Should all enums be immutable?

[Trass3r]

Am 07.04.2011, 18:03 Uhr, schrieb Simen kjaeraas <simen.kjaras at gmail.com>:

> On Wed, 06 Apr 2011 21:42:35 +0200, Trass3r <un at known.com> wrote:
>
>> Am 06.04.2011, 20:40 Uhr, schrieb Simen kjaeraas  
>> <simen.kjaras at gmail.com>:
>>> Yup. In theory, any immutable Foo declared at module scope (and likely
>>> also those at static class/struct scope) could be elided by the linker
>>> if it not used anywhere, but Optlink does not do that.
>>
>> Ok, I guess the hack works by replacing every occurrence of the  
>> constant identifier in the AST with the initializer part of "enum foo =  
>> initializer;"?
>>
>> So there would be no reason to keep enum manifest constants if this  
>> Optlink bug was fixed?
>
> Correct on both counts.

I'm still confused about this.
Don't we need enum to have things like PI not consume memory?
Which directly brings me to the following:
http://www.digitalmars.com/d/2.0/enum.html states that enum ident = val;  
is just shorthand for enum {ident = val}.
Yet it suggests these are different things by calling only these special  
cases "manifest constants" and stating that SUCH declarations are not  
lvalues. Even though normal enums aren't lvalues either:

enum {a = 5, b}
pragma(msg, &b);
Error: constant 6 is not an lvalue

And also dmd handles them specially by treating enum as a storage class  
(STC_manifest) in this case.