Update SA1214 for readonly members

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp8/OrderingRules/SA1214CSharp8UnitTests.cs

@@ -3,9 +3,55 @@
 
 namespace StyleCop.Analyzers.Test.CSharp8.OrderingRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp7.OrderingRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.OrderingRules.SA1214ReadonlyElementsMustAppearBeforeNonReadonlyElements,
+        StyleCop.Analyzers.OrderingRules.ElementOrderCodeFixProvider>;
 
     public partial class SA1214CSharp8UnitTests : SA1214CSharp7UnitTests
     {
+        [Fact]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        public async Task VerifyReadonlyStructMethodsAreIgnoredAsync()
+        {
+            var testCode = @"
+public struct S
+{
+    public void M1() { }
+    public readonly void M2() { }
+    public void M3() { }
+}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        [Fact]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        public async Task VerifyReadonlyAccessorsAreIgnoredAsync()
+        {
+            var testCode = @"
+public struct S
+{
+    private int backingField;
+
+    public int Value
+    {
+        readonly get => this.backingField;
+        set => this.backingField = value;
+    }
+
+    public int Other
+    {
+        get => this.backingField;
+        set => this.backingField = value;
+    }
+}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

documentation/SA1214.md

@@ -23,6 +23,8 @@ A readonly field is positioned beneath a non-readonly field.
 
 A violation of this rule occurs when a readonly field is positioned beneath a non-readonly field.
 
+This rule applies only to field declarations. The C# 8 readonly members feature (e.g. readonly struct methods, properties, indexers, or accessors) is not covered by SA1214; those members follow the normal ordering rules for their declaration kind.
+
 ## How to fix violations
 
 To fix an instance of this violation, place all readonly fields above all non-readonly fields.