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build:
LIBGIT2_NO_VENDOR
to force to use system libgit2
Specify `LIBGIT2_NO_VENDOR` to force to use system libgit2. Due to the additive nature of Cargo features, if some crate in the dependency graph activates `vendored` feature, there is no way to revert it back. This env var serves as a workaround for this purpose. An alternative is having `no_vendored` feature. We still need to modify `build.rs` to make both `vendored` and `no_vendored` play nice with each other (or bail out as they are mutual-exclusive). However, there is no way to activate a Cargo feature via environment variable (see rust-lang/cargo#4829). Altering environment variables may be the only way to interact with some external build systems. It is also pretty common that people don't want to vendor anything and rather to see the build fail than vendoring. Some prior arts: * `OPENSSL_NO_VENDOR` in `openssl` crate, which serves the exact purpose of `LIBGIT2_NO_VENDOR` [^1]. * Crate `ssh2` has a similar `LIBSSH2_SYS_USE_PKG_CONFIG`, but it doesn't force to do so. It still falls back to vendored lib [^2]. * Crate `curl` has a feature `force-system-lib-on-osx` to trump all others features. It was created primarily for Rust releases [^3]. More motivations why this is something we want: * We have no control over transitive dependencies that accidentally activate the vendored feature. And there is no way to turn it off. * If the build environment has the wrong system libgit2, we want build to always fail because that is a real mistake. For example, libgit2 expects a range 1.6.4..1.7.0 and it silently goes vendoring if nothing found. We may want an explicit error like "your system needs a newer libgit2 to proceed". [^1]: https://github.com/sfackler/rust-openssl/blob/50787ed35bf9efa9dd3cbb1993a2564014b67489/openssl/src/lib.rs#L65 [^2]: https://github.com/alexcrichton/ssh2-rs/blob/d9a1dfac4b8c09c5437eb477606b82aa4f67b092/libssh2-sys/build.rs#L22-L33 [^3]: https://github.com/alexcrichton/curl-rust/blob/431babf1dffe205641793353d3d57fdd36fe8534/curl-sys/build.rs#L15-L20
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