Statically linking ffmpeg-sys on Amazon Linux fails with undefined references - rust

Question

My project depends on the ffmpeg-sys crate which is configured to build statically, as follows:
[dependencies.ffmpeg-sys]
version = "3.4.1"
default-features = false
features = ["avcodec", "avformat", "swresample", "build", "static"]
My project consists of a single simple file:
extern crate ffmpeg_sys;
use ffmpeg_sys::av_register_all;
fn main() {
unsafe { av_register_all() };
println!("Hello, world!");
}
When compiling with cargo build I get the following error:
Compiling sample v0.1.0 (file:///home/ec2-user/sample)
error: linking with 'cc' failed: exit code: 1 | = note: "cc"
"-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-L"
[... elided for clarity ...]
In function 'sample::main::hbbb19855251826d6':
/home/ec2-user/sample/src/main.rs:6: undefined reference to 'av_register_all'
collect2: error: ld returned 1 exit status
The required static libraries, libavformat.a and friends, are found in the target/build/debug folder, showing that ffmpeg-sys successfully compiled the libraries.
Here's the rustc command that is failing:
Caused by: process didn't exit successfully: 'rustc --crate-name
sample src/main.rs --crate-type bin --emit=dep-info,link -C
debuginfo=2 -C metadata=250bf40eb277d05a -C
extra-filename=-250bf40eb277d05a --out-dir
/home/ec2-user/sample/target/debug/deps -C
incremental=/home/ec2-user/sample/target/debug/incremental -L
dependency=/home/ec2-user/sample/target/debug/deps --extern
ffmpeg_sys=/home/ec2-user/sample/target/debug/deps/libffmpeg_sys-fa3ff87f80f2d27e.rlib
-L native=/home/ec2-user/sample/target/debug/build/ffmpeg-sys-0b3c813f29a9a20e/out/dist/lib'
(exit code: 1)
libffmpeg_sys-fa3ff87f80f2d27e.rlib is 207M and I assume therefore contains all the statically compiled ffmpeg code.
This only happens when I build on an Amazon Linux instance. Compiling on my regular Fedora 28 desktop results in a working binary.
How would I go about figuring out the root cause of this error?

Answer 1

I solved this problem by building llvm 6.0.1 and then rebuilding with LIBCLANG_PATH set to point to the newer version.
It would appear that rustc has a minimum version requirement on libclang.so, but I could not find an official source documenting that. The version installed on amazon-linux is 3.6.2 which is evidently too old.