Anlayzer to warn against multiple evaluations of LINQ deferred execution query in a function

[bretehlert]

We recently ran across this code:

var succeeded = flatResults.Where(r => r.Value.Outcome.IsSuccessful() || r.Value.Outcome.IsDisabledOrInactive()).Select(r => r.Key.Code);

var failed = flatResults.Where(r => !succeeded.Contains(r.Key.Code));
if (failed.Any())
{
	foreach (var failedTask in failed)
	{
		ReportNudgeFailed(new[] { failedTask.Key.Code }, failedTask.Value, 0);
	}
}

if (succeeded.Any())
{
	ReportNudgeSucceeded(succeeded);
}

The result of this code will be that the LINQ expression to calculate succeeded will be executed many times. We also execute the expression to calculate failed multiple times, there is no point in doing an Any() before the enumeration.

Add a rule or set of rules to warn against multiple evaluations of a LINQ query within the same function.

[brian-reichle]

This can get a little tricky to detect, particularly if the field that Any() is called on gets reused for different collections.

foreach (var entity in entities)
{
    var stuff = entity.Stuff;
    stuff = stuff.Where(x => x.IsMagic);

    if (stuff.Any())
    {
        // ...
    }
}

At the very least, this would probably require the flow graph stuff, which would require that we upgrade the version of Roslyn we build against to somewhere around 2.6.0 (i think).