Move backpointer check into BytecodeDecoder

src/vstc/bytecode_decoder.rs

@@ -71,9 +71,14 @@ impl BytecodeDecoder {
         let from_pos = self.pos;
         let pos = self.decode_pos();
         
+        if pos < from_pos {
+          if self.clone_at(pos).decode_type() != BytecodeType::Function {
+            std::panic!("Invalid: non-function pointer that points backwards");
+          }
+        }
+
         return vs_value::VsPointer::new(
           &self.data,
-          from_pos,
           pos,
         );
       },
@@ -123,4 +128,8 @@ impl BytecodeDecoder {
     // size of the bytecode
     return self.decode_byte() as usize;
   }
+
+  pub fn clone_at(&self, pos: usize) -> BytecodeDecoder {
+    return BytecodeDecoder { data: self.data.clone(), pos: pos };
+  }
 }

src/vstc/vs_value.rs

@@ -69,21 +69,7 @@ pub struct VsPointer {
 }
 
 impl VsPointer {
-  pub fn new(bytecode: &Rc<Vec<u8>>, from_pos: usize, pos: usize) -> Val {
-    if pos < from_pos {
-      let mut bd = BytecodeDecoder {
-        data: bytecode.clone(),
-        pos: pos,
-      };
-
-      let byte = bd.decode_byte();
-
-      if byte != BytecodeType::Function as u8 {
-        // Prevent circular objects
-        std::panic!("Invalid: non-function pointer that points backwards");
-      }
-    }
-
+  pub fn new(bytecode: &Rc<Vec<u8>>, pos: usize) -> Val {
     return Rc::new(VsPointer {
       bytecode: bytecode.clone(),
       pos: pos,