Trouble with crates duplicated between the sysroot and the project

[RalfJung]

See #217 for all the details: seems like sometimes there is a problem when a crate occurs both in the sysroot and as a dependency of the project itself. But unfortunately, @roblabla was not able to reproduce this problem, so it is not entirely clear what is really going on.

[RalfJung]

rust-lang/wg-cargo-std-aware#34 might or might not be related.

[roblabla]

I have come across this bug again while updating a custom libstd to rust nightly 2019-11-25.

I'm currently working on making a minimal reproducer, but I'm very afraid that whatever reproducer I manage to create will be dependent on the rustc version used, making it hard to use as a xargo test...

[roblabla]

While investigating, I discovered that rust does inject custom metadata when building the libstd to avoid this very issue. See here. We should probably be setting that env variable to "std", just like bootstrap does.

As such, I 100% think #217 is the proper fix here.

[roblabla]

https://github.com/roblabla/xargo-reproducer Running xargo build --target x86_64-unknown-linux-mygnu will cause the error.

[roblabla]

Unfortunately, this requires a fairly elaborate setup to reproduce (roblabla/xargo-reproducer@abe5612 in my minimal reproducer, we need to inject a dependency in libcore). Not sure how I'd be able to recreate these conditions in a xargo smoke test.