Cargo doesn't pass target features from config specified for a particular target cpu architecture

[EdvardD]

Problem

I'm specifying Rust flags for target features in .cargo/config.toml and they aren't propagated to CARGO_CFG_TARGET_FEATURE env variable in build.rs if I protect flags with the given target_arch ([target.'cfg(target_arch="x86_64")']). It works correctly if I put flags under [build] section.

Steps

I put together this simple example: https://github.com/EdvardD/cargo-build-rs-rustflags.

To reproduce: git clone https://github.com/EdvardD/cargo-build-rs-rustflags.git && cd cargo-build-rs-rustflags && ./test.sh

Possible Solution(s)

No response

Notes

$ cat test.sh 
#!/bin/bash -e

rustup show | head -n1
rm -rf target
cargo build
cat ./target/debug/build/*/output

$ ./test.sh 
Default host: x86_64-unknown-linux-gnu
   Compiling somelib v0.1.0 (/home/edvard/cargo-build-rs-rustflags/somelib)
   Compiling cargo-build-rs-rustflags v0.1.0 (/home/edvard/cargo-build-rs-rustflags)
    Finished dev [unoptimized + debuginfo] target(s) in 0.60s
fxsr,sse,sse2

Version

cargo 1.59.0 (49d8809dc 2022-02-10)
release: 1.59.0
commit-hash: 49d8809dc2d3e6e0d5ec634fcf26d8e2aab67130
commit-date: 2022-02-10
host: x86_64-unknown-linux-gnu
libgit2: 1.3.0 (sys:0.13.23 vendored)
libcurl: 7.80.0-DEV (sys:0.4.51+curl-7.80.0 vendored ssl:OpenSSL/1.1.1l)
os: Ubuntu 21.10 (impish) [64-bit]

[ehuss]

The problem is that cargo is unable to use the rustflags from config for computing the cfg values because that could change which [target] tables are included from the config. Cargo could potentially do that in a loop until it converges to a fixed point, but I think that's unlikely.

At a minimum, I think there could be better documentation for this.

Closing as a duplicate of #6858.