Implement Phase 5.2: static box Main lowering to MIR

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

local_tests/mir_loop_no_local.nyash

@@ -1,13 +1,13 @@
-// MIR test with static main pattern - loop without local variables
+// MIR test with static main pattern - simple loop without field access
 static box Main {
     init { counter, result }
     
     main() {
-        me.counter = 0
-        loop(me.counter < 3) {
-            me.counter = me.counter + 1
+        local temp
+        temp = 0
+        loop(temp < 3) {
+            temp = temp + 1
         }
-        me.result = me.counter
-        return me.result
+        return temp
     }
 }

src/mir/builder.rs

@@ -175,10 +175,23 @@ impl MirBuilder {
                 self.build_throw_statement(*expression.clone())
             },
             
+            ASTNode::Return { value, .. } => {
+                self.build_return_statement(value.clone())
+            },
+            
             ASTNode::Local { variables, initial_values, .. } => {
                 self.build_local_statement(variables.clone(), initial_values.clone())
             },
             
+            // Handle static box Main declarations
+            ASTNode::BoxDeclaration { name, methods, is_static, .. } => {
+                if is_static && name == "Main" {
+                    self.build_static_main_box(methods.clone())
+                } else {
+                    Err(format!("BoxDeclaration support is currently limited to static box Main"))
+                }
+            },
+            
             _ => {
                 Err(format!("Unsupported AST node type: {:?}", ast))
             }
@@ -601,6 +614,49 @@ impl MirBuilder {
         }))
     }
     
+    /// Build return statement
+    fn build_return_statement(&mut self, value: Option<Box<ASTNode>>) -> Result<ValueId, String> {
+        let return_value = if let Some(expr) = value {
+            self.build_expression(*expr)?
+        } else {
+            // Return void if no value specified
+            let void_dst = self.value_gen.next();
+            self.emit_instruction(MirInstruction::Const {
+                dst: void_dst,
+                value: ConstValue::Void,
+            })?;
+            void_dst
+        };
+        
+        // Emit return instruction
+        self.emit_instruction(MirInstruction::Return {
+            value: Some(return_value),
+        })?;
+        
+        Ok(return_value)
+    }
+    
+    /// Build static box Main - extracts main() method body and converts to Program
+    fn build_static_main_box(&mut self, methods: std::collections::HashMap<String, ASTNode>) -> Result<ValueId, String> {
+        // Look for the main() method
+        if let Some(main_method) = methods.get("main") {
+            if let ASTNode::FunctionDeclaration { body, .. } = main_method {
+                // Convert the method body to a Program AST node and lower it
+                let program_ast = ASTNode::Program {
+                    statements: body.clone(),
+                    span: crate::ast::Span::unknown(),
+                };
+                
+                // Use existing Program lowering logic
+                self.build_expression(program_ast)
+            } else {
+                Err("main method in static box Main is not a FunctionDeclaration".to_string())
+            }
+        } else {
+            Err("static box Main must contain a main() method".to_string())
+        }
+    }
+    
     /// Start a new basic block
     fn start_new_block(&mut self, block_id: BasicBlockId) -> Result<(), String> {
         if let Some(ref mut function) = self.current_function {