Add optional variable replacement in comments.

Author: sharpjs

CHANGES.md

@@ -7,6 +7,8 @@ Most lines should begin with one of these words:
 ## [Unreleased](https://github.com/sharpjs/Prequel/compare/release/1.0.3..HEAD)
 - Change license from ISC to MIT.
 - Add .NET 8.0 target framework.
+- Add optional variable replacement in comments.  Set the `SqlCmdPreprocessor`
+    property `EnableVariableReplacementInComments` to `true` to enable this feature.
 - Fix `-` not recognized in `$(…)` syntax.
 
 <!--

Prequel.Tests/SqlCmdPreprocessorTests.cs

@@ -221,19 +221,38 @@ public void Process_VariableReplacement_Normal(string eol, string eof)
 
     [Test]
     [TestCaseSource(nameof(EolEofCases))]
-    public void Process_VariableReplacement_WithComments(string eol, string eof)
+    public void Process_VariableReplacement_WithComments_Default(string eol, string eof)
     {
         var preprocessor = new SqlCmdPreprocessor
         {
             Variables = { ["foo"] = "bar" }
         };
 
         var batches = preprocessor.Process(
-            Lines(eol, eof, "a $(FOO) b /* $(FOO) */ $(FOO) -- $(FOO)")
+            Lines(eol, eof, "-- $(FOO)", "a $(FOO) b /* $(FOO) */ $(FOO) -- $(FOO)")
         );
 
         batches.Should().Equal(
-            Lines(eol, eof, "a bar b /* $(FOO) */ bar -- $(FOO)")
+            Lines(eol, eof, "-- $(FOO)", "a bar b /* $(FOO) */ bar -- $(FOO)")
+        );
+    }
+
+    [Test]
+    [TestCaseSource(nameof(EolEofCases))]
+    public void Process_VariableReplacement_WithComments_ReplacementEnabled(string eol, string eof)
+    {
+        var preprocessor = new SqlCmdPreprocessor
+        {
+            Variables = { ["foo"] = "bar" },
+            EnableVariableReplacementInComments = true
+        };
+
+        var batches = preprocessor.Process(
+            Lines(eol, eof, "-- $(FOO)", "a $(FOO) b /* $(FOO) */ $(FOO) -- $(FOO)")
+        );
+
+        batches.Should().Equal(
+            Lines(eol, eof, "-- bar", "a bar b /* bar */ bar -- bar")
         );
     }
 

Prequel/SqlCmdPreprocessor.cs

@@ -63,6 +63,13 @@ public SqlCmdPreprocessor()
     public IDictionary<string, string> Variables
         => _variables;
 
+    /// <summary>
+    ///   Gets or sets whether the preprocessor performs variable replacement
+    ///   in comments.  The default value is <see langword="false"/> to match
+    ///   SQLCMD behavior.
+    /// </summary>
+    public bool EnableVariableReplacementInComments { get; set; }
+
     /// <summary>
     ///   Preprocesses the specified text.
     /// </summary>
@@ -151,7 +158,11 @@ private IEnumerable<string> ProcessCore([DisallowNull] Input? input)
                 default:
                 case '-':
                 case '/':
-                    // Comments are verbatim
+                    // Variable expansion requires switch to builder mode
+                    if (EnableVariableReplacementInComments && HasVariableReplacement(match.Value))
+                        return BuildNextBatch(input, start, match);
+
+                    // Other comments are verbatim
                     continue;
 
                 // Quoted
@@ -211,8 +222,11 @@ private IEnumerable<string> ProcessCore([DisallowNull] Input? input)
                 default:
                 case '-':
                 case '/':
-                    // Comments are verbatim
-                    builder.Append(match.Value);
+                    // Comments are subject to variable replacement if enabled
+                    if (EnableVariableReplacementInComments)
+                        PerformVariableReplacement(match.Value);
+                    else
+                        builder.Append(match.Value);
                     break;
 
                 // Quoted