Allow formulae to set an env for their completion generation about brew

[MikeMcQuaid]

Thanks for the issue!

This API seems reasonable although unclear if it would be a major improvement over just ENV["whatever"] = "something" in the formula itself.

Do you have any homebrew/core formulae that would need this?

[jasonkarns]

This API seems reasonable although unclear if it would be a major improvement over just ENV["whatever"] = "something" in the formula itself.

I didn't even think of that! Hrm, the simplicity of that might be enough to not bother with changing the completion-helper api.

One downside is the "blast radius" of the ENV= approach extends beyond only the completion command. But in practice, I assume that helper is typically invoked at the very end of install anyway, so it's probably fine. (Though I'm not certain how much stuff homebrew does after #install is invoked that could be affected. It might be weird if #post_install operates in an environment mutated elsewhere?)

Do you have any homebrew/core formulae that would need this?
The only two formulae I ran into are both private taps. Though I haven't looked in core for other potential candidates.

Honestly, I think I'm satisfied to close this and just set ENV as you suggested. I'll try that in our taps and circle back.

[cho-m]

One downside is the "blast radius" of the ENV= approach extends beyond only the completion command.

There is also with_env, e.g.

with_env(VAR1: ..., VAR2: ...) do
  generate_completions_from_executable(...)
end

Though I'm not certain how much stuff homebrew does after #install is invoked that could be affected.

Should be pretty much nothing. Specifically, looks like just writing metadata files which shouldn't have a major impact by ENV modification (EDIT: you can get the tab to recording a different ENV.compiler, but that isn't normal usage).

brew/Library/Homebrew/build.rb

Lines 179 to 187 in c3094ac

	formula.install
	stdlibs = detect_stdlibs
	tab = Tab.create(formula, ENV.compiler, stdlibs.first)
	tab.write
	# Find and link metafiles
	formula.prefix.install_metafiles formula.buildpath
	formula.prefix.install_metafiles formula.libexec if formula.libexec.exist?

[jasonkarns]

@cho-m oh yeah, that's perfect.

Closing in favor of 'with_env'. thank you both!