From d3aa444755f7498fe710b64a3ed3a132446b61cd Mon Sep 17 00:00:00 2001
From: Aaron O'Mullan <aaron.omullan@gmail.com>
Date: Sat, 2 Apr 2022 00:10:42 +0200
Subject: [PATCH] cleanup(serde_v8): simpler ZeroCopyBuf (#14095)

---
 serde_v8/magic/zero_copy_buf.rs | 81 ++++++++++-----------------------
 1 file changed, 25 insertions(+), 56 deletions(-)

diff --git a/serde_v8/magic/zero_copy_buf.rs b/serde_v8/magic/zero_copy_buf.rs
index c63d4ba668..dcd969aea0 100644
--- a/serde_v8/magic/zero_copy_buf.rs
+++ b/serde_v8/magic/zero_copy_buf.rs
@@ -1,8 +1,8 @@
 // Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
 
-use std::cell::Cell;
 use std::ops::Deref;
 use std::ops::DerefMut;
+use std::ops::Range;
 
 use super::transl8::FromV8;
 
@@ -19,9 +19,8 @@ use super::transl8::FromV8;
 /// `let copy = Vec::from(&*zero_copy_buf);`
 #[derive(Clone)]
 pub struct ZeroCopyBuf {
-  backing_store: v8::SharedRef<v8::BackingStore>,
-  byte_offset: usize,
-  byte_length: usize,
+  store: v8::SharedRef<v8::BackingStore>,
+  range: Range<usize>,
 }
 
 unsafe impl Send for ZeroCopyBuf {}
@@ -29,21 +28,16 @@ unsafe impl Send for ZeroCopyBuf {}
 impl ZeroCopyBuf {
   pub fn from_buffer(
     buffer: v8::Local<v8::ArrayBuffer>,
-    byte_offset: usize,
-    byte_length: usize,
+    range: Range<usize>,
   ) -> Result<Self, v8::DataError> {
-    let backing_store = buffer.get_backing_store();
-    match backing_store.is_shared() {
-      true => Err(v8::DataError::BadType {
+    let store = buffer.get_backing_store();
+    if store.is_shared() {
+      return Err(v8::DataError::BadType {
         actual: "shared ArrayBufferView",
         expected: "non-shared ArrayBufferView",
-      }),
-      false => Ok(Self {
-        backing_store,
-        byte_offset,
-        byte_length,
-      }),
+      });
     }
+    Ok(Self { store, range })
   }
 
   pub fn from_view(
@@ -53,7 +47,17 @@ impl ZeroCopyBuf {
     let buffer = view.buffer(scope).ok_or(v8::DataError::NoData {
       expected: "view to have a buffer",
     })?;
-    Self::from_buffer(buffer, view.byte_offset(), view.byte_length())
+    let (offset, len) = (view.byte_offset(), view.byte_length());
+    Self::from_buffer(buffer, offset..offset + len)
+  }
+
+  fn as_slice(&self) -> &[u8] {
+    unsafe { &*(&self.store[self.range.clone()] as *const _ as *const [u8]) }
+  }
+
+  #[allow(clippy::cast_ref_to_mut)]
+  fn as_slice_mut(&mut self) -> &mut [u8] {
+    unsafe { &mut *(&self.store[self.range.clone()] as *const _ as *mut [u8]) }
   }
 }
 
@@ -65,7 +69,7 @@ impl FromV8 for ZeroCopyBuf {
     if value.is_array_buffer() {
       value
         .try_into()
-        .and_then(|b| Self::from_buffer(b, 0, b.byte_length()))
+        .and_then(|b| Self::from_buffer(b, 0..b.byte_length()))
     } else {
       value
         .try_into()
@@ -78,59 +82,24 @@ impl FromV8 for ZeroCopyBuf {
 impl Deref for ZeroCopyBuf {
   type Target = [u8];
   fn deref(&self) -> &[u8] {
-    unsafe {
-      get_backing_store_slice(
-        &self.backing_store,
-        self.byte_offset,
-        self.byte_length,
-      )
-    }
+    self.as_slice()
   }
 }
 
 impl DerefMut for ZeroCopyBuf {
   fn deref_mut(&mut self) -> &mut [u8] {
-    unsafe {
-      get_backing_store_slice_mut(
-        &self.backing_store,
-        self.byte_offset,
-        self.byte_length,
-      )
-    }
+    self.as_slice_mut()
   }
 }
 
 impl AsRef<[u8]> for ZeroCopyBuf {
   fn as_ref(&self) -> &[u8] {
-    &*self
+    self.as_slice()
   }
 }
 
 impl AsMut<[u8]> for ZeroCopyBuf {
   fn as_mut(&mut self) -> &mut [u8] {
-    &mut *self
+    self.as_slice_mut()
   }
 }
-
-unsafe fn get_backing_store_slice(
-  backing_store: &v8::SharedRef<v8::BackingStore>,
-  byte_offset: usize,
-  byte_length: usize,
-) -> &[u8] {
-  let cells: *const [Cell<u8>] =
-    &backing_store[byte_offset..byte_offset + byte_length];
-  let bytes = cells as *const [u8];
-  &*bytes
-}
-
-#[allow(clippy::mut_from_ref)]
-unsafe fn get_backing_store_slice_mut(
-  backing_store: &v8::SharedRef<v8::BackingStore>,
-  byte_offset: usize,
-  byte_length: usize,
-) -> &mut [u8] {
-  let cells: *const [Cell<u8>] =
-    &backing_store[byte_offset..byte_offset + byte_length];
-  let bytes = cells as *const _ as *mut [u8];
-  &mut *bytes
-}