Build script is using code to compile Windows target instead of WASM target

[FaceFTW]

This is a bug related to a build.rs file using conditional compilation macros to change it's behavior based on the target being compiled to (either WASM or Windows). The minimal example I was able to create is found in this repository here: https://github.com/FaceFTW/cfg-bug

Originally, I found the issue in this repository and had a hacky workaround in CI: https://github.com/FaceFTW/rust-pipes

I tried this:
Compiling the library target of the project for the wasm32-unknown-unknown target with Cargo using the following build script:

build.rs

use std::io;

#[cfg(not(target_os = "windows"))]
fn main() -> io::Result<()> {
    println!("WASM Build script is being run");
    dbg!(cfg!(target_os = "unknown"));
    Ok(())
}

#[cfg(all(target_os = "windows"))]
fn main() -> io::Result<()> {
    println!("Windows Build Script is being Run");
    dbg!(cfg!(target_os = "unknown"));

    Ok(())
}

I expected to see this happen:
When running the following command, I should be able to find the following output from the build script indicating the build script is using the specific code for WASM targets:

cargo +nightly build --lib --release --target wasm32-unknown-unknown -vv

[cfg-bug 0.1.0] WASM Build Script is being Run
[cfg-bug 0.1.0] [build.rs:13:5] cfg!(target_os = "unknown") = true

Instead, this happened:
When running the same command, I instead see that the build script is using code for Windows targets. The expected target from my understanding of the triple is unknown in wasm32-unknown-unknown.

cargo +nightly build --lib --release --target wasm32-unknown-unknown -vv

[cfg-bug 0.1.0] Windows Build Script is being Run
[cfg-bug 0.1.0] [build.rs:13:5] cfg!(target_os = "unknown") = false

Meta

rustc --version --verbose:

rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: x86_64-pc-windows-msvc
release: 1.83.0
LLVM version: 19.1.1

