Display source of error in message

Fluid.Tests/ErrorMessagesTests.cs

@@ -27,5 +27,17 @@ public void WrongTemplateShouldRenderErrorMessage(string source, string expected
             Assert.False(_parser.TryParse(source, out var _, out var error));
             Assert.StartsWith(expected, error); // e.g., message then 'at (1:15)'
         }
+
+        [Theory]
+        [InlineData("  {% assign a 'b' %}")]
+        [InlineData("{% assign a = 'b' %}\n  {% assign a 'b' %}")]
+        [InlineData("  {% assign a 'b' %}\n  {% assign a = 'b' %}")]
+        [InlineData("  {% assign a 'b' %}\n\r  {% assign a = 'b' %}")]
+        [InlineData("  {% assign a 'b' %}\n{% assign a = 'b' %}\n  {% assign a 'b' %}")]
+        public void ErrorMessageContainsLineSource(string source)
+        {
+            Assert.False(_parser.TryParse(source, out var _, out var error));
+            Assert.Contains("Source:\n  {% assign a 'b' %}", error);
+        }
     }
 }

Fluid/FluidParserExtensions.cs

@@ -19,7 +19,14 @@ public static IFluidTemplate Parse(this FluidParser parser, string template)
 
             if (parlotError != null)
             {
-                throw new ParseException($"{parlotError.Message} at {parlotError.Position}");
+                // Extract line with error
+                var start = parlotError.Position.Offset - 1;
+                var end = parlotError.Position.Offset;
+                while (start > 0 && template[start] != '\n' && template[start] != '\r') start--;
+                while (end < template.Length && template[end] != '\n') end++;
+                var source = template.Substring(start, end - start).Trim('\n', '\r');
+
+                throw new ParseException($"{parlotError.Message} at {parlotError.Position}\nSource:\n{source}");
             }
 
             if (!success)