Add Inst::Dupe

Duplicates the top of the stack.

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

src/vm/inst.rs

@@ -21,6 +21,9 @@ pub enum Inst {
     /// Pops the top item off of the stack and stores it in a name.
     Store(Name),
 
+    /// Duplicate the top of the stack.
+    Dupe,
+
     /// Get an attribute from the top item of the stack.
     GetAttr(Arc<String>),
 
@@ -60,6 +63,7 @@ impl Inst {
             Inst::Push(ptr) => format!("push <{:#x}>", (ptr as *const _ as usize)),
             Inst::Load(name) => format!("load {name:?}"),
             Inst::Store(name) => format!("store {name:?}"),
+            Inst::Dupe => format!("dupe"),
             Inst::GetAttr(name) => format!("getattr {name}"),
             Inst::Pop => format!("pop"),
             Inst::Call(argc) => format!("call {argc}"),

src/vm/mod.rs

@@ -75,6 +75,11 @@ impl Vm {
         self.stack_mut().pop()
     }
 
+    /// Peek at the top value of the stack.
+    pub fn peek_stack(&self) -> Option<&ObjPtr> {
+        self.stack().last()
+    }
+
     pub fn condition(&self) -> bool {
         self.condition
     }
@@ -163,11 +168,18 @@ impl Vm {
                 let value = self.pop_stack().expect("Stack underflow");
                 self.store(name, Some(value));
             }
+            Inst::Dupe => {
+                let obj = self
+                    .peek_stack()
+                    .cloned()
+                    .expect("Inst::Dupe executed, but there was no value on top of the stack.");
+                self.push_stack(obj);
+            }
             Inst::GetAttr(name) => {
                 let name = std::sync::Arc::clone(name);
                 let obj = self
                     .pop_stack()
-                    .expect("Inst::GetAttr, but there was no value on top of the stack.");
+                    .expect("Inst::GetAttr executed, but there was no value on top of the stack.");
                 let value = obj.get(&name);
                 if let Some(value) = value {
                     self.push_stack(value);