Add initial draft of crates policy.

procedures/crates.md

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+# Third-party and out-of-tree crates
+This document describe the guidelines for creating out-of-tree crates for use in the compiler and
+using third-party crates within the compiler. These guidelines were originally discussed at the
+Rust All Hands 2019 by the compiler team and others; and then at the
+[01/03/2019 steering meeting][steering_meeting].
+
+[steering_meeting]: https://github.com/rust-lang/compiler-team/blob/master/minutes/steering-meeting/2019.03.01.md
+
+## Out-of-tree crates
+One of the primary goals of this policy is to ensure that there is consistency in how out-of-tree
+crates used in the compiler are set up (at least, those maintained by the compiler team and living
+in `rust-lang`) and that the experience is uniform across `rust-lang/rust` and these crates.
+
+### When should parts of the compiler be extracted into an out-of-tree crate?
+This is left to the discretion of compiler team members but should be discussed with the rest of
+the team, either through raising the question at the weekly triage meeting or asynchronously using
+an FCP in an issue. If the crate is a product of a working group, there should already be agreement
+within the working group that an out-of-tree crate is suitable.
+
+When considering creating an out-of-tree crate, it is worth balancing how general the crate should
+be with the increased maintenance burden that this may bring if widely used.
+
+### Where should compiler crates live?
+Out-of-tree compiler crates should be hosted in the `rust-lang` organization - this simplifies
+integration with external infrastructure tooling and will inherit existing team permissions on
+GitHub. It should be made clear in any documentation that the compiler team and any appropriate
+working groups are responsible for the crate. It is not recommended to start with a prototype in
+another organization or personal repository.
+
+### Can existing out-of-tree crates from personal accounts or other organizations be transferred?
+Yes, this is encouraged. In order to do this, discuss this with the team and familiarize yourself
+with [the GitHub documentation for repository transfers][repo_transfers] and then arrange to perform
+the transfer. Once a transfer is complete, a redirect will exist in the original account or
+organization and this will conflict with the names of any new forks of the transferred repository -
+an email to GitHub is required to resolve this.
+
+[repo_transfers]: https://help.github.com/en/articles/transferring-a-repository
+
+### Who owns these crates?
+It is desired that a compiler team member or working group has loose ownership over a crate so
+that there are clear owners who are responsible for making sure that new versions are published and
+that pull requests are reviewed.
+
+### What should these crates be named?
+Crate naming will be discussed when new out-of-tree crates are proposed to the compiler team.
+
+Crate naming will differ on a case-by-case basis. Crates that are inherently tied to the
+compiler would benefit from a name that is prefixed with `rustc_`. This is an indicator of how
+stable the crate may be to prospective users. Other crates, which are more general-purpose, will
+have names that are disentangled from the compiler.
+
+### Are there any limitations on the review policy for out-of-tree crates?
+Generally, the working groups and team members that are primarily free to maintain the crate using
+whatever practices are best suited to their group, however, there are some limitations so that there
+is some uniformity across the compiler and out-of-tree crates:
+
+- Every compiler team member must be able to review and approve PRs.
+- Where possible, only active participants in the crate (or related working group) need be on the
+  highfive rotation for the crate.
+- Working group members (or regular contributors to the crate) who cannot approve PRs on rustc can
+  be allowed approve PRs to the crate.
+- Major pull requests should have multiple reviewers.
+
+### What is required of an out-of-tree crate?
+It is required that out-of-tree crates must:
+
+- Be dual-licensed with Apache 2.0 and MIT (as the compiler is) unless otherwise
+  appropriate (ports of code from other project, etc).
+- Abide by Rust's code of conduct.
+- Specify that the crate is maintained by the Rust compiler team and any appropriate working groups.
+    - In particular, this should detail the expected level of maintenance and stability for any
+      prospective users.
+    - This should also link to the working group details in this repository.
+- Be added to the list at the bottom of this page.
+- Follow semantic versioning.
+- Use `@bors` and `@rust-highfive`.
+
+
+### Is there a requirement for community infrastructure for an out-of-tree crate?
+There is no requirement that community infrastructure (such as Zulip servers/streams) be created for
+out-of-tree crates. This may be desirable if an out-of-tree crate gains a large community of
+contributors and users, but otherwise, the working group or compiler team streams should be used
+initially.
+
+Linkifiers for auto-linking to issues and PRs on the primary Rust Zulip server can be added on
+request.
+
+### Are there any recommendations for working with out-of-tree crates?
+Recommendations for working with out-of-tree crates will be documented in the rustc-guide, see
+[rust-lang/rustc-guide#285][guide_issue] for progress.
+
+[guide_issue]: https://github.com/rust-lang/rustc-guide/issues/285
+
+---
+
+In summary, the process for establishing an out-of-tree crate is as follows:
+
+1. Where appropriate, discuss and confirm the need within the working group for the out-of-tree
+   crate.
+2. Consult with the compiler team to confirm the need for the out-of-tree crate and kick-off the
+   naming bikeshed.
+3. Create a new repository in the `rust-lang` organization (permissions should be available for all
+   compiler team members).
+4. Add a README describing the intended purpose of the crate, which team and working group are
+   responsible (link to their page in this repository) and the intended level of maintenance and
+   stability.
+
+   > This crate is developed and maintained by the [Rust compiler team](..) for use within
+   > `rustc`, in particular, it is the responsibility of the
+   > [`.template`](../working-groups/.template) working group. This crate [will have regular
+   > breaking changes and provides no stability guarantees|is intended to remain stable and have
+   > limited breaking changes].
+5. Include the [LICENSE-APACHE][apache] and [LICENSE-MIT][mit] files from `rust-lang/rust`.
+6. Include or link the [CODE_OF_CONDUCT][coc] file from `rust-lang/rust`.
+7. Consult with the infrastructure team to set up `@bors` and `@rust-highfive` and a process for
+   publishing under the "The Rust Project Developers" crates.io account.
+8. Perform any initial development required before integration with rustc.
+9. Publish initial version, following semantic versioning.
+10. Add the crate as a dependency to the appropriate in-tree crate and start using.
+
+[apache]: https://github.com/rust-lang/rust/blob/master/LICENSE-APACHE
+[coc]: https://github.com/rust-lang/rust/blob/master/CODE_OF_CONDUCT.md
+[mit]: https://github.com/rust-lang/rust/blob/master/LICENSE-MIT
+
+## Third-party crates
+It is sometimes desirable to use the functionality of an existing third-party crate in the compiler.
+
+### When can a third-party crate be added as a compiler dependency?
+It is desirable that a third-party crate being included in the compiler is well-maintained and that,
+where possible, a compiler team member is added as a maintainer. You should consulting with the
+rest of the compiler team before making this decision.
+
+### What about third-party dependencies to out-of-tree crates?
+The same policies apply to all compiler-team-maintained crates used in the compiler.
+
+## List of out-of-tree crates
+This section contains the list of existing out-of-tree, compiler team-maintained crates:
+
+  - [`rust-lang-nursery/chalk`](https://github.com/rust-lang-nursery/chalk/)
+  - [`rust-lang-nursery/polonius`](https://github.com/rust-lang-nursery/polonius/)