From b532465ce293b01b0654fc509b4f3a322d2eab52 Mon Sep 17 00:00:00 2001
From: Mofei Zhang <mofei.zhang@utexas.edu>
Date: Tue, 28 Feb 2023 16:36:00 -0500
Subject: [PATCH] [inference] Capturing an immutable value should be a read

---

Following #1216:

If a value is known to be immutable, then it doesn't need to be considered
'captured' since no mutation should occur.

Couldn't figure out a unit test in which this specific fix matters, but we need
this to fix test output of #1273

cc. @gsathya, would love some feedback / eyes on this. This makes sense for
Primitives in particular (which are always read / copied in rval position), but
I'm not as familiar with edge cases for other immutable values especially around
lambdas.
---
 .../forget/src/Inference/InferReferenceEffects.ts   |  3 ++-
 .../hir/destructure-capture-global.expect.md        | 13 +++++--------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/compiler/forget/src/Inference/InferReferenceEffects.ts b/compiler/forget/src/Inference/InferReferenceEffects.ts
index fb379f1d2b..1f851f5530 100644
--- a/compiler/forget/src/Inference/InferReferenceEffects.ts
+++ b/compiler/forget/src/Inference/InferReferenceEffects.ts
@@ -308,7 +308,8 @@ class InferenceState {
       case Effect.Capture: {
         if (
           valueKind === ValueKind.Frozen ||
-          valueKind === ValueKind.MaybeFrozen
+          valueKind === ValueKind.MaybeFrozen ||
+          valueKind === ValueKind.Immutable
         ) {
           effect = Effect.Read;
         } else {
diff --git a/compiler/forget/src/__tests__/fixtures/hir/destructure-capture-global.expect.md b/compiler/forget/src/__tests__/fixtures/hir/destructure-capture-global.expect.md
index 499ebec08f..9f0d022a43 100644
--- a/compiler/forget/src/__tests__/fixtures/hir/destructure-capture-global.expect.md
+++ b/compiler/forget/src/__tests__/fixtures/hir/destructure-capture-global.expect.md
@@ -15,18 +15,15 @@ function component(a) {
 ```javascript
 let someGlobal = {};
 function component(a) {
-  const $ = React.unstable_useMemoCache(3);
-  const t0 = someGlobal;
+  const $ = React.unstable_useMemoCache(2);
   const c_0 = $[0] !== a;
-  const c_1 = $[1] !== t0;
   let x;
-  if (c_0 || c_1) {
-    x = { a: a, someGlobal: t0 };
+  if (c_0) {
+    x = { a: a, someGlobal: someGlobal };
     $[0] = a;
-    $[1] = t0;
-    $[2] = x;
+    $[1] = x;
   } else {
-    x = $[2];
+    x = $[1];
   }
   return x;
 }