dtolnay/cxx 1.0.0 on GitHub

extern "C" extern block in the FFI module is no longer accepted, write extern "C++" instead
extern "C++" block must be written unsafe extern "C++" if it contains at least one safe-to-call function
rust::Slice<T> in C++ has been renamed to rust::Slice<const T>; support for &mut [T] post-1.0 will use the rust::Slice<T> name
&mut T where T is an opaque C++ type is no longer ever exposed to Rust, only Pin<&mut T>
Related to the previous: UniquePtr binding no longer implements DerefMut for opaque C++ types
If a function's implementation is unsafe in Rust, extern "Rust" block will enforce that it's listed as unsafe fn in the extern block too
Opaque Rust types are required to be Unpin in addition to Sized
Opaque Rust types are required to be local to the current crate (for now; this may soon be lifted again)
Handwritten Unpin impl for opaque C++ types are ruled out
Minimum supported rustc version is raised from 1.43 to 1.48