Preserve &[T]'s Rust representation in rust::Slice

Author: dtolnay

include/cxx.h

@@ -180,10 +180,9 @@ class Slice final
   void swap(Slice &) noexcept;
 
 private:
-  // Not necessarily ABI compatible with &[T]. Codegen will translate to
-  // cxx::rust_slice::RustSlice which matches this layout.
-  void *ptr;
-  std::size_t len;
+  void *ptr() const noexcept;
+
+  std::array<std::uintptr_t, 2> repr;
 };
 
 template <typename T>
@@ -489,37 +488,42 @@ constexpr unsafe_bitcopy_t unsafe_bitcopy{};
 #ifndef CXXBRIDGE1_RUST_SLICE
 #define CXXBRIDGE1_RUST_SLICE
 template <typename T>
-Slice<T>::Slice() noexcept
-    : ptr(reinterpret_cast<void *>(align_of<T>())), len(0) {}
+Slice<T>::Slice() noexcept {
+  void sliceInit(void *, const void *, std::size_t);
+  sliceInit(this, reinterpret_cast<void *>(align_of<T>()), 0);
+}
 
 template <typename T>
-Slice<T>::Slice(T *s, std::size_t count) noexcept
-    : ptr(const_cast<typename std::remove_const<T>::type *>(s)), len(count) {}
+Slice<T>::Slice(T *s, std::size_t count) noexcept {
+  void sliceInit(void *, const void *, std::size_t);
+  sliceInit(this, const_cast<typename std::remove_const<T>::type *>(s), count);
+}
 
 template <typename T>
 T *Slice<T>::data() const noexcept {
-  return reinterpret_cast<T *>(this->ptr);
+  return reinterpret_cast<T *>(this->ptr());
 }
 
 template <typename T>
 std::size_t Slice<T>::size() const noexcept {
-  return this->len;
+  std::size_t sliceLen(const void *);
+  return sliceLen(this);
 }
 
 template <typename T>
 std::size_t Slice<T>::length() const noexcept {
-  return this->len;
+  return this->size();
 }
 
 template <typename T>
 bool Slice<T>::empty() const noexcept {
-  return this->len == 0;
+  return this->size() == 0;
 }
 
 template <typename T>
 T &Slice<T>::operator[](std::size_t n) const noexcept {
   assert(n < this->size());
-  auto pos = static_cast<char *>(this->ptr) + size_of<T>() * n;
+  auto pos = static_cast<char *>(this->ptr()) + size_of<T>() * n;
   return *reinterpret_cast<T *>(pos);
 }
 
@@ -540,7 +544,7 @@ T &Slice<T>::front() const noexcept {
 template <typename T>
 T &Slice<T>::back() const noexcept {
   assert(!this->empty());
-  return (*this)[this->len - 1];
+  return (*this)[this->size() - 1];
 }
 
 template <typename T>
@@ -659,22 +663,28 @@ bool Slice<T>::iterator::operator>=(const iterator &other) const noexcept {
 template <typename T>
 typename Slice<T>::iterator Slice<T>::begin() const noexcept {
   iterator it;
-  it.pos = this->ptr;
+  it.pos = this->ptr();
   it.stride = size_of<T>();
   return it;
 }
 
 template <typename T>
 typename Slice<T>::iterator Slice<T>::end() const noexcept {
   iterator it = this->begin();
-  it.pos = static_cast<char *>(it.pos) + it.stride * this->len;
+  it.pos = static_cast<char *>(it.pos) + it.stride * this->size();
   return it;
 }
 
 template <typename T>
 void Slice<T>::swap(Slice &rhs) noexcept {
   std::swap(*this, rhs);
 }
+
+template <typename T>
+void *Slice<T>::ptr() const noexcept {
+  void *slicePtr(const void *);
+  return slicePtr(this);
+}
 #endif // CXXBRIDGE1_RUST_SLICE
 
 #ifndef CXXBRIDGE1_RUST_BOX

src/cxx.cc

@@ -45,6 +45,12 @@ bool cxxbridge1$str$from(rust::Str *self, const char *ptr,
                          std::size_t len) noexcept;
 const char *cxxbridge1$str$ptr(const rust::Str *self) noexcept;
 std::size_t cxxbridge1$str$len(const rust::Str *self) noexcept;
+
+// rust::Slice
+void cxxbridge1$slice$new(void *self, const void *ptr,
+                          std::size_t len) noexcept;
+void *cxxbridge1$slice$ptr(const void *self) noexcept;
+std::size_t cxxbridge1$slice$len(const void *self) noexcept;
 } // extern "C"
 
 namespace rust {
@@ -274,6 +280,16 @@ std::ostream &operator<<(std::ostream &os, const Str &s) {
   return os;
 }
 