Backtrace

   Compiling cfg-bug v0.1.0 (C:\Users\awest\Workspaces\cfg-bug)
     Running `set CARGO=\\?\C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe&& set CARGO_CRATE_NAME=build_script_build&& set CARGO_MANIFEST_DIR=C:\Users\awest\Workspaces\cfg-bug&& set CARGO_MANIFEST_PATH=C:\Users\awest\Workspaces\cfg-bug\Cargo.toml&& set CARGO_PKG_AUTHORS=""&& set CARGO_PKG_DESCRIPTION=""&& set CARGO_PKG_HOMEPAGE=""&& set CARGO_PKG_LICENSE=""&& set CARGO_PKG_LICENSE_FILE=""&& set CARGO_PKG_NAME=cfg-bug&& set CARGO_PKG_README=""&& set CARGO_PKG_REPOSITORY=""&& set CARGO_PKG_RUST_VERSION=""&& set CARGO_PKG_VERSION=0.1.0&& set CARGO_PKG_VERSION_MAJOR=0&& set CARGO_PKG_VERSION_MINOR=1&& set CARGO_PKG_VERSION_PATCH=0&& set CARGO_PKG_VERSION_PRE=""&& set CARGO_PRIMARY_PACKAGE=1&& set PATH="C:\Users\awest\Workspaces\cfg-bug\target\release\deps;C:\Program Files\PowerShell\7;C:\Program Files\Common Files\Oracle\Java\javapath;C:\Program Files\Microsoft\jdk-11.0.25.9-hotspot\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files\dotnet\;C:\Program Files\PowerShell\7\;%NVM_HOME%;%NVM_SYMLINK%;C:\Program Files\Docker\Docker\resources\bin;C:\Program Files\Git\cmd;C:\Program Files\Git LFS;C:\Program Files\CMake\bin;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\PuTTY\;C:\Program Files (x86)\gnupg\bin;C:\Program Files (x86)\Gpg4win\..\GnuPG\bin;C:\Program Files (x86)\dotnet-core-uninstall\;C:\Program Files (x86)\Windows Kits\10\Windows Performance Toolkit\;C:\Users\awest\AppData\Local\Programs\Python\Python312\Scripts\;C:\Users\awest\AppData\Local\Programs\Python\Python312\;C:\Users\awest\AppData\Local\Programs\Python\Launcher\;C:\Users\awest\AppData\Local\Programs\Python\Python311\Scripts\;C:\Users\awest\AppData\Local\Programs\Python\Python311\;C:\Users\awest\.cargo\bin;C:\Users\awest\AppData\Local\Microsoft\WindowsApps;C:\Users\awest\AppData\Local\Microsoft\WinGet\Links;C:\Users\awest\AppData\Roaming\nvm;C:\Program Files\nodejs;C:\Users\awest\AppData\Local\Programs\Microsoft VS Code\bin;C:\Users\awest\.local\bin;C:\Users\awest.local\bin\vim"&& C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\rustc.exe --crate-name build_script_build --edition=2021 build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=261 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debug-assertions=off --check-cfg cfg(docsrs) --check-cfg "cfg(feature, values())" -C metadata=5a1ee0679c54dc42 -C extra-filename=-5a1ee0679c54dc42 --out-dir C:\Users\awest\Workspaces\cfg-bug\target\release\build\cfg-bug-5a1ee0679c54dc42 -C strip=debuginfo -L dependency=C:\Users\awest\Workspaces\cfg-bug\target\release\deps`
     Running `set CARGO=\\?\C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe&& set CARGO_CFG_PANIC=abort&& set CARGO_CFG_TARGET_ABI=""&& set CARGO_CFG_TARGET_ARCH=wasm32&& set CARGO_CFG_TARGET_ENDIAN=little&& set CARGO_CFG_TARGET_ENV=""&& set CARGO_CFG_TARGET_FAMILY=wasm&& set CARGO_CFG_TARGET_FEATURE=mutable-globals,sign-ext&& set CARGO_CFG_TARGET_HAS_ATOMIC=16,32,64,8,ptr&& set CARGO_CFG_TARGET_OS=unknown&& set CARGO_CFG_TARGET_POINTER_WIDTH=32&& set CARGO_CFG_TARGET_VENDOR=unknown&& set CARGO_ENCODED_RUSTFLAGS=""&& set CARGO_MANIFEST_DIR=C:\Users\awest\Workspaces\cfg-bug&& set CARGO_MANIFEST_PATH=C:\Users\awest\Workspaces\cfg-bug\Cargo.toml&& set CARGO_PKG_AUTHORS=""&& set CARGO_PKG_DESCRIPTION=""&& set CARGO_PKG_HOMEPAGE=""&& set CARGO_PKG_LICENSE=""&& set CARGO_PKG_LICENSE_FILE=""&& set CARGO_PKG_NAME=cfg-bug&& set CARGO_PKG_README=""&& set CARGO_PKG_REPOSITORY=""&& set CARGO_PKG_RUST_VERSION=""&& set CARGO_PKG_VERSION=0.1.0&& set CARGO_PKG_VERSION_MAJOR=0&& set CARGO_PKG_VERSION_MINOR=1&& set CARGO_PKG_VERSION_PATCH=0&& set CARGO_PKG_VERSION_PRE=""&& set DEBUG=false&& set HOST=x86_64-pc-windows-msvc&& set NUM_JOBS=16&& set OPT_LEVEL=3&& set OUT_DIR=C:\Users\awest\Workspaces\cfg-bug\target\wasm32-unknown-unknown\release\build\cfg-bug-4e9b01acbcb67e7b\out&& set PATH="C:\Users\awest\Workspaces\cfg-bug\target\release\deps;C:\Users\awest\Workspaces\cfg-bug\target\release;C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\x86_64-pc-windows-msvc\lib;C:\Program Files\PowerShell\7;C:\Program Files\Common Files\Oracle\Java\javapath;C:\Program Files\Microsoft\jdk-11.0.25.9-hotspot\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files\dotnet\;C:\Program Files\PowerShell\7\;%NVM_HOME%;%NVM_SYMLINK%;C:\Program Files\Docker\Docker\resources\bin;C:\Program Files\Git\cmd;C:\Program Files\Git LFS;C:\Program Files\CMake\bin;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\PuTTY\;C:\Program Files (x86)\gnupg\bin;C:\Program Files (x86)\Gpg4win\..\GnuPG\bin;C:\Program Files (x86)\dotnet-core-uninstall\;C:\Program Files (x86)\Windows Kits\10\Windows Performance Toolkit\;C:\Users\awest\AppData\Local\Programs\Python\Python312\Scripts\;C:\Users\awest\AppData\Local\Programs\Python\Python312\;C:\Users\awest\AppData\Local\Programs\Python\Launcher\;C:\Users\awest\AppData\Local\Programs\Python\Python311\Scripts\;C:\Users\awest\AppData\Local\Programs\Python\Python311\;C:\Users\awest\.cargo\bin;C:\Users\awest\AppData\Local\Microsoft\WindowsApps;C:\Users\awest\AppData\Local\Microsoft\WinGet\Links;C:\Users\awest\AppData\Roaming\nvm;C:\Program Files\nodejs;C:\Users\awest\AppData\Local\Programs\Microsoft VS Code\bin;C:\Users\awest\.local\bin;C:\Users\awest.local\bin\vim"&& set PROFILE=release&& set RUSTC=C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\rustc.exe&& set RUSTDOC=C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\rustdoc.exe&& set TARGET=wasm32-unknown-unknown&& C:\Users\awest\Workspaces\cfg-bug\target\release\build\cfg-bug-5a1ee0679c54dc42\build-script-build`
