Does not respect +toolchain argument to cargo.

[adlerd]

Running cargo +nightly build builds with rustup's "nightly" toolchain.

However, xargo seems to ignore this option; when it's passed, xargo doesn't even attempt to build libcore:

# rustup default stable
# xargo +nightly build --target simple
<...>
error[E0463]: can't find crate for `core`
  |
  = note: the `simple` target may not be installed
<...>

[adlerd]

Now that I think about it, cargo proper probably doesn't know about the +toolchain either; it's handled by the rustup stub. But since rustup doesn't know about xargo, but xargo knows about rustup, it seems xargo should be the one interpreting and redirecting the flag: xargo +toolchain -> rustup run toolchain xargo, which works fine.

[japaric]

The thing is that Xargo does nothing (it doesn't build a sysroot) when invoked with a stable / beta toolchain. The problem here is that Xargo uses the rustc_version crate, which knows nothing about rustup, to get information (e.g. what channel is being used) about the toolchain.

To fix this we could special case the +$channel argument and use the rustc_version::version_meta_for function with the output of rustc +$channel -Vv instead of vanilla rustc_version::version_meta.