vcpkg build-dependency now unconditionally pulled in for all targets

[decathorpe]

It looks like since openssl-sys 0.9.91, the vcpkg dependency is now hard-required, even on targets where it is unused. Would it be possible to cfg-gate the build-dependency and usage of vcpkg in build.rs? For example, it does not make sense for Linux distributions to package the vcpkg crate, but openssl-sys now can't be built without it.

Looking at the PR that introduced this change (#1982), it sounds like this is a workaround for host != target environment? If there's no other way to resolve this than to make the build-dependency unconditional, then I'll likely need to apply some downstream patches for openssl-sys for the builds in Fedora Linux.

[sfackler]

Yes, it needs to be pulled in for cross-compile workflows.

[decathorpe]

Ok :( That's unfortunate. I'll drop the dependency with a downstream patch then. Thanks for confirming!

[alex]

As a long term action item, it seems like maybe this scenario could be addressed by making cargo more flexible in declaring host or target build deps?

[sfackler]

Yeah more powerful dependency filtering would cover it.

[micolous]

It looks like since openssl-sys 0.9.91, the vcpkg dependency is now hard-required, even on targets where it is unused.

vcpkg (and vcpkg-rs) can also build for non-Windows targets, which may or may not be cross-compiled.

While rust-openssl doesn't support that today, it absolutely could be used to build for a Linux target at some point in the future if someone was so inclined (that someone is unlikely to be me, though).

So while I acknowledge the minor absurdity of the situation for a Linux distributor, even if you could resolve the dependency loop to filter it out by target on a Cargo level, it would lead to a similar situation to what I'm trying to unwrap with the assumption that you wouldn't use vcpkg to build for *-pc-windows-gnu.

If you're building in cargo, it's only used as a build dependency, so doesn't bloat anyone's binaries, much like providing OpenSSL via pkg-config.

Would it be possible to cfg-gate the build-dependency and usage of vcpkg in build.rs? For example, it does not make sense for Linux distributions to package the vcpkg crate, but openssl-sys now can't be built without it.

To turn around your suggestion: it does not make sense to have a hard dependency on pkg-config on *-pc-windows-msvc targets... we should cfg-gate that too! Linux users can just add a --cfg flag for that. 😉

See also: #1062