How to specify crates dependencies versions

Each crate (e.g., tokio-util, tokio-stream, etc.) should specify dependencies according to these rules:

1. The listed version should be the oldest version that the crate works with (e.g., if tokio-util works with tokio version 1.44 but not 1.43, then tokio-util should specify version 1.44 for its tokio dependency). We don't require users to use the latest version unnecessarily.
2. When a crate starts using a newer feature in a dependency, the version should be bumped to the version that introduced it.
3. If a crate depends on an unreleased feature in a dependency, it may use path = dependency to specify this. Since path dependencies must be removed during the release of the crate, this ensures that it can't be released until the dependency has a new version.

Consider the following example from tokio-stream:

[dependencies]
futures-core = { version = "0.3.0" }
pin-project-lite = "0.2.11"
tokio = { path = "../tokio", features = ["sync"] }

In this case, local development of tokio-stream uses the local version of tokio via the path dependency. This means that it's currently not possible to release tokio-stream. Once a new version of tokio is released, the path dependency will be removed from tokio-stream. As mentioned before, this version should only be bumped when adding a new feature in the crate that relies on a newer version.