Necessities

Public dependencies of a stable crate are stable (C-STABLE)

A crate cannot be stable (>=1.0.0) without all of its public dependencies being stable.

Public dependencies are crates from which types are used in the public API of the current crate.

#![allow(unused)]
fn main() {
pub fn do_my_thing(arg: other_crate::TheirThing) { /* ... */ }
}

A crate containing this function cannot be stable unless other_crate is also stable.

Be careful because public dependencies can sneak in at unexpected places.

#![allow(unused)]
fn main() {
pub struct Error {
    private: ErrorImpl,
}

enum ErrorImpl {
    Io(io::Error),
    // Should be okay even if other_crate isn't
    // stable, because ErrorImpl is private.
    Dep(other_crate::Error),
}

// Oh no! This puts other_crate into the public API
// of the current crate.
impl From<other_crate::Error> for Error {
    fn from(err: other_crate::Error) -> Self {
        Error { private: ErrorImpl::Dep(err) }
    }
}
}

Crate and its dependencies have a permissive license (C-PERMISSIVE)

The software produced by the Rust project is dual-licensed, under either the MIT or Apache 2.0 licenses. Crates that simply need the maximum compatibility with the Rust ecosystem are recommended to do the same, in the manner described herein. Other options are described below.

These API guidelines do not provide a detailed explanation of Rust's license, but there is a small amount said in the Rust FAQ. These guidelines are concerned with matters of interoperability with Rust, and are not comprehensive over licensing options.

To apply the Rust license to your project, define the license field in your Cargo.toml as:

[package]
name = "..."
version = "..."
authors = ["..."]
license = "MIT OR Apache-2.0"

Then add the files LICENSE-APACHE and LICENSE-MIT in the repository root, containing the text of the licenses (which you can obtain, for instance, from choosealicense.com, for Apache-2.0 and MIT).

And toward the end of your README.md:

## License

Licensed under either of

 * Apache License, Version 2.0
   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license
   ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.

Besides the dual MIT/Apache-2.0 license, another common licensing approach used by Rust crate authors is to apply a single permissive license such as MIT or BSD. This license scheme is also entirely compatible with Rust's, because it imposes the minimal restrictions of Rust's MIT license.

Crates that desire perfect license compatibility with Rust are not recommended to choose only the Apache license. The Apache license, though it is a permissive license, imposes restrictions beyond the MIT and BSD licenses that can discourage or prevent their use in some scenarios, so Apache-only software cannot be used in some situations where most of the Rust runtime stack can.

The license of a crate's dependencies can affect the restrictions on distribution of the crate itself, so a permissively-licensed crate should generally only depend on permissively-licensed crates.