[dmd-internals] Why do AttribDeclarations exist?

[Don Clugston]

On 17 February 2012 10:49, Daniel Murphy <yebblies at gmail.com> wrote:
> Attributes don't always apply to everything inside them - eg a const
> declaration doesn't make everything parsed inside it automatically
> const, and this is sorted out during semantic as it should be.

Yes, but that's dealt with in StorageDeclaration::semantic, which, as
far as I can tell, does nothing which couldn't be done better in the
parser.
In particular, I do not think the parser should be creating
declarations with attributes which it knows are wrong.
The attributes relevant for a declaration are always known when that
declaration is parsed.

Interestingly declarations involving type inference don't work in the same way.
For example
deprecated auto x = 5;
is marked as deprecated immediately.
whereas
deprecated int x = 5;
is not.

I think the existing approach is the reason the compiler has accepts
nonsensical combinations of attributes without complaint.

> And if
> you pass it to the top-level declarations only, the same problem will
> exist if you try and forward-reference sub-declarations.  Maybe this
> could be done in a pre-semantic pass, though I'm not sure if it should
> be.

There's no problem there, it isn't possible to reach a sub-declaration
of an aggregate type without going through its parent.
I would envisage that each level of nesting would have its own
PendingAttribs, based on the one it received from its parent.

> I think the PendingAttribs approach might work for deprecated, but
> will it for the others?  Being able to pass a bunch of attributes all
> nested declarations without modifying semantic gives an easy way to
> implement new attributes (even user-defined, if we ever go there) that
> would simplify some things such as adding a message to deprecated.

But it doesn't work. The declaration needs to know its attributes, in
the case where it is forward referenced.
I've patched a few of these bugs before, and generally, the semantic
needs to be modified to take the pending attributes into account.

>
> On Fri, Feb 17, 2012 at 7:48 PM, Don Clugston <dclugston at googlemail.com> wrote:
>> I found this why looking at bug 7482, and it seems to be a fundamental
>> problem. Surely other people have hit this before.
>>
>> Something like:
>> deprecated int x;
>> is parsed as  StorageDeclaration( STCdeprecated, new VarDeclaration(x))
>>
>> note that the VarDeclaration is initially created without the
>> 'deprecated' storage class.
>> Then, when semantic is run on all the declarations in the module, it
>> runs on StorageDeclaration, which then runs semanticNewSc on the
>> VarDeclaration.
>>
>> The problem is, if 'x' is forward referenced, the storage declaration
>> hasn't run yet. It gets found with the original, wrong storage class.
>> I don't understand why these AttribDeclarations exist at all. Why
>> isn't the whole thing dealt with in the parser?
>>
>> (ie, create a PendingAttribs struct, and pass it to all the children
>> in a block).
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