Bug 702464 – [Future EAPI] Cross-flag USE dependencies

• Product: Gentoo Hosted Projects
• Status: CONFIRMED ---

[Description Michał Górny 2019-12-10 14:22:21 UTC]

I'd like to propose adding two new operators, or rather extending the existing USE dependency operators with an 'arrow' syntax to allow dependee flag name to be different.

Killer rationale: we have 2-style and 4-style USE dependencies.  We inevitably need to have 8-style USE dependencies, and then we're good till EAPI 16.


New syntax
==========

The new USE dependency syntax I'd like to see is (square brackets indicate optional parts):

  [!]a?>[-]b

meaning 'if a is enabled (disabled) on this package, require b to be enabled (disabled) on the other package', and:

  [!]a=>[-]b

meaning the above + 'and vice versa'.


Expanded forms
==============

  a?>b    <=>  a? ( ...[b] )
  !a?>b   <=> !a? ( ...[b] )
  a?>-b   <=>  a? ( ...[-b] )
  !a?->-b <=> !a? ( ...[-b] )

  a=>b    <=>  a? ( ...[b] )   !a? ( ...[-b] )
  !a=>b   <=> !a? ( ...[b] )    a? ( ...[-b] )
  a=>-b   <=>  a? ( ...[-b] )  !a? ( ...[b] )
  !a=>-b  <=> !a? ( ...[-b] )   a? ( ...[b] )

Naturally, the => forms are equivalent in pairs.


Equivalent same-flag forms
==========================

  a?  <=> a?>a
  !a? <=> !a?>-a
  a=  <=> a=>a
  !a= <=> !a=>-a

While they're obviously redundant, I believe that at least '!a?' is less readable (counter-intuitive) than the expanded form.


Use cases
=========

I can think of two primary USE cases for it.

Firstly, python-single-r1 ebuilds with new API could be simplified.  Currently, they do:

  pst1? ( ...[pt1] )
  pst2? ( ...[pt2] )
  ...

With the new syntax, they could generate a PYTHON_MULTI_USEDEP having:

  pst1?>pt1,pst2?>pt2,...

Secondly, static-enabled ebuilds.  They currently do, usually via a gross hack:

  dev-libs/foo
  static? ( dev-libs/foo[static-libs(+)] )

The new syntax would allow:

  dev-libs/foo[static?>static-libs(+)]