Locate a parse failure in a CR-only metadata file on its real line [patch]

SourceGeneratorToolkit.Test/GeneratorBaseTests.cs

@@ -78,6 +78,7 @@ public void AMalformedMetadataFileIsReportedRatherThanSwallowed()
 	[TestMethod]
 	[DataRow("\n", DisplayName = "LF")]
 	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\r", DisplayName = "CR")]
 	public void AMalformedMetadataFileIsReportedAtTheBadLine(string newLine)
 	{
 		string json = string.Join(newLine, "{", "  \"things\":", "    [1,,2]", "}");
@@ -90,7 +91,7 @@ public void AMalformedMetadataFileIsReportedAtTheBadLine(string newLine)
 		Assert.AreEqual(2, span.StartLinePosition.Line);
 
 		// System.Text.Json reports the position it had reached, which is the first comma of the pair.
-		Assert.AreEqual(json.Split('\n')[2].IndexOf(",,", StringComparison.Ordinal), span.StartLinePosition.Character);
+		Assert.AreEqual("    [1,,2]".IndexOf(",,", StringComparison.Ordinal), span.StartLinePosition.Character);
 	}
 
 	[TestMethod]

SourceGeneratorToolkit/MetadataFile.cs

@@ -223,17 +223,31 @@ private Location LocationOf(JsonException ex)
 		}
 
 		long bytes = ex.BytePositionInLine ?? 0;
-		while (bytes > 0 && index < Text.Length && Text[index] is not '\r' and not '\n')
+		while (bytes > 0 && index < Text.Length && !EndsLine(index))
 		{
 			int width = CharactersAt(index);
 			bytes -= Utf8Bytes(index, width);
 			index += width;
 		}
 
-		int length = index < Text.Length && Text[index] is not '\r' and not '\n' ? 1 : 0;
+		int length = index < Text.Length && !EndsLine(index) ? 1 : 0;
 		return LocationAt(index, length);
 	}
 
+	/// <summary>
+	/// Says whether the character at an offset ends a line as System.Text.Json counts lines: a
+	/// <c>\n</c>, or the <c>\r</c> of a <c>\r\n</c> pair.
+	/// </summary>
+	/// <param name="index">The offset of the character.</param>
+	/// <returns><see langword="true"/> when the character ends the line.</returns>
+	/// <remarks>
+	/// A lone <c>\r</c> is not a line break to System.Text.Json, so in a file with CR-only line
+	/// endings it reports every position on line zero, measured from the start of the file. Stopping
+	/// at that <c>\r</c> would clamp the position to the end of the first line.
+	/// </remarks>
+	private bool EndsLine(int index) =>
+		Text[index] == '\n' || (Text[index] == '\r' && index + 1 < Text.Length && Text[index + 1] == '\n');
+
 	/// <summary>
 	/// Finds where a line starts, counting lines by <c>\n</c> line feeds as System.Text.Json does.
 	/// </summary>