perf(stringbuilder): reuse unshared reset buffer

Author: mizchi

builtin/stringbuilder_buffer.mbt

@@ -16,6 +16,7 @@
 struct StringBuilder {
   mut data : FixedArray[UInt16]
   mut len : Int
+  mut buffer_is_shared : Bool
 }
 
 ///|
@@ -33,7 +34,7 @@ struct StringBuilder {
 pub fn StringBuilder::StringBuilder(size_hint? : Int = 0) -> StringBuilder {
   let initial = if size_hint < 1 { 1 } else { (size_hint + 1) / 2 }
   let data : FixedArray[UInt16] = FixedArray::make(initial, 0)
-  { data, len: 0 }
+  { data, len: 0, buffer_is_shared: false }
 }
 
 ///|
@@ -84,6 +85,7 @@ fn StringBuilder::grow(self : StringBuilder, required : Int) -> Unit {
     len=self.len,
   )
   self.data = new_data
+  self.buffer_is_shared = false
 }
 
 ///|
@@ -184,6 +186,9 @@ pub fn StringBuilder::to_string(self : StringBuilder) -> String {
   if self.len == 0 {
     ""
   } else if self.len == self.data.length() {
+    // This conversion can reuse `data` without copying, so future resets must
+    // detach before the buffer is written again.
+    self.buffer_is_shared = true
     unsafe_fixedarray_uint16_to_string(self.data)
   } else {
     let data = FixedArray::make_and_blit(
@@ -210,9 +215,14 @@ pub impl Show for StringBuilder with fn to_string(self) {
 ///|
 /// Resets the string builder to an empty state.
 pub fn StringBuilder::reset(self : StringBuilder) -> Unit {
-  self.data = FixedArray::make(
-    self.data.length(),
-    (Default::default() : UInt16),
-  )
+  // A full buffer may have been returned directly from `to_string`. Retain the
+  // backing storage in every other case, since it only contains value types.
+  if self.buffer_is_shared {
+    self.data = FixedArray::make(
+      self.data.length(),
+      (Default::default() : UInt16),
+    )
+    self.buffer_is_shared = false
+  }
   self.len = 0
 }

builtin/stringbuilder_wbtest.mbt

@@ -0,0 +1,28 @@
+// Copyright 2026 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+///|
+#cfg(not(target="js"))
+test "StringBuilder::reset reuses an unshared buffer" {
+  let builder = StringBuilder(size_hint=64)
+  let backing = builder.data
+  builder.write_string("hello")
+  let first = builder.to_string()
+  builder.reset()
+  builder.write_string("bye")
+  assert_true(first == "hello")
+  assert_true(backing[0] == ('b' : UInt16))
+  assert_true(backing[1] == ('y' : UInt16))
+  assert_true(backing[2] == ('e' : UInt16))
+}