Improve StringBuilder UTF-16 blit reuse and reset safety

builtin/stringbuilder_buffer.mbt

@@ -62,15 +62,26 @@ fn StringBuilder::grow_if_necessary(
   self.data = new_data
 }
 
+///|
+fn FixedArray::unsafe_blit_from_string(
+  self : FixedArray[UInt16],
+  dst_offset : Int,
+  str : String,
+  str_offset : Int,
+  len : Int,
+) -> Unit {
+  let end_str_offset = str_offset + len
+  for i = str_offset, j = dst_offset; i < end_str_offset; i = i + 1, j = j + 1 {
+    self[j] = str.unsafe_get(i)
+  }
+}
+
 ///|
 /// Writes a string to the StringBuilder.
 pub impl Logger for StringBuilder with write_string(self, str) {
   let str_len = str.length()
   self.grow_if_necessary(self.len + str_len)
-  // TODO: add an intrinsic to copy UTF-16 code units from String directly.
-  for i in 0..<str_len {
-    self.data[self.len + i] = str.unsafe_get(i)
-  }
+  self.data.unsafe_blit_from_string(self.len, str, 0, str_len)
   self.len += str_len
 }
 
@@ -118,10 +129,12 @@ pub impl Logger for StringBuilder with write_view(
 ) -> Unit {
   let str_len = str.length()
   self.grow_if_necessary(self.len + str_len)
-  // TODO: add an intrinsic to copy UTF-16 code units from StringView directly.
-  for i in 0..<str_len {
-    self.data[self.len + i] = str.unsafe_get(i)
-  }
+  self.data.unsafe_blit_from_string(
+    self.len,
+    str.data(),
+    str.start_offset(),
+    str_len,
+  )
   self.len += str_len
 }
 
@@ -146,5 +159,6 @@ pub impl Show for StringBuilder with output(self, logger) {
 ///|
 /// Resets the string builder to an empty state.
 pub fn StringBuilder::reset(self : StringBuilder) -> Unit {
+  self.data = FixedArray::make(self.data.length(), UInt16::default())
   self.len = 0
 }

builtin/stringbuilder_test.mbt

@@ -36,6 +36,17 @@ test "is_empty method" {
   inspect(buf.is_empty(), content="true")
 }
 
+///|
+test "reset does not mutate returned string" {
+  let buf = StringBuilder::new(size_hint=10)
+  buf.write_string("hello")
+  let first = buf.to_string()
+  buf.reset()
+  buf.write_string("bye")
+  inspect(first, content="hello")
+  inspect(buf.to_string(), content="bye")
+}
+
 ///|
 test {
   let data = ["a", "b", "c", "hello world"]