Excerpts from Herbert Valerio Riedel's message of 2016-07-13 23:40:06 -0700:
I.e. write up a specification/proposal outlining motivation (i.e. what problem does this solve), specify what the changes are exactly (syntax & semantics), what the consequences are, and so on.
Then we inevitably need some criteria to decide whether a proposal is accepted and approved for implementation. This could be formally in the hands of the core library committee together with the Hackage trustees (since we do have quite some experience with .cabal files and care a lot about the Hackage ecosystem).
Why can't we release experimental changes to the Cabal specification? Neither setup-depends nor convenience libraries went through any formal proposal mechanism. If a feature is good people will start using it and we will have to continue supporting it. If a feature is bad/not useful we can yank it from the next release; anyone using an experimental feature like this isn't trying to maximize their Cabal compatibility anyway. And any change to the format with cabal-install can't parse is not going to be supportable via custom-setup anyway. So how about this script, which is how web standards work: Cabal changes need to be informally proposed and discussed. But the advancer can also write a PR and get it merged in as an experimental extension, to be trialed for the next major release series. Eventually it gets standardized. Edward