optimize: avoid generating drop glue for StepResult

fix-vm/src/lib.rs

@@ -38,10 +38,10 @@ impl From<Box<Error>> for VmError {
 }
 
 impl VmError {
-    fn into_step_result<'gc>(self) -> StepResult<'gc> {
+    fn into_error(self) -> Box<Error> {
         match self {
             VmError::Catchable(_) => todo!("Check for tryEval catch frames"),
-            VmError::Uncatchable(e) => StepResult::Done(Err(e)),
+            VmError::Uncatchable(e) => e,
         }
     }
 }
@@ -128,7 +128,7 @@ impl<T: VmContext> VmContextExt for T {
 pub(crate) enum StepResult<'gc> {
     Continue,
     ForceThunk(ForceInfo<'gc>),
-    Done(Result<fix_common::Value>),
+    Done,
 }
 
 pub(crate) struct ForceInfo<'gc> {
@@ -159,6 +159,9 @@ pub struct Vm<'gc> {
     pub(crate) empty_attrs: Value<'gc>,
 
     pub(crate) force_mode: ForceMode,
+
+    #[collect(require_static)]
+    pub(crate) result: Option<Result<fix_common::Value>>,
 }
 
 pub(crate) enum OperandData {
@@ -264,9 +267,28 @@ impl<'gc> Vm<'gc> {
             empty_attrs: Value::new_gc(Gc::new(mc, AttrSet::default())),
 
             force_mode,
+
+            result: None,
         }
     }
 
+    #[inline(always)]
+    pub(crate) fn finish_ok(&mut self, val: fix_common::Value) -> StepResult<'gc> {
+        self.result = Some(Ok(val));
+        StepResult::Done
+    }
+
+    #[inline(always)]
+    pub(crate) fn finish_err(&mut self, err: Box<Error>) -> StepResult<'gc> {
+        self.result = Some(Err(err));
+        StepResult::Done
+    }
+
+    #[inline(always)]
+    pub(crate) fn finish_vm_err(&mut self, err: VmError) -> StepResult<'gc> {
+        self.finish_err(err.into_error())
+    }
+
     #[inline(always)]
     pub(crate) fn push_stack(&mut self, val: Value<'gc>) {
         self.stack.push(val);
@@ -339,7 +361,7 @@ impl<'gc> Vm<'gc> {
                 }
                 ThunkState::Apply { .. } => todo!("force apply"),
                 ThunkState::Blackhole => {
-                    StepResult::Done(Err(Error::eval_error("infinite recursion encountered")))
+                    self.finish_err(Error::eval_error("infinite recursion encountered"))
                 }
             }
         } else {
@@ -526,7 +548,11 @@ impl<'gc> Vm<'gc> {
                     self.env = info.env;
                     self.with_env = info.with_env;
                 }
-                StepResult::Done(result) => return Action::Done(result),
+                StepResult::Done => {
+                    return Action::Done(
+                        self.result.take().expect("StepResult::Done without result"),
+                    );
+                }
             }
         }
     }