Add letter casing mismatch validation and start triggering SYSLIB1021

src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/LoggerMessageGenerator.Parser.cs

@@ -248,6 +248,13 @@ public IReadOnlyList<LoggerClass> GetLogClasses(IEnumerable<ClassDeclarationSynt
                                         keepMethod = false;
                                     }
 
+                                    bool casingValid = ValidateTemplatesCasingConsistency(lm.TemplateList);
+                                    if (!casingValid)
+                                    {
+                                        Diag(DiagnosticDescriptors.InconsistentTemplateCasing, method.Identifier.GetLocation(), method.Identifier.ToString());
+                                        keepMethod = false;
+                                    }
+
                                     if (lm.Name[0] == '_')
                                     {
                                         // can't have logging method names that start with _ since that can lead to conflicting symbol names
@@ -788,6 +795,36 @@ private static bool ExtractTemplates(string? message, Dictionary<string, string>
                 return success;
             }
 
+            /// <summary>
+            /// Validates that templates list does not contains templates with mixed casing like {hello} and {Hello}
+            /// </summary>
+            /// <returns>Value indicating lack of mixed casing within templates list</returns>
+            private static bool ValidateTemplatesCasingConsistency(List<string> templates)
+            {
+                int count = templates.Count;
+
+                for (int i = 1; i < count; i++)
+                {
+                    string templateI = templates[i];
+
+                    for (int j = 0; j < i; j++)
+                    {
+                        string templateJ = templates[j];
+
+                        bool matchIgnoreCase = StringComparer.OrdinalIgnoreCase.Compare(templateI, templateJ) == 0;
+                        bool matchExact = StringComparer.Ordinal.Compare(templateI, templateJ) == 0;
+
+                        if (matchIgnoreCase && !matchExact)
+                        {
+                            return false;
+                        }
+
+                    }
+                }
+
+                return true;
+            }
+
             /// <summary>
             /// Searches for the next brace index in the message.
             /// </summary>

src/libraries/Microsoft.Extensions.Logging.Abstractions/tests/Microsoft.Extensions.Logging.Generators.Tests/LoggerMessageGeneratorParserTests.cs

@@ -283,6 +283,21 @@ partial class C
             Assert.Equal(DiagnosticDescriptors.ShouldntMentionLoggerInMessage.Id, diagnostics[1].Id);
         }
 
+        [Fact]
+        public async Task MixedCasing()
+        {
+            IReadOnlyList<Diagnostic> diagnostics = await RunGenerator(@"
+                partial class C
+                {
+                    [LoggerMessage(EventId = 0, Level = LogLevel.Debug, Message = ""M1 {par1} {PAr1} {a}"")]
+                    static partial void M1(ILogger logger, int par1, int a);
+                }
+            ");
+
+            Assert.Single(diagnostics);
+            Assert.Equal(DiagnosticDescriptors.InconsistentTemplateCasing.Id, diagnostics[0].Id);
+        }
+
         [Fact]
         public async Task DoubleLogLevel_InAttributeAndAsParameterButMissingInTemplate_ProducesDiagnostic()
         {