Hello everyone,

If anyone has druntime + dmd + how context pointers work expertise, could you please help with reviewing this PR: https://github.com/dlang/druntime/pull/3638 ?

Thank you,
RazvanN

On Thursday, 9 December 2021 at 09:13:57 UTC, RazvanN wrote:

Like that even? oO I figured we'd just wait for @kinke to take a closer look :D

...I mean, it looks like this would have to be ported into Phobos as well, seeing as there's still this weird code duplication for those symbols going on. Though I would rather see those things removed from Phobos and aliased into core, deprecated, then deleted for good.

But I could add the Phobos change too if there's more eyes there.

On Thursday, 9 December 2021 at 09:47:10 UTC, Stanislav Blinov wrote:

Something like this is already in Phobos, under the name std.traits.hasNested.

However, the behavior is not exactly the same. The Phobos version recurses into classes, even though they are reference types, leading to results like the following:

void example()
{
    import std.traits;

    class Nested {}
    static struct NotNested
    {
        Nested nested;
    }

    pragma(msg, hasNested!NotNested); // true
}

I'm not really sure what we should do about this. The Phobos behavior seems obviously incorrect to me, but "fixing" it is a potentially-breaking change. Maybe the right thing to do is to add the fixed version to Phobos as std.traits.hasContextPointers and mark hasNested as outdated/unrecommended in the documentation. Thoughts?

On Thursday, 9 December 2021 at 15:06:24 UTC, Paul Backus wrote:

void example()
{
    import std.traits;

    class Nested {}
    static struct NotNested
    {
        Nested nested;
    }

    pragma(msg, hasNested!NotNested); // true
}

This shan't be in Phobos, it needs to be in druntime. At the moment we have the whole gang of emplace, move, etc. duplicated between the two :\ These things need to reside in core.lifetime and threabout, usable without Phobos. Phobos could public alias if need be, but certainly not the other way around :)

On Thursday, 9 December 2021 at 15:52:46 UTC, Stanislav Blinov wrote:

Yes, of course; I assumed that went without saying. My point is: we probably do not want to have duplicate implementations of this in two different places with slightly different behavior--which is where we're currently headed.

On Thursday, 9 December 2021 at 18:04:26 UTC, Paul Backus wrote:

Yup, deduplication is next on the list!

On Thursday, 9 December 2021 at 15:52:46 UTC, Stanislav Blinov wrote:

They've been moved from Phobos to druntime - but not 1:1, that's why the Phobos versions still exist. IIRC, the only reason is that the Phobos version additionally checks for self-references, which brings in an awful lot of code: https://github.com/dlang/druntime/pull/2442#issuecomment-452136871

I'd argue that self-references are invalid anyway (but not detected by the compiler) and problematic for implicit moves by the compiler - at least as long we don't have move ctors to fix up such self references. From https://dlang.org/spec/struct.html:

Wrt. the review, please bear with me, I'm pretty busy at the moment.

On Thursday, 9 December 2021 at 21:02:23 UTC, kinke wrote:

Read: interior pointers.