Add analyzer to check for closing over loop variable

[danroth27]

A lot of users are stumbling over the C# behavior of closing a delegate over a loop variable:

@for (var i = 1; i < 4; i++)
{
    <button class="btn btn-primary"
            onclick="@(e => UpdateHeading(e, i))"> // OOPS!
        Button #@i
    </button>
}

We should consider providing an analyzer that detects this case and warns.

[SQL-MisterMagoo]

I'd like to have a try at this, but would need a bit of guidance.

Would this need to go into the Razor folder with the "RZ" diagnostics in ComponentDiagnosticFactory ?
https://github.com/dotnet/aspnetcore/tree/master/src/Razor/Microsoft.AspNetCore.Razor.Language/src/Components

Or is it actually a more general diagnostic that should move up a folder into the RazorDiagnosticFactory as it could be useful in Razor Pages as well as Components?

Thanks

[SQL-MisterMagoo]

@danroth27 Realised I should probably tag someone 😄

[pranavkm]

@SQL-MisterMagoo this would likely go here: https://github.com/dotnet/aspnetcore/tree/master/src/Components/Analyzers/src.

[SQL-MisterMagoo]

@pranavkm Thanks - I'll concentrate on that area

[SQL-MisterMagoo]

@pranavkm @danroth27 Would you want a code fix along with the analyzer?

[SQL-MisterMagoo]

@pranavkm Do you have any tips on how I can test a new analyzer in a project (not unit tests/e2e tests, but manually to see if everything is working like the error list in VS looks correct etc...)?

[pranavkm]

Dupe of #25252.