Improve method.isConst() checks in InvokeMethodVirtualNode

- Check `method.isConst()` every time a method is resolved
  instead of once per node instance (`isConstChecked`).
  Given that `needsConst` only happens in very specific circumstances,
  I'm not entirely sure if every resolved method needs to be checked.
  However, it's a cleaner solution in any case, and `method.isConst()`
  is a fast check that also never happens on the `evalCached` fast path.
- Do not check for const-ness of `FunctionN.apply` methods.
  This check seems unnecessary and would always fail if triggered.
  (According to `base.pkl`, none of the `FunctionN.apply` methods is const.)
- Remove unnecessary Truffle boundaries for modifier checks,
  which are just bitwise operations.
- Improve const/import docs.

docs/modules/language-reference/pages/index.adoc

@@ -989,8 +989,8 @@ Because a `local` property is added to the lexical scope, but not (observably) t
 ====
 
 An _import clause_ defines a local property in the containing module.
-This means `import "someModule.pkl"` is equivalent to `local someModule = import("someModule.pkl")`.
+This means `import "someModule.pkl"` is effectively `const local someModule = import("someModule.pkl")`.
-Also, `import "someModule.pkl" as otherName` is equivalent to `local otherName = import("someModule.pkl")`.
+Also, `import "someModule.pkl" as otherName` is effectively `const local otherName = import("someModule.pkl")`.
 ====
 
 [[fixed-properties]]

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InvokeMethodVirtualNode.java

@@ -16,7 +16,6 @@
 package org.pkl.core.ast.expression.member;
 
 import com.oracle.truffle.api.CompilerDirectives;
-import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
 import com.oracle.truffle.api.RootCallTarget;
 import com.oracle.truffle.api.dsl.Cached;
 import com.oracle.truffle.api.dsl.ImportStatic;
@@ -44,7 +43,6 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
   @Children private final ExpressionNode[] argumentNodes;
   private final MemberLookupMode lookupMode;
   private final boolean needsConst;
-  @CompilationFinal private boolean isConstChecked;
 
   protected InvokeMethodVirtualNode(
       SourceSection sourceSection,
@@ -101,10 +99,9 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
   protected Object evalFunction(
       VirtualFrame frame,
       VmFunction receiver,
-      VmClass receiverClass,
+      @SuppressWarnings("unused") VmClass receiverClass,
       @Cached("create()") IndirectCallNode callNode) {
 
-    checkConst(receiverClass);
     var args = new Object[2 + argumentNodes.length];
     args[0] = receiver.getThisValue();
     args[1] = receiver;
@@ -125,7 +122,6 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
       @Cached("resolveMethod(receiverClass)") ClassMethod method,
       @Cached("create(method.getCallTarget(sourceSection))") DirectCallNode callNode) {
 
-    checkConst(method);
     var args = new Object[2 + argumentNodes.length];
     args[0] = receiver;
     args[1] = method.getOwner();
@@ -145,8 +141,6 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
       @Cached("create()") IndirectCallNode callNode) {
 
     var method = resolveMethod(receiverClass);
-    checkConst(method);
-
     var args = new Object[2 + argumentNodes.length];
     args[0] = receiver;
     args[1] = method.getOwner();
@@ -161,7 +155,10 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
 
   protected ClassMethod resolveMethod(VmClass receiverClass) {
     var method = receiverClass.getMethod(methodName);
-    if (method != null) return method;
+    if (method != null) {
+      checkConst(method);
+      return method;
+    }
 
     CompilerDirectives.transferToInterpreter();
 
@@ -174,17 +171,10 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
         .build();
   }
 
-  private void checkConst(VmClass receiverClass) {
-    checkConst(resolveMethod(receiverClass));
-  }
-
   private void checkConst(ClassMethod method) {
-    if (needsConst && !isConstChecked) {
+    if (needsConst && !method.isConst()) {
-      CompilerDirectives.transferToInterpreterAndInvalidate();
+      CompilerDirectives.transferToInterpreter();
-      if (!method.isConst()) {
+      throw exceptionBuilder().evalError("methodMustBeConst", methodName.toString()).build();
-        throw exceptionBuilder().evalError("methodMustBeConst", methodName.toString()).build();
-      }
-      isConstChecked = true;
     }
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/primary/ResolveVariableNode.java

@@ -194,7 +194,7 @@ public final class ResolveVariableNode extends ExpressionNode {
       }
     }
 
-    // Assuming this method exists at all, it must be a method accessible through `this`.
+    // Assuming this property exists at all, it must be a property accessible through `this`.
     ///
     // Reading a property off of implicit `this` needs a const check if this node is not in a const
     // scope.

pkl-core/src/main/java/org/pkl/core/ast/member/Member.java

@@ -15,7 +15,6 @@
  */
 package org.pkl.core.ast.member;
 
-import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.VmModifier;
 import org.pkl.core.runtime.Identifier;
@@ -76,42 +75,34 @@ public abstract class Member {
   /** For use in user-facing messages. Non-null iff getName() is non-null. */
   public abstract @Nullable String getCallSignature();
 
-  @TruffleBoundary
   public final boolean isLocal() {
     return VmModifier.isLocal(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isConst() {
     return VmModifier.isConst(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isFixed() {
     return VmModifier.isFixed(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isHidden() {
     return VmModifier.isHidden(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isExternal() {
     return VmModifier.isExternal(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isClass() {
     return VmModifier.isClass(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isTypeAlias() {
     return VmModifier.isTypeAlias(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isImport() {
     return VmModifier.isImport(modifiers);
   }
@@ -120,27 +111,22 @@ public abstract class Member {
     return VmModifier.isGlob(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isAbstract() {
     return VmModifier.isAbstract(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isType() {
     return VmModifier.isType(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isLocalOrExternalOrHidden() {
     return VmModifier.isLocalOrExternalOrHidden(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isConstOrFixed() {
     return VmModifier.isConstOrFixed(modifiers);
   }
 
-  @TruffleBoundary
   public final boolean isLocalOrExternalOrAbstract() {
     return VmModifier.isLocalOrExternalOrAbstract(modifiers);
   }