Allow specifying dependencies per feature

[sdroege]

Most feature-dependency correspondences can currently be modeled by making the dependency optional but having the feature depend on this.

This is not possible (from what I see) if "feature-a" and "feature-b" depend on the same crate but in different versions, as each crate can only be specified once.

A syntax like the following would seem consistent for that case (and could also provide another way of specifying feature-dependant dependencies in general).

[target.'cfg(feature = "feature-a")'.dependencies]
foobar = "0.1"

[target.'cfg(feature = "feature-b")'.dependencies]
foobar = "0.2"

[alexcrichton]

I think that this may also largely be supported by #5653? May make it a bit easier to specify as well!

[updogliu]

@alexcrichton There might be dependencies that are relevant only for a certain feature. Is there a way to express that?

Edit:
There is already an example in the doc:
https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section

[mmstick]

Encountered a similar issue where a workspace is split into multiple sub-crates, and each rely on specifying shared features of the main crate dependency. Something like this doesn't seem to work at all:

[features]
default = [ "fwupd", "system76" ]
fwupd = []
system76 = []

[target.'cfg(all(not(feature = "fwupd"), feature = "system76"))'.dependencies]
firmware-manager = { path = "../", default-features = false, features = [ "system76" ] }

[target.'cfg(all(feature = "fwupd", not(feature = "system76")))'.dependencies]
firmware-manager = { path = "../", default-features = false, features = [ "fwupd" ] }

[target.'cfg(all(feature = "fwupd", feature = "system76"))'.dependencies]
firmware-manager = { path = "../", default-features = false, features = [ "fwupd", "system76" ] }

cargo build --manifest-path gtk/Cargo.toml --no-default-features --features system76

[sergiimk]

Hopefully this will save someone an hour of googling.

If you have a library a that depends on b, and you want to support two or more major versions of b that are slightly incompatible:

In a's Cargo.toml:

[dependencies]
b-5 = { package = "b", version = "^5", optional = true }
b-6 = { package = "b", version = "^6", optional = true }

[features]
default = ["use-b-6"]
use-b-5 = ["b-5"] 
use-b-6 = ["b-6"]

Create a b_shim.rs in a:

#[cfg(feature = "use-b-5")]
pub use b_5::*;
#[cfg(feature = "use-b-6")]
pub use b_6::*;

Instead of use b::foo import via crate::b_shim::foo.

Application that uses a can now chose to use it with older b by:

[dependencies]
b = "^5"
a = { version = "*", default-features = false, features = ["use-b-5"] }

This is more work than it should be....

[epage]

With package renaming, we've satisfied the main request and there is a great example just above of that.

As for this case

[features]
default = [ "fwupd", "system76" ]
fwupd = []
system76 = []

[target.'cfg(all(not(feature = "fwupd"), feature = "system76"))'.dependencies]
firmware-manager = { path = "../", default-features = false, features = [ "system76" ] }

[target.'cfg(all(feature = "fwupd", not(feature = "system76")))'.dependencies]
firmware-manager = { path = "../", default-features = false, features = [ "fwupd" ] }

[target.'cfg(all(feature = "fwupd", feature = "system76"))'.dependencies]
firmware-manager = { path = "../", default-features = false, features = [ "fwupd", "system76" ] }

that can be done as

[features]
default = [ "fwupd", "system76" ]
fwupd = ["dep:firmware-manager", "firmware-manager/fwupd"]
system76 = ["dep:firmware-manager", "firmware-manager/system76"]

[dependencies]
firmware-manager = { path = "../", default-features = false, optional = true }

As this seems resolved, I'm closing it. If there is a reason to re-open this, let us know!

[nathanielsimard]

I need to use something like that but for a different use case. I need to activate a feature flag on a dependency when that dependency is activated and the other feature flag is activated as well:

[features]
flag1 = ["crate1"]
flag2 = ["crate2"]
someflag = []

[target.'cfg(all(feature = "flag1",  feature = "someflag"))'.dependencies]
crate1 = { ..., features = ["someflag"] }

[target.'cfg(all(feature = "flag1",  not(feature = "someflag")))'.dependencies]
crate1 = { ..., features = [] }

[target.'cfg(all(feature = "flag2",  feature = "someflag"))'.dependencies]
crate2 = { ..., features = ["someflag"] }

[target.'cfg(all(feature = "flag2",  not(feature = "someflag")))'.dependencies]
crate2 = { ..., features = ["someflag"] }

If I do something like this:

[features]
flag1 = ["crate1"]
flag2 = ["crate2"]
someflag = ["crate1/someflag", "crate2/someflag"]

Then both crates are going to be activated when only the flag someflag is activated.

[epage]

Is weak features what you need?

someflag = ["crate1?/someflag", "crate2?/someflag"]

[nathanielsimard]

@epage Yes exactly! I didn't know it even existed, thanks a lot 🙏

[Sytten]

There is still a use case that is not covered with the weak feature. @epage

[features]
default = ["myfeature"]
myfeature = []
simd = []

[target.'cfg(all(feature = "simd",  feature = "myfeature"))'.dependencies]
base64-simd = "0.8.0"

[target.'cfg(all(not(feature = "simd"),  feature = "myfeature"))'.dependencies]
base64 = "0.22.0"

I know you can do myfeature and myfeature-simd, but it really is not that nice especially if you have multiple features that can have a simd variant.

[epage]

Note that we have

• [target.'cfg(...)'] doesn't expose all keys (like test and features) #8170
• Automatically activate a dependency if its used elsewhere in dependency graph #1555
• Allow arbitrary Horn clauses for inter-feature dependencies. #6789

which in various ways cover that case I believe.