cfg aliases

[kvark]

Describe the problem you are trying to solve

As seen in gfx-rs/wgpu#290 (comment), cfg parameters can be quite long:

[target.'cfg(all(target_os = "windows", not(target_arch = "wasm32")))'.dependencies]
gfx-backend-dx12 = { version = "0.3", optional = true }

[target.'cfg(all(any(target_os = "windows", all(unix, not(any(target_os = "macos", target_os = "ios")))), not(target_arch = "wasm32")))'.dependencies]
gfx-backend-vulkan = { version = "0.3", optional = true }

[target.'cfg(any(all(target_os = "macos", not(target_arch = "wasm32"), all(target_arch = "aarch64", target_os = "ios"))))'.dependencies]
gfx-backend-metal = { version = "0.3", optional = true }

If such a dependency is not optional, using it has to be guarded by a similar cfg statement in the code:

#[cfg(any(all(target_os = "macos", not(target_arch = "wasm32"), all(target_arch = "aarch64", target_os = "ios"))))]
gfx_backend_metal::Instance::create("", 1);

Describe the solution you'd like

It is very inconvenient and error prone. It would be great if instead we could define a set of configurations to be used across Cargo config as well as the code. Something like this:

[configurations]
"metal" = 'any(all(target_os = "macos", not(target_arch = "wasm32"), all(target_arch = "aarch64", target_os = "ios")))'

[target.cfg("metal").dependencies]
gfx_backend_metal = "0.3"

And then in Rust code:

#[cfg("metal")]
gfx_backend_metal::Instance::create("", 1);

Notes

Interestingly, configuration aliases are already there, just hard-coded to just "windows" and "unix": https://doc.rust-lang.org/reference/conditional-compilation.html#unix-and-windows

This issue can be seen as making this more generic.

[kvark]

A real-world example on how unfortunate for us to not have this feature: gfx-rs/wgpu#333

[tarcieri]

I'd be a big fan of this as I'm dealing with the following in the polyval crate:

#[cfg(all(
    target_feature = "pclmulqdq",
    target_feature = "sse2",
    target_feature = "sse4.1",
    any(target_arch = "x86", target_arch = "x86_64")
))]

I see there was a pre-RFC for it:

https://internals.rust-lang.org/t/pre-rfc-cfg-attribute-alias/7404

@Manishearth suggested this syntax, which looks great to me:

#![cfg_alias(foo, any(bar, baz))]

#[cfg(foo)] pub mod bar;

[kvark]

I wonder if there is a way here that would support using these aliases in Cargo.toml as well, but I suppose that's too much to ask :) Having at least cfg_alias would be wonderful.

[alexcrichton]

It's worth pointing out that you can already do this today with build scripts. You'll need to read the env vars yourself, but you can then print out cargo:rustc-cfg=... and make the aliases yourself.

[tarcieri]

@alexcrichton neat! Thanks for the tip

[Pzixel]

@alexcrichton wouldn't it only work for just one crate? For instance how can we create an alias for target_feature = "pclmulqdq", target_feature = "sse2" and share it across a dozen crates? It would probably nice to have it as a cargo command-line argument or something like this. I'm not having an good example how could it be implemented right now but I can define the problem: it's quite inconvinient when you have to retype the same cfg in every crate.