Struct ContextCell 

pub struct ContextCell<T> { /* private fields */ }

A thread-local cell storing a raw pointer to the current wide event.

This type provides safe methods for setting, getting, and saving/restoring the pointer, which is used by the generated guard to manage nested scopes.

Safety Invariants

The raw pointer stored in this cell is sound because:

1. The guard owns the event. The WideEvent<K> is a field of the guard, which is on the stack (or in the scope() future’s state). The guard outlives any code that calls current() within the same scope.
2. Set on creation, cleared on drop. The guard’s new sets the pointer; its Drop restores the previous pointer (or null) before the inner ScopedGuard::Drop runs.
3. Nested scopes save/restore the previous pointer via replace and the prev_ptr field on the guard.
4. No cross-thread access. thread_local! means each thread has its own cell. For async, tokio::task_local! moves with the task across threads.

The &'static lifetime returned by accessors is a “valid for the duration of the guard” lifetime — the standard pattern used by thread_local accessors (e.g., tracing’s Span::current()).

Why UnsafeCell instead of Cell?

ContextCell needs to be Sync (so a static reference to it is usable from any thread, with the pointer contents thread-local via thread_local! mechanics). Cell<T> is !Sync. UnsafeCell<T> is Sync for any T (including raw pointers).

Internally we use UnsafeCell<*mut T> so the cell is Sync without needing an unsafe impl Sync shim, and the Stacked Borrows model under miri doesn’t see the cell’s get as a “shared” access that would conflict with a later &mut deref of the stored pointer.

Implementations

impl<T> ContextCell<T>

pub const fn new() -> Self

Create a new cell initialized to a null pointer.

pub fn get_ptr(&self) -> *mut T

Returns the stored raw pointer (null if unset).

pub fn replace(&self, ptr: *mut T) -> *mut T

Sets the stored pointer. Returns the previous pointer.

pub fn restore(&self, ptr: *mut T)

Restores a previously saved pointer (typically from replace or clear).

Trait Implementations

impl<T> Default for ContextCell<T>

fn default() -> Self

Returns the “default value” for a type.

impl<T> Send for ContextCell<T>

impl<T> Sync for ContextCell<T>