Spread a string into its code points array (#2389)

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);

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": []
+}

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("-"));

tests/harness/corpus-costs/string-spread.ts.json

@@ -0,0 +1 @@
+{ "plain": 4500, "san": 4300 }