rustc is not building for custom targets

[DeepeshWR]

rustc is not building for custom targets with the current nightly branch. I suspect this is related to the following commit

As mentioned in the commit, I tried passing the -Zunstable-options flag using the following methods:

export RUSTFLAGS="-Zunstable-options"
export RUSTFLAGS_BOOTSTRAP="-Zunstable-options"
config.set("rust", "rustflags", e(["-Zunstable-options",]))

However, none of these approaches worked. The failure occurs during the stage1 build. My understanding
is that the stage1 build may be stripping or ignoring these flags.

Am I missing something here, or is there a different way to ensure -Zunstable-options is honoured during
the stage1 build?

The error I see is:

Created at: src/bootstrap/src/core/builder/mod.rs:1403:23
Executed at: src/bootstrap/src/core/build_steps/compile.rs:1143:25

--- STDERR vvv
error: error loading target specification: custom targets are unstable and require `-Zunstable-options`
  |
  = help: run `rustc --print target-list` for a list of built-in targets

So, once this commit landed in stable releases, how are custom targets expected to be built using the stable channel? Are there any alternative mechanisms or future plans to support this use case?
The full log is attached below for reference.

rust.log

[bjorn3]

It is now necessary to pass -Z json-target-spec to cargo for custom targets to work. RUSTFLAGS doesn't work as it is ignored for the rustc invocation that cargo uses to learn basic information about a target like the file extensions and supported crate types.

[DeepeshWR]

@bjorn3 Thanks for the info! As you suggested, I tried passing -Z json-target-spec to cargo. However, this flag seems to be new, and stage0 cargo doesn’t recognize it. I got the following error:

error: unknown -Z flag specified: json-target-spec

Do I need to wait until stage0 cargo is updated to support this flag, or is there any workaround?

[bjorn3]

I think you will have to wait until stage0 cargo is updated.

[ehuss]

@DeepeshWR Do you have reproduction steps? Can you say more about your configuration? Such as, what is your bootstrap.toml? Is x86_64-oe-linux-gnu a JSON target? Are you using RUST_TARGET_PATH? I'm not quite sure how to recreate your issue.

[ehuss]

Ok, I have one potential reproduction, which is to set build.host to include a JSON target, along with building stage=2.

Still curious on your config, though.

[DeepeshWR]

@ehuss Thanks for the follow-up.

Target details :

1. x86_64-oe-linux-gnu is a JSON target
2. The JSON specs are provided by Yocto and available via RUST_TARGET_PATH

x86_64-oe-linux-gnu.json :

{
  "llvm-target": "x86_64-oe-linux-gnu",
  "arch": "x86_64",
  "os": "linux",
  "env": "gnu",
  "linker": "x86_64-oe-linux-gcc",
  ...
}

config.toml :

[build]
build = "x86_64-unknown-linux-gnu"
host = ["x86_64-oe-linux-gnu"]
target = ["x86_64-oe-linux-gnu"]
rustc = ".../recipe-sysroot-native/usr/bin/rustc"
cargo = ".../recipe-sysroot-native/usr/bin/cargo"

[rust]
channel = "nightly"
download-rustc = false

The JSON target x86_64-oe-linux-gnu is being used as both host and target.

Failure point :
The failure happens during the stage1 compiler build, when bootstrap invokes :

stage1/bin/rustc \
  --target x86_64-oe-linux-gnu \
  --print=file-names \
  --crate-type=proc-macro \
  -

This fails with :
error: error loading target specification: custom targets are unstable and require -Zunstable-options``

As mentioned earlier, passing -Z json-target-spec via RUSTFLAGS does not work. I also tried supplying the flag directly to Cargo, as suggested by @bjorn3, but the stage0 Cargo used for bootstrapping does not yet recognize it. I am waiting for stage0 Cargo to be updated.

The failure is triggered immediately when stage1 invokes rustc for the JSON target specified in build.host; stage2 has not yet started at this point.