First draft on making src/grammar the grammar

Author: nagisa

text/0000-grammar-is-canonical.md

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+- Feature Name: grammar
+- Start Date: 2015-10-21
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+[src/grammar]: https://github.com/rust-lang/rust/tree/master/src/grammar
+
+Grammar of the Rust language should not be rustc implementation-defined. We have a formal grammar
+at [src/grammar] which is to be used as the canonical and formal representation of the Rust
+language.
+
+# Motivation
+[motivation]: #motivation
+[#1228]: https://github.com/rust-lang/rfcs/blob/master/text/1228-placement-left-arrow.md
+[#1219]: https://github.com/rust-lang/rfcs/blob/master/text/1219-use-group-as.md
+[#1192]: https://github.com/rust-lang/rfcs/blob/master/text/1192-inclusive-ranges.md
+
+In many RFCs proposing syntactic changes ([#1228], [#1219] and [#1192] being some of more recently
+merged RFCs) the changes are described rather informally, discussions contain a lot of guess-work
+and are hard to work with in general, both when implementing and discussing them.
+
+Making [src/grammar] to be the canonical grammar and demanding for description of syntactic changes
+to be presented in terms of changes to the formal grammar should greatly simplify both the
+discussion and implementation of the RFCs. Using a formal grammar also allows us to discover and
+rule out existence of various issues with the grammar changes (e.g. grammar ambiguities) during
+design phase rather than implementation phase or, even worse, after the stabilisation.
+
+# Detailed design
+[design]: #detailed-design
+[A-grammar]: https://github.com/rust-lang/rust/issues?utf8=✓&q=is:issue+is:open+label:A-grammar
+
+Sadly, the [grammar][src/grammar] in question is [not quite equivalent][A-grammar] to the
+implementation in rustc yet. We cannot possibly hope to catch all the quirks in the rustc parser
+implementation, therefore something else needs to be done.
+
+This RFC proposes following approach to making [src/grammar] the canonical Rust language grammar:
+
+1. Fix the already known discrepancies between implementation and [src/grammar];
+2. Make [src/grammar] a [semi-canonical grammar];
+3. After a period of time transition [src/grammar] to a [fully-canonical grammar].
+
+## Semi-canonical grammar
+[semi-canonical grammar]: #semi-canonical-grammar
+
+Once all known discrepancies between the [src/grammar] and rustc parser implementation are
+resolved, [src/grammar] enters the state of being semi-canonical grammar of the Rust language.
+
+Semi-canonical means that all new development involving syntax changes are made and discussed in
+terms of changes to the [src/grammar] and [src/grammar] is in general regarded to as the canonical
+grammar except when new discrepancies are discovered. These discrepancies must be swiftly resolved,
+but resolution will depend on what kind of discrepancy it is:
+
+1. For syntax changes/additions introduced after [src/grammar] gained the semi-canonical state, the
+   [src/grammar] is canonical;
+2. For syntax that was present before [src/grammar] gained the semi-canonical state, in most cases
+   the implementation is canonical.
+
+This process is sure to become ambiguous over time as syntax is increasingly adjusted (it is harder
+to “blame” syntax changes compared to syntax additions), therefore the resolution process of
+discrepancies will also depend more on a decision from the Rust team.
+
+## Fully-canonical grammar
+[fully-canonical grammar]: #fully-canonical-grammar
+
+After some time passes, [src/grammar] will transition to the state of fully canonical grammar.
+After [src/grammar] transitions into this state, for any discovered discrepancies the
+rustc parser implementation must be adjusted to match the [src/grammar], unless decided otherwise
+by the RFC process.
+
+## RFC process changes for syntactic changes and additions
+
+Once the [src/grammar] enters semi-canonical state, all RFCs must describe syntax additions and
+changes in terms of the formal [src/grammar]. Discussion about these changes are also expected (but
+not necessarily will) to become more formal and easier to follow.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This RFC introduces a period of ambiguity during which neither implementation nor [src/grammar] are
+truly canonical representation of the Rust language. This will be less of an issue over time as
+discrepancies are resolved, but its an issue nevertheless.
+
+# Alternatives
+[alternatives]: #alternatives
+
+One alternative would be to immediately make [src/grammar] a fully-canonical grammar of the Rust
+language at some arbitrary point in the future.
+
+Another alternative is to simply forget idea of having a formal grammar be the canonical grammar of
+the Rust language.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+How much time should pass between [src/grammar] becoming semi-canonical and fully-canonical?