Support custom target-spec

[boozook]

Could be great to support custom target-specs (json files).

As we all know, env TARGET contains rustc's "short target name", not exactly target-triple. It can be triple only if builtin target requested. So cc couldn't use it directly if some custom target requested, e.g. for --target=my-spec.json env TARGET will be "my-spec".

How to do it:

1. change there target type - wrap into something like enum Target { Builtin(...), Custom(...) }
2. probe target to determine is it builtin or not and get details:
   • we have env RUSTC by cargo
   • $RUSTC --print cfg --target {target-short-name}.json
   • $RUSTC --print target-spec-json --target {target-short-name}.json -Zunstable-options (nightly only) From this step we can get llvm-target

But actually we already have almost all we need by cargo's env:

• CARGO_CFG_TARGET_ABI
• CARGO_CFG_TARGET_ARCH
• CARGO_CFG_TARGET_FEATURE
• CARGO_CFG_RELOCATION_MODEL
• other vars starting with CARGO_CFG_TARGET_ excluding _VENDOR and _OS, and probably _ENV.

_Note, CARGO_CFG_TARGET_FEATURE contains features added in the my-spec.json too, so it really useful._

So, could be great to support target-json-specs, use info from them. Also to respect and use all available data from vars CARGO_CFG_TARGET_ - translate and pass to compiler if possible.

Example of env vars given by cargo

_In my case, my target.json contains `"llvm-target": "thumbv7em-none-eabihf"`, uses it as base target, but overrides features and some more._ ``` TARGET: "my-spec" CARGO_CFG_OVERFLOW_CHECKS: "" CARGO_CFG_PANIC: "abort" CARGO_CFG_RELOCATION_MODEL: "pic" CARGO_CFG_TARGET_ABI: "eabihf" CARGO_CFG_TARGET_ARCH: "arm" CARGO_CFG_TARGET_ENDIAN: "little" CARGO_CFG_TARGET_ENV: "elf" CARGO_CFG_TARGET_FEATURE: "dsp,mclass,thumb-mode,thumb2,v5te,v6,v6k,v6t2,v7,neon" CARGO_CFG_TARGET_HAS_ATOMIC: "16,32,8,ptr" CARGO_CFG_TARGET_HAS_ATOMIC_EQUAL_ALIGNMENT: "16,32,8,ptr" CARGO_CFG_TARGET_HAS_ATOMIC_LOAD_STORE: "16,32,8,ptr" CARGO_CFG_TARGET_OS: "myneatos" CARGO_CFG_TARGET_POINTER_WIDTH: "32" CARGO_CFG_TARGET_THREAD_LOCAL: "" CARGO_CFG_TARGET_VENDOR: "custom" CARGO_CFG_UB_CHECKS: "" CARGO_ENCODED_RUSTFLAGS: "" RUSTC: "path/to/rustc" ```

Probably related issue: https://github.com/rust-lang/cc-rs/issues/994

[NobodyXu]

Also to respect and use all available data from vars CARGO_CFGTARGET - translate and pass to compiler if possible.

Yeah that sounds reasonable.

Supporting target spec json query though is a bit more complicated, I suppose we can add the API to pass that information to cc, and do the parsing in another crate, since it would require JSON parsing and maybe nightly compiler.

[boozook]

That could be really helpful!

[boozook]

Also things like that is not work for target with custom names, definitely. Imagine I'll name my target arch-foo-bar-like-msvc or more funny same-but-not-msvc :))) , or better for realistic example msvc-plugin-for-arch-gnu :).

That things could be good practice to determine by compiler (print), or by cargo (env), or directly from json.

Anyway about that concert msvc - I suppose will be better to get it from CARGO_CFG_TARGET_ENV - if value is known, so ok; otherwise just ignoring unknown value.

Wow, just search in the lib.rs for target.contains. I see 125 findings.

---

Also cc fails on target-names that doesn't contains dash, e.g. "strangecpu", trying to split it by -.

[NobodyXu]

Wow, just search in the lib.rs for target.contains. I see 125 findings.

Yeah it is a bit messy