Do not report MA0028 for concatenation of constant strings and chars

docs/Rules/MA0028.md

@@ -11,6 +11,8 @@ new StringBuilder().Append("a" + 10);
 new StringBuilder().Append("a").Append(10);
 ```
 
+The concatenation of constant strings and chars (e.g. `" " + ConstChar + " "`) is not reported, as the compiler merges it into a single string.
+
 ```csharp
 new StringBuilder().Append("a");
 

src/Meziantou.Analyzer/Rules/OptimizeStringBuilderUsageAnalyzer.cs

@@ -357,7 +357,16 @@ private static bool IsEquivalentAppendJoinOverload(IMethodSymbol joinMethod, IMe
 
         private static bool IsConstString(IOperation operation)
         {
-            return TryGetConstStringValue(operation, out _);
+            if (TryGetConstStringValue(operation, out _))
+                return true;
+
+            // The compiler merges the constant chars and strings of a concatenation into a single string (e.g. " " + 'c' + " ")
+            return operation switch
+            {
+                IConversionOperation { IsImplicit: true, Operand.ConstantValue: { HasValue: true, Value: char } } => true,
+                IBinaryOperation { OperatorKind: BinaryOperatorKind.Add } binaryOperation when binaryOperation.Type.IsString() => IsConstString(binaryOperation.LeftOperand) && IsConstString(binaryOperation.RightOperand),
+                _ => false,
+            };
         }
 
         private static bool TryGetConstStringValue(IOperation operation, [NotNullWhen(true)] out string? value)

tests/Meziantou.Analyzer.Test/Rules/OptimizeStringBuilderUsageAnalyzerTests.cs

@@ -264,6 +264,10 @@ void A()
     [InlineData(@"$""abc{""test""}""")]
     [InlineData(@"""abc"" + ""test""")]
     [InlineData(@"$""abc{""test""}"" + ""test""")]
+    [InlineData(@"""abc"" + 'd'")]
+    [InlineData(@"'a' + ""bc""")]
+    [InlineData(@""" "" + 'c' + "" """)]
+    [InlineData(@"""a"" + ('b' + ""c"")")]
     public Task Append_NoDiagnostic(string text)
     {
         var test = CreateTest();
@@ -281,10 +285,28 @@ void A()
         return test.RunAsync();
     }
 
+    [Fact]
+    public Task Append_ConcatConstantChar_NoDiagnostic()
+    {
+        var test = CreateTest();
+        test.TestCode = """
+            using System.Text;
+            class Test
+            {
+                public const char Const = 'C';
+
+                string A() => new StringBuilder().Append(" " + Const + " ").AppendLine(" " + Const + " ").ToString();
+            }
+            """;
+
+        return test.RunAsync();
+    }
+
     [Theory]
     [InlineData(@"$""a{1}""")]
     [InlineData(@"""a"" + 10")]
     [InlineData(@"10 + 20 + ""a""")]
+    [InlineData(@"""a"" + 'b' + 10")]
     [InlineData(@"""""")]
     [InlineData(@""""" + """"")]
     [InlineData(@""""".Substring(0, 10)")]