1 files changed, 77 insertions, 0 deletions
diff --git a/third_party/rust/tokio/src/util/bit.rs b/third_party/rust/tokio/src/util/bit.rs
new file mode 100644
index 0000000000..a43c2c2d36
--- /dev/null
+++ b/third_party/rust/tokio/src/util/bit.rs
@@ -0,0 +1,77 @@
+use std::fmt;
+
+#[derive(Clone, Copy, PartialEq)]
+pub(crate) struct Pack {
+    mask: usize,
+    shift: u32,
+}
+
+impl Pack {
+    /// Value is packed in the `width` least-significant bits.
+    pub(crate) const fn least_significant(width: u32) -> Pack {
+        let mask = mask_for(width);
+
+        Pack { mask, shift: 0 }
+    }
+
+    /// Value is packed in the `width` more-significant bits.
+    pub(crate) const fn then(&self, width: u32) -> Pack {
+        let shift = pointer_width() - self.mask.leading_zeros();
+        let mask = mask_for(width) << shift;
+
+        Pack { mask, shift }
+    }
+
+    /// Width, in bits, dedicated to storing the value.
+    pub(crate) const fn width(&self) -> u32 {
+        pointer_width() - (self.mask >> self.shift).leading_zeros()
+    }
+
+    /// Max representable value.
+    pub(crate) const fn max_value(&self) -> usize {
+        (1 << self.width()) - 1
+    }
+
+    pub(crate) fn pack(&self, value: usize, base: usize) -> usize {
+        assert!(value <= self.max_value());
+        (base & !self.mask) | (value << self.shift)
+    }
+
+    /// Packs the value with `base`, losing any bits of `value` that fit.
+    ///
+    /// If `value` is larger than the max value that can be represented by the
+    /// allotted width, the most significant bits are truncated.
+    pub(crate) fn pack_lossy(&self, value: usize, base: usize) -> usize {
+        self.pack(value & self.max_value(), base)
+    }
+
+    pub(crate) fn unpack(&self, src: usize) -> usize {
+        unpack(src, self.mask, self.shift)
+    }
+}
+
+impl fmt::Debug for Pack {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            fmt,
+            "Pack {{ mask: {:b}, shift: {} }}",
+            self.mask, self.shift
+        )
+    }
+}
+
+/// Returns the width of a pointer in bits.
+pub(crate) const fn pointer_width() -> u32 {
+    std::mem::size_of::<usize>() as u32 * 8
+}
+
+/// Returns a `usize` with the right-most `n` bits set.
+pub(crate) const fn mask_for(n: u32) -> usize {
+    let shift = 1usize.wrapping_shl(n - 1);
+    shift | (shift - 1)
+}
+
+/// Unpacks a value using a mask & shift.
+pub(crate) const fn unpack(src: usize, mask: usize, shift: u32) -> usize {
+    (src & mask) >> shift
+}