Add new feature flag "static"

[NobodyXu]

Resolved #158

Signed-off-by: Jiahao XU Jiahao_XU@outlook.com

[NobodyXu]

Not sure why the CI for windows i686 failed.

[gyscos]

I think we should make it an error to have both pkg-config and static features, rather than silently dropping one.

[NobodyXu]

I prefer static to override pkg-config, since it is possible for one dependency to request pkg-config and another to request static.

In this situation, it is not always possible to modify or it maybe hard to remove that and I would prefer statically linked to dynamically linked zstd.

[gyscos]

If a dependency asks for pkg-config, we should honor this decision and not discard it.

Though in general asking for pkg-config is not something re-usable libraries should be doing, it's more of a end-of-the-line binary decision.

[gyscos]

Note that static linking is already the default.
I believe the only way to build dynamically right now is using the pkg-config feature, which is a conscious decision.

• If you're not using this feature right now, then you are statically linking already.
• If you are using this feature right now and it leads to dynamic linking, then something explicitly asked for dynamic linking. It would be safer to find where this came from and fix that, than to silently disregard this request.

The main reason to add a static feature (and it may be a valid one!) is to detect if the situation ever changes: if some dependency starts requiring pkg-config, rather than silently switching to dynamic linking, it would at least block compilation to make the change visible.

Also note that pkg-config does not have to result in dynamic linking. It will try static linking if the libzstd.a file is available, but if it's not - for example the archlinux zstd package does not ship static libraries - it will fallback to dynamic linking.

In that case, it may be better to detect when pkg-config fails to enable static linking (it uses a private function to check that), and only then trigger a build failure.
Now - how to detect this failure? We can either re-implement this function locally? Talk to the pkg_config maintainers and make it public? There doesn't seem to be an easy-to-use field in the resulting Library that would indicate this. :^S

[NobodyXu]

@gyscos I just realized that pkg_config function in build.rs set pkg_config::Config::statik to true, so maybe we don't need this new feature afterall?

[gyscos]

Also note that pkg-config does not have to result in dynamic linking. It will try static linking if the libzstd.a file is available, but if it's not - for example the archlinux zstd package does not ship static libraries - it will fallback to dynamic linking.

It's an unfortunate decision, as it does not appear to make this very visible to the caller.

Seems like there's some demand to change things, but nothing done so far:

rust-lang/pkg-config-rs#68
rust-lang/pkg-config-rs#102
rust-lang/pkg-config-rs#37