Move MethodInst stack mangling to MethodInst::call

When a MethodInst is called as a function in the VM, it needs to push
its `self_binding` member to the stack. Previously, we were downcasting
(if possible) to MethodInst in the VM, but really, we are calling
`MethodInst::call` anyway, so it makes more sense to do
MethodInst-specific stuff in the MethodInst-specific function.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj.rs

@@ -1079,10 +1079,16 @@ impl Display for MethodInst {
 
 impl Obj for MethodInst {
     fn arity(&self) -> Option<Argc> {
-        self.function.borrow().arity()
+        // Subtract one from the arity - this is because the VM uses arity() to check against the
+        // number of arguments passed.
+        self.function.borrow().arity().map(|arity| arity - 1)
     }
 
-    fn call(&self, vm: &mut Vm, argc: Argc) {
+    fn call(&self, vm: &mut Vm, mut argc: Argc) {
+        let self_pos = vm.stack().len() - (argc as usize);
+        vm.stack_mut().insert(self_pos, self.self_binding.clone());
+        argc += 1;
+
         self.function.borrow().call(vm, argc)
     }
 

src/vm.rs

@@ -131,6 +131,11 @@ impl Vm {
         &self.stack
     }
 
+    /// Get the stack, mutably.
+    pub fn stack_mut(&mut self) -> &mut Vec<ObjP> {
+        &mut self.stack
+    }
+
     /// Current stack frame.
     pub fn frame(&self) -> &Frame {
         self.frames.last().unwrap()
@@ -281,12 +286,12 @@ impl Vm {
                     }
                 }
                 Op::Call(argc) => {
-                    let argc = argc as usize;
+                    let argc = argc;
-                    let index = self.stack.len() - argc - 1;
+                    let index = self.stack.len() - (argc as usize) - 1;
                     let fun_ptr = Ptr::clone(&self.stack[index]);
 
                     let arity = if let Some(arity) = fun_ptr.borrow().arity() {
-                        arity as usize
+                        arity
                     } else {
                         // TODO Vm::run, Op::Call - throw an exception when the value isn't
                         //                          callable
@@ -297,20 +302,7 @@ impl Vm {
                         );
                     };
 
-                    // Methods with bound "self" parameter
+                    // Check arity
-                    // argc may be mutated
-                    let mut argc = argc;
-                    if let Some(method) = fun_ptr.borrow().as_any().downcast_ref::<MethodInst>() {
-                        // shift all of the arguments over by one
-                        // (duplicate the last item on the stack and then shift everyone else over)
-                        self.stack
-                            .insert(self.stack.len() - argc, Ptr::clone(method.self_binding()));
-                        // also increment argc since we're specifying another arg
-                        argc += 1;
-                    }
-                    // remove mutability
-                    let argc = argc;
-
                     if arity != argc {
                         // TODO Vm::run, Op::Call - throw an exception when the number of arguments
                         //                          does not match the function's arity