How to build std Rust programs with customized musl libc.

工具链：

• Rust nightly
• xargo(rust-src)

  cargo install xargo
  rustup component add rust-src

• musl-gcc:

  # For macOS:
  brew install FiloSottile/musl-cross/musl-cross
  # For ubuntu
  sudo apt install musl-dev

首先定制并编译musl，之后在build/lib目录下应该能找到libc.a,crt*.o等文件。

这里我们需要做一步Hack，在lib中创建一个空的libunwind.a，以骗过Rust编译器：

cp libm.a libunwind.a

接下来，在项目目录下：

rustup override set nightly

添加文件.cargo/config：

[target.x86_64-unknown-linux-musl]
linker = "rust-lld"
rustflags = [
    "-C", "target-feature=-mmx,-sse,+soft-float",
    "-C", "panic=abort",
    "-L", "<path/to/musl/lib>"
]

添加文件Xargo.toml：

[target.x86_64-unknown-linux-musl.dependencies]
std = {}

编译：

# For Linux:
export TARGET_CC=musl-gcc
# For macOS:
export TARGET_CC=x86_64-linux-musl-gcc

xargo build --target x86_64-unknown-linux-musl