Is your first example really equivalent to Box<str>? I would have expected something like using BoxStr = std::unique_ptr<Str>; where Str is defined as

Is your first example really equivalent to Box<str>? I would have expected something like using BoxStr = std::unique_ptr<Str>; where Str is defined as struct Str { size_t len; char data[]; }; The difference is that the len is stored on the heap, and the data is stored inline with the length. Unfortunately C++ does not support flexible array members so this syntax is not actually valid. Edit: Never mind, after reading the article Rust does use the above representation because Box holds a “fat” pointer to str, which stores it’s length on the stack. So BoxStr is the correct equivalent, because &[u8] is not equivalent to u8*, it’s equivalent to std::span<u8>.