Decorators, Annotations, Macros, AST transforms…

[Dicebot]

There is no such thing as transformation of existing entity on 
compile-time in D. Despite having similar syntax Python 
decorators don't really have much in common with User-Defined 
Attributes. Attributes can never change symbol/type they are 
attached to, they only represent additional information available 
at compile-time.

Right now idiomatic D way to do such thing is via two-step 
declaration and mixin:

```pythonEntry.d
private string generateFromImpl(alias func)()
{
     import std.string : format;
     import std.array : join;
     import std.traits : ParameterIdentifierTuple;

     string code;

     // signature
     code ~= "extern(C) ";
     code ~= cloneFunctionDeclaration!func(
         __traits(identifier, func)[$-4, $] // new name, XImpl -> X
     );

     // body
     code ~= format(
         q{
             {
                 initializeIfNoAlreadyDone();
                 %s(%s);
             }
          },
          fullyQualifiedName!func,
          [ ParameterIdentifierTuple!func ].join(",")
     );
}

public mixin template pythonEntry(alias func)
{
     mixin(generateFromImpl!func());
}
```

```main.d
import pythonEntry;

auto nameImpl() {
     // ...
}

mixin pythonEntry!nameImpl; // will add `name` function you want
```

As you can see it is not very obvious. In practice it is even 
much more complicated as I have skipped implementation of 
cloneFunctionDeclaration - one that I use for vibe.d takes quite 
a lot of space - 
https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/internal/meta/codegen.d#L177

One can also use module introspection + UDA to automatically do 
such mixin for all function attrbiuted with @pythonentry but it 
is even more code so I'd prefer to not paste it here :)

Unfortunately Phobos is currently lacking some very common code 
generation helpers used in idiomatic D metaprogramming. I plan to 
work on improvements in that direction after tuple situation is 
taken care of.

P.S. If my talk proposal for DConf is going to be accepted, it 
will cover this very topic, extensively :P