feat(mangen): clap/env is itself a feature of mangen

[AndreasBackx]

This adds env as a feature to clap_mangen and enables it by default to keep the default behaviour. I wish there was a way to make it automatically enable the feature if it detects the used library/binary uses clap/env, though I'm unsure of how to exactly do that. I found this StackOverflow post, though it would be somewhat non-trivial and I would appreciate some input.

Fixes #3355.

[epage]

I had missed this when fixing some of the other feature flags. In my opinion, stable but optional features should have no-op reflection functions when disabled, so instead we'd offer a version of get_env that returns None and is_hide_env_set returns true when env is not set. That'd make it so clap_mangen needs to do nothing extra.

I'm also tempted to just enable it. It doesn't really buy us much in terms of compile times or binary size and as we modularize, it'll shifting to being optional but without feature flags which aren't ideal to work with.

[thalesfragoso]

I wish there was a way to make it automatically enable the feature if it detects the used library/binary uses clap/env

It's (kinda) possible with a build script, see cargo:rustc-cfg

It doesn't make it possible to enable a cargo feature, but you can still achieve conditional compilation with that. Not sure if adding a build script is worth though.

[epage]

Build scripts bring in their own build-time performance issues so I lean towards keeping things simple until the next MSRV bump where we get the new feature features.