[Vala] .gir and .metadata file warnings and errors

[Jim Nelson <jim yorba org>]

We've had a lot of trouble recently with building Geary due to some 
flux with the WebKitGTK bindings and headers. Depending on the version 
of WebKitGTK we're compiling against, we get errors like this:

WebKit-3.0.gir:27733.7-27735.16: error: overriding method 
`WebKit.DOM.TextTrackCue.dispatch_event' is incompatible with base 
method `WebKit.DOM.EventTarget.dispatch_event': incompatible return 
type.

Which appears to be a problem with the WebKitGTK .gir, but causes valac 
to issue an error and exit. (Note that our code doesn't use the 
TextTrackCue class.)

We use .metadata files to get around this problem:

DOMTextTrackCue.dispatch_event type="void"

However, if the symbol is unavailable in the .gir file (this particular 
symbol was not available in earlier versions of WebKitGTK) valac issues 
this warning:

WebKit-3.0.metadata:16.1-16.16: warning: metadata never used

... which is treated as an error because we have fatal warnings enabled.

There's multiple issues here, and not all of them Vala's, but I bring 
them up because I think there may be one or two things Vala can do to 
make life easier.  I'm hoping to hear what others think before filing a 
ticket.

First, it seems that detecting an error in a .gir file shouldn't 
necessarily cause a compiler error.  The broader problem (in my mind) 
is that the .gir file was generated in the first place, but so be it.  
I think Vala should issue a warning in this situation, not an error.  
But more specifically, I would prefer Vala not issue either a warning 
or an error unless the faulty binding was causing a code generation 
problem, i.e. the symbol was actually used by the .vala code.

Second, is there any way Vala can ignore the .metadata problem, or at 
least offer a command-line switch to enable/disable the warning?  I 
would really prefer not to have to disable fatal warnings to get around 
this problem.

I think the general problem is that there should be validation tools 
that are stringent about checking (and producing) bindings and binding 
metadata, but compilers should be looser about checking unless the 
error or ambiguity is a problem for generating code.

-- Jim