Always split host and build environments

[jakirkham]

Historically there was only build and run environments (particularly in conda-build 2 where it was last seen). In conda-build 3 the new host environment was added. The idea being that packagers want to separate and isolate build tools like compilers, CMake, autotools, etc. that only need to be used on the command line from libraries they want to link to in the build. This way packages don't accidentally link to libraries pulled in by build tools (like the openssl package that cmake depends on).

As this was a significant change at the time, splitting build and host only happened under certain circumstances (like when a compiler was added to build). Again this made since as many recipes still used build to mean host as host was a new thing.

However as time has passed and recipes/tooling/knowledge has evolved it is understood now that build should be reserved for build tools and recipes should use host for anything they are linking to as part of the build. Given this, it may make sense to switch to always splitting host and build as this is increasingly the safer way to build things. For an example of where not splitting these causes problems please see issue ( conda-forge/openssl-feedstock#87 ). After all packagers already have tools like merge_build_host to combine the two together if that is really intended ( though maybe we can document that key better #2890 ).

[jezdez]

Let's discuss this during the community meeting and figure out what we can do about aligning the different ways how conda-forge and the Anaconda build system relies on this. This may be something that we should codify in a CEP.

[jakirkham]

It's worth noting this is already the intended behavior. Please see PR ( #2579 ). However it doesn't seem to always be working. IOW there is likely a bug somewhere.

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Thanks!

[jakirkham]

(bump)