Phase 25.1b: VM undefined-value diagnostics and builder SSA helpers

src/mir/builder.rs

@@ -451,11 +451,33 @@ impl MirBuilder {
                 _ => (None, None, None),
             };
 
+            // Extract function name before mutable borrow to avoid borrowck error
+            let current_fn_name = function.signature.name.clone();
+
             if let Some(block) = function.get_block_mut(block_id) {
                 // Invariant: Call must always carry a Callee (unified path).
                 if let MirInstruction::Call { callee, .. } = &instruction {
                     if callee.is_none() {
                         return Err("builder invariant violated: MirInstruction::Call.callee must be Some (unified call)".into());
+                    } else if std::env::var("NYASH_BUILDER_TRACE_RECV").ok().as_deref() == Some("1") {
+                        use crate::mir::definitions::call_unified::Callee;
+                        if let Some(Callee::Method { box_name, method, receiver: Some(r), .. }) = callee {
+                            let names: Vec<String> = self
+                                .variable_map
+                                .iter()
+                                .filter(|(_, &vid)| vid == *r)
+                                .map(|(k, _)| k.clone())
+                                .collect();
+                            eprintln!(
+                                "[builder/recv-trace] fn={} bb={:?} method={}.{} recv=%{} aliases={:?}",
+                                current_fn_name,
+                                self.current_block,
+                                box_name,
+                                method,
+                                r.0,
+                                names
+                            );
+                        }
                     }
                 }
                 if utils::builder_debug_enabled() {