Analyzer to guard against comparing struct to null

[AmadeusW]

Describe the problem you are trying to solve

We checked if a variable of struct type is null. The check was negative, and we took incorrect code path.
We should have checked if the variable is default. This would have resulted in positive check, and we would have taken correct code path.

I would like to guard against such with an analyzer.
Does an analyzer like this already exist?

Describe suggestions on how to achieve the rule

1. Look for comparison between value type and null (== null or != null)
2. The fix is to replace null with default

I guess implementation could be similar to one of CA2242

Additional context

Sample scenario

struct MyStructWithObject
{
    object MyObject {get;}
    public myStruct(object o)
    {
        MyObject = o ?? throw new ArgumentNullException(nameof(o));
    }
}

// Code which triggers NullReferenceException
GetText(default);

// Analyzer should raise a warning in this method
string GetText(MyStructWithObject s)
{
    if (s == null) // Evaluates to false, branch not taken
    {
        return string.Empty;
    }
    return s.MyObject.ToString(); // Throws NullReferenceException
}

// Fixed
string GetText(MyStructWithObject s)
{
    if (s == default) // Fixed
    {
        return string.Empty;
    }
    return s.MyObject.ToString();
}

[mavasani]

Tagging @jmarolf @gafter @jaredpar - I believe this is one of the diagnostic planned for compiler warning wave. If not, this should certainly be a good candidate for warning level analyzer diagnostic that ships with the .NET SDK.

[jmarolf]

I believe this is exactly encompasses dotnet/roslyn#45744 and is already checked into roslyn master

[mavasani]

Great, closing this as external!

[AmadeusW]

Thanks @jmarolf! I read through the PR you linked and its linked PRs, and I'm curious how to enable this.

• Will compiler shipping with VS 16.8 allow me to use <AnalysisLevel>5</AnalysisLevel>?
• Is there a way to opt into seeing CS8073 on a build machine which hasn't been updated to VS 16.8? Is <Features>strict</Features>the way to go?
• Additionally, do I need to enable nullability to see CS8073?

[jmarolf]

<WarningLevel>5</WarningLevel> is probably the way to go. I have no idea how CoreXT targets work so I am unable to speculate what other msbuild options will function correctly. <AnalysisLevel>5</AnalysisLevel> is an alias for this that also enable .NET platform analyzers. If all you care about is getting the new warnings from the compiler setting WarningLevel to 5 should do what you want.