An Optimization That's Impossible in Rust

I never heard of "German strings" in place of the small string optimization before! And there had been way too many proposals to add such types to the standard library [1] [2] [3] [4]. My understanding is that:

1. Both `Vec<u8>` and `String` should share the same layout as part of the standard library's contract.

2. Unlike `String`, the equivalent optimization for `Vec<T>` is more questionable especially when T might be any type. C++ standard libraries frequently do implement multiple layouts for std::string but not for std::vector<T> for the same reason.

3. Rust never allows an implicit copying of `String` in the first place, which greatly benefits from the SSO, so the lack of SSO in Rust is less important compared to C++.

[1] https://github.com/rust-lang/rust/issues/4991

[2] https://github.com/rust-lang/rust/issues/20198

[3] https://github.com/rust-lang/rfcs/pull/2990

[4] https://github.com/rust-lang/unsafe-code-guidelines/issues/4...