Add require-cas feature

[taiki-e]

Provide better error message if dependents require atomic CAS but the end user forgets to use single-core cfg or critical-section feature.

Context: mvdnes/spin-rs#151

cc @zesterer

Example (with spin-rs)

Enable require-cas feature on spin (taiki-e/spin-rs@990fa92)

- portable-atomic = { version = "1", optional = true, default-features = false }
+ # Enable require-cas feature to provide a better error message if the end user forgets to use cfg.
+ portable-atomic = { version = "1", optional = true, default-features = false, features = ["require-cas"] }

Then, when building spin without single-core cfg or CS feature, the following error message will be displayed.

# without cfg
$ cargo build --target thumbv6m-none-eabi --no-default-features --features spin_mutex,portable_atomic
error: dependents require atomic CAS but not available on this target by default;
       consider using portable_atomic_unsafe_assume_single_core cfg or critical-section feature.
       see <https://docs.rs/portable-atomic/latest/portable_atomic/#optional-cfg> for more.
   --> /Users/taiki/.cargo/git/checkouts/portable-atomic-1ec81f4f47ac5a29/dd55758/src/lib.rs:426:1
    |
426 | / compile_error!(
427 | |     "dependents require atomic CAS but not available on this target by default;\n\
428 | |     consider using portable_atomic_unsafe_assume_single_core cfg or critical-section feature.\n\
429 | |     see <https://docs.rs/portable-atomic/latest/portable_atomic/#optional-cfg> for more."
430 | | );
    | |_^

error: could not compile `portable-atomic` (lib) due to previous error

# with cfg
$ RUSTFLAGS='--cfg portable_atomic_unsafe_assume_single_core' cargo build --target thumbv6m-none-eabi --no-default-features --features spin_mutex,portable_atomic
    Finished dev [unoptimized + debuginfo] target(s) in 0.96s

Maybe we can improve more on the error messages.

[taiki-e]

Published in 1.3.0.