Validate diagnostic ID in Experimental attribute

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7824,4 +7824,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_CollectionExpressionImmutableArray" xml:space="preserve">
     <value>This version of '{0}' cannot be used with collection expressions.</value>
   </data>
+  <data name="ERR_InvalidExperimentalDiagID" xml:space="preserve">
+    <value>The diagnosticId argument to the 'Experimental' attribute must be a valid identifier</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2279,6 +2279,8 @@ internal enum ErrorCode
         WRN_CollectionExpressionRefStructSpreadMayAllocate = 9209,
         ERR_CollectionExpressionImmutableArray = 9210,
 
+        ERR_InvalidExperimentalDiagID = 9211,
+
         #endregion
 
         // Note: you will need to do the following after adding warnings:

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -2409,6 +2409,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.WRN_CollectionExpressionRefStructMayAllocate:
                 case ErrorCode.WRN_CollectionExpressionRefStructSpreadMayAllocate:
                 case ErrorCode.ERR_CollectionExpressionImmutableArray:
+                case ErrorCode.ERR_InvalidExperimentalDiagID:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEAssemblySymbol.cs

@@ -320,23 +320,11 @@ internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
             {
                 if (_lazyObsoleteAttributeData == ObsoleteAttributeData.Uninitialized)
                 {
-                    Interlocked.CompareExchange(ref _lazyObsoleteAttributeData, computeObsoleteAttributeData(), ObsoleteAttributeData.Uninitialized);
+                    var experimentalData = PrimaryModule.Module.TryDecodeExperimentalAttributeData(Assembly.Handle, new MetadataDecoder(PrimaryModule));
+                    Interlocked.CompareExchange(ref _lazyObsoleteAttributeData, experimentalData, ObsoleteAttributeData.Uninitialized);
                 }
 
                 return _lazyObsoleteAttributeData;
-
-                ObsoleteAttributeData? computeObsoleteAttributeData()
-                {
-                    foreach (var attrData in GetAttributes())
-                    {
-                        if (attrData.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
-                        {
-                            return attrData.DecodeExperimentalAttribute();
-                        }
-                    }
-
-                    return null;
-                }
             }
         }
     }

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEModuleSymbol.cs

@@ -866,23 +866,11 @@ internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
             {
                 if (_lazyObsoleteAttributeData == ObsoleteAttributeData.Uninitialized)
                 {
-                    Interlocked.CompareExchange(ref _lazyObsoleteAttributeData, computeObsoleteAttributeData(), ObsoleteAttributeData.Uninitialized);
+                    var experimentalData = _module.TryDecodeExperimentalAttributeData(Token, new MetadataDecoder(this));
+                    Interlocked.CompareExchange(ref _lazyObsoleteAttributeData, experimentalData, ObsoleteAttributeData.Uninitialized);
                 }
 
                 return _lazyObsoleteAttributeData;
-
-                ObsoleteAttributeData? computeObsoleteAttributeData()
-                {
-                    foreach (var attrData in GetAttributes())
-                    {
-                        if (attrData.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
-                        {
-                            return attrData.DecodeExperimentalAttribute();
-                        }
-                    }
-
-                    return null;
-                }
             }
         }
     }

src/Compilers/CSharp/Portable/Symbols/Symbol_Attributes.cs

@@ -223,7 +223,14 @@ protected void DecodeWellKnownAttribute(ref DecodeWellKnownAttributeArguments<At
             {
                 // Do not use '{FullName}'. This is reserved for compiler usage.
                 arguments.Diagnostics.DiagnosticBag.Add(ErrorCode.ERR_ExplicitReservedAttr, arguments.AttributeSyntaxOpt.Location, AttributeDescription.CompilerFeatureRequiredAttribute.FullName);
-                return;
+            }
+            else if (arguments.Attribute.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
+            {
+                if (!SyntaxFacts.IsValidIdentifier((string?)arguments.Attribute.CommonConstructorArguments[0].ValueInternal))
+                {
+                    var attrArgumentLocation = arguments.Attribute.GetAttributeArgumentSyntaxLocation(parameterIndex: 0, arguments.AttributeSyntaxOpt);
+                    arguments.Diagnostics.DiagnosticBag.Add(ErrorCode.ERR_InvalidExperimentalDiagID, attrArgumentLocation);
+                }
             }
 
             DecodeWellKnownAttributeImpl(ref arguments);