Build fails with environment variable OUT_DIR not defined

[pka]

Release build fails for me with:

   Compiling mime_guess v2.0.0-alpha.4
error: environment variable `OUT_DIR` not defined
  --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/mime_guess-2.0.0-alpha.4/src/lib.rs:16:18
   |
16 | include!(concat!(env!("OUT_DIR"), "/mime_types_generated.rs"));
   |                  ^^^^^^^^^^^^^^^

error: couldn't read "/root/.cargo/registry/src/github.com-1ecc6299db9ec823/mime_guess-2.0.0-alpha.4/src/0/mime_types_generated.rs": No such file or directory (os error 2)
  --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/mime_guess-2.0.0-alpha.4/src/lib.rs:16:1
   |
16 | include!(concat!(env!("OUT_DIR"), "/mime_types_generated.rs"));
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Build failed, waiting for other jobs to finish...
error: Could not compile `mime_guess`.

According to rust-lang/cargo#3368 replacing env!("OUT_DIR") with env::var("OUT_DIR") should solve the issue.

[abonander]

This invocation isn't in a build script, however. It's when building the main library.

[abonander]

@pka what is the output of cargo -V for you?

[pka]

$ cargo -V
cargo 0.16.0-nightly (3568be9 2016-11-26)
$ rustc --version
rustc 1.26.0-nightly (e026b59cf 2018-03-03)

Versions from https://hub.docker.com/r/schickling/rust/

[abonander]

Wow, that Cargo version is crazy outdated! It's not picking up the implicit build script because it didn't look for them then. Why isn't the container using a version more appropriate to that release of Rust?

[pka]

I made a PR to update the Docker container.
Thanks for investigating and sorry for the noise.