Warn when a heading contains an inline applies_to

src/Elastic.Markdown/Diagnostics/ProcessorDiagnosticExtensions.cs

@@ -14,10 +14,36 @@ public static class ProcessorDiagnosticExtensions
 {
 	private static string CreateExceptionMessage(string message, Exception? e) => message + (e != null ? Environment.NewLine + e : string.Empty);
 
+	public static void EmitError(this BlockProcessor processor, string message, int? line = null, int? column = null, int? length = null) =>
+		processor.Emit(Severity.Error, message, line, column, length);
+
+	public static void EmitWarning(this BlockProcessor processor, string message, int? line = null, int? column = null, int? length = null) =>
+		processor.Emit(Severity.Warning, message, line, column, length);
+
+	public static void EmitHint(this BlockProcessor processor, string message, int? line = null, int? column = null, int? length = null) =>
+		processor.Emit(Severity.Hint, message, line, column, length);
+
+	public static void Emit(this BlockProcessor processor, Severity severity, string message, int? line = null, int? column = null, int? length = null)
+	{
+		var context = processor.GetContext();
+		if (context.SkipValidation)
+			return;
+		var d = new Diagnostic
+		{
+			Severity = severity,
+			File = processor.GetContext().MarkdownSourcePath.FullName,
+			Column = column ?? 1,
+			Line = line ?? processor.LineIndex + 1,
+			Message = message,
+			Length = length ?? processor.Line.Length + 1
+		};
+		context.Build.Collector.Write(d);
+	}
+
+
 	public static void EmitError(this InlineProcessor processor, int line, int column, int length, string message) =>
 		processor.Emit(Severity.Error, line, column, length, message);
 
-
 	public static void EmitWarning(this InlineProcessor processor, int line, int column, int length, string message) =>
 		processor.Emit(Severity.Warning, line, column, length, message);
 

src/Elastic.Markdown/Myst/InlineParsers/HeadingBlockWithSlugParser.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information
 
 using System.Text.RegularExpressions;
+using Elastic.Markdown.Diagnostics;
 using Elastic.Markdown.Myst.Roles.Icons;
 using Markdig;
 using Markdig.Helpers;
@@ -32,13 +33,18 @@ public void Setup(MarkdownPipeline pipeline, IMarkdownRenderer renderer) { }
 public class HeadingBlockWithSlugParser : HeadingBlockParser
 {
 	private static readonly Regex IconSyntax = IconParser.IconRegex();
+	private static readonly Regex AppliesToSyntax = HeadingAppliesToParser.AppliesToSyntaxRegex();
 
 	public override bool Close(BlockProcessor processor, Block block)
 	{
 		if (block is not HeadingBlock headingBlock)
 			return base.Close(processor, block);
 
 		var text = headingBlock.Lines.Lines[0].Slice.AsSpan();
+
+		if (AppliesToSyntax.IsMatch(text))
+			processor.EmitWarning("Do not use inline 'applies_to' annotations with headings. Use a section 'applies_to' annotation instead.");
+
 		// Remove icon syntax from the heading text
 		var cleanText = IconSyntax.Replace(text.ToString(), "").Trim();
 		headingBlock.SetData("header", cleanText);
@@ -79,3 +85,9 @@ public static partial class HeadingAnchorParser
 	[GeneratedRegex(@"\$\$\$[^\$]+\$\$\$", RegexOptions.IgnoreCase, "en-US")]
 	public static partial Regex InlineAnchors();
 }
+
+public static partial class HeadingAppliesToParser
+{
+	[GeneratedRegex(@"\{applies_to\}`[^`]*`", RegexOptions.Compiled)]
+	public static partial Regex AppliesToSyntaxRegex();
+}