User Defined Attributes

[Walter Bright]

On 11/6/2012 8:23 AM, bearophile wrote:
> Supporting annotations for function arguments is probably an important sub-feature.

It would be a significant extension, and so I'd like to see a compelling use 
case first.


> Yesterday I was discussing about the bug-prone nature of foreach loops on a
> struct array, and one of the solutions I've suggested was a user-defined
> annotation for the programmer to denote that she wants to modify just the copy:
>
> struct Foo {}
> Foo[10] foos;
> foreach (@copy f; foos) { ... }
>
> With UDA syntax:
>
> foreach ([Copy] f; foos) { ... }
> Or:
> foreach (@(Copy) f; foos) { ... }
>
> But I think there's no way to tell the compiler to give a compile-time error if
> such annotation is not present there (unless there's "ref").

User defined attributes cannot invent new semantics for the language. And 
besides, 'ref' already does what you suggest.