[Issue 9238] Support rvalue references

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9238


Jonathan M Davis <jmdavisProg at gmx.com> changed:

           What    |Removed                     |Added
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                 CC|                            |jmdavisProg at gmx.com


--- Comment #5 from Jonathan M Davis <jmdavisProg at gmx.com> 2012-12-29 12:08:17 PST ---
I'm against the in ref idea.

1. It subverts what in currently does. You _really_ don't want to require that
scope be used in this case. Once scope has actually been fixed to actually
check for escaping references, you'll either end up with conflicting behavior
with in depending on whether it's ref or not, or you'll end up with scope's
restrictions on it, which would be horrendously over-restrictive. Not to
mention, I'd argue that in is already too overloaded as it is. Too many people
use it because they like they idea that it's the opposite of out without taking
into account that it means not only const but _scope_. We should _not_
encourage it's use further, let alone give it a conflicting meaning. It's
causing enough trouble as it is.

2. I think that that the fact that auto ref allows you to accept both rvalues
and lvalues without const is very valuable. Yes, that means that if the
function actually mutates the parameter, then lvalue arguments will get mutated
whereas the change to rvalue arguments will be lost, but it means that you can
get the efficiency benefit without requiring const. And given how restrictive
D's const is, lots of people are avoiding it, and there are plenty of
legitimate use cases where you _can't_ use it.

So, I'd strongly argue for using auto ref such that

void foo(auto ref int param);

became

void foo(ref param);

and

foo(bar());

gets lowered to something like

auto _temp = bar();
foo(bar());

Then if you _don't_ want foo to be able to mutate its argument, you use const

void foo(auto ref const int param);

and if you don't care, you don't have to. And of course, if you _want_ it to
mutate the argument, then you just use plain ref.

I am _extremely_ leery of overloading in any further, and we do _not_ want to
these types of parameters to have have scope on them.

Honestly, if it were up to me, we'd make in on function parameters outright
illegal, since I think that overloading it like we already have is confusing
and is going to cause a lot of problems once scope is fixed. Let's not make the
problem worse.

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