Add projection utilities to Yoke

[Manishearth]

Fixes #829

Discovered another compiler bug in the process 🙃 (rust-lang/rust#86702)

I also figured out a better workaround for rust-lang/rust#84937 and used it here: there's a separate version of this function with explicit capturing that can be used until we can use closures here.

Fallible try_ versions of this can be added if necessary.

[Manishearth]

The ICE is due to rust-lang/rust#86703 .

[mcy]

Is this list complete? I'm struggling to convince myself it is. Here's a proof sketch that feels right but which I'm not willing to bet on:

If Yoke<Y> is destroyed, it must not invalidate Yoke<P>. The yoked data will still be alive, because of CloneableCart, so it remains to check that we don't keep any of the underlying bookkeeping data that Y would be entitled to destroy. We can't hold onto any data that lives only for 'y (the lifetime of the Y object) so data owned by Y is out. So we can only copy data from Y, so it remains to check that copying non-'static data is ok, which reduces to checking if copying references is ok.

If the reference has the special for<'a> yoke lifetime, that means it's attached to the cart, so it can be copied, since it's the same yoke lifetime on both sides. If the reference has some other lifetime, like if we were converting Y<'static, 'b> -> P<'static>, then we can only copy it if P can express 'b separately from the yoke lifetime, since the borrowed content is not yoked.

Now, the part I'm not sure on is that I've referred to "the" yoke lifetime, but one could conceive of multiple yoke lifetimes being active simultaneously? Like can you wind up in a situation where you have something silly like

struct NestedYokeable<'y> {
  inner: Box<Yoke<NestedYokeable<'y>, Cart>>,
  ref: &'y i32,
}

Could you sneak yoked content intended for the outer yoke into the inner yoke, thus breaking the guarantee we otherwise get from CloneableCart? Can a value of this type even be constructed safely?

[Manishearth]

If you sneak in other lifetimes they're like adding more existential parameters, essentially.

Note that the Yokeable used in a Yoke must be 'static, you can only sneak in other lifetimes via the cart. Yokes only have one "the" yoke lifetime because of Yokeable, all other lifetimes are in the cart and would constrain both sides in an acceptable way.

[mcy]

I guess you could do

y.project(|y, _| y.inner.as_ref().clone())

but that's fine, since the lifetimes match up anyways (since you've cloned).

I think I'm pretty convinced this is correct. Huzzah.

[sffc]

Just some bikeshedding.

[sffc]

Nice! (Although we will need to fix the coverage CI before this is mergeable)

[Manishearth]

Yeah the only fix is to ignore that doctest, which I did

[codecov-commenter]

Codecov Report

Merging #833 (b2f2e84) into main (67bd340) will decrease coverage by 0.08%.
The diff coverage is 13.33%.

@@            Coverage Diff             @@
##             main     #833      +/-   ##
==========================================
- Coverage   74.80%   74.71%   -0.09%     
==========================================
  Files         198      198              
  Lines       12762    12777      +15     
==========================================
  Hits         9546     9546              
- Misses       3216     3231      +15     

Impacted Files	Coverage Δ
provider/core/src/data_provider/test.rs	88.68% <0.00%> (-1.23%)	⬇️
provider/core/src/hello_world.rs	76.56% <0.00%> (-3.77%)	⬇️
provider/core/src/marker/macros.rs	72.72% <0.00%> (-27.28%)	⬇️
utils/yoke/src/lib.rs	100.00% <ø> (ø)
utils/yoke/src/trait_hack.rs	0.00% <ø> (ø)
utils/yoke/src/yoke.rs	88.88% <ø> (ø)
utils/yoke/src/yokeable.rs	81.81% <ø> (ø)
utils/yoke/src/zero_copy_from.rs	88.23% <ø> (ø)
provider/core/src/serde.rs	44.44% <66.66%> (-2.62%)	⬇️
provider/core/src/dynutil.rs	38.88% <0.00%> (ø)

---

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[coveralls]

Pull Request Test Coverage Report for Build 0660b94ac44c19e2bb9274e1eef7c053647cd2f4-PR-833

• 2 of 15 (13.33%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 74.789%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
provider/core/src/serde.rs	2	3	66.67%
provider/core/src/data_provider/test.rs	0	3	0.0%
provider/core/src/hello_world.rs	0	3	0.0%
provider/core/src/marker/macros.rs	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
provider/core/src/serde.rs	2	44.44%

Totals
Change from base Build 67bd340dd7cb6e1a958ceb36f8f1d4e73c63742e:	-0.09%
Covered Lines:	9674
Relevant Lines:	12935

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💛 - Coveralls