Render source snippet on location (#291)

This is something I implemented in my project's test implementation and found very helpful on large source generator samples - rendering the actual source snippet as an output comment, with squiggly lines underlining the problematic section, similar to what you get when compiling in the console.

Unfortunately, only one test in this repo uses locations from actual source trees, so it might be harder to see the benefits, but even this modified example should give an idea of what it looks like.

src/Tests/SyntaxLocationTests.ConsistentLocation.verified.txt

@@ -1 +1,7 @@
-: (1,0)-(1,14)
+/*
+class Foo {
+void Bar() { }
+^^^^^^^^^^^^^^
+}
+*/
+theFile.cs: (1,0)-(1,14)

src/Tests/SyntaxLocationTests.cs

@@ -11,7 +11,7 @@ void Bar() { }
             """;
 
         var source = SourceText.From(sourceText);
-        var syntaxTree = SyntaxFactory.ParseSyntaxTree(source);
+        var syntaxTree = SyntaxFactory.ParseSyntaxTree(source, path: "theFile.cs");
 
         var methodDeclarationSyntax = syntaxTree
             .GetRoot()

src/Verify.SourceGenerators/Converters/LocationConverter.cs

@@ -1,6 +1,37 @@
 class LocationConverter :
     WriteOnlyJsonConverter<Location>
 {
-    public override void Write(VerifyJsonWriter writer, Location value) =>
-        writer.Serialize(value.GetMappedLineSpan());
-}
+    const int contextLines = 1;
+
+    public override void Write(VerifyJsonWriter writer, Location value)
+    {
+        var lineSpan = value.GetMappedLineSpan();
+
+        // Pretty-print the error with the source code snippet.
+        if (value.SourceTree is { } source)
+        {
+            var comment = new StringBuilder().AppendLine();
+            var lines = source.GetText().Lines;
+            var startLine = Math.Max(lineSpan.StartLinePosition.Line - contextLines, 0);
+            var endLine = Math.Min(lineSpan.EndLinePosition.Line + contextLines, lines.Count - 1);
+            for (var lineIdx = startLine; lineIdx <= endLine; lineIdx++)
+            {
+                var line = lines[lineIdx];
+                // print the source line
+                comment.AppendLine(line.ToString());
+                // print squiggly line highlighting the location
+                if (line.Span.Intersection(value.SourceSpan) is { } intersection)
+                {
+                    comment
+                        .Append(' ', intersection.Start - line.Start)
+                        .Append('^', intersection.Length)
+                        .AppendLine();
+                }
+            }
+            writer.WriteComment(comment.ToString());
+            writer.WriteWhitespace("\n");
+        }
+
+        writer.Serialize(lineSpan);
+    }
+}