Ignore conditional block when condition is CancellationToken.IsCancellationRequested

[skarllot]

If the asynchronous method has a conditional block testing if CancellationToken is canceled that block should be ignored.

public ValueTask Test(CancellationToken cancellationToken)
{
    if (cancellationToken.IsCancellationRequested)
    {
        // ...
        return ValueTask.FromCanceled(cancellationToken);
    }
    // ...
}

[lsoft]

@skarllot not sure that ISG should care about this issue. Imagine more complex if statements like if (cancellationToken.IsCancellationRequested && ...). removing conditional subtree is not an option here, ISG will have to rewrite source code in a complex way like replacing cancellationToken.IsCancellationRequested with false. Not sure ISG should perform such complex rewriting, let's wait for @virzak resolution.

I guess this case is the case for #if SYNC_ONLY.

[skarllot]

On second thought I believe you are right, I prefer using compiler conditional.

[virzak]

Great issue.

The source generator has already picked an aggressive/advanced way of dealing with such issues. The goal is to make is as close to the code one would write by hand.

For example if it sees IProgress<int> used like this:

if (true)
{
    progress.Report(1);
}

the entire if block will be dropped.

This approach did land us in a bit of a hot water, for example in #45 (comment)

I think that the example above should be dropped without forcing the user to invoke SYNC_ONLY, just because of how common that scenario can be.

When combined with && it should be dropped as well.
When negated (!) the body should get unrolled.

If there's while instead of if, the block should be dropped for ct.IsCancellationRequested and be made while (true) for !ct.IsCancellationRequested

Looking at code occurrences under dotnet org, the most common uses are:

• while (!ct.IsCancellationRequested)
• if (ct.IsCancellationRequested)

[virzak]

This fix is available in 1.3.35. Let me know how this works out.

At the end not unrolling the if block because you could have a duplicate variable:

if (true)
{
    var myVar = 1;
}

if (true)
{
    var myVar = 1;
}

Taking either myVar out of scope would cause a compilation issue.