fix(serde_v8): update bytes::Bytes layout assumptions (#15718)

Cargo.lock

@@ -398,9 +398,9 @@ checksum = "14c189c53d098945499cdfa7ecc63567cf3886b3332b312a5b4585d8d3a6a610"
 
 [[package]]
 name = "bytes"
-version = "1.1.0"
+version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c4872d67bab6358e59559027aa3b9157c53d9358c51423c17554809a8858e0f8"
+checksum = "ec8a7b6a70fde80372154c65702f00a0f56f3e1c36abbc6c440484be248856db"
 
 [[package]]
 name = "cache_control"

serde_v8/Cargo.toml

@@ -13,7 +13,7 @@ description = "Rust to V8 serialization and deserialization"
 path = "lib.rs"
 
 [dependencies]
-bytes = "1"
+bytes = "=1.2.1"
 derive_more = "0.99.17"
 serde = { version = "1.0.136", features = ["derive"] }
 serde_bytes = "0.11"

serde_v8/magic/rawbytes.rs

@@ -26,11 +26,25 @@ impl RawBytes {
   }
 }
 
+// Validate some bytes::Bytes layout assumptions at compile time.
+const _: () = {
+  assert!(
+    core::mem::size_of::<RawBytes>() == core::mem::size_of::<bytes::Bytes>(),
+  );
+  assert!(
+    core::mem::align_of::<RawBytes>() == core::mem::align_of::<bytes::Bytes>(),
+  );
+};
+
 #[allow(unused)]
 pub(crate) struct Vtable {
   /// fn(data, ptr, len)
   pub clone: unsafe fn(&AtomicPtr<()>, *const u8, usize) -> bytes::Bytes,
   /// fn(data, ptr, len)
+  ///
+  /// takes `Bytes` to value
+  pub to_vec: unsafe fn(&AtomicPtr<()>, *const u8, usize) -> Vec<u8>,
+  /// fn(data, ptr, len)
   pub drop: unsafe fn(&mut AtomicPtr<()>, *const u8, usize),
 }
 
@@ -55,6 +69,7 @@ mod tests {
   const STATIC_VTABLE: Vtable = Vtable {
     clone: static_clone,
     drop: static_drop,
+    to_vec: static_to_vec,
   };
 
   unsafe fn static_clone(
@@ -65,6 +80,15 @@ mod tests {
     from_static(std::slice::from_raw_parts(ptr, len)).into()
   }
 
+  unsafe fn static_to_vec(
+    _: &AtomicPtr<()>,
+    ptr: *const u8,
+    len: usize,
+  ) -> Vec<u8> {
+    let slice = std::slice::from_raw_parts(ptr, len);
+    slice.to_vec()
+  }
+
   unsafe fn static_drop(_: &mut AtomicPtr<()>, _: *const u8, _: usize) {
     // nothing to drop for &'static [u8]
   }

serde_v8/magic/v8slice.rs

@@ -146,6 +146,7 @@ impl From<V8Slice> for bytes::Bytes {
 const V8SLICE_VTABLE: rawbytes::Vtable = rawbytes::Vtable {
   clone: v8slice_clone,
   drop: v8slice_drop,
+  to_vec: v8slice_to_vec,
 };
 
 unsafe fn v8slice_clone(
@@ -161,6 +162,18 @@ unsafe fn v8slice_clone(
   rawbytes::RawBytes::new_raw(ptr, len, data.cast(), &V8SLICE_VTABLE)
 }
 
+unsafe fn v8slice_to_vec(
+  data: &rawbytes::AtomicPtr<()>,
+  ptr: *const u8,
+  len: usize,
+) -> Vec<u8> {
+  let rc = Rc::from_raw(*data as *const V8Slice);
+  std::mem::forget(rc);
+  // NOTE: `bytes::Bytes` does bounds checking so we trust its ptr, len inputs
+  // and must use them to allow cloning Bytes it has sliced
+  Vec::from_raw_parts(ptr as _, len, len)
+}
+
 unsafe fn v8slice_drop(
   data: &mut rawbytes::AtomicPtr<()>,
   _: *const u8,