Import 'futures-executor' rust crate version 0.3.4

Bug: b/151760391
Test: m crosvm.experimental
Change-Id: Ibc1d910237f83f47a4896aba84707fbaac365c10

src/enter.rs

diff --git a/src/enter.rs b/src/enter.rs
new file mode 100644
index 0000000..5895a9e
--- /dev/null
+++ b/src/enter.rs
@@ -0,0 +1,80 @@
+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 tasks, 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);
+        });
+    }
+}