refactor: avoid capturing this in delegate using local self pointer

[gorsing]

To avoid implicit capturing of this in a delegate,
replaced direct field access with access via a local pointer self cast to Impl*.

Old similar and alternative #10816

This is a preventive fix for a potential Phobos error related to delegate lifetime,
as seen in: dlang/dmd#21514

No functional behavior change is intended.

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Testing this PR locally

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dub run digger -- build "master + phobos#10821"

[0xEAB]

Thanks for your contribution, @gorsing :)

Would you mind adding a unittest that showcases the advantage of this change? (Assuming that this is possible. If not, don’t bother.) Doing so could also potentially prevent accidental breakage caused by future changes.

[gorsing]

@0xEAB Changes aimed at eliminating leaks in closures
The behavior has not changed.
Unfortunately, I haven’t been able to come up with a visual test yet.
Perhaps dlang/dmd#21514 will be adopted in some form

[gorsing]

Hi @CyberShadow , how long merge will be review?

[CyberShadow]

This introduces more complexity in an already fairly-complex bit of code. I can't approve this in good faith.

The cast to void* and back is especially scary. I wouldn't envy anyone who would need to look at this code in the future, after this patch.

Could we do something else here? For example:

static struct HeaderReceiver
{
    // add any necessary state here...
    // ...
    void receiveHeader(in char[] header)
    {
        //  ... put logic here ...
    }
}
HeaderReceiver headerReceiver;
headerReceiver.stateVar1 = ...; // maybe even put &this here if really needed
curl.onReceiveHeader = &headerReceiver.receiveHeader;
// ...