Invalid Cargo.toml when using dep: namespaced features

[Anders429]

Below is an attempt at creating a minimal reproducible example of this bug.

In rustc 1.60.0, namespaced dependencies were added that made the following Cargo.toml manifest possible:

[package]
name = "foo"
version = "0.1.0"
edition = "2021"

[dependencies]
hashbrown = "0.12.0"
rayon = {version = "1.5.1", optional = true}

[dev-dependencies]
trybuild = "1.0.61"

[features]
rayon = ["dep:rayon", "hashbrown/rayon"]

However, when adding a simple trybuild test, like the following:

tests/trybuild.rs

#[test]
fn test() {
    trybuild::TestCases::new().compile_fail("tests/ui/test.rs");
}

With tests/ui/test.rs simply containing:

fn main() {}

Running the tests results in the following:

$ cargo t

[...]

running 1 test
error: failed to parse manifest at `[...]/target/tests/foo/Cargo.toml`

Caused by:
  optional dependency `rayon` is not included in any feature
  Make sure that `dep:rayon` is included in one of features in the [features] table.
test test ... FAILED

[...]

If you look at target/tests/foo/Cargo.toml, it is as follows:

[package]
name = "foo-tests"
version = "0.0.0"
edition = "2021"
publish = false

[features]
rayon = ["foo/rayon"]
[dependencies.foo]
path = "[...]/foo/"
default-features = false

[dependencies.hashbrown]
version = "0.12.0"

[dependencies.rayon]
version = "1.5.1"
optional = true

[[bin]]
name = "foo-tests"
path = "main.rs"

[[bin]]
name = "trybuild000"
path = "[...]/foo/tests/ui/test.rs"

[workspace]

It seems the problem is that rayon = ["foo/rayon"] causes cargo to not use an implicit rayon = ["dep:rayon"], which means dep:rayon is never included in any feature, hence the error. Changing the line to rayon = ["foo/rayon", "dep:rayon"] solves the problem and allows the crate to compile.

I'm not exactly sure how to go about solving this generally. It seems all of the dependencies and features are included to allow the test crate to use them. I wonder if it would work to copy the list of additional features to enable for each feature into the new Cargo.toml file that is generated. This would retain the dep: features, while also not enabling any extra features, since those same features are already being enabled by the original crate itself.

[jhpratt]

I'd love it if this were fixed! I recently updated time to use weak and namespaced dependencies, and now I'm looking at switching back to compiletest_rs, which is unfortunate.

[jhpratt]

Well that was quick. Thank you @dtolnay!

[jhpratt]

Actually, it looks like this does miss one case. If a feature has the same name as the namespaced feature, it fails with the same error.

[features]
rand = ["dep:rand"]

[dependencies]
rand = { version = "0.8", optional = true }

Caused by:
  feature `rand` includes `dep:rand`, but `rand` is not an optional dependency
  A non-optional dependency of the same name is defined; consider adding `optional = true` to its definition.

[dtolnay]

That should work. Is it maybe if an optional dependency has the same name as a non-optional dev-dependency?

[jhpratt]

Yeah, that is the case. I was typing that off-hand without thinking that it could be more complicated. Specifically I was testing the current head of time-rs/time (with RUSTFLAGS="--cfg __ui_tests").

Edit: Just tried 1.0.68, and it works. Thanks again!

[Anders429]

@dtolnay thanks, #195 resolves my use-case as well. Much appreciated! :)