Method return types: Non-nullable reference, nullable reference, struct and Nullable<struct>.

NullableTest.cs

using System;
using System.Linq;
using Xunit;

namespace Something
{
    public class NullableTest
    {
        public struct FooS
        {
        }
        public class Foo
        {
            public Foo NonNullable() => new Foo();
            public Foo? Nullable() => null;
            public FooS Struct() => new FooS();
            public FooS? NullableStruct() => null;
        }

        [Fact]
        public void NonNullableReturnTypeMustHaveNullableFlagsOf1()
        {
            var sut = typeof(Foo).GetMethod(nameof(Foo.NonNullable));
            Assert.Equal(typeof(Foo), sut.ReturnType);

            var attr = sut.ReturnTypeCustomAttributes
                .GetCustomAttributes(true)
                .Single(a => a.GetType().Name.Equals("NullableAttribute"));
            var flag = attr.GetType().GetField("NullableFlags").GetValue(attr);
            Assert.Equal(new byte[] { 1 }, flag);
        }

        [Fact]
        public void NullableReturnTypeMustHaveNullableFlagsOf2()
        {
            var sut = typeof(Foo).GetMethod(nameof(Foo.Nullable));
            Assert.Equal(typeof(Foo?), sut.ReturnType);

            var attr = sut.ReturnTypeCustomAttributes
                .GetCustomAttributes(true)
                .Single(a => a.GetType().Name.Equals("NullableAttribute"));
            var flag = attr.GetType().GetField("NullableFlags").GetValue(attr);
            Assert.Equal(new byte[] { 2 }, flag);
        }

        [Fact]
        public void StructReturnTypeHaveNoNullableAttribute()
        {
            var sut = typeof(Foo).GetMethod(nameof(Foo.Struct));
            Assert.Equal(typeof(FooS), sut.ReturnType);

            var hasAttr = sut.ReturnTypeCustomAttributes
                .GetCustomAttributes(true)
                .Any(a => a.GetType().Name.Equals("NullableAttribute"));
            Assert.False(hasAttr);
        }

        [Fact]
        public void NullableStructReturnTypeIsNullableGenericDefinition()
        {
            var sut = typeof(Foo).GetMethod(nameof(Foo.NullableStruct));
            Assert.Equal(typeof(FooS?), sut.ReturnType);
            Assert.Equal(typeof(Nullable<>), sut.ReturnType.GetGenericTypeDefinition());
        }
    }
}

• System.Runtime.CompilerServices.NullableAttribute Attribute will be created dynamically in each assemblies.
• The attribute will be attached to the ReturnTypeCustomAttributes of all methods that returns reference type.
• The attribute has NullableFlags field with type of byte[]. { 1 } for non-nullable reference, { 2 } for nullable reference.
• Both struct and Nullable return type doesn't have the attribute.