Report unused anchors

[remcohaszing]

Is your enhancement related to a problem? Please describe.

If an anchor isn’t referenced, it’s unused. The language server protocol explicitly supports unused ranges using the Diagnostic#tags. Monaco editor / VS Code also support this by rendering them as faded.

Describe the solution you would like

I would like the yaml validation to report unused anchors in addition to JSON schema validation errors.

I suggest to report them using a diagnostic with the DiagnosticTag.Deprecated tag without a severity.

Describe alternatives you have considered

N/A

Additional context

I already fiddled a bit myself, but I couldn’t find a way to get the anchor range.

import { visit, isNode, Node, isAlias } from 'yaml';

// Added in https://github.com/redhat-developer/yaml-language-server/blob/13adb0e02199ce673e582bb5387577f9936a66a1/src/languageservice/services/yamlValidation.ts#L89
const anchors = new Set<Node>();
const usedAnchors = new Set<Node>();
visit(currentYAMLDoc.internalDocument, (key, node) => {
  if (!isNode(node)) {
    return;
  }
  if (node.anchor) {
    anchors.add(node);
  }
  if (isAlias(node)) {
    usedAnchors.add(node.resolve(currentYAMLDoc.internalDocument));
  }
});
for (const anchor of anchors) {
  if (!usedAnchors.has(anchor)) {
    console.dir(anchor);
  }
}

This finds unused anchors, but I don’t know if this is the right approach. I hope it helps.