fix(serde_v8): avoid creating unsound slice reference (#16189)

This commit fixes one ocurrence of unsoundness by using the newly added
API (`v8::String::write_utf8_uninit`).

See also
[`clippy:uninit_vec`](https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec).

Note that it is not actually a bug. Avoiding unsoundness improves our
code quality.

1 files changed, 16 insertions, 21 deletions

diff --git a/serde_v8/de.rs b/serde_v8/de.rs
index 87e3fb1f6..554b2c366 100644
--- a/serde_v8/de.rs
+++ b/serde_v8/de.rs
@@ -733,28 +733,26 @@ fn to_utf8_fast(
   scope: &mut v8::HandleScope,
 ) -> Option<String> {
   // Over-allocate by 20% to avoid checking string twice
-  let len = s.length();
-  let capacity = (len as f64 * 1.2) as usize;
+  let str_chars = s.length();
+  let capacity = (str_chars as f64 * 1.2) as usize;
   let mut buf = Vec::with_capacity(capacity);
+
   let mut nchars = 0;
-  let data = buf.as_mut_ptr();
-  let length = s.write_utf8(
+  let bytes_len = s.write_utf8_uninit(
     scope,
-    // SAFETY: we're essentially providing the raw internal slice/buffer owned by the Vec
-    // which fulfills all of from_raw_parts_mut's safety requirements besides "initialization"
-    // and since we're operating on a [u8] not [T] we can safely assume the slice's values
-    // are sufficiently "initialized" for writes
-    unsafe { std::slice::from_raw_parts_mut(data, capacity) },
+    buf.spare_capacity_mut(),
     Some(&mut nchars),
     v8::WriteOptions::NO_NULL_TERMINATION
       | v8::WriteOptions::REPLACE_INVALID_UTF8,
   );
-  if nchars < len {
+
+  if nchars < str_chars {
     return None;
   }
-  // SAFETY: write_utf8 guarantees `length` bytes are initialized & valid utf8
+
+  // SAFETY: write_utf8_uninit guarantees `bytes_len` bytes are initialized & valid utf8
   unsafe {
-    buf.set_len(length);
+    buf.set_len(bytes_len);
     Some(String::from_utf8_unchecked(buf))
   }
 }
@@ -765,21 +763,18 @@ fn to_utf8_slow(
 ) -> String {
   let capacity = s.utf8_length(scope);
   let mut buf = Vec::with_capacity(capacity);
-  let data = buf.as_mut_ptr();
-  let length = s.write_utf8(
+
+  let bytes_len = s.write_utf8_uninit(
     scope,
-    // SAFETY: we're essentially providing the raw internal slice/buffer owned by the Vec
-    // which fulfills all of from_raw_parts_mut's safety requirements besides "initialization"
-    // and since we're operating on a [u8] not [T] we can safely assume the slice's values
-    // are sufficiently "initialized" for writes
-    unsafe { std::slice::from_raw_parts_mut(data, capacity) },
+    buf.spare_capacity_mut(),
     None,
     v8::WriteOptions::NO_NULL_TERMINATION
       | v8::WriteOptions::REPLACE_INVALID_UTF8,
   );
-  // SAFETY: write_utf8 guarantees `length` bytes are initialized & valid utf8
+
+  // SAFETY: write_utf8_uninit guarantees `bytes_len` bytes are initialized & valid utf8
   unsafe {
-    buf.set_len(length);
+    buf.set_len(bytes_len);
     String::from_utf8_unchecked(buf)
   }
 }