Why are properties of concrete types not assignable?

[Sean Kelly]

I was trying to compose a complex number today and was reminded that 
this does not work:

     creal c;
     c.re = 1.0;
     c.im = 2.0;

Why is this?  I can't imagine it's a technical limitation, and I can 
think of a few instances where this is actually a useful feature 
(conversion from a string, for example), but there must be some reason 
for the current behavior.

The same goes for the array ptr property, though its behavior seems 
somewhat more understandable:

     char[] buf;
     buf.ptr = null; // fails

I suppose that buf.ptr is just syntactic sugar for &buf[0], and it is 
not assignable so the length property is always accurate?  And the .len 
struct member is not exposed to avoid confusing with .length?  This 
seems reasonable, though I've encountered a few instances where I wanted 
to manually manipulate these properties to avoid the risk of 
reallocation and wished they were assignable.  But slicing is powerful 
enough that direct member manipulation really isn't necessary, so 
perhaps it's safer to keep things the way they are for arrays.


Sean