Allow init accessors in readonly contexts

This allows `init` properties on readonly structs, readonly properties in structs, and allows `init` accessors to be declared readonly themselves. Fixes https://github.com/dotnet/roslyn/issues/47612.

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

@@ -373,7 +373,7 @@ internal virtual bool IsExplicitInterfaceImplementation
         /// Indicates whether the method is effectively readonly,
         /// by either the method or the containing type being marked readonly.
         /// </summary>
-        internal virtual bool IsEffectivelyReadOnly => (IsDeclaredReadOnly || ContainingType?.IsReadOnly == true) && IsValidReadOnlyTarget;
+        internal virtual bool IsEffectivelyReadOnly => (IsDeclaredReadOnly || ContainingType?.IsReadOnly == true) && IsValidReadOnlyTarget && !IsInitOnly;
 
         protected bool IsValidReadOnlyTarget => !IsStatic && ContainingType.IsStructType() && MethodKind != MethodKind.Constructor;
 

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertyAccessorSymbol.cs

@@ -673,7 +673,7 @@ private void CheckModifiers(Location location, bool hasBody, bool isAutoProperty
                 // Static member '{0}' cannot be marked 'readonly'.
                 diagnostics.Add(ErrorCode.ERR_StaticMemberCantBeReadOnly, location, this);
             }
-            else if (LocalDeclaredReadOnly && _isAutoPropertyAccessor && MethodKind == MethodKind.PropertySet)
+            else if (LocalDeclaredReadOnly && _isAutoPropertyAccessor && MethodKind == MethodKind.PropertySet && !_usesInit)
             {
                 // Auto-implemented accessor '{0}' cannot be marked 'readonly'.
                 diagnostics.Add(ErrorCode.ERR_AutoSetterCantBeReadOnly, location, this);

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs

@@ -161,7 +161,7 @@ private enum Flags : byte
 
                 bool isGetterOnly = hasGetAccessor && !hasSetAccessor;
 
-                if (isAutoPropertyWithGetSyntax && !IsStatic && !isGetterOnly)
+                if (isAutoPropertyWithGetSyntax && !IsStatic && !isGetterOnly && !isInitOnly)
                 {
                     if (ContainingType.IsReadOnly)
                     {

