Raw FIR: generate Unit at the end of empty lambda

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -669,9 +669,14 @@ class RawFirBuilder(val session: FirSession, val stubMode: Boolean) {
                     FirLabelImpl(this@RawFirBuilder.session, expression, it.asString())
                 }
                 val bodyExpression = literal.bodyExpression.toFirExpression("Lambda has no body")
-                if (destructuringBlock is FirBlock && bodyExpression is FirBlockImpl) {
-                    for ((index, statement) in destructuringBlock.statements.withIndex()) {
-                        bodyExpression.statements.add(index, statement)
+                if (bodyExpression is FirBlockImpl) {
+                    if (bodyExpression.statements.isEmpty()) {
+                        bodyExpression.statements.add(FirUnitExpression(this@RawFirBuilder.session, expression))
+                    }
+                    if (destructuringBlock is FirBlock) {
+                        for ((index, statement) in destructuringBlock.statements.withIndex()) {
+                            bodyExpression.statements.add(index, statement)
+                        }
                     }
                 }
                 body = FirSingleExpressionBlock(this@RawFirBuilder.session, bodyExpression.toReturn())

compiler/fir/psi2fir/testData/rawBuilder/expressions/lambda.kt

@@ -26,4 +26,8 @@ fun bar(): Int {
 fun test(list: List<Int>) {
     val map = mutableMapOf<Int, String>()
     list.forEach { map.getOrPut(it, { mutableListOf() }) += "" }
-}
+}
+
+val simple = { }
+
+val another = { 42 }

compiler/fir/psi2fir/testData/rawBuilder/expressions/lambda.txt

@@ -81,3 +81,19 @@ FILE: lambda.kt
         }
         )
     }
+    public? final? val simple: <implicit> = fun <implicit>.<anonymous>(): <implicit> {
+        ^  {
+            Unit
+        }
+
+    }
+
+        public? get(): <implicit>
+    public? final? val another: <implicit> = fun <implicit>.<anonymous>(): <implicit> {
+        ^  {
+            Int(42)
+        }
+
+    }
+
+        public? get(): <implicit>