Provide comparison operators for rust::Str and rust::String

[dtolnay]

The C++ types should come with operators for ==, !=, <, >, <=, >=.

[alexmaco]

While looking into adding this, I noticed that a few trivial functions of alloc::String are routed via FFI, e.g. cxxbridge1$string$len for String::len. So long as Vec has a guaranteed memory layout, and String is a Vec<u8>, then it should be possible to get the length out of rust::String::repr[1], and similarly for the pointer.

I bring this up because, if this would work reliably, then all these operators could be implemented with just an equality and a comparison function that operates on 2 string slices (technically two pointer + length pairs, since &str is not FFI safe), in addition to removing the accessors mentioned above.

alloc::String is not yet marked as repr_transparent, pending rust-lang/rust#70473.

Do you think having repr_transparent on String is really a requirement in practice for this case, or should this go ahead before the issue above is implemented ? Because personally, I can't see how the layout of String could differ from Vec<u8> in practice, given its implementation.

[dtolnay]

Vec does not have a guaranteed layout.