loopform(hints): detect up to 2 assigned vars in loop body (no break/continue) and emit LoopCarrier hint; add smoke for two-vars case

src/mir/builder.rs

@@ -182,6 +182,10 @@ impl MirBuilder {
     pub(crate) fn hint_scope_leave(&mut self, id: u32) { self.hint_sink.scope_leave(id); }
     #[inline]
     pub(crate) fn hint_join_result<S: Into<String>>(&mut self, var: S) { self.hint_sink.join_result(var.into()); }
+    #[inline]
+    pub(crate) fn hint_loop_carrier<S: Into<String>>(&mut self, vars: impl IntoIterator<Item = S>) {
+        self.hint_sink.loop_carrier(vars.into_iter().map(|s| s.into()).collect::<Vec<_>>());
+    }
 
     // moved to builder_calls.rs: lower_method_as_function
 

src/mir/loop_builder.rs

@@ -143,6 +143,39 @@ impl<'a> LoopBuilder<'a> {
         condition: ASTNode,
         body: Vec<ASTNode>,
     ) -> Result<ValueId, String> {
+        // Pre-scan body for simple carrier pattern (up to 2 assigned variables, no break/continue)
+        fn collect_assigns(n: &ASTNode, vars: &mut Vec<String>, has_ctrl: &mut bool) {
+            match n {
+                ASTNode::Assignment { target, .. } => {
+                    if let ASTNode::Variable { name, .. } = target.as_ref() {
+                        if !vars.iter().any(|v| v == name) {
+                            vars.push(name.clone());
+                        }
+                    }
+                }
+                ASTNode::Break { .. } | ASTNode::Continue { .. } => { *has_ctrl = true; }
+                ASTNode::If { then_body, else_body, .. } => {
+                    let tp = ASTNode::Program { statements: then_body.clone(), span: crate::ast::Span::unknown() };
+                    collect_assigns(&tp, vars, has_ctrl);
+                    if let Some(eb) = else_body {
+                        let ep = ASTNode::Program { statements: eb.clone(), span: crate::ast::Span::unknown() };
+                        collect_assigns(&ep, vars, has_ctrl);
+                    }
+                }
+                ASTNode::Program { statements, .. } => {
+                    for s in statements { collect_assigns(s, vars, has_ctrl); }
+                }
+                _ => {}
+            }
+        }
+        let mut assigned_vars: Vec<String> = Vec::new();
+        let mut has_ctrl = false;
+        for st in &body { collect_assigns(st, &mut assigned_vars, &mut has_ctrl); }
+        if !has_ctrl && !assigned_vars.is_empty() && assigned_vars.len() <= 2 {
+            // Emit a carrier hint (no-op sink by default; visible with NYASH_MIR_TRACE_HINTS=1)
+            self.parent_builder.hint_loop_carrier(assigned_vars.clone());
+        }
+
         // 1. ブロックの準備
         let preheader_id = self.current_block()?;
         let (header_id, body_id, after_loop_id) =

tools/test/smoke/mir/hints_loop_carrier_two_vars_smoke.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root=$(cd "$(dirname "$0")"/../../../.. && pwd)
+bin="$root/target/release/nyash"
+src="apps/tests/macro/loopform/two_vars.nyash"
+
+if [ ! -x "$bin" ]; then
+  echo "nyash binary not found at $bin; build first (cargo build --release)" >&2
+  exit 1
+fi
+
+export NYASH_MIR_TRACE_HINTS=1
+
+out=$({ "$bin" --backend vm "$src" 1>/dev/null; } 2>&1 || true)
+
+# Check the LoopCarrier hint contains both variable names (order-agnostic)
+echo "$out" | rg -q "\[mir\]\[hint\] LoopCarrier\((i,sum|sum,i)\)" || {
+  echo "[FAIL] missing LoopCarrier(i,sum) hint" >&2
+  printf '%s\n' "$out" | tail -n 80 >&2
+  exit 2
+}
+
+echo "[OK] MIR hints LoopCarrier(two vars) trace smoke passed"
+