Cross compile std

[japaric]

For starters, with a few restrictions

• Xargo won't compile std for the host
• No custom std source
• The Cargo features of std can't be tweaked

These issues need to be addressed in rust-lang/rust:

• LLVM assertion when cargo building std with panic=abort rust-lang/rust#37252
• Error cargo building std as a dylib rust-lang/rust#36501

But there are probably others. The easiest way to test this feature today is by compiling a binary Cargo project with the following Cargo.toml.

[dependencies]
std = "$(rustc --print sysroot)/lib/rustlib/src/rust/src/libstd"

[japaric]

I would strongly suggest to people that want this feature to instead push for "std aware" Cargo (rust-lang/rfcs#1133) though. That's the proper solution.

[japaric]

I think the only remaining blocker for this is rust-lang/rust#37975

[HybridEidolon]

Any way we can get rust-lang/rust#37975 retried on CI?

[japaric]

@HybridEidolon You'll have to convince Alex 😄. It'd be best if someone with access to a mac tested the PR to verify if the issue is real.

[HybridEidolon]

The patch I put on my last comment of that issue makes the tests pass under makefile builds, which makes tests pass for mac.