Where there are two or more packages which we wish to pull-in at the same time using the same USE flag for features which we don't want to both be enabled, the end-user choices are currently: * Perform the build in two steps: `USE="foo" emerge bar && USE="-foo" emerge baz`; * Update `/etc/portage/package.use(/*)` and add the USE flag for one package only. This can get more unwieldy as more packages are wanted to be pulled-in at once. It would appear be beneficial (especially in a world of containers and CI) to be able to achieve the above in one step, and without requiring additional data to be written to `/etc/portage/`. If there is currently no way to achieve this, then my proposal is to re-use existing ebuild syntax to allow the user to run a command such as: ``` emerge dev-libs/gobject-introspection 'dev-libs/glib[introspection]' sys-libs/libblockdev ``` … to atomically install `glib` with introspection support and `libblockdev` without, from a single invocation. (As per ebuild syntax, multiple USE flags could be specified on a per-package basis by separating them with commas inside the square brackets)
