The good error message provided by `pkg-config` itself is suppressed and replaced by a rust-level error message that isn't always accurate or useful

[eli-schwartz]

I tried to build a package which links to libgit2-sys. It is a bit finicky about which system libgit2 version to use. I can upgrade or downgrade libgit2 as needed, however. I just need to know what the actual problem is...

This is the error message I got:

warning: libgit2-sys@0.18.0+1.9.0: failed to probe system libgit2: 

error: failed to run custom build command for `libgit2-sys v0.18.0+1.9.0`

Caused by:
  process didn't exit successfully: `/var/tmp/portage/dev-vcs/git-absorb-0.6.17/work/git-absorb-0.6.17/target/release/build/libgit2-sys-3dc86e5cce7dbd79/build-script-build` (exit status: 101)
  --- stdout
  cargo:rustc-check-cfg=cfg(libgit2_vendored,)
  cargo:rerun-if-env-changed=LIBGIT2_NO_VENDOR
  cargo:rerun-if-env-changed=LIBGIT2_NO_PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG_x86_64-unknown-linux-gnu
  cargo:rerun-if-env-changed=PKG_CONFIG_x86_64_unknown_linux_gnu
  cargo:rerun-if-env-changed=HOST_PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG
  cargo:rerun-if-env-changed=LIBGIT2_STATIC
  cargo:rerun-if-env-changed=LIBGIT2_DYNAMIC
  cargo:rerun-if-env-changed=PKG_CONFIG_ALL_STATIC
  cargo:rerun-if-env-changed=PKG_CONFIG_ALL_DYNAMIC
  cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64-unknown-linux-gnu
  cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64_unknown_linux_gnu
  cargo:rerun-if-env-changed=HOST_PKG_CONFIG_PATH
  cargo:rerun-if-env-changed=PKG_CONFIG_PATH
  cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64-unknown-linux-gnu
  cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64_unknown_linux_gnu
  cargo:rerun-if-env-changed=HOST_PKG_CONFIG_LIBDIR
  cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64-unknown-linux-gnu
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64_unknown_linux_gnu
  cargo:rerun-if-env-changed=HOST_PKG_CONFIG_SYSROOT_DIR
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR
  cargo:warning=failed to probe system libgit2: 
  pkg-config exited with status code 1
  > PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 x86_64-pc-linux-gnu-pkg-config --libs --cflags libgit2 libgit2 >= 1.9.0 libgit2 < 1.10.0

  The system library `libgit2` required by crate `libgit2-sys` was not found.
  The file `libgit2.pc` needs to be installed and the PKG_CONFIG_PATH environment variable must contain its parent directory.
  The PKG_CONFIG_PATH environment variable is not set.

  HINT: if you have installed the library, try setting PKG_CONFIG_PATH to the directory containing `libgit2.pc`.


  --- stderr
  thread 'main' panicked at /var/tmp/portage/dev-vcs/git-absorb-0.6.17/work/cargo_home/gentoo/libgit2-sys-0.18.0+1.9.0/build.rs:45:13:
  The environment variable `LIBGIT2_NO_VENDOR` has been set but no compatible system libgit2 could be found.
  The build is now aborting. To disable, unset the variable or use `LIBGIT2_NO_VENDOR=0`.

  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Most of this can be ignored, and seems to come from cargo's handling of "build-script". The critical bit is cargo:warning, which tells me that pkg-config failed, and helpfully reports a command-line I can run to get more details:

PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 x86_64-pc-linux-gnu-pkg-config --libs --cflags libgit2 libgit2 >= 1.9.0 libgit2 < 1.10.0

This doesn't work

bash: 1.10.0: No such file or directory

which is apparently a bit of an edge case caused by:

pkg-config-rs/src/lib.rs

Lines 230 to 236 in 91fcbdb

	/// Output a command invocation that can be copy-pasted into the terminal.
	/// `Command`'s existing debug implementation is not used for that reason,
	/// as it can sometimes lead to output such as:
	/// `PKG_CONFIG_ALLOW_SYSTEM_CFLAGS="1" PKG_CONFIG_ALLOW_SYSTEM_LIBS="1" "pkg-config" "--libs" "--cflags" "mylibrary"`
	/// Which cannot be copy-pasted into terminals such as nushell, and is a bit noisy.
	/// This will look something like:
	/// `PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 pkg-config --libs --cflags mylibrary`

Side note: Maybe rust Command needs to borrow python's shlex module:

>>> command = ['PKG_CONFIG_ALLOW_SYSTEM_LIBS=1', 'PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1', 'x86_64-pc-linux-gnu-pkg-config', '--libs', '--cflags', 'libgit2', 'libgit2 >= 1.9.0', 'libgit2 < 1.10.0']
>>> print(shlex.join(command))
PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 x86_64-pc-linux-gnu-pkg-config --libs --cflags libgit2 'libgit2 >= 1.9.0' 'libgit2 < 1.10.0'

Anyways, I run the real command:

$ PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 x86_64-pc-linux-gnu-pkg-config \
         --libs --cflags libgit2 'libgit2 >= 1.9.0' 'libgit2 < 1.10.0'
Package dependency requirement 'libgit2 >= 1.9.0' could not be satisfied.
Package 'libgit2' has version '1.8.4', required version is '>= 1.9.0'

pkg-config tells me quite clearly what the problem is. It also does something similar if I try to ask for "doesnt-exist":

$ x86_64-pc-linux-gnu-pkg-config --libs --cflags doesnt-exist
Package doesnt-exist was not found in the pkg-config search path.
Perhaps you should add the directory containing `doesnt-exist.pc'
to the PKG_CONFIG_PATH environment variable
Package 'doesnt-exist' not found

This new error message is very similar to the error message pkg-config-rs gives, except that pkg-config itself knows when to give a better error message for partially matched version bounds.