        .find(|cgu| cgu.items().iter().any(|(_, (linkage, _))| *linkage == Linkage::External))
+    {
+        cgu
+    } else {
+        // If there are no CGUs that have externally linked items,
+        // then we just pick the first CGU as a fallback.
+        &mut codegen_units[0]
+    };
+    dead_code_cgu.make_code_coverage_dead_code_cgu();
+}
+
 fn characteristic_def_id_of_mono_item<'tcx>(
     tcx: TyCtxt<'tcx>,
     mono_item: MonoItem<'tcx>,