Cannot install on stock macOS

[lilyball]

cargo-tree won't build on stock macOS due to a transitive dependency on openssl-sys which I believe comes from cargo. The issue is that macOS ships with a very old version of OpenSSL that openssl-sys does not support. I'm pretty sure this can be worked around with brew install openssl but I don't use Homebrew so that doesn't work for me.

A quick look at https://github.com/rust-lang/cargo/blob/master/Cargo.toml indicates that there's a vendored-openssl feature flag, it might be a good idea to opt into that feature flag when building cargo-tree on macOS (or at least to re-expose it so I can select it myself).

[sfackler]

Sure - seems reasonable to add a vendored-openssl Cargo feature here.

[dtolnay]

It should be possible to select Cargo's vendored-openssl feature without cargo-tree specifically re-exposing it:

cargo install cargo-tree --features cargo/vendored-openssl

[lilyball]

Huh, that's not documented very well.

Incidentally, I think cargo's openssl feature actually isn't the source of this after all. It's marked optional and I was really confused as to why it was being picked up. It's not. It's the git2 dependency, which by default includes openssl.

Incidentally, I don't see any way, as an end user, of being able to say "turn off default features for this dependency". cargo install cargo-tree --no-default-features is going to ensure cargo-tree itself doesn't have default features but I wouldn't expect that to turn off default features on its dependencies (though the documentation isn't explicit, so it certainly could - EDIT: it doesn't).

That said, --features cargo/vendored-openssl seems to work. I guess it turns on the feature for openssl overall even though the dependency is coming via git2 instead of cargo. I bet I could write --features openssl/vendored instead if I wanted to.

[lilyball]

Ok so, short term: Update the README to specify that on macOS you can say --features openssl/vendored.

Long term: Figure out how to disable the default features on git2. I don't know if you can just re-declare the git2 dependency with features disabled, or what. Ideally cargo would have a feature that disables default features on git2, but I'm not sure if there's actually a way to specify that. I wish you could just write something like --features git2/-default to turn off the default feature.