https://github.com/dotnet/coreclr/pull/25883

Program.cs

using System.Collections.Generic;
using System.Runtime.CompilerServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public class Program
{
    static void Main() => BenchmarkRunner.Run<Benchmark<string>>();
}

public class Benchmark<T>
{
    [Benchmark]
    public IEqualityComparer<T> DirectCast() => (IEqualityComparer<T>)MyComparer.Default;

    [Benchmark]
    public IEqualityComparer<T> UnsafeAs() => Unsafe.As<IEqualityComparer<T>>(MyComparer.Default);

    [Benchmark]
    public IEqualityComparer<T> InvariantWrapperCast() => ((Wrapper<IEqualityComparer<T>>)(object)MyComparer.DefaultWrapper).Value;
}

class MyComparer : IEqualityComparer<string>
{
    public static IEqualityComparer<string> Default { get; } = new MyComparer();

    public static Wrapper<IEqualityComparer<string>> DefaultWrapper { get; } = new Wrapper<IEqualityComparer<string>>(Default);

    public bool Equals(string x, string y) => string.Equals(x, y);

    public int GetHashCode(string obj) => obj?.GetHashCode() ?? 0;
}

class Wrapper<T>
{
    public T Value { get; }

    public Wrapper(T value) => Value = value;
}

results.md

BenchmarkDotNet=v0.11.5, OS=Windows 10.0.18362
Intel Core i5-2300 CPU 2.80GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET Core SDK=3.0.100-preview6-012264
  [Host]     : .NET Core 3.0.0-preview6-27804-01 (CoreCLR 4.700.19.30373, CoreFX 4.700.19.30308), 64bit RyuJIT
  DefaultJob : .NET Core 3.0.0-preview6-27804-01 (CoreCLR 4.700.19.30373, CoreFX 4.700.19.30308), 64bit RyuJIT

Method	Mean	Error	StdDev	Median
DirectCast	72.1164 ns	0.4131 ns	0.3864 ns	71.9603 ns
UnsafeAs	0.0026 ns	0.0067 ns	0.0062 ns	0.0000 ns
InvariantWrapperCast	2.3557 ns	0.0189 ns	0.0168 ns	2.3577 ns

// * Warnings *
ZeroMeasurement
  Benchmark<String>.UnsafeAs: Default -> The method duration is indistinguishable from the empty method duration