src/Compilers/CSharp/Test/Semantic/Semantics/InitOnlyMemberTests.cs

@@ -3774,5 +3774,218 @@ public static void Main()
             var setter = (RetargetingMethodSymbol)property.SetMethod;
             Assert.True(setter.IsInitOnly);
         }
+
+        [Fact]
+        [WorkItem(47612, "https://github.com/dotnet/roslyn/issues/47612")]
+        public void InitOnlyOnReadonlyStruct_AutoProp()
+        {
+            var comp = CompileAndVerify(new[] { IsExternalInitTypeDefinition, @"
+var s = new S { I = 1 };
+System.Console.Write(s.I);
+
+public readonly struct S
+{
+    public int I { get; init; }
+}
+" }, expectedOutput: "1"); ;
+
+            comp.VerifyIL("<top-level-statements-entry-point>", @"
+{
+  // Code size       31 (0x1f)
+  .maxstack  2
+  .locals init (S V_0, //s
+                S V_1)
+  IL_0000:  ldloca.s   V_1
+  IL_0002:  initobj    ""S""
+  IL_0008:  ldloca.s   V_1
+  IL_000a:  ldc.i4.1
+  IL_000b:  call       ""void S.I.init""
+  IL_0010:  ldloc.1
+  IL_0011:  stloc.0
+  IL_0012:  ldloca.s   V_0
+  IL_0014:  call       ""int S.I.get""
+  IL_0019:  call       ""void System.Console.Write(int)""
+  IL_001e:  ret
+}
+");
+        }
+
+        [Fact]
+        [WorkItem(47612, "https://github.com/dotnet/roslyn/issues/47612")]
+        public void InitOnlyOnReadonlyStruct_ManualProp()
+        {
+            var comp = CompileAndVerify(new[] { IsExternalInitTypeDefinition, @"
+var s = new S { I = 1 };
+System.Console.Write(s.I);
+
+public readonly struct S
+{
+    private readonly int i;
+    public int I { get => i; init => i = value; }
+}
+" }, expectedOutput: "1"); ;
+
+            comp.VerifyIL("<top-level-statements-entry-point>", @"
+{
+  // Code size       31 (0x1f)
+  .maxstack  2
+  .locals init (S V_0, //s
+                S V_1)
+  IL_0000:  ldloca.s   V_1
+  IL_0002:  initobj    ""S""
+  IL_0008:  ldloca.s   V_1
+  IL_000a:  ldc.i4.1
+  IL_000b:  call       ""void S.I.init""
+  IL_0010:  ldloc.1
+  IL_0011:  stloc.0
+  IL_0012:  ldloca.s   V_0
+  IL_0014:  call       ""int S.I.get""
+  IL_0019:  call       ""void System.Console.Write(int)""
+  IL_001e:  ret
+}
+");
+        }
+
+        [Fact]
+        [WorkItem(47612, "https://github.com/dotnet/roslyn/issues/47612")]
+        public void InitOnlyOnReadonlyProperty_AutoProp()
+        {
+            var comp = CompileAndVerify(new[] { IsExternalInitTypeDefinition, @"
+var s = new S { I = 1 };
+System.Console.Write(s.I);
+
+public struct S
+{
+    public readonly int I { get; init; }
+}
+" }, expectedOutput: "1"); ;
+
+            comp.VerifyIL("<top-level-statements-entry-point>", @"
+{
+  // Code size       31 (0x1f)
+  .maxstack  2
+  .locals init (S V_0, //s
+                S V_1)
+  IL_0000:  ldloca.s   V_1
+  IL_0002:  initobj    ""S""
+  IL_0008:  ldloca.s   V_1
+  IL_000a:  ldc.i4.1
+  IL_000b:  call       ""readonly void S.I.init""
+  IL_0010:  ldloc.1
+  IL_0011:  stloc.0
+  IL_0012:  ldloca.s   V_0
+  IL_0014:  call       ""readonly int S.I.get""
+  IL_0019:  call       ""void System.Console.Write(int)""
+  IL_001e:  ret
+}
+");
+        }
+
+        [Fact]
+        [WorkItem(47612, "https://github.com/dotnet/roslyn/issues/47612")]
+        public void InitOnlyOnReadonlyProperty_ManualProp()
+        {
+            var comp = CompileAndVerify(new[] { IsExternalInitTypeDefinition, @"
+var s = new S { I = 1 };
+System.Console.Write(s.I);
+
+public struct S
+{
+    private readonly int i;
+    public readonly int I { get => i; init => i = value; }
+}
+" }, expectedOutput: "1"); ;
+
+            comp.VerifyIL("<top-level-statements-entry-point>", @"
+{
+  // Code size       31 (0x1f)
+  .maxstack  2
+  .locals init (S V_0, //s
+                S V_1)
+  IL_0000:  ldloca.s   V_1
+  IL_0002:  initobj    ""S""
+  IL_0008:  ldloca.s   V_1
+  IL_000a:  ldc.i4.1
+  IL_000b:  call       ""readonly void S.I.init""
+  IL_0010:  ldloc.1
+  IL_0011:  stloc.0
+  IL_0012:  ldloca.s   V_0
+  IL_0014:  call       ""readonly int S.I.get""
+  IL_0019:  call       ""void System.Console.Write(int)""
+  IL_001e:  ret
+}
+");
+        }
+
+        [Fact]
+        [WorkItem(47612, "https://github.com/dotnet/roslyn/issues/47612")]
+        public void InitOnlyOnReadonlyInit_AutoProp()
+        {
+            var comp = CompileAndVerify(new[] { IsExternalInitTypeDefinition, @"
+var s = new S { I = 1 };
+System.Console.Write(s.I);
+
+public struct S
+{
+    public int I { get; readonly init; }
+}
+" }, expectedOutput: "1"); ;
+
+            comp.VerifyIL("<top-level-statements-entry-point>", @"
+{
+  // Code size       31 (0x1f)
+  .maxstack  2
+  .locals init (S V_0, //s
+                S V_1)
+  IL_0000:  ldloca.s   V_1
+  IL_0002:  initobj    ""S""
+  IL_0008:  ldloca.s   V_1
+  IL_000a:  ldc.i4.1
+  IL_000b:  call       ""readonly void S.I.init""
+  IL_0010:  ldloc.1
+  IL_0011:  stloc.0
+  IL_0012:  ldloca.s   V_0
+  IL_0014:  call       ""readonly int S.I.get""
+  IL_0019:  call       ""void System.Console.Write(int)""
+  IL_001e:  ret
+}
+");
+        }
+
+        [Fact]
+        [WorkItem(47612, "https://github.com/dotnet/roslyn/issues/47612")]
+        public void InitOnlyOnReadonlyInit_ManualProp()
+        {
+            var comp = CompileAndVerify(new[] { IsExternalInitTypeDefinition, @"
+var s = new S { I = 1 };
+System.Console.Write(s.I);
+
+public struct S
+{
+    private readonly int i;
+    public int I { get => i; readonly init => i = value; }
+}
+" }, expectedOutput: "1"); ;
+
+            comp.VerifyIL("<top-level-statements-entry-point>", @"
+{
+  // Code size       31 (0x1f)
+  .maxstack  2
+  .locals init (S V_0, //s
+                S V_1)
+  IL_0000:  ldloca.s   V_1
+  IL_0002:  initobj    ""S""
+  IL_0008:  ldloca.s   V_1
+  IL_000a:  ldc.i4.1
+  IL_000b:  call       ""readonly void S.I.init""
+  IL_0010:  ldloc.1
+  IL_0011:  stloc.0
+  IL_0012:  ldloca.s   V_0
+  IL_0014:  call       ""int S.I.get""
+  IL_0019:  call       ""void System.Console.Write(int)""
+  IL_001e:  ret
+}
+");
+        }
     }
 }