+void sliceInit(void *self, const void *ptr, std::size_t len) {
+  cxxbridge1$slice$new(self, ptr, len);
+}
+
+void *slicePtr(const void *self) noexcept { return cxxbridge1$slice$ptr(self); }
+
+std::size_t sliceLen(const void *self) noexcept {
+  return cxxbridge1$slice$len(self);
+}
+
 // Rust specifies that usize is ABI compatible with C's uintptr_t.
 // https://rust-lang.github.io/unsafe-code-guidelines/layout/scalars.html#isize-and-usize
 // However there is no direct Rust equivalent for size_t. C does not guarantee

src/rust_slice.rs

@@ -1,36 +1,37 @@
 use core::mem;
-use core::ptr::NonNull;
+use core::ptr::{self, NonNull};
 use core::slice;
 
-// Not necessarily ABI compatible with &[T]. Codegen performs the translation.
 #[repr(C)]
-#[derive(Copy, Clone)]
 pub struct RustSlice {
-    pub(crate) ptr: NonNull<()>,
-    pub(crate) len: usize,
+    pub(crate) repr: NonNull<[()]>,
 }
 
 impl RustSlice {
-    pub fn from_ref<T>(s: &[T]) -> Self {
+    pub fn from_ref<T>(slice: &[T]) -> Self {
+        let ptr = ptr::slice_from_raw_parts::<()>(slice.as_ptr().cast(), slice.len());
         RustSlice {
-            ptr: NonNull::from(s).cast::<()>(),
-            len: s.len(),
+            repr: unsafe { NonNull::new_unchecked(ptr as *mut _) },
         }
     }
 
-    pub fn from_mut<T>(s: &mut [T]) -> Self {
+    pub fn from_mut<T>(slice: &mut [T]) -> Self {
+        let ptr = ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().cast(), slice.len());
         RustSlice {
-            len: s.len(),
-            ptr: NonNull::from(s).cast::<()>(),
+            repr: unsafe { NonNull::new_unchecked(ptr) },
         }
     }
 
     pub unsafe fn as_slice<'a, T>(self) -> &'a [T] {
-        slice::from_raw_parts(self.ptr.as_ptr().cast::<T>(), self.len)
+        let ptr = self.repr.as_ptr();
+        let len = self.repr.as_ref().len();
+        slice::from_raw_parts(ptr.cast(), len)
     }
 
     pub unsafe fn as_mut_slice<'a, T>(self) -> &'a mut [T] {
-        slice::from_raw_parts_mut(self.ptr.as_ptr().cast::<T>(), self.len)
+        let ptr = self.repr.as_ptr();
+        let len = self.repr.as_ref().len();
+        slice::from_raw_parts_mut(ptr.cast(), len)
     }
 }
 

src/symbols/mod.rs

@@ -1,4 +1,5 @@
 mod exception;
+mod rust_slice;
 mod rust_str;
 mod rust_string;
 mod rust_vec;

src/symbols/rust_slice.rs

@@ -0,0 +1,22 @@
+use crate::rust_slice::RustSlice;
+use core::mem::MaybeUninit;
+use core::ptr::{self, NonNull};
+
+#[export_name = "cxxbridge1$slice$new"]
+unsafe extern "C" fn slice_new(this: &mut MaybeUninit<RustSlice>, ptr: *const (), len: usize) {
+    let ptr = ptr::slice_from_raw_parts(ptr, len);
+    let rust_slice = RustSlice {
+        repr: NonNull::new_unchecked(ptr as *mut _),
+    };
+    ptr::write(this.as_mut_ptr(), rust_slice);
+}
+
+#[export_name = "cxxbridge1$slice$ptr"]
+unsafe extern "C" fn slice_ptr(this: &RustSlice) -> *const () {
+    this.repr.as_ptr().cast()
+}
+
+#[export_name = "cxxbridge1$slice$len"]
+unsafe extern "C" fn slice_len(this: &RustSlice) -> usize {
+    this.repr.as_ref().len()
+}