From 9724f0d7947ac4c40c14729aee8d70b6a0b7bcf7 Mon Sep 17 00:00:00 2001
From: Manish Goregaokar <manishsmail@gmail.com>
Date: Fri, 30 Apr 2021 14:25:04 -0700
Subject: [PATCH] Add yoke crate

---
 Cargo.lock                 |  4 ++++
 Cargo.toml                 |  1 +
 utils/yoke/Cargo.toml      |  9 +++++++++
 utils/yoke/src/cart.rs     | 11 +++++++++++
 utils/yoke/src/lib.rs      | 11 +++++++++++
 utils/yoke/src/yoke.rs     | 36 ++++++++++++++++++++++++++++++++++++
 utils/yoke/src/yokeable.rs | 29 +++++++++++++++++++++++++++++
 7 files changed, 101 insertions(+)
 create mode 100644 utils/yoke/Cargo.toml
 create mode 100644 utils/yoke/src/cart.rs
 create mode 100644 utils/yoke/src/lib.rs
 create mode 100644 utils/yoke/src/yoke.rs
 create mode 100644 utils/yoke/src/yokeable.rs

diff --git a/Cargo.lock b/Cargo.lock
index 2a00eef0141..a31e1832c7b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2441,6 +2441,10 @@ dependencies = [
  "icu_benchmark_macros",
 ]
 
+[[package]]
+name = "yoke"
+version = "0.1.0"
+
 [[package]]
 name = "zerovec"
 version = "0.2.0"
diff --git a/Cargo.toml b/Cargo.toml
index 539f4985679..50bad3f890d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -29,6 +29,7 @@ members = [
     "utils/litemap",
     "utils/pattern",
     "utils/writeable",
+    "utils/yoke",
     "utils/zerovec",
 ]
 
diff --git a/utils/yoke/Cargo.toml b/utils/yoke/Cargo.toml
new file mode 100644
index 00000000000..9e5c6cd1ea6
--- /dev/null
+++ b/utils/yoke/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "yoke"
+version = "0.1.0"
+authors = ["Manish Goregaokar <manishsmail@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/utils/yoke/src/cart.rs b/utils/yoke/src/cart.rs
new file mode 100644
index 00000000000..539f346411c
--- /dev/null
+++ b/utils/yoke/src/cart.rs
@@ -0,0 +1,11 @@
+use std::rc::Rc;
+
+pub trait Cart {
+    fn as_bytes(&self) -> &[u8];
+}
+
+impl Cart for Rc<[u8]> {
+    fn as_bytes(&self) -> &[u8] {
+        &self
+    }
+}
diff --git a/utils/yoke/src/lib.rs b/utils/yoke/src/lib.rs
new file mode 100644
index 00000000000..72e590280c7
--- /dev/null
+++ b/utils/yoke/src/lib.rs
@@ -0,0 +1,11 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+mod cart;
+mod yoke;
+mod yokeable;
+
+pub use cart::Cart;
+pub use yoke::Yoke;
+pub use yokeable::Yokeable;
diff --git a/utils/yoke/src/yoke.rs b/utils/yoke/src/yoke.rs
new file mode 100644
index 00000000000..d0a41bcd26c
--- /dev/null
+++ b/utils/yoke/src/yoke.rs
@@ -0,0 +1,36 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+use crate::{Cart, Yokeable};
+
+pub struct Yoke<Y: for<'a> Yokeable<'a>, C: Cart> {
+    // must be the first field for drop order
+    // this will have a 'static lifetime parameter, that parameter is a lie
+    yokeable: Y,
+    cart: C,
+}
+
+impl<Y: for<'a> Yokeable<'a>, C: Cart> Yoke<Y, C> {
+    pub fn new(cart: C, yokeable: Y) -> Self {
+        Self { yokeable, cart }
+    }
+    pub fn get<'a>(&'a self) -> &'a <Y as Yokeable<'a>>::Output {
+        self.yokeable.transform()
+    }
+
+    pub fn backing_cart(&self) -> &C {
+        &self.cart
+    }
+    pub fn attach_to_cart<F>(cart: C, f: F) -> Self
+    where
+        for<'de> F: FnOnce(&'de [u8]) -> <Y as Yokeable<'de>>::Output,
+    {
+        let bytes = cart.as_bytes();
+        let deserialized = f(bytes);
+        Self {
+            yokeable: unsafe { Y::make(deserialized) },
+            cart,
+        }
+    }
+}
diff --git a/utils/yoke/src/yokeable.rs b/utils/yoke/src/yokeable.rs
new file mode 100644
index 00000000000..7f067e2a0f7
--- /dev/null
+++ b/utils/yoke/src/yokeable.rs
@@ -0,0 +1,29 @@
+use std::borrow::{Cow, ToOwned};
+use std::mem;
+
+pub unsafe trait Yokeable<'a>: 'static {
+    type Output: 'a + Sized; // MUST be `Self` with the lifetime swapped
+                             // used by `SharedData::get()`
+    fn transform(&'a self) -> &'a Self::Output;
+
+    // Used for zero-copy deserialization. Safety constraint: `Self` must be
+    // destroyed before the data `Self::Output` was deserialized
+    // from is
+    unsafe fn make(from: Self::Output) -> Self;
+}
+
+unsafe impl<'a, T: 'static + ToOwned + ?Sized> Yokeable<'a> for Cow<'static, T>
+where
+    <T as ToOwned>::Owned: Sized,
+{
+    type Output = Cow<'a, T>;
+    fn transform(&'a self) -> &'a Cow<'a, T> {
+        self
+    }
+
+    unsafe fn make(from: Cow<'a, T>) -> Self {
+        debug_assert!(mem::size_of::<Cow<'a, T>>() == mem::size_of::<Self>());
+        // i hate this
+        mem::transmute_copy(&from)
+    }
+}