RFC: Unreserve proc

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+- Feature Name: `unreserve_proc`
+- Start Date: 2018-04-26
+- RFC PR: [rust-lang/rfcs#2420](https://github.com/rust-lang/rfcs/pull/2420)
+- Rust Issue: N/A. Already implemented.
+
+# Summary
+[summary]: #summary
+
+The keyword `proc` gets unreserved.
+
+# Motivation
+[motivation]: #motivation
+
+We are currently not using `proc` as a keyword for anything in the language.
+Currently, `proc` is a reserved keyword for future use. However, we have
+no intention of using the keyword for anything in the future, and as such,
+we want to unreserve it so that rustaceans can use it as identifiers.
+
+In the specific case of `proc`, it is a useful identifier for many things.
+In particular, it is useful when dealing with processes, OS internals and
+kernel development.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+See the [reference-level-explanation].
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+[list of reserved keywords]: https://doc.rust-lang.org/book/second-edition/appendix-01-keywords.html#keywords-currently-in-use
+
+The keyword `proc` is removed from the [list of reserved keywords] and is no
+longer reserved. This is done immediately and on edition 2015.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The only drawback is that we're not able to use `proc` as a keyword in the
+future, without a reservation in a new edition, if we realize that we made
+a mistake.
+
+[arrow]: https://downloads.haskell.org/~ghc/7.8.1/docs/html/users_guide/arrow-notation.html
+
+The keyword `proc` could be used for some [`Arrow` notation][arrow] as used in
+Haskell. However, `proc` notation is rarely used in Haskell since `Arrow`s are
+not generally understood; and if something is not well understood by one of the
+most academically inclined of communities of users, it is doubly a bad fit for
+Rust which has a community mixed with users used to FP, systemsy and dynamically
+checked programming languages. Moreover, `Arrow`s would most likely require HKTs
+which we might not get.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+There's only one alternative: Not doing anything.
+
+Previously, this used to be the keyword used for `move |..| { .. }` closures,
+but `proc` is no longer used for anything.
+
+Not unreserving this keyword would make the word unavailable for use as an
+identifier.
+
+# Prior art
+[prior-art]: #prior-art
+
+Not applicable.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+There are none.