Update SA1401 for readonly members

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp8/MaintainabilityRules/SA1401CSharp8UnitTests.cs

@@ -3,9 +3,44 @@
 
 namespace StyleCop.Analyzers.Test.CSharp8.MaintainabilityRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp7.MaintainabilityRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopDiagnosticVerifier<
+        StyleCop.Analyzers.MaintainabilityRules.SA1401FieldsMustBePrivate>;
 
     public partial class SA1401CSharp8UnitTests : SA1401CSharp7UnitTests
     {
+        [Theory]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        [InlineData("public")]
+        [InlineData("internal")]
+        [InlineData("protected")]
+        [InlineData("protected internal")]
+        public async Task TestClassWithReadonlyFieldAsync(string accessModifier)
+        {
+            var testCode = $@"public class Foo
+{{
+    {accessModifier} readonly string [|bar|];
+}}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        [Fact]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        public async Task VerifyReadonlyStructMembersAreIgnoredAsync()
+        {
+            var testCode = @"
+public struct S
+{
+    public readonly int Property { get; }
+    public readonly void Method() { }
+}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

documentation/SA1401.md

@@ -25,7 +25,9 @@ A violation of this rule occurs whenever a field in a class is given non-private
 
 Fields located within C# structs are allowed to have any access level.
 
-Fields that are static and readonly will not raise a violation. These kinds of fields are commonly used to represent a constant value when the `const` keyword cannot be used, and therefore they are exempt from this rule.
+Fields that are static and readonly will not raise a violation. These kinds of fields are commonly used to represent a constant value when the `const` keyword cannot be used, and therefore they are exempt from this rule. Instance fields are not exempt just because they are readonly; they still need to be private to satisfy this rule.
+
+The C# 8 `readonly` modifier for struct members (methods, properties, indexers, and events) does not affect this rule because it only considers fields.
 
 ## How to fix violations