Automatically activate a dependency if its used elsewhere in dependency graph

[cgaebel]

Cargo should be able to express the idea of: "I want to depend on this package iff whoever depends on me is already depending on that package." This is especially useful because rust doesn't have orphan instances.

Specifically, I want to add a quickcheck Arbitrary instance to Iobuf. However, I don't want to add the dependency on Quickcheck for people not using it (non-test executables, for most people). It would be nice if it "just worked", perhaps in a new section:

[optional-dependencies]
quickcheck = "*"

And then anyone that depends on iobuf can have the quickcheck instances built for tests by including quickcheck in their dev-dependencies.

[alexcrichton]

Are you familiar with inline table syntax for TOML? I think this is much more ergonomic than a new section:

[dependencies]
quickcheck = { optional = true }

[cgaebel]

I didn't know about that syntax, but it sounds good to me. I care much more about the "magic, just-works" behavior than the syntax. :)

[alexcrichton]

Ok, in that case I'm going to close this as the inline table syntax should be much more ergonomic than a whole new section.

[cgaebel]

Wait, this feature exists already? How do I express "I want to depend on this package iff whoever depends on me is already depending on that package." ?

[alexcrichton]

Ah I believe I have misread.

[erickt]

While this doesn't happen automatically, I believe this can currently be expressed with:

crate foo:

[dependencies]
quickcheck = { version = "*", optional = true }

crate bar:

[features]
with-quickcheck = ["quickcheck", "foo/quickcheck"]

[dependencies]
foo = "*"
quickcheck { version = "*", optional = true }

Then for the downstream, if they're using quickcheck, they can opt into bar using quickcheck via:

[features]
with-quickcheck = ["quickcheck", "bar/with-quickcheck"]

[dependencies]
bar = "*"
quickcheck = { version = "*", optional = true }

[Storyyeller]

I think this would be very useful. When writing a library that defines a new type, it often makes sense to impl traits from other popular libraries for interoperability. But I don't want to pull in all those extra dependencies if the user isn't using them.

[mehcode]

This would be awesome. 💯

[dependencies]
diesel = { version = "0.10.0", features = ["postgres", "uuid"] }
diesel_codegen = { version = "0.10.0", features = ["postgres", "dotenv"] }
uuid = { version = "0.4.0", features = ["serde"] }
serde = "0.9"
dotenv = "*"

Wouldn't it be simply beautiful if the above could instead be:

[dependencies]
diesel = { version = "0.10.0", features = ["postgres"] }
diesel_codegen = { version = "0.10.0", features = ["postgres"] }
uuid = "0.4.0"
serde = "0.9"
dotenv = "*"

Automatic use of features (the optional crate kind) that I have as explicit dependencies would be a very ergonomic addition.

[alexcrichton]

I believe this feature has an associated RFC now: rust-lang/rfcs#1787