Unify constant expressions, mark them as safe, and allow proper pointer arithmetic

[mahkoh]

• Ensure that, if a constant expression is accepted in one position, then
  it's also accepted in every other position that expects constant
  expressions.
• Don't force the user to write unsafe { } in constant expressions.
• Allow arbitrary pointer arithmetic in constant expressions. This includes
  dereferencing the pointer as long as the value is not being read.

Rendered

[mahkoh]

Here are the implementations of offset_of and align_of:

macro_rules! offset_of {
    ($T:ty, $f:ident) => {
        unsafe { 
            &(*(0u as *const $T)).$f as *const _ as uint
        }
    }
}

macro_rules! align_of {
    ($T:ty) => {{
        #[repr(C)]
        struct TMP {
            _b: u8,
            t: $T,
        }
        offset_of!(TMP, t)
    }}
}

This means that, with this, no special-case CTFE is necessary to get the size and alignment of objects. (You still have to store the expressions if type parameters are involved.)

[aturon]

Ping @brson