C# 12: Support for collection expressions.

[michaelnebel]

In this PR we introduce extractor, QL library and dataflow support for collection expressions.
Example of collection expressions are [1, x, 3] and [2, .. y] in

var x = 6;
int[] y = [1, x, 3];
int[] z = [2, .. y];

Microsoft has documented collection expressions here. However, it is worth noting that even though the documentation states that collection expressions are also fully supported for inline arrays - but this appears not to be the case - which is indicated here.

[michaelnebel]

DCA
(1) Performance looks good on average.
(2) A lot of extraction errors have been removed from the MS repositories (it looks like the collection expression feature has been used quite a lot already). There is some discrepancy in the alerts - especially in the repositories that uses the collection expression features. I still need to look into some of these.

[tamasvajk]

LGTM. The DB stats seem to be manually added. Should the stats be generated to have real numbers?

[michaelnebel]

LGTM. The DB stats seem to be manually added. Should the stats be generated to have real numbers?

Yes, they are manually added.
@hvitved is in the process of figuring out whether we need the DB stats at all. I will postpone updating stats unless we run into performance issues, or if it turns out that they can't be scrapped all together or if I manage to finish all the C# 12 updates before we identify, whether stats can be scrapped :-D

[hvitved]

LGTM. One suggestion.

[hvitved]

I think it is better to break this up into two steps:

For [..x], add a local flow step from x to ..x, and another local flow step from ..x to [..x]. We can then implement expectsContent on the ..x node, so that flow will only be allowed when we are in fact tracking flow inside a collection.

[michaelnebel]

Wow, that is a really nice catch @hvitved !!!

[michaelnebel]

DCA (1) Performance looks good on average. (2) A lot of extraction errors have been removed from the MS repositories (it looks like the collection expression feature has been used quite a lot already). There is some discrepancy in the alerts - especially in the repositories that uses the collection expression features. I still need to look into some of these.

I did some spot checking on the alerts changes on dotnet/roslyn.

• All removed cs/unused-collection were false positives.
• Checked several cs/useless-assignment-of-local and these were false positives as well as the query couldn’t detect uses of variables in collection expressions.
• The added alerts to cs/call-to-object-tostring are true positives.

[michaelnebel]

DCA looks good.