[cfg-bug 0.1.0] Windows Build Script is being Run
[cfg-bug 0.1.0] [build.rs:13:5] cfg!(target_os = "unknown") = false
     Running `set CARGO=\\?\C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\cargo.exe&& set CARGO_CRATE_NAME=cfg_bug&& set CARGO_MANIFEST_DIR=C:\Users\awest\Workspaces\cfg-bug&& set CARGO_MANIFEST_PATH=C:\Users\awest\Workspaces\cfg-bug\Cargo.toml&& set CARGO_PKG_AUTHORS=""&& set CARGO_PKG_DESCRIPTION=""&& set CARGO_PKG_HOMEPAGE=""&& set CARGO_PKG_LICENSE=""&& set CARGO_PKG_LICENSE_FILE=""&& set CARGO_PKG_NAME=cfg-bug&& set CARGO_PKG_README=""&& set CARGO_PKG_REPOSITORY=""&& set CARGO_PKG_RUST_VERSION=""&& set CARGO_PKG_VERSION=0.1.0&& set CARGO_PKG_VERSION_MAJOR=0&& set CARGO_PKG_VERSION_MINOR=1&& set CARGO_PKG_VERSION_PATCH=0&& set CARGO_PKG_VERSION_PRE=""&& set CARGO_PRIMARY_PACKAGE=1&& set OUT_DIR=C:\Users\awest\Workspaces\cfg-bug\target\wasm32-unknown-unknown\release\build\cfg-bug-4e9b01acbcb67e7b\out&& set PATH="C:\Users\awest\Workspaces\cfg-bug\target\release\deps;C:\Program Files\PowerShell\7;C:\Program Files\Common Files\Oracle\Java\javapath;C:\Program Files\Microsoft\jdk-11.0.25.9-hotspot\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files\dotnet\;C:\Program Files\PowerShell\7\;%NVM_HOME%;%NVM_SYMLINK%;C:\Program Files\Docker\Docker\resources\bin;C:\Program Files\Git\cmd;C:\Program Files\Git LFS;C:\Program Files\CMake\bin;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\PuTTY\;C:\Program Files (x86)\gnupg\bin;C:\Program Files (x86)\Gpg4win\..\GnuPG\bin;C:\Program Files (x86)\dotnet-core-uninstall\;C:\Program Files (x86)\Windows Kits\10\Windows Performance Toolkit\;C:\Users\awest\AppData\Local\Programs\Python\Python312\Scripts\;C:\Users\awest\AppData\Local\Programs\Python\Python312\;C:\Users\awest\AppData\Local\Programs\Python\Launcher\;C:\Users\awest\AppData\Local\Programs\Python\Python311\Scripts\;C:\Users\awest\AppData\Local\Programs\Python\Python311\;C:\Users\awest\.cargo\bin;C:\Users\awest\AppData\Local\Microsoft\WindowsApps;C:\Users\awest\AppData\Local\Microsoft\WinGet\Links;C:\Users\awest\AppData\Roaming\nvm;C:\Program Files\nodejs;C:\Users\awest\AppData\Local\Programs\Microsoft VS Code\bin;C:\Users\awest\.local\bin;C:\Users\awest.local\bin\vim"&& C:\Users\awest\.rustup\toolchains\stable-x86_64-pc-windows-msvc\bin\rustc.exe --crate-name cfg_bug --edition=2021 src\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=261 --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no --check-cfg cfg(docsrs) --check-cfg "cfg(feature, values())" -C metadata=fd6989a280b3860f -C extra-filename=-fd6989a280b3860f --out-dir C:\Users\awest\Workspaces\cfg-bug\target\wasm32-unknown-unknown\release\deps --target wasm32-unknown-unknown -C strip=debuginfo -L dependency=C:\Users\awest\Workspaces\cfg-bug\target\wasm32-unknown-unknown\release\deps -L dependency=C:\Users\awest\Workspaces\cfg-bug\target\release\deps`
    Finished `release` profile [optimized] target(s) in 1.31s```

</p>
</details>

[FaceFTW]

@rustbot label +A-cfg

[jieyouxu]

AFAICT this is cargo's build.rs implementation and not rustc. Transferring this to the cargo issue tracker for further triage (T-cargo please send it back here if it's a rustc concern).

@rustbot transfer cargo

[weihanglo]

To run your build script on your host machine, Cargo always compiles it to your host platform, which is x86_64-pc-windows-msvc. cfg! value is determined at "build script compile time" and resolved to x86_64-pc-windows-msvc.

To get the target platform you main package compile to at "build script execution time", you'll need to read either TARGET or CARGO_CFG_TARGET_OS environment variable via std::env::var{_os}, which will be wasm32-unknown-unknown. See Environment variables Cargo sets for build scripts

If you do want to compile your build script to a certain target platform, see the unstable feature artifact dependencies.

[weihanglo]

As this seems to be a misuse of cfg! when other environment variables set at build script execution time ought to be used, I am going to close this. If you have more questions, feel free to ask here or on forums like https://users.rust-lang.org/.

[FaceFTW]

Thanks for the quick response/triage. I'll add a quick README to the example repository with a reference to this information in case anyone stumbles upon it in the future.