Handle invalid dotnet_diagnostic entry without ID part in configuration code fix

Fixes #45446

src/EditorFeatures/CSharpTest/Diagnostics/Configuration/ConfigureSeverity/DotNetDiagnosticSeverityBasedSeverityConfigurationTests.cs

@@ -193,6 +193,40 @@ public async Task ConfigureEditorconfig_InvalidRule_None()
         <AnalyzerConfigDocument FilePath=""z:\\.editorconfig"">[*.{vb,cs}]
 dotnet_diagnostic.XYZ1111.severity = none
 
+# XYZ0001: Title
+dotnet_diagnostic.XYZ0001.severity = none
+</AnalyzerConfigDocument>
+    </Project>
+</Workspace>";
+
+                await TestInRegularAndScriptAsync(input, expected, CodeActionIndex);
+            }
+
+            [ConditionalFact(typeof(IsEnglishLocal)), Trait(Traits.Feature, Traits.Features.CodeActionsConfiguration)]
+            [WorkItem(45446, "https://github.com/dotnet/roslyn/issues/45446")]
+            public async Task ConfigureEditorconfig_MissingRule_None()
+            {
+                var input = @"
+<Workspace>
+    <Project Language=""C#"" AssemblyName=""Assembly1"" CommonReferences=""true"">
+        <Document FilePath=""z:\\file.cs"">
+[|class Program1 { }|]
+        </Document>
+        <AnalyzerConfigDocument FilePath=""z:\\.editorconfig"">[*.{vb,cs}]
+dotnet_diagnostic.severity = none
+</AnalyzerConfigDocument>
+    </Project>
+</Workspace>";
+
+                var expected = @"
+<Workspace>
+    <Project Language=""C#"" AssemblyName=""Assembly1"" CommonReferences=""true"">
+        <Document FilePath=""z:\\file.cs"">
+[|class Program1 { }|]
+        </Document>
+        <AnalyzerConfigDocument FilePath=""z:\\.editorconfig"">[*.{vb,cs}]
+dotnet_diagnostic.severity = none
+
 # XYZ0001: Title
 dotnet_diagnostic.XYZ0001.severity = none
 </AnalyzerConfigDocument>

src/Features/Core/Portable/CodeFixes/Configuration/ConfigurationUpdater.cs

@@ -488,10 +488,13 @@ private async Task<Solution> ConfigureAsync()
                                         key.EndsWith(SeveritySuffix, StringComparison.Ordinal))
                                     {
                                         var diagIdLength = key.Length - (DiagnosticOptionPrefix.Length + SeveritySuffix.Length);
-                                        var diagId = key.Substring(DiagnosticOptionPrefix.Length, diagIdLength);
-                                        if (string.Equals(diagId, _diagnostic.Id, StringComparison.OrdinalIgnoreCase))
+                                        if (diagIdLength > 0)
                                         {
-                                            textChange = new TextChange(curLine.Span, $"{key} = {_newSeverity}{commentValue}");
+                                            var diagId = key.Substring(DiagnosticOptionPrefix.Length, diagIdLength);
+                                            if (string.Equals(diagId, _diagnostic.Id, StringComparison.OrdinalIgnoreCase))
+                                            {
+                                                textChange = new TextChange(curLine.Span, $"{key} = {_newSeverity}{commentValue}");
+                                            }
                                         }
                                     }