Allow publishing crates with nightly features

[matklad]

closes #5427.

cc @rust-lang/cargo: I remember a vigorous debate over publishing crates with nightly Cargo features, but I can't recollect our exact plan of action. The discussion is logged here: https://paper.dropbox.com/doc/Unstable-Cargo-features-JBYMdsUYcO3FyW8Ubkjoz.

I think we just need to allow to publish crates with unstable cargo features, for the same reason we allow unstable rust features: you need explicit opt-in, even for deps. This is covered by Cargo tests:

cargo/tests/testsuite/cargo_features.rs

Lines 115 to 215 in 9f09778

	#[test]
	fn nightly_feature_requires_nightly() {
	let p = project("foo")
	.file(
	"Cargo.toml",
	r#"
	cargo-features = ["test-dummy-unstable"]
	[package]
	name = "a"
	version = "0.0.1"
	authors = []
	im-a-teapot = true
	"#,
	)
	.file("src/lib.rs", "")
	.build();
	assert_that(
	p.cargo("build").masquerade_as_nightly_cargo(),
	execs().with_status(0).with_stderr(
	"\
	[COMPILING] a [..]
	[FINISHED] [..]
	",
	),
	);
	assert_that(
	p.cargo("build"),
	execs().with_status(101).with_stderr(
	"\
	error: failed to parse manifest at `[..]`
	Caused by:
	the cargo feature `test-dummy-unstable` requires a nightly version of Cargo, \
	but this is the `stable` channel
	",
	),
	);
	}
	#[test]
	fn nightly_feature_requires_nightly_in_dep() {
	let p = project("foo")
	.file(
	"Cargo.toml",
	r#"
	[package]
	name = "b"
	version = "0.0.1"
	authors = []
	[dependencies]
	a = { path = "a" }
	"#,
	)
	.file("src/lib.rs", "")
	.file(
	"a/Cargo.toml",
	r#"
	cargo-features = ["test-dummy-unstable"]
	[package]
	name = "a"
	version = "0.0.1"
	authors = []
	im-a-teapot = true
	"#,
	)
	.file("a/src/lib.rs", "")
	.build();
	assert_that(
	p.cargo("build").masquerade_as_nightly_cargo(),
	execs().with_status(0).with_stderr(
	"\
	[COMPILING] a [..]
	[COMPILING] b [..]
	[FINISHED] [..]
	",
	),
	);
	assert_that(
	p.cargo("build"),
	execs().with_status(101).with_stderr(
	"\
	error: failed to load source for a dependency on `a`
	Caused by:
	Unable to update [..]
	Caused by:
	failed to parse manifest at `[..]`
	Caused by:
	the cargo feature `test-dummy-unstable` requires a nightly version of Cargo, \
	but this is the `stable` channel
	",
	),
	);
	}

.

I am not sure if we have ever implemented crates.io side of validation?

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[matklad]

ping @rust-lang/cargo, @joshtriplett and @aturon :)

This is blocking clippy folks right now I think (@oli-obk has more details).

I personally am 100% fine with allowing publishing crates with unstable features, but I want to make sure the team is OK with this as well :)

[aturon]

@matklad I agree with this move and your rationale. My only concern would be for unstable Cargo features that affect the publication process/metadata, but we can handle that case-by-case.

[matklad]

My only concern would be for unstable Cargo features that affect the publication process/metadata, but we can handle that case-by-case.

Double checked that current unstable features do not affect publication, and it seems to be the case. Moreover, any feature which affects metadata/publication needs some special handling from crates.io, so, in some sense, such features are crates.io features and not Cargo features.

@bors r+

[bors]

📌 Commit 39a3709 has been approved by matklad

[bors]

⌛ Testing commit 39a3709 with merge e2348c2...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: matklad
Pushing e2348c2 to master...