C# 14: Support extension types.

[michaelnebel]

In this PR we introduce support for extension types (blocks). The feature is explained in detail here.
The feature generalises extensions to operators and properties.

Here is a small example of an declaration and how it can be invoked.

public static class MyExtensions
{
    extension(string s)
    {
        public bool IsValid() { return s is not null; }
    }
}

public class A
{
    public void M()
    {
        var s = "Hello World";
        s.IsValid(); // Call to extension method.
        MyExtensions.IsValid(s) // Call to compiler generated static method.
    }
}

It turns out that Roslyn generates a static method corresponding to the extension method. To avoid extracting multiple methods with identical bodies (which would further complicate the QL implementation) we

• Only extract the IsValid() method, which has the qualified name MyExtensions.extension(string).IsValid.
• Add a parameter s to IsValid() corresponding to the receiver parameter s. This needs to be synthesised as we can't re-use the parameter from the extension declaration.
• Replace invocations of MyExtensions.IsValid with MyExtensions.extensions(string).IsValid.

DCA looks good.

• Performance appears to be un-affected.
• Nice reduction in the number of missing call targets for a range of projects (and an increase in the number of alerts).
• There are some "new" extraction errors for the dotnet/runtime project. These extraction errors are because some of the extensions (that we now extract) adds user-defined compound assignment operators (described here), which are not supported yet.

[michaelnebel]

@hvitved : The PR is ready for review. However, I haven't added any upgrade/downgrade scripts yet in case the review leads to a change in the DB scheme. Review on commit by commit basis is encouraged.

[hvitved]

Impressive work! Some mostly minor comments

[hvitved]

This ought to use the FQN of the type being extended, i.e. TestExtensions+extension(System.Object)

[michaelnebel]

@hvitved Added some commits which addresses your comments, added DB upgrade/downgrade scripts (not intending to make the DB downgrade script delete all the extension members and types from the DB). Also made a minor modification to one of the QL4QL queries.

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[michaelnebel]

The last DCA run looks a bit skewed in terms of alert diff - this is most likely because one of the DCA variants for mono is still listed as "running". However, the other DCA numbers look similar to the first execution (before addressing the review comments). I think we should accept it as is it is.

[hvitved]

🎉