Do not swallow a trailing comment into the argument

src/DiffEngine.Tests/InlinePatcherTests.cs

@@ -123,6 +123,27 @@ public async Task ASnapshotBeforeAVerbatimStringOpeningOnAnEscapedQuoteIsStillFo
     const string q3 = "\"\"\"";
     const string q4 = "\"\"\"\"";
 
+    /// <summary>
+    /// A comment between the argument and the closing paren. A line comment's span includes the
+    /// newline that ends it, so trimming trailing whitespace first ate that newline and the
+    /// comment then looked like part of the argument - which is not a string literal.
+    /// </summary>
+    [Test]
+    public async Task ATrailingCommentIsNotPartOfTheArgument()
+    {
+        var source = Method(
+            "        await Snapshot(\"old\" // note\n" +
+            "            );");
+
+        var status = TryApply(source, 5, InlinePatchMode.Set, "\"old\"", "new", out var newSource, out var reason);
+
+        await Assert.That(status).IsEqualTo(PatchStatus.Applied);
+        await Assert.That(reason).IsEmpty();
+        // The literal is replaced and the comment is left where it was, rather than being
+        // swallowed into the argument
+        await Assert.That(newSource).Contains("Snapshot(\"new\" // note");
+        await Assert.That(newSource).DoesNotContain("\"old\"");
+    }
     [Test]
     public async Task ReplaceRegularLiteral()
     {

src/DiffEngine/Inline/InlinePatcher.cs

@@ -915,16 +915,19 @@ static void TrimSpan(string source, SourceScan scan, ref int start, ref int end)
 
         while (end > start)
         {
-            if (char.IsWhiteSpace(source[end - 1]))
+            // The comment is asked about before the whitespace, because a line comment's span
+            // includes the newline that ends it. Trimming first ate that newline, after which
+            // nothing ended at `end` any more and the comment stayed inside the argument
+            if (scan.TryGetCommentEndingAt(end, out var commentStart) &&
+                commentStart >= start)
             {
-                end--;
+                end = commentStart;
                 continue;
             }
 
-            if (scan.TryGetCommentEndingAt(end, out var commentStart) &&
-                commentStart >= start)
+            if (char.IsWhiteSpace(source[end - 1]))
             {
-                end = commentStart;
+                end--;
                 continue;
             }