Shifting the unsafety guarantees around

src/cache/intern.rs

@@ -8,6 +8,7 @@ cfg_if! {
         use std::borrow::ToOwned;
         use std::cell::RefCell;
         use crate::JsValue;
+        use crate::convert::IntoWasmAbi;
         use uluru::{LRUCache, Entry};
 
 
@@ -33,13 +34,14 @@ cfg_if! {
             cache.find(|p| p.key == key).map(|x| &x.value)
         }
 
-        pub(crate) fn get_str(s: &str) -> Option<JsValue> {
+        /// This returns the raw index of the cached JsValue, so you must take care
+        /// so that you don't use it after it is freed.
+        pub(crate) fn unsafe_get_str(s: &str) -> Option<<JsValue as IntoWasmAbi>::Abi> {
             CACHE.with(|cache| {
                 let mut cache = cache.entries.borrow_mut();
 
                 if let Some(value) = get_js_string(&mut cache, s) {
-                    // This is safe because the cache values are never removed
-                    Some(value._unsafe_clone())
+                    Some(value.into_abi())
 
                 } else {
                     None

src/convert/slices.rs

@@ -129,8 +129,11 @@ cfg_if! {
     if #[cfg(feature = "enable-interning")] {
         #[inline]
         fn get_cached_str(x: &str) -> Option<WasmSlice> {
-            // This uses 0 for the ptr as an indication that it is a JsValue and not a str
-            crate::cache::intern::get_str(x).map(|x| WasmSlice { ptr: 0, len: x.into_abi() })
+            // This is safe because the JsValue is immediately converted into a JS string,
+            // so use-after-free cannot occur.
+            //
+            // This uses 0 for the ptr as an indication that it is a JsValue and not a str.
+            crate::cache::intern::unsafe_get_str(x).map(|x| WasmSlice { ptr: 0, len: x })
         }
 
     } else {

src/lib.rs

@@ -125,11 +125,6 @@ impl JsValue {
         }
     }
 
-    #[inline]
-    fn _unsafe_clone(&self) -> JsValue {
-        Self::_new(self.idx)
-    }
-
     /// Creates a new JS value which is a string.
     ///
     /// The utf-8 string provided is copied to the JS heap and the string will