Run cargo expand outside the normal target directory

When crates should get expanded, then cargo is run again from within cbindgen.
When cbindgen is started from a build.rs file, this means that cargo is starting
another cargo run.

Cargo locks the directory it is running on. If two cargos spawn by each other
run with the same target directory, then they both want to accquire a lock and
hence deadlock.

This commit fixes the problem with running the spawned cargo at a different
directory. Please note that this will always be the same directory, hence
any subsequent runs will be faster (as you would exepct it to be).

This is part of mozilla#347.

src/bindgen/cargo/cargo_expand.rs

@@ -6,7 +6,7 @@ use std::env;
 use std::error;
 use std::fmt;
 use std::io;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::process::Command;
 use std::str::{from_utf8, Utf8Error};
 
@@ -75,6 +75,12 @@ pub fn expand(
         cmd.env("CARGO_TARGET_DIR", _temp_dir.unwrap().path());
     } else if let Ok(ref path) = env::var("CARGO_EXPAND_TARGET_DIR") {
         cmd.env("CARGO_TARGET_DIR", path);
+    } else if let Ok(ref path) = env::var("OUT_DIR") {
+        // When cbindgen was started programatically from a build.rs file, Cargo is running and
+        // locking the default target directory. In this case we need to use another directory,
+        // else we would end up in a deadlock. If Cargo is running `OUT_DIR` will be set, so we
+        // can use a directory relative to that.
+        cmd.env("CARGO_TARGET_DIR", PathBuf::from(path).join("expanded"));
     }
 
     cmd.arg("rustc");