Add validation for Params* attributes on init-only properties, fix #1877

Author: AndreyAkinshin

src/BenchmarkDotNet/Extensions/ReflectionExtensions.cs

@@ -19,6 +19,20 @@ internal static bool HasAttribute<T>(this MemberInfo? memberInfo) where T : Attr
 
         internal static bool IsNullable(this Type type) => Nullable.GetUnderlyingType(type) != null;
 
+        public static bool IsInitOnly(this PropertyInfo propertyInfo)
+        {
+            var setMethodReturnParameter = propertyInfo.SetMethod?.ReturnParameter;
+            if (setMethodReturnParameter == null)
+                return false;
+
+            var isExternalInitType = typeof(System.Runtime.CompilerServices.Unsafe).Assembly
+                .GetType("System.Runtime.CompilerServices.IsExternalInit");
+            if (isExternalInitType == null)
+                return false;
+
+            return setMethodReturnParameter.GetRequiredCustomModifiers().Contains(isExternalInitType);
+        }
+
         /// <summary>
         /// returns type name which can be used in generated C# code
         /// </summary>

src/BenchmarkDotNet/Validators/ParamsValidator.cs

@@ -46,6 +46,12 @@ private IEnumerable<ValidationError> Validate(Type type)
                     yield return new ValidationError(TreatsWarningsAsErrors,
                         $"Unable to use {name} with {attributeString} because it's a {modifier} field. Please, remove the {modifier} modifier.");
                 }
+
+                if (memberInfo is PropertyInfo propertyInfo && propertyInfo.IsInitOnly())
+                {
+                    yield return new ValidationError(TreatsWarningsAsErrors,
+                        $"Unable to use {name} with {attributeString} because it's init-only. Please, provide a public setter.");
+                }
             }
         }
     }

tests/BenchmarkDotNet.Tests/Validators/ParamsValidatorTests.cs

@@ -176,5 +176,31 @@ public class PropMultiple4 : Base
             [ParamsSource(nameof(Source))]
             public bool Input { get; set; }
         }
+
+#if NET5_0_OR_GREATER
+
+        [Fact] public void InitOnly1Test() => Check<InitOnly1>(nameof(InitOnly1.Input), "init-only", P);
+        [Fact] public void InitOnly2Test() => Check<InitOnly2>(nameof(InitOnly2.Input), "init-only", Pa);
+        [Fact] public void InitOnly3Test() => Check<InitOnly3>(nameof(InitOnly3.Input), "init-only", Ps);
+
+        public class InitOnly1 : Base
+        {
+            [Params(false, true)]
+            public bool Input { get; init; }
+        }
+
+        public class InitOnly2 : Base
+        {
+            [ParamsAllValues]
+            public bool Input { get; init; }
+        }
+
+        public class InitOnly3 : Base
+        {
+            [ParamsSource(nameof(Source))]
+            public bool Input { get; init; }
+        }
+
+#endif
     }
 }