RFC: For custom string types, add ability to define newtyped unsized vectors once DST gets implemented

[Kimundi]

Right now, the DST proposal is still not implemented/decide upon. However, once it gets implemented I'd like to propose the following roadmap:

1. Implement ability to declare newtypes of at least unsized vectors (syntax just arbitrary):

   struct vec<T>[T];
   

   Those types would work the same way as a bare [T] works: They don't have a compile-time known size, and are specially represented by an slice or an owned pointer:

   let vec_slice: &'static vec<uint> = &'static vec[1, 2, 3];
   let owned_vec: ~vec<i8>           = ~vec[4, 5, 6];
   

2. Remove str from the compiler and replace with an library type:

   struct str[u8];
   

   This would allow to clean up a lot of library types:

   struct str_ascii[u8];
   struct str_c[u8];
   
   struct str_utf16[u16]; // Servo seems to need utf16
   

3. Rewrite string library to share as much code with traits and generics as possibly for those types.

[annevk]

To be clear, Servo doesn't need utf-16, it needs 16-bit code units. utf-16 has the same value space as utf-8, 16-bit code units don't.

[Kimundi]

Ah, you're right. Still, the ability to newtype [u16] as str_usc2 or so could be useful.

[alexcrichton]

Closing for now. DST is still very up-in-the-air (but maybe getting close soon?).

This can be revisited if and when we get DST.