3 files changed, 127 insertions, 0 deletions
diff --git a/third_party/rust/pin-utils/src/lib.rs b/third_party/rust/pin-utils/src/lib.rs
new file mode 100644
index 0000000000..90434b5b71
--- /dev/null
+++ b/third_party/rust/pin-utils/src/lib.rs
@@ -0,0 +1,16 @@
+//! Utilities for pinning
+
+#![no_std]
+#![warn(missing_docs, missing_debug_implementations)]
+#![deny(bare_trait_objects)]
+#![allow(unknown_lints)]
+
+#![doc(html_root_url = "https://docs.rs/pin-utils/0.1.0-alpha.2")]
+
+#[doc(hidden)]
+pub mod core_reexport {
+    pub use core::*;
+}
+
+#[macro_use] mod stack_pin;
+#[macro_use] mod projection;
diff --git a/third_party/rust/pin-utils/src/projection.rs b/third_party/rust/pin-utils/src/projection.rs
new file mode 100644
index 0000000000..8abc99bbce
--- /dev/null
+++ b/third_party/rust/pin-utils/src/projection.rs
@@ -0,0 +1,88 @@
+/// A pinned projection of a struct field.
+///
+/// To make using this macro safe, three things need to be ensured:
+/// - If the struct implements [`Drop`], the [`drop`] method is not allowed to
+///   move the value of the field.
+/// - If the struct wants to implement [`Unpin`], it has to do so conditionally:
+///   The struct can only implement [`Unpin`] if the field's type is [`Unpin`].
+/// - The struct must not be `#[repr(packed)]`.
+///
+/// ```
+/// use pin_utils::unsafe_pinned;
+/// use std::marker::Unpin;
+/// use std::pin::Pin;
+///
+/// struct Foo<T> {
+///     field: T,
+/// }
+///
+/// impl<T> Foo<T> {
+///     unsafe_pinned!(field: T);
+///
+///     fn baz(mut self: Pin<&mut Self>) {
+///         let _: Pin<&mut T> = self.field(); // Pinned reference to the field
+///     }
+/// }
+///
+/// impl<T: Unpin> Unpin for Foo<T> {} // Conditional Unpin impl
+/// ```
+///
+/// Note that borrowing the field multiple times requires using `.as_mut()` to
+/// avoid consuming the `Pin`.
+///
+/// [`Unpin`]: core::marker::Unpin
+/// [`drop`]: Drop::drop
+#[macro_export]
+macro_rules! unsafe_pinned {
+    ($f:tt: $t:ty) => (
+        fn $f<'__a>(
+            self: $crate::core_reexport::pin::Pin<&'__a mut Self>
+        ) -> $crate::core_reexport::pin::Pin<&'__a mut $t> {
+            unsafe {
+                $crate::core_reexport::pin::Pin::map_unchecked_mut(
+                    self, |x| &mut x.$f
+                )
+            }
+        }
+    )
+}
+
+/// An unpinned projection of a struct field.
+///
+/// This macro is unsafe because it creates a method that returns a normal
+/// non-pin reference to the struct field. It is up to the programmer to ensure
+/// that the contained value can be considered not pinned in the current
+/// context.
+///
+/// Note that borrowing the field multiple times requires using `.as_mut()` to
+/// avoid consuming the `Pin`.
+///
+/// ```
+/// use pin_utils::unsafe_unpinned;
+/// use std::pin::Pin;
+///
+/// struct Bar;
+/// struct Foo {
+///     field: Bar,
+/// }
+///
+/// impl Foo {
+///     unsafe_unpinned!(field: Bar);
+///
+///     fn baz(mut self: Pin<&mut Self>) {
+///         let _: &mut Bar = self.field(); // Normal reference to the field
+///     }
+/// }
+/// ```
+#[macro_export]
+macro_rules! unsafe_unpinned {
+    ($f:tt: $t:ty) => (
+        fn $f<'__a>(
+            self: $crate::core_reexport::pin::Pin<&'__a mut Self>
+        ) -> &'__a mut $t {
+            unsafe {
+                &mut $crate::core_reexport::pin::Pin::get_unchecked_mut(self).$f
+            }
+        }
+    )
+}
diff --git a/third_party/rust/pin-utils/src/stack_pin.rs b/third_party/rust/pin-utils/src/stack_pin.rs
new file mode 100644
index 0000000000..997a5b9a32
--- /dev/null
+++ b/third_party/rust/pin-utils/src/stack_pin.rs
@@ -0,0 +1,23 @@
+/// Pins a value on the stack.
+///
+/// ```
+/// # use pin_utils::pin_mut;
+/// # use core::pin::Pin;
+/// # struct Foo {}
+/// let foo = Foo { /* ... */ };
+/// pin_mut!(foo);
+/// let _: Pin<&mut Foo> = foo;
+/// ```
+#[macro_export]
+macro_rules! pin_mut {
+    ($($x:ident),*) => { $(
+        // Move the value to ensure that it is owned
+        let mut $x = $x;
+        // Shadow the original binding so that it can't be directly accessed
+        // ever again.
+        #[allow(unused_mut)]
+        let mut $x = unsafe {
+            $crate::core_reexport::pin::Pin::new_unchecked(&mut $x)
+        };
+    )* }
+}