[IR generator] Better support for fields' default value in base classes

- Allows using those with the incoming IR implementation printer.
- Moves the logic to common tree generator, so other trees can specify
default values in base classes as well, if needed.

^KT-65773 In Progress

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/IrTree.kt

@@ -587,6 +587,16 @@ object IrTree : AbstractTreeBuilder() {
         +field("isConst", boolean)
         +field("isLateinit", boolean)
         +field("initializer", expression, nullable = true)
+        +field("isAssignable", boolean, mutable = false) {
+            defaultValueInBase = "true"
+            withGetter = true
+            additionalImports.add(setValue)
+            kDoc = """
+            Variables are assignable by default. This means that they can be used in [${setValue.typeName}].
+            Variables are assigned in the IR even though they are not 'var' in the input. Hence
+            the separate assignability flag.
+            """.trimIndent()
+        }
     }
     val packageFragment: Element by element(Declaration) {
         ownsChildren = false
@@ -608,8 +618,9 @@ object IrTree : AbstractTreeBuilder() {
         }
         +field("packageFqName", type<FqName>())
         +field("fqName", type<FqName>()) {
-            baseGetter = "packageFqName"
+            defaultValueInBase = "packageFqName"
             customSetter = "packageFqName = value"
+            withGetter = true
             deprecation = Deprecated(
                 "Please use `packageFqName` instead",
                 ReplaceWith("packageFqName"),

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/model/Field.kt

@@ -12,8 +12,6 @@ sealed class Field(
     override val name: String,
     override var isMutable: Boolean,
 ) : AbstractField<Field>(), AbstractFieldWithDefaultValue<Field> {
-    var baseGetter: String? = null
-
     sealed class UseFieldAsParameterInIrFactoryStrategy {
 
         data object No : UseFieldAsParameterInIrFactoryStrategy()
@@ -31,28 +29,26 @@ sealed class Field(
                 UseFieldAsParameterInIrFactoryStrategy.Yes(null)
             }
 
-    override var withGetter: Boolean
-        get() = baseGetter != null
-        set(value) = error("Operation not supported")
 
-    override var defaultValueInImplementation: String? by ::baseGetter
+    override var withGetter: Boolean = false
 
+    override var defaultValueInBase: String? = null
+    override var defaultValueInImplementation: String? = null
     override var defaultValueInBuilder: String?
         get() = null
         set(_) = error("Builders are not supported")
 
+    override var customSetter: String? = null
+
     override val origin: Field
         get() = this
 
-    override var customSetter: String? = null
-
     override fun toString() = "$name: $typeRef"
 
     override val isVolatile: Boolean
         get() = false
 
-    override val isFinal: Boolean
-        get() = baseGetter != null
+    override var isFinal: Boolean = false
 
     override val isParameter: Boolean
         get() = false
@@ -61,7 +57,9 @@ sealed class Field(
 
     override fun updateFieldsInCopy(copy: Field) {
         super.updateFieldsInCopy(copy)
-        copy.baseGetter = baseGetter
+        copy.withGetter = withGetter
+        copy.defaultValueInBase = defaultValueInBase
+        copy.defaultValueInImplementation = defaultValueInImplementation
         copy.customUseInIrFactoryStrategy = customUseInIrFactoryStrategy
         copy.customSetter = customSetter
     }