extensions/markdown-language-features/src/languageFeatures/diagnostics.ts/DiagnosticComputer/validateReferenceLinks
class DiagnosticComputer:	private *validateReferenceLinks(options: DiagnosticOptions, links: readonly MdLink[], definitions: LinkDefinitionSet): Iterable<vscode.Diagnostic> {
		const severity = toSeverity(options.validateReferences);
		if (typeof severity === 'undefined') {
			return [];
		}

		for (const link of links) {
			if (link.href.kind === 'reference' && !definitions.lookup(link.href.ref)) {
				yield new vscode.Diagnostic(
					link.source.hrefRange,
					localize('invalidReferenceLink', 'No link definition found: \'{0}\'', link.href.ref),
					severity);
			}
		}
	}