cfg lottery demo

Why we moved to --cfg flags instead of using feature for non-additive selections.

• Allows the top-level / binary crate to control the setting
• Works well for non-additive selections which otherwise needs compile_error!()
• Sometimes feasible not let transitive dependencies to be burdened with configuration
• Ofcourse as with everything there are tradeoffs!
• TBC: Blog entry coming up!

Setup

$ cargo tree -i lib_with_cfgs

lib_with_cfgs v0.1.0 (./crates/lib_with_cfgs)
├── naughty_lib v0.1.0 (./crates/naughty_lib)
│   └── wheelie_bin v0.1.0 (./crates/wheelie_bin)
└── okay_lib v0.1.0 (./crates/okay_lib)
    └── wheelie_bin v0.1.0 (./crates/wheelie_bin)

Naughty lib

Don't let the naughty lib get what it wants!

$ cat crates/naughty_lib/build.rs

fn main() {
    println!("cargo:rustc-cfg=lib_selection=\"what_naughty_lib_wanted\"");
}

Sensible defaults

Set some sensible defaults..

$ cat crates/lib_with_cfgs/build.rs

fn main() {
    #[cfg(all(
        not(lib_selection = "what_good_bin_wanted"),
        not(lib_selection = "what_naughty_lib_wanted")
    ))]
    println!("cargo:rustc-cfg=lib_selection=\"sensible_default\"");
}

$ env RUSTFLAGS cargo run

Sensible defaults!

Various Ways

env RUSTFLAGS='--cfg lib_selection="what_good_bin_wanted"' cargo run

Good bin got what it wanted!

Also .cargo/config [build] section.

Real World

• curve25519_dalek_[bits|backend]: https://github.com/dalek-cryptography/curve25519-dalek/