Validate diagnostic ID in Experimental attribute

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7809,4 +7809,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ExpectedInterpolatedString" xml:space="preserve">
     <value>Expected interpolated string</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

@@ -2275,6 +2275,8 @@ internal enum ErrorCode
         ERR_InterceptorGlobalNamespace = 9206,
         ERR_InterceptableMethodMustBeOrdinary = 9207,
 
+        ERR_InvalidExperimentalDiagID = 9208,
+
         #endregion
 
         // Note: you will need to do the following after adding warnings:

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

@@ -2404,6 +2404,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.WRN_Experimental:
                 case ErrorCode.ERR_ExpectedInterpolatedString:
                 case ErrorCode.ERR_InterceptorGlobalNamespace:
+                case ErrorCode.ERR_InvalidExperimentalDiagID:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

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

@@ -226,6 +226,15 @@ protected void DecodeWellKnownAttribute(ref DecodeWellKnownAttributeArguments<At
                 return;
             }
 
+            if (arguments.Attribute.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute)
+                && !SyntaxFacts.IsValidIdentifier((string?)arguments.Attribute.CommonConstructorArguments[0].ValueInternal))
+            {
+                Debug.Assert(arguments.AttributeSyntaxOpt is not null);
+                var attrArgumentLocation = arguments.Attribute.GetAttributeArgumentSyntaxLocation(parameterIndex: 0, arguments.AttributeSyntaxOpt);
+                arguments.Diagnostics.DiagnosticBag.Add(ErrorCode.ERR_InvalidExperimentalDiagID, attrArgumentLocation);
+                return;
+            }
+
             DecodeWellKnownAttributeImpl(ref arguments);
         }
 #nullable disable

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -14875,6 +14875,88 @@ public static void M() { }
             var comp = CreateCompilation(src, options: TestOptions.DebugExe.WithGeneralDiagnosticOption(ReportDiagnostic.Suppress));
             comp.VerifyDiagnostics();
         }
+
+        [Fact]
+        public void ExperimentalWithWhitespaceDiagnosticID()
+        {
+            var dir = Temp.CreateDirectory();
+            var src = dir.CreateFile("test.cs").WriteAllText("""
+C.M();
+
+[System.Diagnostics.CodeAnalysis.Experimental(" ")]
+public class C
+{
+    public static void M() { }
+}
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    public sealed class ExperimentalAttribute : Attribute
+    {
+        public ExperimentalAttribute(string diagnosticId) { }
+    }
+}
+""");
+            var analyzerConfig = dir.CreateFile(".editorconfig").WriteAllText("""
+[*.cs]
+dotnet_diagnostic.CS9208.severity = warning
+""");
+            var cmd = CreateCSharpCompiler(null, dir.Path, new[] {
+                "/nologo",
+                "/t:exe",
+                "/preferreduilang:en",
+                "/analyzerconfig:" + analyzerConfig.Path,
+                src.Path });
+
+            Assert.Equal(analyzerConfig.Path, Assert.Single(cmd.Arguments.AnalyzerConfigPaths));
+
+            var outWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var exitCode = cmd.Run(outWriter);
+            Assert.Equal(1, exitCode);
+            Assert.StartsWith("test.cs(3,2): error CS9208: The diagnosticId argument to the 'Experimental' attribute must be a valid identifier",
+                outWriter.ToString());
+        }
+
+        [Fact]
+        public void ExperimentalWithValidDiagnosticID()
+        {
+            var dir = Temp.CreateDirectory();
+            var src = dir.CreateFile("test.cs").WriteAllText("""
+C.M();
+
+[System.Diagnostics.CodeAnalysis.Experimental("DiagID")]
+public class C
+{
+    public static void M() { }
+}
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    public sealed class ExperimentalAttribute : Attribute
+    {
+        public ExperimentalAttribute(string diagnosticId) { }
+    }
+}
+""");
+            var analyzerConfig = dir.CreateFile(".editorconfig").WriteAllText("""
+[*.cs]
+dotnet_diagnostic.DiagID.severity = warning
+""");
+            var cmd = CreateCSharpCompiler(null, dir.Path, new[] {
+                "/nologo",
+                "/t:exe",
+                "/preferreduilang:en",
+                "/analyzerconfig:" + analyzerConfig.Path,
+                src.Path });
+
+            Assert.Equal(analyzerConfig.Path, Assert.Single(cmd.Arguments.AnalyzerConfigPaths));
+
+            var outWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var exitCode = cmd.Run(outWriter);
+            Assert.Equal(0, exitCode);
+            Assert.StartsWith("test.cs(1,1): warning DiagID: 'C' is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.",
+                outWriter.ToString());
+        }
     }
 
     [DiagnosticAnalyzer(LanguageNames.CSharp, LanguageNames.VisualBasic)]