From 9ec2e28b6a35026f3e77cb8e41a488f3851a53d0 Mon Sep 17 00:00:00 2001
From: nyash-codex <codex@nyash.local>
Date: Wed, 17 Dec 2025 22:32:55 +0900
Subject: [PATCH] feat(joinir): add Pattern2 post-loop early return step

---
 .../pattern2_lowering_orchestrator.rs         |  5 +-
 .../joinir/patterns/pattern2_steps/mod.rs     |  2 +-
 .../post_loop_early_return_step_box.rs        | 52 +++++++++++++++++++
 3 files changed, 57 insertions(+), 2 deletions(-)
 create mode 100644 src/mir/builder/control_flow/joinir/patterns/pattern2_steps/post_loop_early_return_step_box.rs

diff --git a/src/mir/builder/control_flow/joinir/patterns/pattern2_lowering_orchestrator.rs b/src/mir/builder/control_flow/joinir/patterns/pattern2_lowering_orchestrator.rs
index e41d0a19..fc097f9f 100644
--- a/src/mir/builder/control_flow/joinir/patterns/pattern2_lowering_orchestrator.rs
+++ b/src/mir/builder/control_flow/joinir/patterns/pattern2_lowering_orchestrator.rs
@@ -11,6 +11,7 @@ use super::pattern2_steps::apply_policy_step_box::ApplyPolicyStepBox;
 use super::pattern2_steps::emit_joinir_step_box::EmitJoinIRStepBox;
 use super::pattern2_steps::gather_facts_step_box::GatherFactsStepBox;
 use super::pattern2_steps::merge_step_box::MergeStepBox;
+use super::pattern2_steps::post_loop_early_return_step_box::PostLoopEarlyReturnStepBox;
 use super::pattern2_steps::promote_step_box::PromoteStepBox;
 
 pub(crate) struct Pattern2LoweringOrchestrator;
@@ -54,6 +55,8 @@ impl Pattern2LoweringOrchestrator {
             skeleton,
         )?;
 
-        MergeStepBox::merge(builder, emitted.join_module, emitted.boundary, debug)
+        let out = MergeStepBox::merge(builder, emitted.join_module, emitted.boundary, debug)?;
+        PostLoopEarlyReturnStepBox::maybe_emit(builder, promoted.inputs.post_loop_early_return.as_ref())?;
+        Ok(out)
     }
 }
diff --git a/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/mod.rs b/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/mod.rs
index f91d95eb..4b6cce6d 100644
--- a/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/mod.rs
+++ b/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/mod.rs
@@ -10,5 +10,5 @@ pub(crate) mod apply_policy_step_box;
 pub(crate) mod emit_joinir_step_box;
 pub(crate) mod gather_facts_step_box;
 pub(crate) mod merge_step_box;
+pub(crate) mod post_loop_early_return_step_box;
 pub(crate) mod promote_step_box;
-
diff --git a/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/post_loop_early_return_step_box.rs b/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/post_loop_early_return_step_box.rs
new file mode 100644
index 00000000..30ff174d
--- /dev/null
+++ b/src/mir/builder/control_flow/joinir/patterns/pattern2_steps/post_loop_early_return_step_box.rs
@@ -0,0 +1,52 @@
+//! PostLoopEarlyReturnStepBox (Phase 107)
+//!
+//! Responsibility: emulate `return i` inside the loop by emitting a post-loop
+//! early-return guard. This keeps JoinIR lowering "break-only" while preserving
+//! source semantics for the recognized family.
+
+use crate::ast::{ASTNode, BinaryOperator, Span};
+use crate::mir::builder::MirBuilder;
+
+pub(crate) struct PostLoopEarlyReturnStepBox;
+
+impl PostLoopEarlyReturnStepBox {
+    pub(crate) fn maybe_emit(
+        builder: &mut MirBuilder,
+        plan: Option<
+            &crate::mir::builder::control_flow::joinir::patterns::policies::balanced_depth_scan_policy::PostLoopEarlyReturnPlan,
+        >,
+    ) -> Result<(), String> {
+        let Some(plan) = plan else { return Ok(()); };
+
+        // if i < n { return i }
+        let cond = ASTNode::BinaryOp {
+            operator: BinaryOperator::Less,
+            left: Box::new(ASTNode::Variable {
+                name: plan.loop_counter_name.clone(),
+                span: Span::unknown(),
+            }),
+            right: Box::new(ASTNode::Variable {
+                name: plan.bound_name.clone(),
+                span: Span::unknown(),
+            }),
+            span: Span::unknown(),
+        };
+
+        let ret_stmt = ASTNode::Return {
+            value: Some(Box::new(ASTNode::Variable {
+                name: plan.loop_counter_name.clone(),
+                span: Span::unknown(),
+            })),
+            span: Span::unknown(),
+        };
+
+        builder.build_statement(ASTNode::If {
+            condition: Box::new(cond),
+            then_body: vec![ret_stmt],
+            else_body: None,
+            span: Span::unknown(),
+        })?;
+
+        Ok(())
+    }
+}