`CARGO_CFG_TARGET_HAS_ATOMIC{,_LOAD_STORE}` are empty strings (with Rust nightly)

[vadorovsky]

Problem

In the current Rust nightly, the cfg for atomics usually looks like:

❯ rustc +nightly --print cfg | grep target_has_atomic
target_has_atomic
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="ptr"
target_has_atomic_equal_alignment="16"
target_has_atomic_equal_alignment="32"
target_has_atomic_equal_alignment="64"
target_has_atomic_equal_alignment="8"
target_has_atomic_equal_alignment="ptr"
target_has_atomic_load_store
target_has_atomic_load_store="16"
target_has_atomic_load_store="32"
target_has_atomic_load_store="64"
target_has_atomic_load_store="8"
target_has_atomic_load_store="ptr"

The configs are defined both with values (e.g. target_has_atomic="16" to indicate that the target supports atomics for 16 bit types) and without values (e.g. target_has_atomic to indicate that the target supports atomics in general, for at least one type / size in bytes).

That change in rustc was made in this PR: rust-lang/rust#106856

However, that change made the CARGO_CFG_TARGET_HAS_ATOMIC{,_LOAD_STORE} environment variables empty.

Steps

1. Create a new Rust project with the following build.rs:

fn main() {
    eprintln!(
        "CARGO_CFG_TARGET_THREAD_LOCAL: {}",
        std::env::var("CARGO_CFG_TARGET_THREAD_LOCAL").unwrap()
    );
    eprintln!(
        "CARGO_CFG_TARGET_HAS_ATOMIC: {}",
        std::env::var("CARGO_CFG_TARGET_HAS_ATOMIC").unwrap()
    );
    eprintln!(
        "CARGO_CFG_TARGET_HAS_ATOMIC_LOAD_STORE: {}",
        std::env::var("CARGO_CFG_TARGET_HAS_ATOMIC_LOAD_STORE").unwrap()
    );
    eprintln!(
        "CARGO_CFG_TARGET_HAS_ATOMIC_EQUAL_ALIGNMENT: {}",
        std::env::var("CARGO_CFG_TARGET_HAS_ATOMIC_EQUAL_ALIGNMENT").unwrap()
    );
}

2. See the following output:

[rust-108201 0.1.0] CARGO_CFG_TARGET_THREAD_LOCAL: 
[rust-108201 0.1.0] CARGO_CFG_TARGET_HAS_ATOMIC: 
[rust-108201 0.1.0] CARGO_CFG_TARGET_HAS_ATOMIC_LOAD_STORE: 
[rust-108201 0.1.0] CARGO_CFG_TARGET_HAS_ATOMIC_EQUAL_ALIGNMENT: 16,32,64,8,ptr

The expected, correct output would be rather:

[rust-108201 0.1.0] CARGO_CFG_TARGET_THREAD_LOCAL: 16,32,64,8,ptr
[rust-108201 0.1.0] CARGO_CFG_TARGET_HAS_ATOMIC: 16,32,64,8,ptr
[rust-108201 0.1.0] CARGO_CFG_TARGET_HAS_ATOMIC_LOAD_STORE: 16,32,64,8,ptr
[rust-108201 0.1.0] CARGO_CFG_TARGET_HAS_ATOMIC_EQUAL_ALIGNMENT: 16,32,64,8,ptr

Possible Solution(s)

When there is a cfg in Rustc which is defined both with and without value, I think we should keep the values as an environment variables.

For example, for the following cfg:

target_has_atomic
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="ptr"

Cargo should produce the following env variable:

CARGO_CFG_TARGET_HAS_ATOMIC: 16,32,64,8,ptr

Notes

No response

Version

cargo 1.67.1 (or current master) + rustc nightly