Add support for const object members (#678)

This adds support for adding the `const` modifier to object members.

Such object members are also required to have the `local` modifier applied.

This follows SPICE-0011.

pkl-core/src/main/java/org/pkl/core/ast/VmModifier.java

@@ -72,7 +72,7 @@ public final class VmModifier {
   public static final int VALID_PROPERTY_MODIFIERS =
       ABSTRACT | LOCAL | HIDDEN | EXTERNAL | FIXED | CONST;
 
-  public static final int VALID_OBJECT_MEMBER_MODIFIERS = LOCAL;
+  public static final int VALID_OBJECT_MEMBER_MODIFIERS = LOCAL | CONST;
 
   public static final int TYPEALIAS_OBJECT_MEMBER = TYPE_ALIAS | CONST;
 

pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java

@@ -721,12 +721,22 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
       @Nullable TypeAnnotationContext typeAnnCtx,
       @Nullable ExprContext exprCtx,
       @Nullable List<? extends ObjectBodyContext> bodyCtx) {
-
+    var modifiers =
+        doVisitModifiers(
+            modifierCtxs, VmModifier.VALID_OBJECT_MEMBER_MODIFIERS, "invalidObjectMemberModifier");
+    if (VmModifier.isConst(modifiers) && !VmModifier.isLocal(modifiers)) {
+      @SuppressWarnings("OptionalGetWithoutIsPresent")
+      var constModifierCtx =
+          modifierCtxs.stream().filter((it) -> it.CONST() != null).findFirst().get();
+      throw exceptionBuilder()
+          .evalError("invalidConstObjectMemberModifier")
+          .withSourceSection(createSourceSection(constModifierCtx))
+          .build();
+    }
     return doVisitObjectProperty(
         createSourceSection(ctx),
         createSourceSection(propertyName),
-        doVisitModifiers(
-            modifierCtxs, VmModifier.VALID_OBJECT_MEMBER_MODIFIERS, "invalidObjectMemberModifier"),
+        modifiers,
         propertyName.getText(),
         typeAnnCtx,
         exprCtx,

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -19,6 +19,9 @@ Modifier `{0}` is not applicable to properties.
 invalidObjectMemberModifier=\
 Modifier `{0}` is not applicable to object members.
 
+invalidConstObjectMemberModifier=\
+Modifier `const` can only be applied to object members that are also `local`.
+
 objectMethodMustBeLocal=\
 Method needs a `local` modifier because it is defined in an object, not a class.
 

pkl-core/src/test/files/LanguageSnippetTests/input-helper/modules/Birds2.pkl

@@ -0,0 +1,5 @@
+module Birds2
+
+abstract class Bird
+
+bird: Bird

pkl-core/src/test/files/LanguageSnippetTests/input/basic/constModifier4.pkl

@@ -0,0 +1,15 @@
+const baz = 15
+
+foo {
+  const local bar = baz
+
+  const local bar2 = biz()
+
+  const local function biz() = baz
+
+  qux = bar
+
+  corge = biz()
+
+  corge2 = bar2
+}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constLocalAmendModule.pkl

@@ -0,0 +1,9 @@
+amends ".../input-helper/modules/Birds2.pkl"
+
+const local myBirdName = "Birdy"
+
+local class MyBird extends Bird {
+  name: String = myBirdName
+}
+
+bird = new MyBird {}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constLocalMethod.pkl

@@ -0,0 +1,7 @@
+bar = 15
+
+obj {
+  const local function foo() = bar
+  
+  res = foo()
+}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constLocalProperty.pkl

@@ -0,0 +1,5 @@
+foo {
+  res1 = 15
+  const local qux = res1
+  res2 = qux
+}

pkl-core/src/test/files/LanguageSnippetTests/output/basic/constModifier4.pcf

@@ -0,0 +1,6 @@
+baz = 15
+foo {
+  qux = 15
+  corge = 15
+  corge2 = 15
+}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/const/constLocalAmendModule.pcf

@@ -0,0 +1,3 @@
+bird {
+  name = "Birdy"
+}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/const/constLocalMethod.err

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Cannot reference property `bar` from here because it is not `const`.
+
+x | const local function foo() = bar
+                                 ^^^
+at constLocalMethod#obj.foo (file:///$snippetsDir/input/errors/const/constLocalMethod.pkl)
+
+To fix, do either of:
+1. Add modifier `const` to property `bar`
+2. Self-import this module, and reference this property from the import.
+
+x | res = foo()
+          ^^^^^
+at constLocalMethod#obj.res (file:///$snippetsDir/input/errors/const/constLocalMethod.pkl)
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/const/constLocalProperty.err

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Cannot reference property `res1` from here because it is not `const`.
+
+x | const local qux = res1
+                      ^^^^
+at constLocalProperty#foo.qux (file:///$snippetsDir/input/errors/const/constLocalProperty.pkl)
+
+To fix, do either of:
+1. Add modifier `const` to property `res1`
+2. Self-import this module, and reference this property from the import.
+
+x | res2 = qux
+           ^^^
+at constLocalProperty#foo.res2 (file:///$snippetsDir/input/errors/const/constLocalProperty.pkl)
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/const/constObjectMember.err

@@ -1,5 +1,5 @@
 –– Pkl Error ––
-Modifier `const` is not applicable to object members.
+Modifier `const` can only be applied to object members that are also `local`.
 
 x | const foo = 10
     ^^^^^