Refactor vtable format for upcoming trait_upcasting feature.

[crlf0710]

This modifies vtable format:

1. reordering occurrence order of methods coming from different traits
2. include VPtrs for supertraits where this vtable cannot be directly reused during trait upcasting.
   Also, during codegen, the vtables corresponding to these newly included VPtr will be requested and generated.

For the cases where this vtable can directly used, now the super trait vtable has exactly the same content to some prefix of this one.

r? @bjorn3
cc @RalfJung
cc @rust-lang/wg-traits

[rust-highfive]

Some changes occured to the CTFE / Miri engine

cc @rust-lang/miri

Some changes occured to rustc_codegen_cranelift

cc @bjorn3

[crlf0710]

So it seems there's still something wrong with the subst_supertrait related part...

[RalfJung]

reordering components

To be clear, the 3 default components are still in the same place? If those move around, we'll have to adjust some CTFE and Miri tests.

[RalfJung]

Also, so far nothing is using those new components yet, right?

[crlf0710]

1. Yes, the three default components location are not changed at all.
2. Yes, currently there's nothing that reads the newly added VPtrs embedded within the vtable. This is not covered by any tests yet, will be covered by the trait_upcasting unit tests.
3. Some methods will be moved to using another slot within the vtable. I think this is covered by existing unit tests.

[RalfJung]

Okay, 👍 from the Miri/CTFE side then.

Yes, currently there's nothing that reads the newly added VPtrs embedded within the vtable. This is not covered by any tests yet, will be covered by the trait_upcasting unit tests.

If this required codegen changes (which I assume it will), then Miri will also need updating. I'm happy to help with that. :)

[crlf0710]

@RalfJung It will. Let's resume this discussion on Zulip: https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/object.20upcasting .

[crlf0710]

Blocked on #86475 and after that this change will be smaller.

[bors]

☔ The latest upstream changes (presumably #86475) made this pull request unmergeable. Please resolve the merge conflicts.

[crlf0710]

#86475 has landed and this is unblocked.

[nikomatsakis]

r=me but:

• can we include the examples from the documentation as tests?
• it'd be good to see a test with methods that cannot be called (because of irreconcilable where clauses)

[nikomatsakis]

@bors delegate+

[bors]

✌️ @crlf0710 can now approve this pull request

[crlf0710]

@bors r=nikomatsakis

[bors]

📌 Commit fbb353a has been approved by nikomatsakis

[bors]

⌛ Testing commit fbb353a with merge 48e11d475136cc2161215b068c1f6b3182116276...

[bors]

💔 Test failed - checks-actions

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[crlf0710]

@bors retry

[bors]

⌛ Testing commit fbb353a with merge f9b95f9...

[bors]

☀️ Test successful - checks-actions
Approved by: nikomatsakis
Pushing f9b95f9 to master...