Add JMH benchmark for parser structure formatting

Author: DmitryNekrasov

benchmarks/src/jmh/kotlin/ParserStructureConcatBenchmark.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+@file:Suppress("unused")
+
+package kotlinx.datetime
+
+import kotlinx.datetime.format.char
+import kotlinx.datetime.format.optional
+import org.openjdk.jmh.annotations.*
+import org.openjdk.jmh.infra.Blackhole
+import java.util.concurrent.*
+
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+@Fork(1)
+open class ParserStructureConcatBenchmark {
+
+    @Benchmark
+    fun buildFormat(blackhole: Blackhole) {
+        val v = LocalDateTime.Format {
+            year()
+            char('-')
+            monthNumber()
+            char('-')
+            day()
+            char(' ')
+            hour()
+            char(':')
+            minute()
+            optional {
+                char(':')
+                second()
+                optional {
+                    char('.')
+                    secondFraction()
+                }
+            }
+        }
+        blackhole.consume(v)
+    }
+}