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Change upb::Arena to be Send (but still not Sync)
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PiperOrigin-RevId: 626409752
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protobuf-github-bot authored and copybara-github committed Apr 19, 2024
1 parent e98423c commit c413b12
Showing 1 changed file with 9 additions and 5 deletions.
14 changes: 9 additions & 5 deletions rust/upb/arena.rs
Original file line number Diff line number Diff line change
Expand Up @@ -22,11 +22,11 @@ const _CHECK_UPB_MALLOC_ALIGN_AT_LEAST_POINTER_ALIGNED: () =
/// This is an owning type and will automatically free the arena when
/// dropped.
///
/// Note that this type is neither `Sync` nor `Send`. The upb_Arena C object
/// could be understood as being Sync (at least vacuously, as there are no
/// const-ptr functions on upb_Arena's API), but the Rust Arena is
/// necessarily expressed as interior mutability (&self rather than &mut self
/// receivers) See https://doc.rust-lang.org/nomicon/lifetime-mismatch.html and
/// Note that this type is not `Sync` as it implements unsynchronized interior
/// mutability. The upb_Arena C object could be understood as being Sync (at
/// least vacuously under current API since there are not any const upb_Arena*
/// API functions), but the Rust Arena is necessarily expressed as interior
/// mutability (&self rather than &mut self receivers) See https://doc.rust-lang.org/nomicon/lifetime-mismatch.html and
/// https://blog.reverberate.org/2021/12/19/arenas-and-rust.html, and the
/// 'known problems' section of https://rust-lang.github.io/rust-clippy/master/index.html#/mut_from_ref.
#[derive(Debug)]
Expand All @@ -36,6 +36,10 @@ pub struct Arena {
_not_sync: PhantomData<UnsafeCell<()>>,
}

// SAFETY: `Arena` uniquely holds the underlying RawArena and has no
// thread-local data.
unsafe impl Send for Arena {}

impl Arena {
/// Allocates a fresh arena.
#[inline]
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