Cannot declare different version of dependencies in mutually exclusive targets

[wagenet]

Maintainer's update

• This is fixed for registry dependencies but not git dependencies

---

That's a mouthful, but here's an example:

[target.'cfg(not(target_os = "linux"))'.dependencies]
nix = "0.5.0"

[target.'cfg(target_os = "linux")'.dependencies]
nix = "0.4.3"

Unfortunately, right now this isn't allowed as it's considered a duplicate declaration per #2491.

The use case for this is that FreeBSD requires a newer version of nix to work. However, Linuxes with glibc 2.5 require an older version of nix.

My very ugly workaround is to fork nix and rename it. Then I can use cfg to import the correct crate in the source.

[alexcrichton]

Yeah unfortunately Cargo doesn't handle this case very well in general (depending on two versions of a library). If this works at all I'd be pretty surprised!

[wagenet]

It works, but only if the libraries are forked an renamed to avoid actual duplicate names.

[alexcrichton]

Yeah Cargo should definitely have built-in support for that kind of operation.

[TheNeikos]

Interesting, there is this test: https://github.com/rust-lang/cargo/blob/be010b545fef76858ad74a5f4a188e42e1fffb91/tests/cfg.rs#L232

That seems to test exactly that, and it checks for success

[damaex]

I somewhat have an equal problem with the feature-dependencies for mysql-crate:

[target.'cfg(any(unix, macos))'.dependencies.mysql]
version = "*"
default-features = false
features = ["socket"]

[target.'cfg(windows)'.dependencies.mysql]
version = "*"
default-features = false
features = ["pipe"]

[alexcrichton]

@TheNeikos oh interesting! @wagenet maybe this was fixed in nightly at some point? Or perhaps you could gist the exact error?

@damaex ah I think that my actually be an unrelated failure (e.g. another bug in Cargo). Enabling separate features for different targets unfortunately isn't supported right now.

[wagenet]

@alexcrichton looks like you're right about this being fixed on the latest nightly. Thanks!

[wagenet]

Looks like there are still issues around git dependencies. This doesn't work:

[target.'cfg(not(target_os = "linux"))'.dependencies]
nix     = "0.5.0"
pidfile = { git = "https://github.com/wagenet/pidfile-rust", rev = "a0fe92915e419938d8d33f5cefb6e8f50b89d174" }

[target.'cfg(target_os = "linux")'.dependencies]
nix     = "0.4.3"
pidfile = { git = "https://github.com/carllerche/pidfile-rust", rev = "7516edfc9af2c0480e9f187c87de8cf6323c1ee3" }

It complains about pidfile being a duplicate, but not nix.

[wagenet]

Initial work on a PR to fix this here: #3275

[Eh2406]

Note that https://github.com/rust-lang/cargo/issues/6179#issuecomment-431544643 may provide a workaround by using --minimal-cargo-lock to build only for the current platform.

[wagenet]

See #6212.

[alexcrichton]

I don't personally know how we'd solve this today or whether it's easily solvable, but PRs to implement it are of course always welcome!

[RalfJung]

Looks like a duplicate of https://github.com/rust-lang/cargo/issues/2524.

[ehuss]

@RalfJung I don't see this as a duplicate. The issue here is that two different targets using git dependencies does not work. #2524 is about using different features.

[RalfJung]

Sorry for that then, seems I didn't understand what this bug is about.

[Autoparallel]

I have an issue that is very similar to this. Essentially, when you run cargo check or cargo build, cargo will compile dependencies for all the targets provided. For example, I have a case like this:

[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
tokio       ={ version="1", features=["rt", "rt-multi-thread", "macros", "net", "io-std", "fs"] }
# snip

[target.'cfg(target_arch = "wasm32")'.dependencies]
# No use of `tokio`
# snip

There are, of course, other dependencies that change between the wasm32 and non-wasm32 targets, but what I find is that cargo check builds tokio for wasm32.

This is happening given that distinct portions of code are gated with:

#[cfg(not(target_arch = "wasm32"))] pub mod my_mod;
#[cfg(target_arch = "wasm32")] pub mod my_wasm_mod;

---

Now, I can resolve this by compiling targets separately:

cargo check --target wasm32-unknown-unknown

cargo check --target aarch64-apple-ios

But this leaves a lot to be desired. It's not really very easy to then have rust-analyzer use cargo check as we need to adjust its settings to only focus on a single target at a time. We also cannot build both targets with a single command.

What would be great is, like with features as well, we can have cargo check and cargo build only compile the features and dependencies necessary for the specific target in mind.