Why doesn't the rust crate openssl-sys compile?

Question

I've just added an external crate to my Rust project:

[dependencies]
feed = "2.0"

This crate has several dependencies, notably openssl-sys v0.9.10. This one failed when I'm trying to build my project:

$ cargo build 
Compiling unicode-normalization v0.1.4
Compiling openssl-probe v0.1.0
Compiling matches v0.1.4
Compiling log v0.3.7
Compiling unicode-bidi v0.2.5
Compiling libc v0.2.21
Compiling quick-xml v0.4.2
Compiling pkg-config v0.3.9
Compiling rss v0.4.0
Compiling idna v0.1.1
Compiling time v0.1.36
Compiling num-traits v0.1.37
Compiling gcc v0.3.45
Compiling num-integer v0.1.34
Compiling url v1.4.0
Compiling num-iter v0.1.33
Compiling num v0.1.37
Compiling chrono v0.3.0
Compiling openssl-sys v0.9.10
Compiling libz-sys v1.0.13
error: failed to run custom build command for `openssl-sys v0.9.10`
process didn't exit successfully: 
`/home/E3news/flux/target/debug/build/openssl-sys-223aa532c32a251f/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "Failed to run `\"pkg-config\" \"--libs\" \"--cflags\" \"openssl\"`: No such file or directory (os error 2)"', /buildslave/rust-
buildbot/slave/stable-dist-rustc-linux/build/src/libcore/result.rs:868
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Build failed, waiting for other jobs to finish...
error: build failed

I'm on a Debian 8.7 and I use Rust 1.16. I tried a fix by installing libssl-dev on my OS but it doesn't fix the issue.

Any idea?

Answer 1

From the comments:

Have you installed pkg-config and openssl?

---

Yes it works! openssl was already installed but not pkg-config. So:

$ apt install pkg-config
$ cargo clean 
$ cargo build

solved my problem. Thank you @kennytm!