Warn of files with no title

src/Elastic.Markdown/Diagnostics/DiagnosticsChannel.cs

@@ -138,4 +138,26 @@ public virtual async Task StopAsync(CancellationToken cancellationToken)
 			await _started;
 		await Channel.Reader.Completion;
 	}
+
+
+	public void EmitError(string file, string message)
+	{
+		var d = new Diagnostic
+		{
+			Severity = Severity.Error,
+			File = file,
+			Message = message,
+		};
+		Channel.Write(d);
+	}
+	public void EmitWarning(string file, string message)
+	{
+		var d = new Diagnostic
+		{
+			Severity = Severity.Warning,
+			File = file,
+			Message = message,
+		};
+		Channel.Write(d);
+	}
 }

src/Elastic.Markdown/IO/MarkdownFile.cs

@@ -2,6 +2,7 @@
 // Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information
 using System.IO.Abstractions;
+using Elastic.Markdown.Diagnostics;
 using Elastic.Markdown.Myst;
 using Elastic.Markdown.Myst.Directives;
 using Elastic.Markdown.Slices;
@@ -23,8 +24,11 @@ public MarkdownFile(IFileInfo sourceFile, IDirectoryInfo rootPath, MarkdownParse
 		FileName = sourceFile.Name;
 		UrlPathPrefix = context.UrlPathPrefix;
 		MarkdownParser = parser;
+		Collector = context.Collector;
 	}
 
+	public DiagnosticsCollector Collector { get; }
+
 	public string? UrlPathPrefix { get; }
 	private MarkdownParser MarkdownParser { get; }
 	public YamlFrontMatter? YamlFrontMatter { get; private set; }
@@ -60,6 +64,8 @@ public async Task<MarkdownDocument> ParseFullAsync(Cancel ctx)
 			await MinimalParse(ctx);
 
 		var document = await MarkdownParser.ParseAsync(SourceFile, YamlFrontMatter, ctx);
+		if (Title == RelativePath)
+			Collector.EmitWarning(SourceFile.FullName, "Missing yaml front-matter block defining a title or a level 1 header");
 		return document;
 	}
 

tests/Elastic.Markdown.Tests/Directives/DirectiveBaseTests.cs

@@ -59,7 +59,16 @@ protected DirectiveTest(ITestOutputHelper output, [LanguageInjection("markdown")
 		var logger = new TestLoggerFactory(output);
 		FileSystem = new MockFileSystem(new Dictionary<string, MockFileData>
 		{
-			{ "docs/source/index.md", new MockFileData(content) }
+			{ "docs/source/index.md", new MockFileData(string.IsNullOrEmpty(content) || content.StartsWith("---") ? content :
+				// language=markdown
+$"""
+---
+title: Test Document
+---
+
+{content}
+"""
+			)}
 		}, new MockFileSystemOptions
 		{
 			CurrentDirectory = Paths.Root.FullName

tests/Elastic.Markdown.Tests/Directives/YamlFrontMatterTests.cs

@@ -32,3 +32,17 @@ public void ReadsSubstitutions()
 			.And.ContainKey("key");
 	}
 }
+
+public class EmptyFileWarnsNeedingATitle(ITestOutputHelper output) : DirectiveTest(output, "")
+{
+	[Fact]
+	public void ReadsTitle() => File.Title.Should().Be("index.md");
+
+	[Fact]
+	public void ReadsNavigationTitle() => File.NavigationTitle.Should().Be("index.md");
+
+	[Fact]
+	public void WarnsOfNoTitle() =>
+		Collector.Diagnostics.Should().NotBeEmpty()
+			.And.Contain(d=>d.Message.Contains("Missing yaml front-matter block defining a title"));
+}

tests/Elastic.Markdown.Tests/Inline/AnchorLinkTests.cs

@@ -12,14 +12,14 @@ namespace Elastic.Markdown.Tests.Inline;
 
 public abstract class AnchorLinkTestBase(ITestOutputHelper output, [LanguageInjection("markdown")] string content)
 	: InlineTest<LinkInline>(output,
-		$"""
-		## Hello world
+$"""
+## Hello world
 
-		A paragraph
+A paragraph
 
-		{content}
+{content}
 
-		""")
+""")
 {
 	protected override void AddToFileSystem(MockFileSystem fileSystem)
 	{

tests/Elastic.Markdown.Tests/Inline/InlneBaseTests.cs

@@ -69,7 +69,16 @@ protected InlineTest(ITestOutputHelper output, [LanguageInjection("markdown")]st
 		var logger = new TestLoggerFactory(output);
 		FileSystem = new MockFileSystem(new Dictionary<string, MockFileData>
 		{
-			{ "docs/source/index.md", new MockFileData(content) }
+			{ "docs/source/index.md", new MockFileData(string.IsNullOrEmpty(content) || content.StartsWith("---") ? content :
+				// language=markdown
+$"""
+---
+title: Test Document
+---
+
+{content}
+"""
+			)}
 		}, new MockFileSystemOptions
 		{
 			CurrentDirectory = Paths.Root.FullName