WASM: Properly handle nullable exported strings

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/BodyGenerator.kt

@@ -369,7 +369,7 @@ class BodyGenerator(val context: WasmFunctionCodegenContext) : IrElementVisitorV
         // Handle complex exported parameters.
         // TODO: This should live as a separate lowering which creates separate shims for each exported function.
         if (context.irFunction.isExported(context.backendContext) &&
-            expression.value.type == irBuiltIns.stringType) {
+            expression.value.type.getClass() == irBuiltIns.stringType.getClass()) {
 
             body.buildCall(context.referenceFunction(wasmSymbols.exportString))
         }

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/WasmModuleCodegenContextImpl.kt

@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.ir.symbols.IrFunctionSymbol
 import org.jetbrains.kotlin.ir.symbols.IrSimpleFunctionSymbol
 import org.jetbrains.kotlin.ir.types.IrType
 import org.jetbrains.kotlin.ir.types.defaultType
+import org.jetbrains.kotlin.ir.types.getClass
 import org.jetbrains.kotlin.ir.types.isNothing
 import org.jetbrains.kotlin.ir.util.isFunction
 import org.jetbrains.kotlin.ir.util.parentAsClass
@@ -59,7 +60,7 @@ class WasmModuleCodegenContextImpl(
 
     override fun transformExportedResultType(irType: IrType): WasmType? {
         // Exported strings are passed as pointers to the raw memory
-        if (irType == backendContext.irBuiltIns.stringType)
+        if (irType.getClass() == backendContext.irBuiltIns.stringType.getClass())
             return WasmI32
         return with(typeTransformer) { irType.toWasmResultType() }
     }

libraries/stdlib/wasm/internal/kotlin/wasm/internal/ImportExport.kt

@@ -8,7 +8,10 @@ package kotlin.wasm.internal
 // This is called when exported function returns a string. It writes [i32 length, [i16 chars ...]] into a temporary raw memory area and
 // returns pointer to the start of it.
 // Note: currently there is a single temporary raw memory area so it's not possible to export more than one string at a time.
-internal fun exportString(src: String): Int {
+internal fun exportString(src: String?): Int {
+    if (src == null)
+        throw IllegalArgumentException("Exporting null string")
+
     val retAddr = unsafeGetScratchRawMemory(INT_SIZE_BYTES + src.length * CHAR_SIZE_BYTES)
     wasm_i32_store(retAddr, src.length)
     unsafeCharArrayToRawMemory(src.toCharArray(), retAddr + INT_SIZE_BYTES)