commit 46d515c2db59f5089df0dd461d7f70dc41c97570
author Devin Jeanpierre <jeanpierreda@google.com> Tue May 03 02:36:54 2022 -0700
committer Copybara-Service <copybara-worker@google.com> Tue May 03 02:37:31 2022 -0700
tree d3e9abb5d09746f314a73fb6c3274ae1fb41733d
parent b368e68a858ff9acfd7341248aae233316ffb92a

Remove blanket `CtorNew<(x,)>` impl, in favor of manual impls.

The problem: for each T, if `X : CtorNew<T>`, then `X : CtorNew<(T,)>`, then `X : CtorNew<((T,),)>`, then...

This causes infinite overflow on failure to match:

```
error[E0275]: overflow evaluating the requirement `_: Sized`
  --> third_party/crubit/rs_bindings_from_cc/test/cc_std/test.rs:15:22
   |
15 |             let _t = tm::ctor_new(());
   |                      ^^^^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`main`)
   = note: required because of the requirements on the impl of `CtorNew<(_,)>` for `cc_std::tm`
   = note: 128 redundant requirements hidden
   = note: required because of the requirements on the impl of `CtorNew<(((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((_,),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),)>` for `cc_std::tm`
```

However, if we have exactly two impls -- for `T` and `(T,)` (but not `((T,),)` etc.), then the same error becomes:

```
error[E0277]: the trait bound `cc_std::tm: CtorNew<_>` is not satisfied
  --> third_party/crubit/rs_bindings_from_cc/test/cc_std/test.rs:15:22
   |
15 |             let _t = tm::ctor_new(());
   |                      ^^^^^^^^^^^^ the trait `CtorNew<_>` is not implemented for `cc_std::tm`
```

Much, much clearer.

Unfortunately, the only way to do this that I know of is to manually implement the `T` and `(T,)` variants. We can't automatically implement `(T,)` via blanket impl, because that will also implement `((T,),)`, etc.  And we can't start with `(T,)` and automatically implement for `T`, because this produces the same infinite recursion errors even faster (https://play.rust-lang.org/?gist=87e3bbdbe8ea6ac0e2d16f4d3a9a37b2).

So yeah, the solution would seem to be manual impls. Although exactly how to do them is up in the air -- this CL just pastes in a short snippet as an extra item. I'm not super happy with it, but there's no easy way to do it in a more structured way for now.

P.S. I swore I already had implemented this. I can't find it in my CL history, so I must have thrown it away... oops!

PiperOrigin-RevId: 446146280