ipdl syntax changes for types coming from C++

Over the weekend, I landed bug 918651, which changes the syntax for how you inform the IPDL compiler about types defined in C++.  Previously, you did:

include "mozilla/HeaderFile.h";
using typeFromHeaderFile;

The using declaration informs the IPDL compiler that typeFromHeaderFile may appear in places types can normally appear.  The include directive is so the generated headers know what to #include for the C++ compiler to be informed about typeFromHeaderFile.

This scheme has a couple of drawbacks:

  • The header files from the include directives aren’t connected to the using declarations in any way.  Those headers might only include the relevant type(s) incidentally, which doesn’t help in unraveling Gecko’s include dependencies.
  • The generated IPDL headers don’t necessarily need the full definition of typeFromHeaderFile.  For structs or classes, the generated headers can get by with a simple forward declaration.  The full definition is only needed in the generated source files.  The above syntax, however, doesn’t enable any sort of forward declaration magic.

To address both of these issues, the syntax for using declarations was changed.  For structs, you should say:

using struct structFromHeaderFile from "mozilla/HeaderFile.h"

The syntax for classes is similar:

using class classFromHeaderFile from "mozilla/HeaderFile.h"

In these cases, the IPDL compiler will forward-declare the types where appropriate and only #include the header in the generated source files.  Additionally, the compiler is intelligent enough to #include the header in the generated headers if it is required. For instance, if there is a struct or a union defined in the header file that requires a struct or a class from a using declaration, the relevant header will be included in the generated header instead of the generated source file.

Finally, if you need an enum type or a typedef, you should say:

using typeFromHeaderFile from "mozilla/HeaderFile.h"

This case functions similarly to what we had before, except that the header file is now closely associated with the type; ideally, that will encourage people to use the correct header (i.e. the one that defines the type).  While you are able to use this syntax with struct or class types, you should use the using struct or using class syntax, as appropriate, so that forward declarations are generated.

There are still a few instances of include directives for C++ headers in IPDL files; those should be considered a bug, and the include directive for C++ headers should not normally be needed going forward.

This change didn’t completely address the original issue of the bug (touching headers in gfx/ causes source files in netwerk/ to rebuild), but it moved us a lot closer to fixing those sorts of issues.

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