[Suggestion] More deprecation features

[Don]

Stewart Gordon wrote:
> Now that most of the rusty old deprecation bugs have finally been 
> squashed (if you'll excuse the mixed metaphor), here are a few ideas 
> I've had for a while for taking the concept of deprecation further.
> 
> 
> 1. Sometimes it's useful to deprecate something, but keep it for 
> internal use.  So effectively it's private, except if compiling with -d, 
> in which case it will be public.  The notation might look something like
> 
>    private deprecated public void qwert() { ... }
> 
> The error message on trying to use it from outside might look something 
> like
> 
>    qwert.d(42): function qwert is deprecated for public access
> 
> Other combinations of access levels would be similarly allowed, of which 
> these make sense IMM:
> 
>    private deprecated package
>    private deprecated protected *
>    private deprecated public *
>    private deprecated export *
>    package deprecated public *
>    package deprecated export *
>    protected deprecated public
>    protected deprecated export
>    public deprecated export
> 
> Overriding of methods with the asterisked protection settings would be 
> allowed only if the derived class method is also deprecated (or -d is 
> specified).  To declare a method with the same name and parameters in a 
> derived class, without specifying either the deprecated attribute or the 
> -d switch, would be an error.  This is necessary to the principle of 
> deprecation, i.e. code that compiles without -d doesn't change its 
> behaviour when -d is specified, and existing code can still compile.
> 
> Of course, implementing this would affect how attributes are parsed.  I 
> suppose the best idea would be to treat each possible case of the word 
> "deprecated" immediately between two protection attributes as a 
> protection attribute in its own right in terms of the way they override 
> each other.
> 
> 
> 2. A means of deprecating callbacks.  That is, deprecating overriding of 
> a method rather than using it.  This makes sense as callbacks are going 
> to want replacing from time to time, just as callforwards :-) are.  The 
> base class would keep its calls to the method, so that old code will 
> still work, but new or modernised code would not be overriding it anymore.
> 
> (This would be provided at least to some extent by idea 1....)
> 
> 
> 3. Deprecating modules.  Currently, the compiler doesn't allow modules 
> to be declared as deprecated.  A module being deprecated may signify:
> 
> - that the whole API area that it is there to support is deprecated, 
> either because it's an obsolete technology or because it's been 
> superseded by another module
> 
> - that the module has been renamed, and all the old one does is imports 
> the new one for compatibility
> 
> - that it was used for development/testing purposes and is no longer needed
> 
> 
> 4. Deprecated imports.  So effectively, any attempt to use anything from 
> the imported module would throw a deprecation error, unless a 
> non-deprecated import of the same module is also visible from the scope 
> where the use occurs.  This might be to prevent the compiler error that 
> would otherwise be caused by importing a deprecated module for use by 
> deprecated code.  Or to phase out a public import that was figured to be 
> a bad idea.
> 
> 
> Comments?
> 
> Stewart.
> 

Suppose there was version(deprecated), which is set only if -d is used 
on the command line. Wouldn't that let you do most of these things?

Eg, point 3 and 4:

module reallyold;
version(deprecated) {
import anotherdeprecatedmodule;
} else static assert(0, "This module is deprecated");

Sure, it's a bit ugly, but it's simple and would give a lot of 
flexibility. BTW this could be added to D1.0.