Setting up a new Rust Target with a cross compiler

This instructions are can be simplified by providing a precompiled libstd and libc as target. But for now it outlines the process needed to get a standard rust compiler to compile undefined illumos and arch combinations like sparcv9-unknown-illumos aarch64-unknown-illumos and riscv64-unknown-illumos 

Note: if your crate needs an external C Library you will have to fiddle with it a bit to properly set the include and cross build directives that crate supports.

1. Get Rustup via the normal channels (goto https://rustup.rs copy and paste OSX instructions)
2. Install Nightly toolchain rustup toolchain add nightly
3. print an already existing illumos rustc target and adjust to the target CPU rustc +nightly -Z unstable-options --target=aarch64-unknown-freebsd --print target-spec-json 
4. check other OS’s spec files for that CPU to see what is OS specific (most) and what is CPU specific
5. use https://llvm.org/docs/LangRef.html#data-layout to adjust data-layout key according to platform. all other OS’s should use the same layout on that cou architecture if not they are at least equally bugged
6. Set llvm-target to <arch>-unknown-solaris2.11 where arch is the same as used in the arch key in the json
7. put any linker flags that are system independant in pre-link-args  like -std=c99
8. Setup Cross compile capable GCC plus LD and illumos gate sysroot as usual (Checkout solarm-sysroot for packaging details)
9. Use a path from https://doc.rust-lang.org/cargo/reference/config.html to write either user specific cargo overrides or system based ones. (basically override the linker on the desired arch in .cargo/config.toml in the desired location)
10. Patch the dependency tree to use fixed libc crate from https://github.com/Toasterson/libc.git
11. run cargo update -p libc after that to update Cargo.lock
12. export OPENSSL_DIR to point to sysroot if needed (basically if your package links to openssl)
13. Use this invocation to compile your crate cargo +nightly build -Z build-std --target=/path/to/aarch64-unknown-illumos.json

Example files:

aarch64-unknown-illumos.json:

{
  "arch": "aarch64",
  "data-layout": "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128",
  "dynamic-linking": true,
  "eh-frame-header": false,
  "frame-pointer": "always",
  "has-rpath": true,
  "is-like-solaris": true,
  "late-link-args": {
    "gcc": [
      "-lc",
      "-lssp"
    ]
  },
  "limit-rdylib-exports": false,
  "linker-is-gnu": false,
  "llvm-target": "aarch64-unknown-solaris2.11",
  "max-atomic-width": 128,
  "os": "illumos",
  "pre-link-args": {
    "gcc": [
      "-std=c99"
    ]
  },
  "supported-sanitizers": [
    "address",
    "cfi"
  ],
  "target-family": [
    "unix"
  ],
  "target-pointer-width": "64"
}

cross compile cargo config for gcc with custom sysroot

#[build]
#target-dir = "/export/home/vagrant/.cargo/target" # Set this to instruct cargo to place the compile artifacts into the specified directory (usefull for NFS mounted directories)

[target.aarch64-unknown-illumos]
linker = "/opt/solarm/bin/aarch64-unknown-solaris2.11-gcc" # Path of the gcc binary to use as linker (calls our LD in the background but $LD cannot be set)
rustflags = "-Clink-arg=--sysroot=/opt/solarm/braich" # Any linker args that are usually set in $LDFLAGS go here