[pre-RFC] Allowing string literals to be either &'static str or String, similar to numeric literals

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@Gankra Good post.

Regarding capacity: Searching the standard library, I find:

  • foo.capacity() - foo.len() on Vec<u8> in two places (libstd/sys/windows/pipe.rs, libstd/sys_common/io.rs), both as part of unsafe code that passes the uninitialized portion of the Vec to OS read functions;
  • foo.capacity() == foo.len() on Vec<u8> in the same libstd/sys/windows/pipe.rs, and on Vec<T> in libcollections/vec.rs. (In the latter case, capacity ≠ len is taken to mean that an element can be appended without reallocating.)

If capacity() returns 0, the first will overflow, and the second will allow out-of-bounds writes – both really bad. Of course, the stdlib itself can be changed, but external code may include similar logic. So I think capacity() returning 0 is unviable at least for Vec, and probably for String as well.

(But then, I agree that supporting statics in Vec is a bad idea anyway because of the cost on deref_mut; I didn’t think of doing it only for String.)

Also, nit: I don’t think any of pop, truncate, or clear would need to special-case static strings, unless you’re really concerned about capacity() staying the same; the static pointer is still valid with a reduced length.

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