From 4b8ed48a8a05aa4df5a1a89188ef6698caea0c8c Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Tue, 27 Oct 2026 01:19:32 -0500
Subject: [PATCH] Spread a string into its code points array

`[...text]` lowered to an array literal that spread the array of text's
code points: the literal's array copied the code point strings, retaining
each one, and the code point array then released them. An array literal
whose only element is a spread of a string is now the code points array
itself, as `Array.from(text)` already was. Both are fresh arrays.
---
 .../lowering/expressions/array-literals.ts    | 11 ++++++++
 .../tests/corpus/string-spread.ts.json        |  6 +++++
 tests/corpus/string-spread.ts                 | 25 +++++++++++++++++++
 .../corpus-costs/string-spread.ts.json        |  1 +
 4 files changed, 43 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/string-spread.ts.json
 create mode 100644 tests/corpus/string-spread.ts
 create mode 100644 tests/harness/corpus-costs/string-spread.ts.json

diff --git a/packages/compiler/src/frontend/lowering/expressions/array-literals.ts b/packages/compiler/src/frontend/lowering/expressions/array-literals.ts
index 5854a0ae8..e3088f4a2 100644
--- a/packages/compiler/src/frontend/lowering/expressions/array-literals.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/array-literals.ts
@@ -652,6 +652,17 @@ export function lowerArrayLiteral(
     return lowered;
   });
   if (!needsStateBuild) {
+    // `[...text]`: the code points call already builds a fresh array of the
+    // literal's type, so it is the literal's value, as `Array.from(text)` is.
+    const only = elems[0];
+    if (
+      elems.length === 1 &&
+      spreads.length === 1 &&
+      only?.kind === "libCall" &&
+      only.fn === "str.codePoints" &&
+      typeEquals(only.type, type)
+    )
+      return { ...only, loc };
     return { kind: "arrayLit", elems, ...(spreads.length > 0 ? { spreads } : {}), type, loc };
   }
   const out = lowerer.declareHiddenLocal("%arrayLit", type);
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/string-spread.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/string-spread.ts.json
new file mode 100644
index 000000000..2c5cc69e8
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/string-spread.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/string-spread.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/string-spread.ts b/tests/corpus/string-spread.ts
new file mode 100644
index 000000000..6813806f4
--- /dev/null
+++ b/tests/corpus/string-spread.ts
@@ -0,0 +1,25 @@
+// Spreading strings into arrays: code points of each UTF-8 length, astral
+// pairs, lone surrogates, the empty string, and the fresh array each spread
+// returns.
+
+const samples = ["", "a", "abc", "é", "café", "日本語", "😀", "a😀b", "🚀🚀", "\ud800", "a\udc00b", "xé日😀"];
+
+for (const s of samples) {
+  const chars = [...s];
+  console.log(JSON.stringify(s), chars.length, JSON.stringify(chars), chars.join("|"));
+}
+
+// Each spread is a new array: changing one leaves the others and the string.
+const text = "héllo 😀";
+const first = [...text];
+const second = [...text];
+first.reverse();
+first.push("!");
+console.log(first.join(""), second.join(""), first === second, text);
+
+// The spread result is an ordinary array.
+const letters = [..."naïve"];
+letters.sort();
+console.log(letters, letters.length, Array.isArray(letters));
+console.log([..."abc"].map((c) => c.toUpperCase()).join(""));
+console.log([..."日本"].concat([..."語"]).join("-"));
diff --git a/tests/harness/corpus-costs/string-spread.ts.json b/tests/harness/corpus-costs/string-spread.ts.json
new file mode 100644
index 000000000..d7fbd510a
--- /dev/null
+++ b/tests/harness/corpus-costs/string-spread.ts.json
@@ -0,0 +1 @@
+{ "plain": 4500, "san": 4300 }