avoid some string alloc by using AppendJoin

TUnit.Aspire/AspireFixture.cs

@@ -428,13 +428,13 @@ private async Task WaitForResourcesWithFailFastAsync(
 
             var sb = new StringBuilder();
             sb.Append("Resources not ready: [");
-            sb.Append(string.Join(", ", pending.Select(n => $"'{n}'")));
+            sb.AppendJoin(", ", pending.Select(n => $"'{n}'"));
             sb.Append(']');
 
             if (readySet.Count > 0)
             {
                 sb.Append(". Resources ready: [");
-                sb.Append(string.Join(", ", readySet.Select(n => $"'{n}'")));
+                sb.AppendJoin(", ", readySet.Select(n => $"'{n}'"));
                 sb.Append(']');
             }
 

TUnit.Assertions.SourceGenerator/Generators/AssertionMethodGenerator.cs

@@ -780,7 +780,7 @@ private static void GenerateAssertConditionClassForMethod(SourceProductionContex
                 // Extension method - call it like an instance method
                 sourceBuilder.Append($"        var result = actualValue.{methodName}(");
                 var paramList = parameters.Select(p => $"_{p.Name}").ToArray();
-                sourceBuilder.Append(string.Join(", ", paramList));
+                sourceBuilder.AppendJoin(", ", paramList);
                 sourceBuilder.AppendLine(");");
             }
             else if (attributeData.TreatAsInstance)
@@ -829,7 +829,7 @@ private static void GenerateAssertConditionClassForMethod(SourceProductionContex
                     // Instance method on the target type itself
                     sourceBuilder.Append($"        var result = actualValue.{methodName}(");
                     var paramList = parameters.Select(p => $"_{p.Name}").ToArray();
-                    sourceBuilder.Append(string.Join(", ", paramList));
+                    sourceBuilder.AppendJoin(", ", paramList);
                     sourceBuilder.AppendLine(");");
                 }
             }
@@ -854,7 +854,7 @@ private static void GenerateAssertConditionClassForMethod(SourceProductionContex
                     sourceBuilder.AppendLine($"        var instance = new {containingType.ToDisplayString()}();");
                     sourceBuilder.Append($"        var result = instance.{methodName}(");
                     var paramList = parameters.Select(p => $"_{p.Name}").ToArray();
-                    sourceBuilder.Append(string.Join(", ", paramList));
+                    sourceBuilder.AppendJoin(", ", paramList);
                     sourceBuilder.AppendLine(");");
                 }
             }
@@ -863,7 +863,7 @@ private static void GenerateAssertConditionClassForMethod(SourceProductionContex
                 // Default instance method behavior
                 sourceBuilder.Append($"        var result = actualValue.{methodName}(");
                 var paramList = parameters.Select(p => $"_{p.Name}").ToArray();
-                sourceBuilder.Append(string.Join(", ", paramList));
+                sourceBuilder.AppendJoin(", ", paramList);
                 sourceBuilder.AppendLine(");");
             }
         }

TUnit.Core.SourceGenerator/CodeGenerationHelpers.cs

@@ -199,7 +199,7 @@ public static string GenerateAttributeInstantiation(AttributeData attr, Immutabl
                 }
             }
 
-            writer.Append(string.Join(", ", argStrings));
+            writer.AppendJoin(", ", argStrings);
         }
 
         writer.Append(")");
@@ -208,7 +208,7 @@ public static string GenerateAttributeInstantiation(AttributeData attr, Immutabl
         {
             writer.Append(" { ");
             var namedArgs = attr.NamedArguments.Select(na => $"{na.Key} = {TypedConstantParser.GetRawTypedConstantValue(na.Value)}");
-            writer.Append(string.Join(", ", namedArgs));
+            writer.AppendJoin(", ", namedArgs);
             writer.Append(" }");
         }
 
@@ -311,7 +311,7 @@ public static string GenerateTestAttributes(IMethodSymbol methodSymbol)
                 attributeStrings.Add(GenerateAttributeInstantiation(attr));
             }
 
-            writer.Append(string.Join(", ", attributeStrings));
+            writer.AppendJoin(", ", attributeStrings);
             writer.AppendLine(" },");
         }
 

TUnit.Core.SourceGenerator/CodeWriter.cs

@@ -70,6 +70,17 @@ public ICodeWriter Append(string text)
         return this;
     }
 
+    public ICodeWriter AppendJoin<T>(string separator, IEnumerable<T> values)
+    {
+        if (_isNewLine)
+        {
+            _builder.Append(GetIndentation(_indentLevel));
+            _isNewLine = false;
+        }
+        _builder.AppendJoin(separator, values);
+        return this;
+    }
+
     public int IndentLevel => _indentLevel;
 
     /// <summary>

TUnit.Core.SourceGenerator/ICodeWriter.cs

@@ -16,6 +16,11 @@ public interface ICodeWriter : IDisposable
     /// </summary>
     ICodeWriter Append(string text);
 
+    /// <summary>
+    /// Appends the string representation of each element separated by the given separator.
+    /// </summary>
+    ICodeWriter AppendJoin<T>(string separator, IEnumerable<T> values);
+
     /// <summary>
     /// Appends multiple lines of code.
     /// </summary>