So, if I have:

SUBPACKAGES= gtk3 gtk4
USES= gnome=libxml2
USES.gtk3= gnome=gtk30
USES.gtk4= gnome=gtk40

How does that work ?

The idea is that every time a USES is included, it generates a USES_*_DEPENDS
Those prefixed depends are going to be copied to the appropriate *_DEPENDS (without suffix main package, with suffixes for the subpackages)

USES= gnome=libxml2

It will set USES_LIB_DEPENDS= libxml2.so:textproc/libxml2and it will be added to LIB_DEPENDS

USES.gtk3= gnome=gtk30

It will set USES_LIB_DEPENDS= libgtk-3.so:x11-toolkits/gtk30and it will be added to LIB_DEPENDS.gtk3

And so on.

In D43587#994067, @pizzamig wrote:

The idea is that every time a USES is included, it generates a USES_*_DEPENDS
Those prefixed depends are going to be copied to the appropriate *_DEPENDS (without suffix main package, with suffixes for the subpackages)

USES= gnome=libxml2

It will set USES_LIB_DEPENDS= libxml2.so:textproc/libxml2and it will be added to LIB_DEPENDS

USES.gtk3= gnome=gtk30

It will set USES_LIB_DEPENDS= libgtk-3.so:x11-toolkits/gtk30and it will be added to LIB_DEPENDS.gtk3

And so on.

A USES can only be parsed once, there is a guard against it.

So, in my example, the only one to actually do something is USES= gnome=libxml2, the other two are ignored.

Bear in mind that make has no local variables, every variable is global.