fix(Stream): avoid mutable Channel.runFold accumulator in mkUint8Array

.changeset/fix-stream-mkuint8array-mutable-fold.md

@@ -0,0 +1,7 @@
+---
+"effect": patch
+---
+
+Fix `Stream.mkUint8Array` crash in Bun compiled+minified binaries.
+
+Reverted to the pre-beta.59 immutable `Channel.runFold` pattern where each fold iteration returns a new `Uint8Array` instead of mutating a shared accumulator. The beta.59 refactor introduced in-place mutation (`acc.bytes +=`, `acc.arrays.push`) which throws in Bun `--compile --minify` binaries.

packages/effect/src/Stream.ts

@@ -10339,31 +10339,20 @@ export const mkString = <E, R>(self: Stream<string, E, R>): Effect.Effect<string
  * @category Destructors
  */
 export const mkUint8Array = <E, R>(self: Stream<Uint8Array, E, R>): Effect.Effect<Uint8Array, E, R> =>
-  Effect.map(
-    Channel.runFold(
-      self.channel,
-      (): {
-        bytes: number
-        readonly arrays: Array<Uint8Array>
-      } => ({
-        bytes: 0,
-        arrays: []
-      }),
-      (acc, chunk) => {
-        for (let i = 0; i < chunk.length; i++) {
-          acc.bytes += chunk[i].length
-          acc.arrays.push(chunk[i])
-        }
-        return acc
+  Channel.runFold(
+    self.channel,
+    () => new Uint8Array(0),
+    (acc, chunk) => {
+      let chunkLength = 0
+      for (let i = 0; i < chunk.length; i++) {
+        chunkLength += chunk[i].length
       }
-    ),
-    ({ arrays, bytes }) => {
-      const result = new Uint8Array(bytes)
-      let offset = 0
-      for (let i = 0; i < arrays.length; i++) {
-        const array = arrays[i]
-        result.set(array, offset)
-        offset += array.length
+      const result = new Uint8Array(acc.length + chunkLength)
+      result.set(acc, 0)
+      let offset = acc.length
+      for (let i = 0; i < chunk.length; i++) {
+        result.set(chunk[i], offset)
+        offset += chunk[i].length
       }
       return result
     }

packages/effect/test/Stream.test.ts

@@ -205,6 +205,29 @@ describe("Stream", () => {
       }))
   })
 
+  describe("mkUint8Array", () => {
+    it.effect("concatenates chunks into a single Uint8Array", () =>
+      Effect.gen(function*() {
+        const result = yield* Stream.mkUint8Array(
+          Stream.make(new Uint8Array([1, 2]), new Uint8Array([3, 4]))
+        )
+        assert.deepStrictEqual(result, new Uint8Array([1, 2, 3, 4]))
+      }))
+
+    it.effect("works with many chunks (regression: mutable accumulator in Bun compiled binaries)", () =>
+      Effect.gen(function*() {
+        const chunks = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9].map((i) => new Uint8Array([i]))
+        const result = yield* Stream.mkUint8Array(Stream.fromIterable(chunks))
+        assert.deepStrictEqual(result, new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]))
+      }))
+
+    it.effect("handles empty stream", () =>
+      Effect.gen(function*() {
+        const result = yield* Stream.mkUint8Array(Stream.empty)
+        assert.deepStrictEqual(result, new Uint8Array([]))
+      }))
+  })
+
   describe("encoding", () => {
     it.effect("decodeText handles multi-byte characters split across chunks", () =>
       Effect.gen(function*() {