Expose choice of supported tonic version

[dtolnay]

The current way that tokio-vsock tries to support multiple tonic versions is problematic, and is a regression from 0.6.0 (which used version = "0.12.0" for the tonic dependency).

tokio-vsock/Cargo.toml

Line 22 in d52af1b

tonic = { version = ">= 0.5.0, < 0.13.0", optional = true }

This does not mean "tokio-vsock supports every tonic version from 0.5 through 0.12". It means "tokio-vsock supports a single random version of tonic between 0.5 and 0.12 according to Cargo's whim". Cargo does not expose any public way to control what choice of tonic would be used by this dependency.

One possible way the problems would manifest (there are many) is:

• Somebody writes a Cargo.toml depending on tokio-vsock = { version = "0.6", features = ["tonic"] } and tonic = "0.11". In this situation Cargo (probably) resolves the tonic dependency inside tokio-vsock to 0.11 during local development.

• They write Rust code counting on tonic 0.11's Connected impl to exist for VsockStream, such as by calling .connect_info() with use tonic::transport::server::Connected; referring to tonic 0.11.

• They test their code extensively, locally and in CI, and everything works.

• They publish to crates.io as a library.

• Somebody else has a different project already using tonic = "0.12", and they run cargo add to add a new dependency on the library from the previous step.

• The library, despite having been extensively tested in CI, now does not compile because Cargo decided that it depends on tonic 0.11 and tokio-vsock 0.6 while tokio-vsock 0.6 now depends on tonic 0.12, and the necessary Connected impl is absent.

This is a regression from tokio-vsock 0.6.0 because at least in 0.6.0 I could reliably use tokio-vsock with tonic 0.12 and write Rust code counting on that Connected impl to exist. In tokio-vsock master, there is no longer any way to use the Connected impl correctly. It may or may not exist from moment-to-moment depending on what else is going on in the project's dependency graph.

This PR makes tokio-vsock correctly support a range of tonic versions. If built with tokio-vsock = { features = ["tonic012"] } then tonic 0.12's Connected trait is guaranteed to be implemented.

[jalil-salame]

Thanks for catching this! Thankfully we didn't publish this yet c:

We probably need to adjust #55 to use cargo hack instead of -Z direct-minimum-dependencies after this.

[jalil-salame]

Are these features additive? Gut feeling says yes, but I'm not sure

[dtolnay]

Officially they are additive, and docs.rs will build them all.

tokio-vsock/Cargo.toml

Lines 33 to 34 in cdb58ef

	[package.metadata.docs.rs]
	all-features = true

Technically they are not additive, in the sense that it is possible to write contrived Rust code that compiles if a feature is off and does not compile if a feature is on, but this is true of almost every feature in every library.

Example:

// [dependencies]
// tokio-vsock = { features = ["tonic012"] }
// tonic011 = { package = "tonic", version = "0.11" }
// tonic012 = { package = "tonic", version = "0.12" }

use tokio_vsock::VsockStream;
use tonic011::transport::server::Connected as _;
use tonic012::transport::server::Connected as _;

fn repro(vs: VsockStream) {
    let _ = vs.connect_info(); // ambiguous if tokio-vsock/tonic011 enabled
}

[cryo28]

Thanks @dtolnay for doing correctly what I tried to do incorrectly in my earlier PR.

[Tim-Zhang]

Hi, @dtolnay thank you for your contribution to tokio-vsock. I have invited you as a developer. If you are interested, you are welcome to join us.