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

src/mir/builder/ssa/local.rs

@@ -37,6 +37,31 @@ pub fn ensure(builder: &mut MirBuilder, v: ValueId, kind: LocalKind) -> ValueId
         if let Some(&loc) = builder.local_ssa_map.get(&key) {
             return loc;
         }
+
+        // CRITICAL FIX: If `v` is from a pinned slot, check if there's a PHI value for that slot
+        // in the current block's variable_map. If so, use the PHI value directly instead of
+        // emitting a Copy from the old value (which might not be defined in this block).
+        let names_for_v: Vec<String> = builder.variable_map.iter()
+            .filter(|(k, &vid)| vid == v && k.starts_with("__pin$"))
+            .map(|(k, _)| k.clone())
+            .collect();
+
+        if let Some(first_pin_name) = names_for_v.first() {
+            // This value is from a pinned slot. Check if the slot has been updated
+            // (e.g., by a PHI) in the current block.
+            if let Some(&current_val) = builder.variable_map.get(first_pin_name) {
+                if current_val != v {
+                    // The slot has been updated (likely by a PHI). Use the updated value.
+                    if std::env::var("NYASH_LOCAL_SSA_TRACE").ok().as_deref() == Some("1") {
+                        eprintln!("[local-ssa] phi-redirect bb={:?} kind={:?} slot={} %{} -> %{}",
+                            bb, kind, first_pin_name, v.0, current_val.0);
+                    }
+                    builder.local_ssa_map.insert(key, current_val);
+                    return current_val;
+                }
+            }
+        }
+
         let loc = builder.next_value_id();
         // Best-effort: errors are propagated by caller; we ignore here to keep helper infallible
         let _ = builder.emit_instruction(crate::mir::MirInstruction::Copy { dst: loc, src: v });