StubIndex created, but not used

Please verify javadoc com.intellij.extapi.psi.StubBasedPsiElementBase#getStub . Does it apply in your use case?

Thank you, that wasn't called. I've had StubElement, but not StubBasedPsiElement so I've added extends StubBasedPsiElementBase to implementation and now StubBasedPsiElementBase#getStub() method is called.

Problem now is that this falls into

com.intellij.psi.impl.source.SubstrateRef#getStub()

where lies an implementation:

@Nullable
public Stub getStub() {
  return null;
}

So calling element?.stub on the element still results in null.

Do I have to somehow override this method? Or am I still missing some extension?

No, you don't have to worry about that method.

From Javadoc I mentioned in com.intellij.extapi.psi.StubBasedPsiElementBase#getStub, please verify whether these conditions apply in your specific use-case and switch to use getGreenStub().

Note: for most clients (where the logic doesn't crucially differ for stub and AST cases), {@link #getGreenStub()} should be preferred.
* @return the stub that this element is built upon, or null if the element is currently AST-based. The latter can happen
* if the file text was loaded from the very beginning, or if it was loaded via {@link #getNode()} on this or any other element
* in the containing file.

I'm actually more confused by that doc.

From "null if the element is currently AST-based" I understand that I won't get a stub on an element on which the method was called from IDE (like a node on which I triggered CompletionContributor).

So I'd expect that Stub would be loaded when I list all project files and find Stubbed elements inside them, something like:

var vFiles = FileTypeIndex.getFiles(GdFileType.INSTANCE, GlobalSearchScope.allScope(project));
for (VirtualFile vf : virtualFiles) {
        var myFile = (GdFile) PsiManager.getInstance(project).findFile(vf);
        var prop = (GdClassNamingImpl) PsiTreeUtil.findChildOfType(myFile, GdClassNamingImpl.class);
}

But to me that seems to contradict the part: "returns null... if the file text was loaded from the very beginning"

Also getGreenStub only falls into getStub, which returns null:

public Stub getStub() {
  return null;
}

public Stub getGreenStub() {
  return getStub();
}

Seems I've finally found the last missing piece today:

Before calling

PsiTreeUtil.findChildOfType(file, ...)

to get desired elements I've had to first call

file.getStubTree()

Now I'm finally getting some stub elements instead of full AST nodes.

Ok maybe I don't get it in the end... as getGreenStub or getGreenStubTree seemingly randomly throws an exception of ProcessCancel or something.

Is there anywhere tutorial with more detail about StubTrees and how to correctly use them that https://plugins.jetbrains.com/docs/intellij/indexing-and-psi-stubs.html page?

There's actually not much about actuall usage... only on Stub Indices but not on the tree itself.

Sorry for delay.

https://gitlab.com/IceExplosive/gdscript/-/blob/master/src/main/java/gdscript/psi/GdFile.java does not implement necessary IStubFileElementType, please see "The following steps need to be performed..." paragraph here https://plugins.jetbrains.com/docs/intellij/stub-indexes.html

You can use Tools->View PSI Structure dialog available in internal mode (https://plugins.jetbrains.com/docs/intellij/enabling-internal.html) to check stubs are correctly built for your files/language.