feat: Fix is_onebyte and add an uninit write for onebyte (#1255)

Some fixes around one-byte strings:

 - `is_onebyte` was calling the wrong v8 API.
 - We didn't have a way to write one-byte strings with uninitialized buffers
 - (bonus) The test_string method was quite slow making testing a bit of a pain

src/string.rs

@@ -258,6 +258,28 @@ impl String {
     }
   }
 
+  /// Writes the contents of the string to an external [`MaybeUninit`] buffer, as one-byte
+  /// (Latin-1) characters.
+  #[inline(always)]
+  pub fn write_one_byte_uninit(
+    &self,
+    scope: &mut Isolate,
+    buffer: &mut [MaybeUninit<u8>],
+    start: usize,
+    options: WriteOptions,
+  ) -> usize {
+    unsafe {
+      v8__String__WriteOneByte(
+        self,
+        scope,
+        buffer.as_mut_ptr() as *mut u8,
+        start.try_into().unwrap_or(int::max_value()),
+        buffer.len().try_into().unwrap_or(int::max_value()),
+        options,
+      ) as usize
+    }
+  }
+
   /// Writes the contents of the string to an external buffer, as UTF-8.
   #[inline(always)]
   pub fn write_utf8(
@@ -281,7 +303,7 @@ impl String {
     }
   }
 
-  /// Writes the contents of the string to an external buffer, as UTF-8.
+  /// Writes the contents of the string to an external [`MaybeUninit`] buffer, as UTF-8.
   pub fn write_utf8_uninit(
     &self,
     scope: &mut Isolate,
@@ -378,16 +400,17 @@ impl String {
     unsafe { v8__String__IsExternalTwoByte(self) }
   }
 
-  /// True if string is known to contain only one-byte data.
-  /// Doesn't read the string so can return false positives.
+  /// Will return true if and only if string is known for certain to contain only one-byte data,
+  /// ie: Latin-1, a.k.a. ISO-8859-1 code points. Doesn't read the string so can return false
+  /// negatives, and a return value of false does not mean this string is not one-byte data.
   ///
-  /// For a method that will not return false positives at the cost of
+  /// For a method that will not return false negatives at the cost of
   /// potentially reading the entire string, use [`contains_only_onebyte()`].
   ///
   /// [`contains_only_onebyte()`]: String::contains_only_onebyte
   #[inline(always)]
   pub fn is_onebyte(&self) -> bool {
-    unsafe { v8__String__IsExternalOneByte(self) }
+    unsafe { v8__String__IsOneByte(self) }
   }
 
   /// True if the string contains only one-byte data.

tests/test_api.rs

@@ -305,27 +305,35 @@ fn test_string() {
   }
   {
     let scope = &mut v8::HandleScope::new(isolate);
-    let buffer = (0..v8::String::max_length() / 4)
-      .map(|_| '\u{10348}') // UTF8: 0xF0 0x90 0x8D 0x88
-      .collect::<String>();
-    let local = v8::String::new_from_utf8(
-      scope,
-      buffer.as_bytes(),
-      v8::NewStringType::Normal,
-    )
-    .unwrap();
+    let mut buffer = Vec::with_capacity(v8::String::max_length());
+    for _ in 0..buffer.capacity() / 4 {
+      // U+10348 in UTF-8
+      buffer.push(0xF0_u8);
+      buffer.push(0x90_u8);
+      buffer.push(0x8D_u8);
+      buffer.push(0x88_u8);
+    }
+    let local =
+      v8::String::new_from_utf8(scope, &buffer, v8::NewStringType::Normal)
+        .unwrap();
     // U+10348 is 2 UTF-16 code units, which is the unit of v8::String.length().
     assert_eq!(v8::String::max_length() / 2, local.length());
-    assert_eq!(buffer, local.to_rust_string_lossy(scope));
-
-    let too_long = (0..(v8::String::max_length() / 4) + 1)
-      .map(|_| '\u{10348}') // UTF8: 0xF0 0x90 0x8D 0x88
-      .collect::<String>();
-    let none = v8::String::new_from_utf8(
-      scope,
-      too_long.as_bytes(),
-      v8::NewStringType::Normal,
+    assert_eq!(
+      buffer.as_slice(),
+      local.to_rust_string_lossy(scope).as_bytes()
     );
+
+    let mut too_long = Vec::with_capacity(v8::String::max_length() + 4);
+    for _ in 0..too_long.capacity() / 4 {
+      // U+10348 in UTF-8
+      too_long.push(0xF0_u8);
+      too_long.push(0x90_u8);
+      too_long.push(0x8D_u8);
+      too_long.push(0x88_u8);
+    }
+
+    let none =
+      v8::String::new_from_utf8(scope, &too_long, v8::NewStringType::Normal);
     assert!(none.is_none());
   }
   {
@@ -8316,6 +8324,14 @@ fn external_strings() {
   assert!(!gradients.is_external_twobyte());
   assert!(!gradients.is_onebyte());
   assert!(!gradients.contains_only_onebyte());
+
+  // one-byte "internal" test
+  let latin1 = v8::String::new(scope, "latin-1").unwrap();
+  assert!(!latin1.is_external());
+  assert!(!latin1.is_external_onebyte());
+  assert!(!latin1.is_external_twobyte());
+  assert!(latin1.is_onebyte());
+  assert!(latin1.contains_only_onebyte());
 }
 
 #[test]