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diff --git a/third_party/rust/threadbound/src/lib.rs b/third_party/rust/threadbound/src/lib.rs
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+//! [![github]](https://github.com/dtolnay/threadbound)&ensp;[![crates-io]](https://crates.io/crates/threadbound)&ensp;[![docs-rs]](https://docs.rs/threadbound)
+//!
+//! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
+//! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust
+//! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs
+//!
+//! <br>
+//!
+//! [`ThreadBound<T>`] is a wrapper that binds a value to its original thread.
+//! The wrapper gets to be [`Sync`] and [`Send`] but only the original thread on
+//! which the ThreadBound was constructed can retrieve the underlying value.
+//!
+//! [`ThreadBound<T>`]: struct.ThreadBound.html
+//! [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
+//! [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
+//!
+//! # Example
+//!
+//! ```
+//! use std::marker::PhantomData;
+//! use std::rc::Rc;
+//! use std::sync::Arc;
+//! use threadbound::ThreadBound;
+//!
+//! // Neither Send nor Sync. Maybe the index points into a
+//! // thread-local interner.
+//! #[derive(Copy, Clone)]
+//! struct Span {
+//!     index: u32,
+//!     marker: PhantomData<Rc<()>>,
+//! }
+//!
+//! // Error types are always supposed to be Send and Sync.
+//! // We can use ThreadBound to make it so.
+//! struct Error {
+//!     span: ThreadBound<Span>,
+//!     message: String,
+//! }
+//!
+//! fn main() {
+//!     let err = Error {
+//!         span: ThreadBound::new(Span {
+//!             index: 99,
+//!             marker: PhantomData,
+//!         }),
+//!         message: "fearless concurrency".to_owned(),
+//!     };
+//!
+//!     // Original thread can see the contents.
+//!     assert_eq!(err.span.get_ref().unwrap().index, 99);
+//!
+//!     let err = Arc::new(err);
+//!     let err2 = err.clone();
+//!     std::thread::spawn(move || {
+//!         // Other threads cannot get access. Maybe they use
+//!         // a default value or a different codepath.
+//!         assert!(err2.span.get_ref().is_none());
+//!     });
+//!
+//!     // Original thread can still see the contents.
+//!     assert_eq!(err.span.get_ref().unwrap().index, 99);
+//! }
+//! ```
+
+#![doc(html_root_url = "https://docs.rs/threadbound/0.1.5")]
+#![allow(clippy::doc_markdown)]
+
+use std::fmt::{self, Debug};
+use std::thread::{self, ThreadId};
+
+/// ThreadBound is a Sync-maker and Send-maker that allows accessing a value
+/// of type T only from the original thread on which the ThreadBound was
+/// constructed.
+///
+/// Refer to the [crate-level documentation] for a usage example.
+///
+/// [crate-level documentation]: index.html
+pub struct ThreadBound<T> {
+    value: T,
+    thread_id: ThreadId,
+}
+
+unsafe impl<T> Sync for ThreadBound<T> {}
+
+// Send bound requires Copy, as otherwise Drop could run in the wrong place.
+unsafe impl<T: Copy> Send for ThreadBound<T> {}
+
+impl<T> ThreadBound<T> {
+    /// Binds a value to the current thread. The wrapper can be sent around to
+    /// other threads, but no other threads will be able to access the
+    /// underlying value.
+    pub fn new(value: T) -> Self {
+        ThreadBound {
+            value,
+            thread_id: thread::current().id(),
+        }
+    }
+
+    /// Accesses a reference to the underlying value if this is its original
+    /// thread, otherwise `None`.
+    pub fn get_ref(&self) -> Option<&T> {
+        if thread::current().id() == self.thread_id {
+            Some(&self.value)
+        } else {
+            None
+        }
+    }
+
+    /// Accesses a mutable reference to the underlying value if this is its
+    /// original thread, otherwise `None`.
+    pub fn get_mut(&mut self) -> Option<&mut T> {
+        if thread::current().id() == self.thread_id {
+            Some(&mut self.value)
+        } else {
+            None
+        }
+    }
+
+    /// Extracts ownership of the underlying value if this is its original
+    /// thread, otherwise `None`.
+    pub fn into_inner(self) -> Option<T> {
+        if thread::current().id() == self.thread_id {
+            Some(self.value)
+        } else {
+            None
+        }
+    }
+}
+
+impl<T: Default> Default for ThreadBound<T> {
+    fn default() -> Self {
+        ThreadBound::new(Default::default())
+    }
+}
+
+impl<T: Debug> Debug for ThreadBound<T> {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match self.get_ref() {
+            Some(value) => Debug::fmt(value, formatter),
+            None => formatter.write_str("unknown"),
+        }
+    }
+}