Experimentation with trait objects in Yoke

[sffc]

See #667

As discussed, I need a way to deal with trait objects in Yoke. There are two trait objects I care about right now:

• dyn ErasedDataStruct + 'static
• dyn SerdeSeDataStruct<'s> + 's

However, since Yokeable requires Sized, one cannot simply put the trait object as the Y type parameter in Yokeable.

Based on some discussions with @Manishearth, as well as additional experimentation, here's what I came up with.

First, we need a Sized type to represent the trait (say R). We could use plain &dyn R, and we get Yokeable for free thanks to the auto-impl, but this approach does not allow us to set the lifetime parameter of R (needed for SerdeSeDataStruct<'s>). I considered writing a general trait object wrapping struct, but this was too hard for the type system to figure out. So, I ended up writing a custom wrapper struct for each trait. I think this is fine, and it will scale, since we already need to write some boilerplate to support the two existing trait objects.

For example:

struct SerdeSeDataStructWrap<'a>(&'a dyn SerdeSeDataStruct<'a>);

Second, we need a way to transform from a normal Yoke to a trait object Yoke. @Manishearth suggested making Yoke implement the trait itself, and then boxing up the Yoke as the Cart of another Yoke. I took this idea and expressed it as a new trait, which I'm calling DynHelper:

trait DynHelper {
    type Yokeable: for<'a> Yokeable<'a>;
    type Cart: StableDeref;

    fn attach<'de>(
        data: &'de <Self::Cart as Deref>::Target,
    ) -> <Self::Yokeable as Yokeable<'de>>::Output;

    fn make_cart(input: Box<Self>) -> Self::Cart;
}

With this helper trait, it is possible to transform from Yoke<Y, C> to Yoke<RWrap, Box<dyn R>> where Y: R.

I managed to get pretty much everything working. However, I'm still hitting rust-lang/rust#85636. I tried one of the workarounds @Manishearth suggested, but haven't been able to get it to work with a trait that has its own type parameter, like SerdeSeDataStruct<'s>. Could you take a look? I left comments in the code to indicate what compiles and what doesn't compile.

[codecov-commenter]

Codecov Report

Merging #743 (223a1f2) into main (958ee68) will increase coverage by 0.06%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #743      +/-   ##
==========================================
+ Coverage   74.63%   74.69%   +0.06%     
==========================================
  Files         178      181       +3     
  Lines       10715    10765      +50     
==========================================
+ Hits         7997     8041      +44     
- Misses       2718     2724       +6     

Impacted Files	Coverage Δ
utils/yoke/src/lib.rs	100.00% <ø> (ø)
utils/yoke/src/dyn_impls.rs	90.90% <90.90%> (ø)
experimental/provider_ppucd/src/parse_ppucd.rs	92.99% <0.00%> (-0.14%)	⬇️
utils/yoke/src/yokeable.rs	75.00% <0.00%> (ø)
utils/yoke/src/yoke.rs	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 958ee68...223a1f2. Read the comment docs.

[coveralls]

Pull Request Test Coverage Report for Build 4a567d273e8be141de4b0894202e5a2b983f539b-PR-743

• 30 of 33 (90.91%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.06%) to 74.787%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
utils/yoke/src/dyn_impls.rs	30	33	90.91%

Files with Coverage Reduction	New Missed Lines	%
experimental/provider_ppucd/src/parse_ppucd.rs	1	93.0%

Totals
Change from base Build 958ee68fa4f88fd740bb02927674871f1565dba2:	0.06%
Covered Lines:	8169
Relevant Lines:	10923

---

💛 - Coveralls

[sffc]

Subsumed by #745