pass varargs literaly in kps's toAnnotaitonSpec()

This works around an issue where OptIn annotations fail to compile using
the explicit array syntax

Fixes #1831

Author: evant

interop/ksp/src/main/kotlin/com/squareup/kotlinpoet/ksp/Annotations.kt

@@ -39,19 +39,35 @@ public fun KSAnnotation.toAnnotationSpec(omitDefaultValues: Boolean = false): An
     is ParameterizedTypeName -> AnnotationSpec.builder(type)
     else -> error("This is never possible.")
   }
+  val params = (annotationType.resolve().declaration as KSClassDeclaration).primaryConstructor?.parameters.orEmpty()
+    .associateBy { it.name }
   useSiteTarget?.let { builder.useSiteTarget(it.kpAnalog) }
   // TODO support type params once they're exposed https://github.com/google/ksp/issues/753
+  var varargValues: List<*>? = null
   for (argument in arguments) {
     val value = argument.value ?: continue
     val name = argument.name!!.getShortName()
+    val type = params[argument.name]
     if (omitDefaultValues) {
       val defaultValue = this.defaultArguments.firstOrNull { it.name?.asString() == name }?.value
       if (isDefaultValue(value, defaultValue)) { continue }
     }
-    val member = CodeBlock.builder()
-    member.add("%N = ", name)
-    addValueToBlock(value, member, omitDefaultValues)
-    builder.addMember(member.build())
+    if (type?.isVararg == true) {
+      // wait to add varargs to end
+      varargValues = value as List<*>
+    } else {
+      val member = CodeBlock.builder()
+      member.add("%N = ", name)
+      addValueToBlock(value, member, omitDefaultValues)
+      builder.addMember(member.build())
+    }
+    if (varargValues != null) {
+      for (item in varargValues) {
+        val member = CodeBlock.builder()
+        addValueToBlock(item!!, member, omitDefaultValues)
+        builder.addMember(member.build())
+      }
+    }
   }
   return builder.build()
 }

interop/ksp/test-processor/src/main/kotlin/com/squareup/kotlinpoet/ksp/test/processor/exampleAnnotations.kt

@@ -79,3 +79,5 @@ annotation class AnotherAnnotation(val input: String)
 enum class AnnotationEnumValue {
   ONE, TWO, THREE
 }
+
+annotation class AnnotationWithVararg(vararg val args: String)

interop/ksp/test-processor/src/test/kotlin/com/squareup/kotlinpoet/ksp/test/processor/TestProcessorTest.kt

@@ -620,6 +620,51 @@ class TestProcessorTest {
     )
   }
 
+  @Test
+  fun varargArgument() {
+    val compilation = prepareCompilation(
+      kotlin(
+        "Example.kt",
+        """
+           package test
+
+           import com.squareup.kotlinpoet.ksp.test.processor.AnnotationWithVararg
+           import com.squareup.kotlinpoet.ksp.test.processor.ExampleAnnotation
+
+           @RequiresOptIn
+           annotation class MyOptIn
+
+           @ExampleAnnotation
+           @OptIn(MyOptIn::class)
+           @AnnotationWithVararg("one", "two")
+           interface Example
+        """.trimIndent()
+      )
+    )
+
+    val result = compilation.compile()
+    assertThat(result.exitCode).isEqualTo(KotlinCompilation.ExitCode.OK)
+    val generatedFileText = File(compilation.kspSourcesDir, "kotlin/test/TestExample.kt")
+      .readText()
+
+    assertThat(generatedFileText).isEqualTo(
+      """
+      package test
+
+      import com.squareup.kotlinpoet.ksp.test.processor.AnnotationWithVararg
+      import kotlin.OptIn
+
+      @OptIn(MyOptIn::class)
+      @AnnotationWithVararg(
+        "one",
+        "two",
+      )
+      public class TestExample
+
+      """.trimIndent()
+    )
+  }
+
   @Test
   fun regression_1513() {
     val compilation = prepareCompilation(