From 35783c3767d8aa4d2719f686a098bf7782c8f710 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 11 Jun 2025 15:25:40 -0400
Subject: [PATCH] complete revert

---
 .../svelte/src/internal/client/dev/tracing.js | 17 +++++++++++
 .../src/internal/client/dom/blocks/each.js    | 10 +++++++
 .../src/internal/client/reactivity/sources.js |  1 +
 .../src/internal/client/reactivity/types.d.ts |  2 ++
 .../svelte/src/internal/client/runtime.js     | 29 +++++++++++--------
 5 files changed, 47 insertions(+), 12 deletions(-)

diff --git a/packages/svelte/src/internal/client/dev/tracing.js b/packages/svelte/src/internal/client/dev/tracing.js
index 05a4ac3806..65c6379625 100644
--- a/packages/svelte/src/internal/client/dev/tracing.js
+++ b/packages/svelte/src/internal/client/dev/tracing.js
@@ -26,6 +26,23 @@ function log_entry(signal, entry) {
 		return;
 	}
 
+	if (signal.trace) {
+		var previous_captured_signals = captured_signals;
+		var captured = new Set();
+		set_captured_signals(captured);
+
+		try {
+			untrack(signal.trace);
+		} finally {
+			set_captured_signals(previous_captured_signals);
+		}
+
+		if (captured.size > 0) {
+			for (const dep of captured) log_entry(dep);
+			return;
+		}
+	}
+
 	const type = (signal.f & DERIVED) !== 0 ? '$derived' : '$state';
 	const current_reaction = /** @type {Reaction} */ (active_reaction);
 	const dirty = signal.wv > current_reaction.wv || current_reaction.wv === 0;
diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index e8f00546d0..954dcb2214 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -524,6 +524,16 @@ function create_item(
 	var v = reactive ? (mutable ? mutable_source(value) : source(value)) : value;
 	var i = (flags & EACH_INDEX_REACTIVE) === 0 ? index : source(index);
 
+	if (DEV && reactive) {
+		// For tracing purposes, we need to link the source signal we create with the
+		// collection + index so that tracing works as intended
+		/** @type {Value} */ (v).trace = () => {
+			var collection_index = typeof i === 'number' ? index : i.v;
+			// eslint-disable-next-line @typescript-eslint/no-unused-expressions
+			get_collection()[collection_index];
+		};
+	}
+
 	/** @type {EachItem} */
 	var item = {
 		i,
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 4c04c37a67..40a3e4e77f 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -70,6 +70,7 @@ export function source(v, stack) {
 		signal.created = stack ?? get_stack('CreatedAt');
 		signal.updated = null;
 		signal.set_during_effect = false;
+		signal.trace = null;
 	}
 
 	return signal;
diff --git a/packages/svelte/src/internal/client/reactivity/types.d.ts b/packages/svelte/src/internal/client/reactivity/types.d.ts
index f2e7576720..756bb98f09 100644
--- a/packages/svelte/src/internal/client/reactivity/types.d.ts
+++ b/packages/svelte/src/internal/client/reactivity/types.d.ts
@@ -29,6 +29,8 @@ export interface Value<V = unknown> extends Signal {
 	 * increment the write version so that it shows up as dirty when the effect re-runs
 	 */
 	set_during_effect?: boolean;
+	/** A function that retrieves the underlying source, used for each block item signals */
+	trace?: null | (() => void);
 }
 
 export interface Reaction extends Signal {
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index b7a2fc09a5..c06705ce24 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -773,22 +773,27 @@ export function get(signal) {
 		active_reaction !== null &&
 		tracing_expressions.reaction === active_reaction
 	) {
-		var trace = get_stack('TracedAt');
+		// Used when mapping state between special blocks like `each`
+		if (signal.trace) {
+			signal.trace();
+		} else {
+			var trace = get_stack('TracedAt');
 
-		if (trace) {
-			var entry = tracing_expressions.entries.get(signal);
+			if (trace) {
+				var entry = tracing_expressions.entries.get(signal);
 
-			if (entry === undefined) {
-				entry = { traces: [] };
-				tracing_expressions.entries.set(signal, entry);
-			}
+				if (entry === undefined) {
+					entry = { traces: [] };
+					tracing_expressions.entries.set(signal, entry);
+				}
 
-			var last = entry.traces[entry.traces.length - 1];
+				var last = entry.traces[entry.traces.length - 1];
 
-			// traces can be duplicated, e.g. by `snapshot` invoking both
-			// both `getOwnPropertyDescriptor` and `get` traps at once
-			if (trace.stack !== last?.stack) {
-				entry.traces.push(trace);
+				// traces can be duplicated, e.g. by `snapshot` invoking both
+				// both `getOwnPropertyDescriptor` and `get` traps at once
+				if (trace.stack !== last?.stack) {
+					entry.traces.push(trace);
+				}
 			}
 		}
 	}