Update SA1212 for readonly members

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp8/OrderingRules/SA1212CSharp8UnitTests.cs

@@ -3,9 +3,120 @@
 
 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.SA1212PropertyAccessorsMustFollowOrder,
+        StyleCop.Analyzers.OrderingRules.SA1212SA1213CodeFixProvider>;
 
     public partial class SA1212CSharp8UnitTests : SA1212CSharp7UnitTests
     {
+        [Fact]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        public async Task TestPropertyWithReadonlyGetterAfterSetterAsync()
+        {
+            var testCode = @"
+public struct S
+{
+    private int _value;
+
+    public int Value
+    {
+        [|set
+        {
+            _value = value;
+        }|]
+        readonly get
+        {
+            return _value;
+        }
+    }
+}";
+
+            var fixedCode = @"
+public struct S
+{
+    private int _value;
+
+    public int Value
+    {
+        readonly get
+        {
+            return _value;
+        }
+        set
+        {
+            _value = value;
+        }
+    }
+}";
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        [Fact]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        public async Task TestIndexerWithReadonlyGetterAfterSetterAsync()
+        {
+            var testCode = @"
+public struct S
+{
+    private int[] _values;
+
+    public int this[int index]
+    {
+        [|set
+        {
+            _values[index] = value;
+        }|]
+        readonly get
+        {
+            return _values[index];
+        }
+    }
+}";
+
+            var fixedCode = @"
+public struct S
+{
+    private int[] _values;
+
+    public int this[int index]
+    {
+        readonly get
+        {
+            return _values[index];
+        }
+        set
+        {
+            _values[index] = value;
+        }
+    }
+}";
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        [Fact]
+        [WorkItem(3001, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3001")]
+        public async Task TestReadonlyGetterBeforeSetterAsync()
+        {
+            var testCode = @"
+public struct S
+{
+    private int _value;
+
+    public int Value
+    {
+        readonly get => _value;
+        set => _value = value;
+    }
+}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

documentation/SA1212.md

@@ -43,6 +43,23 @@ public string Name
 }
 ```
 
+### C# 8 readonly accessors
+
+Readonly members in structs do not change the ordering requirement. A `readonly` getter still needs to appear before any `set` or `init` accessor to satisfy this rule.
+
+```csharp
+public struct S
+{
+    public int Value
+    {
+        set => this.value = value;
+        readonly get => this.value;
+    }
+}
+```
+
+The preceding property violates SA1212 because the readonly getter comes after the setter. Move the readonly getter before the setter to resolve the warning.
+
 ## How to fix violations
 
 To fix an instance of this violation, place the get accessor before the set accessor.