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#![allow(unsafe_code)]
use std::any::TypeId;
use std::cell::Cell;
use std::ptr::NonNull;
use std::sync::Arc;
thread_local! {
static CURRENT_STATE: Cell<Option<ErasedStateRef>> = Cell::new(None)
}
#[derive(Clone, Copy)]
struct ErasedStateRef {
ptr: NonNull<()>,
id: TypeId,
}
pub fn enter<S, R>(state: &Arc<S>, f: impl FnOnce() -> R) -> R
where
S: Send + Sync + 'static,
{
struct Guard<'a> {
cell: &'a Cell<Option<ErasedStateRef>>,
prev: Option<ErasedStateRef>,
}
impl Drop for Guard<'_> {
fn drop(&mut self) {
self.cell.set(self.prev)
}
}
CURRENT_STATE.with(|cell| {
let prev = cell.replace(Some(ErasedStateRef {
ptr: NonNull::from(state).cast(),
id: TypeId::of::<S>(),
}));
let _guard = Guard { cell, prev };
f()
})
}
pub fn inject<S>() -> Option<Arc<S>>
where
S: Send + Sync + 'static,
{
CURRENT_STATE.with(|cell| {
cell.get()
.filter(|s| s.id == TypeId::of::<S>())
.map(|s| unsafe { Arc::clone(s.ptr.cast::<Arc<S>>().as_ref()) })
})
}