Give a bonus to Drop if the instantiation would be !needs_drop

Author: scottmcm

compiler/rustc_mir_transform/src/cost_checker.rs

@@ -12,6 +12,7 @@ const CONST_SWITCH_BONUS: usize = 10;
 const MULTIPLE_MUT_PENALTY: usize = 50;
 const REF_COPY_BONUS: usize = 5;
 const MANY_PARAMETERS_BONUS: usize = 10;
+const REMOVABLE_DROP_BONUS: usize = 20;
 
 /// Verify that the callee body is compatible with the caller.
 #[derive(Clone)]
@@ -156,13 +157,16 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, _: Location) {
         match &terminator.kind {
             TerminatorKind::Drop { place, unwind, .. } => {
-                // If the place doesn't actually need dropping, treat it like a regular goto.
                 let ty = self.instantiate_ty(place.ty(self.callee_body, self.tcx).ty);
                 if ty.needs_drop(self.tcx, self.param_env) {
                     self.penalty += CALL_PENALTY;
                     if let UnwindAction::Cleanup(_) = unwind {
                         self.penalty += LANDINGPAD_PENALTY;
                     }
+                } else {
+                    // If the place doesn't actually need dropping,
+                    // we'd like to inline it to simplify the CFG
+                    self.bonus += REMOVABLE_DROP_BONUS;
                 }
             }
             TerminatorKind::Call { func, unwind, .. } => {