Support treat warnings as errors while compiling

Null coalescing mutation is not killed when nullability is enforced.

public class Model
    {
        public static Model Create(string? nullable)
        {
            return new Model()
            {
                NotNullable = nullable ?? string.Empty // The actual code
                NotNullable = nullable // the survived mutation which does not actually compile if left like this in the code
            };
        }
        public string NotNullable { get; init; } = string.Empty;
    }

edit:// Updated text, because I made some mistake during initial testing which made me think that it was caused by warnings as errors, after further retesting I found out this was not the case.

[adamnova]

I see, sorry for the confusion. I considered them separately because one is done through .editorconfig and the other one is project configuration. So it was possible these were 2 separate cases and one could be more general than the other, or maybe they don’t overlap at all. I am not sure exactly how this is handled internally.

And yes, without any configuration changes the project will compile just fine in which case, mutant is absolutely valid.

[dupdob]

I think Stryker should adhere to this option (warning as errors): it would mark more mutants as CompileError, but it makes sense.