Adds support to @ signed prefixed parameters

Fixes #60968

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

@@ -206,7 +206,7 @@ private void GenFieldAssignments(LoggerMethod lm, string nestedIndentation)
             {
                 foreach (LoggerParameter p in lm.TemplateParameters)
                 {
-                    _builder.AppendLine($"                {nestedIndentation}this._{p.Name} = {p.Name};");
+                    _builder.AppendLine($"                {nestedIndentation}this._{p.Name} = {(p.NeedsAtSign ? "@" : string.Empty)}{p.Name};");
                 }
             }
 
@@ -265,7 +265,7 @@ private void GenCallbackArguments(LoggerMethod lm)
             {
                 foreach (LoggerParameter p in lm.TemplateParameters)
                 {
-                    _builder.Append($"{p.Name}, ");
+                    _builder.Append($"{(p.NeedsAtSign ? "@" : string.Empty)}{p.Name}, ");
                 }
             }
 
@@ -323,7 +323,7 @@ private void GenParameters(LoggerMethod lm)
                     {
                         _builder.Append($"{p.Qualifier} ");
                     }
-                    _builder.Append($"{p.Type} {p.Name}");
+                    _builder.Append($"{p.Type} {(p.NeedsAtSign ? "@" : string.Empty)}{p.Name}");
                 }
             }
 
@@ -341,7 +341,7 @@ private void GenArguments(LoggerMethod lm)
                         _builder.Append(", ");
                     }
 
-                    _builder.Append($"{p.Type} {p.Name}");
+                    _builder.Append($"{p.Type} {(p.NeedsAtSign ? "@" : string.Empty)}{p.Name}");
                 }
             }
 
@@ -357,7 +357,7 @@ private void GenHolder(LoggerMethod lm)
                         _builder.Append(", ");
                     }
 
-                    _builder.Append(p.Name);
+                    _builder.Append((p.NeedsAtSign ? "@" : string.Empty) + p.Name);
                 }
 
                 _builder.Append(')');

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

@@ -306,6 +306,15 @@ public IReadOnlyList<LoggerClass> GetLogClasses(IEnumerable<ClassDeclarationSynt
                                         foreach (IParameterSymbol paramSymbol in methodSymbol.Parameters)
                                         {
                                             string paramName = paramSymbol.Name;
+                                            bool needsAtSign = false;
+                                            if (paramSymbol.DeclaringSyntaxReferences.Length > 0)
+                                            {
+                                                ParameterSyntax paramSyntax = paramSymbol.DeclaringSyntaxReferences[0].GetSyntax(_cancellationToken) as ParameterSyntax;
+                                                if (paramSyntax != null && !string.IsNullOrEmpty(paramSyntax.Identifier.Text))
+                                                {
+                                                    needsAtSign = paramSyntax.Identifier.Text[0] == '@';
+                                                }
+                                            }
                                             if (string.IsNullOrWhiteSpace(paramName))
                                             {
                                                 // semantic problem, just bail quietly
@@ -339,6 +348,7 @@ public IReadOnlyList<LoggerClass> GetLogClasses(IEnumerable<ClassDeclarationSynt
                                                 Name = paramName,
                                                 Type = typeName,
                                                 Qualifier = qualifier,
+                                                NeedsAtSign = needsAtSign,
                                                 IsLogger = !foundLogger && IsBaseOrIdentity(paramTypeSymbol!, loggerSymbol),
                                                 IsException = !foundException && IsBaseOrIdentity(paramTypeSymbol!, exceptionSymbol),
                                                 IsLogLevel = !foundLogLevel && IsBaseOrIdentity(paramTypeSymbol!, logLevelSymbol),
@@ -740,6 +750,7 @@ internal class LoggerParameter
             public string Name = string.Empty;
             public string Type = string.Empty;
             public string? Qualifier;
+            public bool NeedsAtSign;
             public bool IsLogger;
             public bool IsException;
             public bool IsLogLevel;

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

@@ -0,0 +1,11 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.Extensions.Logging.Generators.Tests.TestClasses
+{
+    internal static partial class AtSymbolTestExtensions
+    {
+        [LoggerMessage(EventId = 0, Level = LogLevel.Information, Message = "M0 {event}")]
+        internal static partial void M0(ILogger logger, string @event);
+    }
+}