feat: delete redundant box in cstr_ffi module

omrf/src/cstr_ffi.rs

@@ -106,11 +106,13 @@ pub type CStrViewVecPtr = *const CStringView;
 
 /// One independent string-vector slot.
 ///
-/// `items` owns the actual `CString` data, each individually boxed so the pointers stay stable
+/// `items` owns the actual `CString` data. Each `CString`'s backing buffer is an independent heap
-/// even when `items` grows. `ptrs` is the parallel pointer array consumed by C/C++; for the
+/// allocation, so the `CStrPtr` values captured in `ptrs` stay stable even when `items` (or the
-/// nullptr-terminated variant a trailing null pointer is appended to `ptrs`.
+/// containing cache) reallocates and merely moves the `CString` headers. `ptrs` is the parallel
+/// pointer array consumed by C/C++; a trailing null pointer is appended for the
+/// nullptr-terminated form.
 struct CStringVec {
-    items: Vec<Box<CString>>,
+    items: Vec<CString>,
     ptrs: Vec<CStrPtr>,
 }
 
@@ -133,9 +135,9 @@ impl CStringVec {
             ptrs: Vec::with_capacity(ptrs_cap),
         };
         for s in items {
-            let boxed = Box::new(CString::new(*s)?);
+            let cs = CString::new(*s)?;
-            obj.ptrs.push(boxed.as_ptr());
+            obj.ptrs.push(cs.as_ptr());
-            obj.items.push(boxed);
+            obj.items.push(cs);
         }
         obj.ptrs.push(std::ptr::null());
         Ok(obj)
@@ -185,10 +187,29 @@ impl CStringViewVec {
     }
 }
 
+/// The thread-local container that owns every output slot produced during a single FFI call.
+///
+/// It holds three independent groups of slots, each of which may accumulate an arbitrary number of
+/// entries across one call:
+///
+/// - `strings`: individual NUL-terminated strings produced by [`push_ffi_string`].
+/// - `string_vecs`: string-vector slots produced by [`push_ffi_string_vec`] /
+///   [`push_ffi_string_vec_with_len`].
+/// - `string_view_vecs`: string-view-vector containers produced by
+///   [`push_ffi_string_view_vec_with_len`]; only the container is owned here, the viewed string
+///   data stays caller-owned.
+///
+/// # Pointer stability
+///
+/// The slots are stored directly (not boxed). Each slot's exposed pointer -- a [`CStrPtr`] for a
+/// `CString`, the head of `ptrs` for a `CStringVec`, or the head of `views` for a
+/// `CStringViewVec` -- addresses an independent heap allocation. Reallocating any of these `Vec`s
+/// only moves the owning headers; the heap buffers behind those pointers stay put, so every
+/// pointer already handed to C/C++ remains valid until [`StringCache::clear`] is called.
 struct StringCache {
-    strings: Vec<Box<CString>>,
+    strings: Vec<CString>,
-    string_vecs: Vec<Box<CStringVec>>,
+    string_vecs: Vec<CStringVec>,
-    string_view_vecs: Vec<Box<CStringViewVec>>,
+    string_view_vecs: Vec<CStringViewVec>,
 }
 
 impl StringCache {
@@ -210,20 +231,20 @@ impl StringCache {
         self.string_view_vecs.clear();
     }
 
-    /// Store one boxed NUL-terminated string and return its stable pointer.
+    /// Store one NUL-terminated string and return its stable pointer.
-    fn push_string(&mut self, s: Box<CString>) -> CStrPtr {
+    fn push_string(&mut self, s: CString) -> CStrPtr {
         let ptr = s.as_ptr();
         self.strings.push(s);
         ptr
     }
 
-    /// Store one boxed string-vector slot.
+    /// Store one string-vector slot.
-    fn push_string_vec(&mut self, sv: Box<CStringVec>) {
+    fn push_string_vec(&mut self, sv: CStringVec) {
         self.string_vecs.push(sv);
     }
 
-    /// Store one boxed string-view-vector slot.
+    /// Store one string-view-vector slot.
-    fn push_string_view_vec(&mut self, svv: Box<CStringViewVec>) {
+    fn push_string_view_vec(&mut self, svv: CStringViewVec) {
         self.string_view_vecs.push(svv);
     }
 }
@@ -258,8 +279,8 @@ pub fn clear_ffi_strings() {
 ///
 /// Returns an error if `s` contains an interior NUL byte.
 pub fn push_ffi_string(s: &str) -> Result<CStrPtr> {
-    let boxed = Box::new(CString::new(s)?);
+    let cs = CString::new(s)?;
-    STRING_CACHE.with(|c| Ok(c.borrow_mut().push_string(boxed)))
+    STRING_CACHE.with(|c| Ok(c.borrow_mut().push_string(cs)))
 }
 
 /// Push one independent string vector (NUL-terminated entries, **nullptr-terminated list**) into
@@ -276,7 +297,7 @@ pub fn push_ffi_string(s: &str) -> Result<CStrPtr> {
 ///
 /// Returns an error if any entry contains an interior NUL byte.
 pub fn push_ffi_string_vec(items: &[&str]) -> Result<CStrVecPtr> {
-    let sv = Box::new(CStringVec::new(items)?);
+    let sv = CStringVec::new(items)?;
     let head = sv.head_ptr();
     STRING_CACHE.with(|c| c.borrow_mut().push_string_vec(sv));
     Ok(head)
@@ -298,7 +319,7 @@ pub fn push_ffi_string_vec(items: &[&str]) -> Result<CStrVecPtr> {
 ///
 /// Returns an error if any entry contains an interior NUL byte.
 pub fn push_ffi_string_vec_with_len(items: &[&str]) -> Result<(CStrVecPtr, usize)> {
-    let sv = Box::new(CStringVec::new(items)?);
+    let sv = CStringVec::new(items)?;
     let head = sv.head_ptr();
     let count = sv.item_count();
     STRING_CACHE.with(|c| c.borrow_mut().push_string_vec(sv));
@@ -347,7 +368,7 @@ pub fn push_ffi_string_view(s: &str) -> (CStrPtr, usize) {
 /// reads (per `Vec::as_ptr` for an empty allocation). The foreign side must drive consumption off
 /// `count` and must not dereference the pointer when `count` is 0.
 pub fn push_ffi_string_view_vec_with_len(items: &[&str]) -> (CStrViewVecPtr, usize) {
-    let svv = Box::new(CStringViewVec::new(items));
+    let svv = CStringViewVec::new(items);
     let head = svv.head_ptr();
     let count = svv.item_count();
     STRING_CACHE.with(|c| c.borrow_mut().push_string_view_vec(svv));