Test string edge cases for Kotlin and Swift (#1602)

* Test string edge cases for Kotlin and Swift

* Kotlin: Throw `CharacterCodingException` on invalid UTF-16 `String`

This exception is produced using
`CharsetEncoder.onUnmappableCharacter(CodingErrorAction.REPORT)`.

fixtures/type-limits/tests/bindings/test_type_limits.kts

@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import uniffi.uniffi_type_limits.*;
+
+// test_strings
+try {
+    takeString("\ud800")
+    throw RuntimeException("Should have thrown an CharacterCodingException exception!")
+} catch (e: java.nio.charset.CharacterCodingException) {
+    // It's okay!
+}
+assert(takeString("") == "")
+assert(takeString("愛") == "愛")
+assert(takeString("💖") == "💖")

fixtures/type-limits/tests/bindings/test_type_limits.swift

@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import Foundation
+import uniffi_type_limits
+
+// test_strings
+do {
+    // strings cannot contain surrogates, "\u{d800}" gives an error.
+    assert(takeString(v: "") == "")
+    assert(takeString(v: "愛") == "愛")
+    assert(takeString(v: "💖") == "💖")
+}

fixtures/type-limits/tests/test_generated_bindings.rs

@@ -1,4 +1,6 @@
 uniffi::build_foreign_language_testcases!(
+    "tests/bindings/test_type_limits.kts",
     "tests/bindings/test_type_limits.py",
     "tests/bindings/test_type_limits.rb",
+    "tests/bindings/test_type_limits.swift",
 );

uniffi_bindgen/src/bindings/kotlin/templates/StringHelper.kt

@@ -19,17 +19,25 @@ public object FfiConverterString: FfiConverter<String, RustBuffer.ByValue> {
         return byteArr.toString(Charsets.UTF_8)
     }
 
+    fun toUtf8(value: String): ByteBuffer {
+        // Make sure we don't have invalid UTF-16, check for lone surrogates.
+        return Charsets.UTF_8.newEncoder().run {
+            onMalformedInput(CodingErrorAction.REPORT)
+            encode(CharBuffer.wrap(value))
+        }
+    }
+
     override fun lower(value: String): RustBuffer.ByValue {
-        val byteArr = value.toByteArray(Charsets.UTF_8)
+        val byteBuf = toUtf8(value)
         // Ideally we'd pass these bytes to `ffi_bytebuffer_from_bytes`, but doing so would require us
         // to copy them into a JNA `Memory`. So we might as well directly copy them into a `RustBuffer`.
-        val rbuf = RustBuffer.alloc(byteArr.size)
-        rbuf.asByteBuffer()!!.put(byteArr)
+        val rbuf = RustBuffer.alloc(byteBuf.limit())
+        rbuf.asByteBuffer()!!.put(byteBuf)
         return rbuf
     }
 
     // We aren't sure exactly how many bytes our string will be once it's UTF-8
-    // encoded.  Allocate 3 bytes per unicode codepoint which will always be
+    // encoded.  Allocate 3 bytes per UTF-16 code unit which will always be
     // enough.
     override fun allocationSize(value: String): Int {
         val sizeForLength = 4
@@ -38,8 +46,8 @@ public object FfiConverterString: FfiConverter<String, RustBuffer.ByValue> {
     }
 
     override fun write(value: String, buf: ByteBuffer) {
-        val byteArr = value.toByteArray(Charsets.UTF_8)
-        buf.putInt(byteArr.size)
-        buf.put(byteArr)
+        val byteBuf = toUtf8(value)
+        buf.putInt(byteBuf.limit())
+        buf.put(byteBuf)
     }
 }

uniffi_bindgen/src/bindings/kotlin/templates/wrapper.kt

@@ -26,6 +26,8 @@ import com.sun.jna.Callback
 import com.sun.jna.ptr.*
 import java.nio.ByteBuffer
 import java.nio.ByteOrder
+import java.nio.CharBuffer
+import java.nio.charset.CodingErrorAction
 import java.util.concurrent.ConcurrentHashMap
 
 {%- for req in self.imports() %}