Put latin1 flag in len field

core/engine/src/string/mod.rs

@@ -175,11 +175,10 @@ impl CodePoint {
 /// The raw representation of a [`JsString`] in the heap.
 #[repr(C)]
 struct RawJsString {
-    /// The UTF-16 length.
-    len: usize,
-
-    // TODO: Put flag in len.
-    flags: usize,
+    /// Contains the flags and Latin1/UTF-16 length.
+    ///
+    /// The latin1 flag is stored in the bottom bit.
+    flags_and_len: usize,
 
     /// The number of references to the string.
     ///
@@ -190,6 +189,23 @@ struct RawJsString {
     data: [u16; 0],
 }
 
+impl RawJsString {
+    const LATIN1_BITFLAG: usize = 1 << 0;
+    const BITFLAG_COUNT: usize = 1;
+
+    const fn is_latin1(&self) -> bool {
+        (self.flags_and_len & Self::LATIN1_BITFLAG) != 0
+    }
+
+    const fn len(&self) -> usize {
+        self.flags_and_len >> Self::BITFLAG_COUNT
+    }
+
+    const fn encode_flags_and_len(len: usize, latin1: bool) -> usize {
+        (len << Self::BITFLAG_COUNT) | (latin1 as usize)
+    }
+}
+
 const DATA_OFFSET: usize = std::mem::size_of::<RawJsString>();
 
 /// A Latin1 or UTF-16–encoded, reference counted, immutable string.
@@ -260,24 +276,22 @@ impl JsString {
                 unsafe {
                     let h = h.as_ptr();
 
-                    if (*h).flags == 0 {
-                        JsStr::utf16(std::slice::from_raw_parts(
+                    if (*h).is_latin1() {
+                        JsStr::latin1(std::slice::from_raw_parts(
                             addr_of!((*h).data).cast(),
-                            (*h).len,
+                            (*h).len(),
                         ))
                     } else {
-                        JsStr::latin1(std::slice::from_raw_parts(
+                        JsStr::utf16(std::slice::from_raw_parts(
                             addr_of!((*h).data).cast(),
-                            (*h).len,
+                            (*h).len(),
                         ))
                     }
                 }
             }
             UnwrappedTagged::Tag(index) => {
                 // SAFETY: all static strings are valid indices on `STATIC_JS_STRINGS`, so `get` should always
                 // return `Some`.
-                //
-                // SAFETY: All strings in `StaticJsStrings` are ascii strings, so this is safe.
                 unsafe { StaticJsStrings::get(index).unwrap_unchecked() }
             }
         }
@@ -518,7 +532,6 @@ impl JsString {
         assert!(position < size);
 
         match self.as_str().variant() {
-            // Fast path for ascii strings.
             JsStrVariant::Latin1(v) => {
                 let code_point = v.get(position).expect("Already checked the size");
                 CodePoint::Unicode(*code_point as char)
@@ -664,9 +677,8 @@ impl JsString {
         unsafe {
             // Write the first part, the `RawJsString`.
             inner.as_ptr().write(RawJsString {
-                len: str_len,
+                flags_and_len: RawJsString::encode_flags_and_len(str_len, latin1),
                 refcount: Cell::new(1),
-                flags: usize::from(latin1),
                 data: [0; 0],
             });
         }
@@ -750,7 +762,7 @@ impl JsString {
                 //   by its signature, so this doesn't outlive `self`.
                 unsafe {
                     let h = h.as_ptr();
-                    (*h).len
+                    (*h).len()
                 }
             }
             UnwrappedTagged::Tag(index) => {
@@ -869,15 +881,15 @@ impl Drop for JsString {
             // All the checks for the validity of the layout have already been made on `alloc_inner`,
             // so we can skip the unwrap.
             let layout = unsafe {
-                if inner.flags == 0 {
+                if inner.is_latin1() {
                     Layout::for_value(inner)
-                        .extend(Layout::array::<u16>(inner.len).unwrap_unchecked())
+                        .extend(Layout::array::<u8>(inner.len()).unwrap_unchecked())
                         .unwrap_unchecked()
                         .0
                         .pad_to_align()
                 } else {
                     Layout::for_value(inner)
-                        .extend(Layout::array::<u8>(inner.len).unwrap_unchecked())
+                        .extend(Layout::array::<u16>(inner.len()).unwrap_unchecked())
                         .unwrap_unchecked()
                         .0
                         .pad_to_align()
@@ -1023,10 +1035,6 @@ impl<const N: usize> PartialEq<[u16; N]> for JsString {
 
 impl PartialEq<str> for JsString {
     fn eq(&self, other: &str) -> bool {
-        // if let Some(s) = self.as_str().as_ascii() {
-        //     return s == other.as_bytes();
-        // }
-
         let utf16 = self.code_points();
         let mut utf8 = other.chars();