libgit2-sys provides no way to use a system provided libgit2 about git2-rs

[alexcrichton]

The libgit2-sys library specifies the links manifest key which allows overrides, but the system library is almost never the right version so that's why the check isn't currently implemented.

[codyps]

I still think this is an issue, perhaps more accurately stated as:

libgit2-sys provides no way to use a system provided libgit2 via pkg-config

Notably, this means that pkg-config would need to be driven by an external program and it's output translated into something like the overrides for links (only to be converted later by cargo back into the original form). That seems rather awkward and unnecessary.

[alexcrichton]

Right, this was explicitly removed in b5334b0 because the depended-upon version of libgit2 is rarely found in the wild, and future versions of libgit2 frequently break ABI compatibility, so it really does need to use a precise version for now until the ABI stabilizes.

[codyps]

I opened this because right now I am packaging a recent enough libgit2, and have to add this nasty patch to git2-rs to get it to be used: https://github.com/jmesmon/meta-rust/blob/master/recipes/cargo/files/git2-rs/0002-libgit2-sys-avoid-the-build-script-it-is-a-disaster.patch

I'd expect it might be a good idea to at least allow some type of override to get the expected pkg-config behavior so packagers can hack up packages a bit less.

[alexcrichton]

Hm ok, how about something like if the LIBGIT2_SYS_USE_PKG_CONFIG environment variable is set, then pkg-config is queried first, but otherwise it's not used at all. Would that work?

[codyps]

That would be fine.

[alexcrichton]

Ok! I've added this in alexcrichton@ed629d7