RFC: Reserve delegate as a keyword in edition 2018

text/0000-reserve-delegate.md

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+- Feature Name: `reserve_delegate`
+- Start Date: 2018-05-03
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+The keyword `delegate` is reserved in edition 2018 and beyond.
+
+# Motivation
+[motivation]: #motivation
+
+[RFC 2393]: https://github.com/rust-lang/rfcs/pull/2393
+
+The motivation for reserving `delegate` is so that have the option to
+later use it for delegation such as proposed in [RFC 2393].
+Reserving `delegate` also gives us flexibility wrt. *"omitting the `impl` block"*
+in the future if we wish.
+
+Furthermore, this RFC is motivated right now by the time constraints to get
+the keyword reserved even if the details of [RFC 2393] or similar proposals
+has not been fully fleshed out.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The word `delegate` is reserved as a keyword in edition 2018.
+This means that code in edition 2018 can't use it directly as an identifier.
+However, you can always use raw identifiers as in `r#delegate` if you need
+to refer to `delegate`, used in a crate from 2015, from an edition 2018 crate.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+[list of keywords]: https://doc.rust-lang.org/book/second-edition/appendix-01-keywords.html
+
+The word `delegate` is reserved as a keyword in edition 2018 and added to the
+[list of keywords].
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+## It might not end up being used
+
+Simply put, a final accepted delegation proposal *might* not end up using the keyword.
+
+## Edition breakage
+
+[sourcegraph]: https://sourcegraph.com/search?q=repogroup:crates+case:yes++%5Cb%28%28let%7Cconst%7Ctype%7C%29%5Cs%2Bdelegate%5Cs%2B%3D%7C%28fn%7Cimpl%7Cmod%7Cstruct%7Cenum%7Cunion%7Ctrait%29%5Cs%2Bdelegate%29%5Cb+max:400
+
+Some code will break when transitioning from edition 2015 to 2018.
+Most of this will be easily fixable with `rustfix`.
+However, transitioning between editions will add some churn,
+therefore, we analyse the extent of the breakage and find that `delegate`:
+
++ is not used as an identifier in the standard library.
++ is not used as the name of a crate.
++ is found 19+ times by [sourcegraph].
+  The extent of breakage is fairly minimal.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+A more frugal option to reserving `delegate` would be to reuse `derive` for
+these purposes. However, the keyword fits less well than `delegate` with respect
+to the user's intent where `delegate` fits quite well.
+
+Another option is to simply not use `delegate`,
+however, we feel confident that we should keep this option open to us right now.
+Not doing so would mean that we couldn't use the word `delegate` as a keyword
+for another 3 years or so.
+
+[keyword policy]: https://paper.dropbox.com/doc/Keyword-policy-SmIMziXBzoQOEQmRgjJPm
+[permalink]: https://gist.github.com/Centril/4c82c19b3cb02cc565622a37d1591785
+
+Furthermore, a recent [keyword policy] ([permalink]), adopted by the language team,
+decided that moving forward, keywords for new features in new editions
+should be real keywords instead of being contextual. The main motivation
+for this was to optimize for maintenance (and reduce technical debt).
+
+# Prior art
+[prior-art]: #prior-art
+
+C# uses the word [`delegate`](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/delegate)
+for something different.
+However, the concept of *"delegation"* is widely used to mean different things
+in different languages.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.