commit be1efd2274002696787611ca41df1b51993d6c38
author Devin Jeanpierre <jeanpierreda@google.com> Tue May 10 16:52:07 2022 -0700
committer Copybara-Service <copybara-worker@google.com> Tue May 10 16:52:40 2022 -0700
tree ed7274bf6c0eb799dac0474b09572290c9cd4e64
parent 39777062a4a8d5472550e2e1fde068abfd5ecb4e

Implement sound `Drop` support in our little pin_project alternative.

Unlike `pin_project`, the function is just marked unsafe to make it sound (as it's not safe to drop a struct). This is maybe a slight ergonomic hit, but for the most part we don't expect people to implement this manually -- in the near term, it's only going to be autogenerated as part of bindings generation. Also, it's not _that_ bad. Worst case, we could define a macro much like `pin_project` does, to make it look like you're defining a safe function.

Otherwise, this follows relatively closely with the approach described in https://docs.rs/pin-project/latest/pin_project/attr.pin_project.html#safety

After this CL, the pin projection implemented here should be sound, without safety holes that would allow UB in safe code. Followup CLs will actually start using this in interop, so that we can use the `ctor!()` macro.

---

Error scenarios and their error messages, tested by hand:

defined PinnedDrop without PinnedDrop enabled:

```
error[E0119]: conflicting implementations of trait `ctor::PinnedDrop` for type `test_pinned_drop::DropStruct`
   --> third_party/crubit/rs_bindings_from_cc/support/ctor_proc_macros_test.rs:201:5
    |
201 |     #[::ctor::recursively_pinned]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `test_pinned_drop::DropStruct`
202 |     struct DropStruct(Rc<Cell<bool>>);
203 |     impl ::ctor::PinnedDrop for DropStruct {
    |     -------------------------------------- first implementation here
    |
    = note: this error originates in the attribute macro `::ctor::recursively_pinned` (in Nightly builds, run with -Z macro-backtrace for more info)
```

forgot to define PinnedDrop, with PinnedDrop enabled:

```
error[E0277]: the trait bound `DropStruct: PinnedDrop` is not satisfied
   --> third_party/crubit/rs_bindings_from_cc/support/ctor_proc_macros_test.rs:201:5
    |
201 |     #[::ctor::recursively_pinned(PinnedDrop)]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `PinnedDrop` is not implemented for `DropStruct`
    |
    = help: the following other types implement trait `PinnedDrop`:
            ...
    = note: this error originates in the attribute macro `::ctor::recursively_pinned` (in Nightly builds, run with -Z macro-backtrace for more info)
```

defined Drop (without PinnedDrop enabled)

```
error[E0119]: conflicting implementations of trait `ctor::macro_internal::DoNotImplDrop` for type `test_pinned_drop::DropStruct`
   --> third_party/crubit/rs_bindings_from_cc/support/ctor_proc_macros_test.rs:201:5
    |
201 |     #[::ctor::recursively_pinned]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: conflicting implementation in crate `ctor`:
            - impl<T> DoNotImplDrop for T
              where T: Drop;
    = note: this error originates in the attribute macro `::ctor::recursively_pinned` (in Nightly builds, run with -Z macro-backtrace for more info)
```

defined Drop (*with* PinnedDrop enabled), instead of PinnedDrop:

```
error[E0119]: conflicting implementations of trait `std::ops::Drop` for type `test_pinned_drop::DropStruct`
   --> third_party/crubit/rs_bindings_from_cc/support/ctor_proc_macros_test.rs:201:5
    |
201 |     #[::ctor::recursively_pinned(PinnedDrop)]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `test_pinned_drop::DropStruct`
...
208 |     impl Drop for DropStruct {
    |     ------------------------ first implementation here
    |
    = note: this error originates in the attribute macro `::ctor::recursively_pinned` (in Nightly builds, run with -Z macro-backtrace for more info)
```

PiperOrigin-RevId: 447860359