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use std::cell::Cell;
use std::fmt;
thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
/// Represents an executor context.
///
/// For more details, see [`enter` documentation](enter()).
pub struct Enter {
_priv: (),
}
/// An error returned by `enter` if an execution scope has already been
/// entered.
pub struct EnterError {
_priv: (),
}
impl fmt::Debug for EnterError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("EnterError").finish()
}
}
impl fmt::Display for EnterError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "an execution scope has already been entered")
}
}
impl std::error::Error for EnterError {}
/// Marks the current thread as being within the dynamic extent of an
/// executor.
///
/// Executor implementations should call this function before beginning to
/// execute a task, and drop the returned [`Enter`](Enter) value after
/// completing task execution:
///
/// ```
/// use futures::executor::enter;
///
/// let enter = enter().expect("...");
/// /* run task */
/// drop(enter);
/// ```
///
/// Doing so ensures that executors aren't
/// accidentally invoked in a nested fashion.
///
/// # Error
///
/// Returns an error if the current thread is already marked, in which case the
/// caller should panic with a tailored error message.
pub fn enter() -> Result<Enter, EnterError> {
ENTERED.with(|c| {
if c.get() {
Err(EnterError { _priv: () })
} else {
c.set(true);
Ok(Enter { _priv: () })
}
})
}
impl fmt::Debug for Enter {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Enter").finish()
}
}
impl Drop for Enter {
fn drop(&mut self) {
ENTERED.with(|c| {
assert!(c.get());
c.set(false);
});
}
}