From 90baec16e68879a2eb1d898f706a34234bae4101 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 6 Jun 2025 19:12:48 -0400
Subject: [PATCH] WIP

---
 .../phases/2-analyze/visitors/AssignmentExpression.js      | 4 ++++
 .../compiler/phases/2-analyze/visitors/MemberExpression.js | 5 +++--
 .../compiler/phases/2-analyze/visitors/UpdateExpression.js | 4 ++++
 .../phases/3-transform/client/visitors/shared/utils.js     | 7 ++++++-
 packages/svelte/src/compiler/phases/nodes.js               | 4 +++-
 packages/svelte/src/compiler/types/index.d.ts              | 4 ++++
 6 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js
index 673c79f2df..39358f72fc 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js
@@ -23,5 +23,9 @@ export function AssignmentExpression(node, context) {
 		}
 	}
 
+	if (context.state.expression) {
+		context.state.expression.has_assignment = true;
+	}
+
 	context.next();
 }
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/MemberExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/MemberExpression.js
index 245a164c71..0a3b386198 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/MemberExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/MemberExpression.js
@@ -15,8 +15,9 @@ export function MemberExpression(node, context) {
 		}
 	}
 
-	if (context.state.expression && !is_pure(node, context)) {
-		context.state.expression.has_state = true;
+	if (context.state.expression) {
+		context.state.expression.has_member_expression = true;
+		context.state.expression.has_state ||= !is_pure(node, context);
 	}
 
 	if (!is_safe_identifier(node, context.state.scope)) {
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js
index 13f4b9019e..ed48e026ac 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js
@@ -21,5 +21,9 @@ export function UpdateExpression(node, context) {
 		}
 	}
 
+	if (context.state.expression) {
+		context.state.expression.has_assignment = true;
+	}
+
 	context.next();
 }
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
index cce6f15396..3b11341712 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
@@ -442,6 +442,12 @@ function is_pure_expression(expression) {
  * @param {ExpressionMetadata} metadata
  */
 export function build_legacy_expression_2(context, expression, metadata) {
+	const value = /** @type {Expression} */ (context.visit(expression));
+
+	if (!metadata.has_call && !metadata.has_member_expression && !metadata.has_assignment) {
+		return value;
+	}
+
 	const sequence = b.sequence([]);
 
 	for (const binding of metadata.dependencies) {
@@ -460,7 +466,6 @@ export function build_legacy_expression_2(context, expression, metadata) {
 		sequence.expressions.push(getter);
 	}
 
-	const value = /** @type {Expression} */ (context.visit(expression));
 	sequence.expressions.push(b.call('$.untrack', b.thunk(value)));
 
 	return sequence;
diff --git a/packages/svelte/src/compiler/phases/nodes.js b/packages/svelte/src/compiler/phases/nodes.js
index 2043747ed0..954f232424 100644
--- a/packages/svelte/src/compiler/phases/nodes.js
+++ b/packages/svelte/src/compiler/phases/nodes.js
@@ -63,7 +63,9 @@ export function create_expression_metadata() {
 	return {
 		dependencies: new Set(),
 		has_state: false,
-		has_call: false
+		has_call: false,
+		has_member_expression: false,
+		has_assignment: false
 	};
 }
 
diff --git a/packages/svelte/src/compiler/types/index.d.ts b/packages/svelte/src/compiler/types/index.d.ts
index fdd6024726..5bcc99e147 100644
--- a/packages/svelte/src/compiler/types/index.d.ts
+++ b/packages/svelte/src/compiler/types/index.d.ts
@@ -285,6 +285,10 @@ export interface ExpressionMetadata {
 	has_state: boolean;
 	/** True if the expression involves a call expression (often, it will need to be wrapped in a derived) */
 	has_call: boolean;
+	/** True if the expression includes a member expression */
+	has_member_expression: boolean;
+	/** True if the expression includes an assignment or an update */
+	has_assignment: boolean;
 }
 
 export interface StateField {