Hm could you give an example of what's broken here? Some simple smoke tests work so I'm a little unclear on what's going wrong.

Works for me on an executable crate but not on a library crate. E.g.:

#Cargo.toml

[project]
name = "foo"
version = "0.5.0"
authors = []
build = "build.rs"

[lib]
name = "foo"
path = "lib.rs"

//lib.rs

include!(concat!(env!("OUT_DIR"), "/hello.rs"))

// build.rs

use std::os;
use std::io::File;
fn main() {
    let dst = Path::new(os::getenv("OUT_DIR").unwrap());
    let mut f = File::create(&dst.join("hello.rs")).unwrap();
    f.write_str("pub fn message() -> &'static str {
                \"Hello, World!\"
    }
").unwrap();
}

$ cargo --version
cargo 0.0.1-pre-nightly (56852db 2014-11-14 23:33:33 +0000)
$ cargo test
     Running target/foo-60d37b257115c1c7

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests foo
lib.rs:3:1: 3:48 error: environment variable `OUT_DIR` not defined
lib.rs:3 include!(concat!(env!("OUT_DIR"), "/hello.rs"))
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib.rs:3:1: 3:48 error: couldn't read 0/hello.rs: couldn't open file (no such file or directory (No such file or directory); path=0/hello.rs; mode=open; access=read)
lib.rs:3 include!(concat!(env!("OUT_DIR"), "/hello.rs"))
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
task '<main>' panicked at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libsyntax/diagnostic.rs:86

Aha, I see what's going on here! It looks like it's just the doc tests that cause a problem.

Did this fix make it into the pre-alpha versions? I'm getting this error when trying to build the "rust-http" module, just downloaded from "https://github.com/chris-morgan/rust-http/archive/master.zip".

cargo build
Compiling http v0.1.0-pre (file:///home/john/projects/rustcode/rust-http)
Compiling rustc-serialize v0.2.7
Compiling gcc v0.1.4
Compiling regex v0.1.10
Compiling matches v0.1.2
Compiling pkg-config v0.1.4
Compiling time v0.1.12
Compiling openssl-sys v0.2.15
Compiling log v0.1.9
Compiling openssl v0.2.15
Compiling url v0.2.16
src/http/server/request.rs:127:21: 127:75 error: couldn't read /home/john/projects/rustcode/rust-http/target/build/http-2b9772103c3d8d3f/out/read_method.rs: IoError { kind: FileNotFound, desc: "couldn't open path as file", detail: Some("no such file or directory (No such file or directory); path=/home/john/projects/rustcode/rust-http/target/build/http-2b9772103c3d8d3f/out/read_method.rs; mode=open; access=read") }
src/http/server/request.rs:127 mod dummy { include!(concat!(env!("OUT_DIR"), "/read_method.rs")); }
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Could not compile http.
To learn more, run the command again with --verbose.

cargo --version
cargo 0.0.1-pre-nightly (8c01b6b 2015-01-08 20:52:43 +0000)

Yes this change should definitely be in that version of Cargo. Perhaps a local bug?

It's a problem with the build script for "rust-http". Just for the record, rust-http has a program which generates "read_method.rs", but into the codegen directory, which is not OUT_DIR. "rust-http" is now abandoned - ("OBSOLETION NOTICE...This library is DEAD...YOU SHOULD NOT USE THIS LIBRARY AT ALL.") So this will not be fixed. Switching to "Hyper" for HTTP use. Sigh.

I think I'm having that same problem, when using OUT_DIR just in tests:

#Cargo.toml

[package]
name = "foo"
version = "0.0.1"
authors = [ ]

//tests/tests.rs

#[test]
fn test() {
    println!(env!("OUT_DIR"));
}

Resulting in:

$ cargo --version
cargo 0.9.0-nightly (8fc3fd8 2016-02-29)
$ cargo test
   Compiling foo v0.0.1 (file:///~~~/dev/sources/foo)
tests/tests.rs:3:5: 3:31 error: environment variable `OUT_DIR` not defined
tests/tests.rs:3     println!(env!("OUT_DIR"));
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
error: Could not compile `foo`.

To learn more, run the command again with --verbose.

I have a workaround: adding a build.rs. It's ok, but that sounds like a problem to me.
If I add a build.rs as follows, that works fine:

#Cargo.toml

[package]
name = "foo"
version = "0.0.1"
authors = [ ]
build = "build.rs"

//build.rs

fn main() { }

Resulting in:

$ cargo test
   Compiling foo v0.0.1 (file:///~~~/dev/sources/foo)
     Running target/debug/tests-695fd20f593eeffc

running 1 test
test test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

@pbougue currently OUT_DIR is only defined if a build script is defined as well I believe

Ok thanks, we'll let it that way then!

currently OUT_DIR is only defined if a build script is defined as well I believe

Oh wow, that wasn't very obvious.