Add RFC to feature gate some slice patterns

Author: brson

active/0000-feature-gate-slice-pats.md

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+- Start Date: 2014-07-14
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Rust's support for pattern matching on sices has grown steadily and incrementally without a lot of oversight,
+and we have concern that Rust is doing too much here, that the complexity is not worth it. This RFC proposes
+to feature gate multiple-element slice matches in the head and middle positions (`[..xs, 0, 0]` and `[0, ..xs, 0]`.
+
+# Motivation
+
+Some general reasons and one specific: first, the implementation of Rust's match machinery is notoriously complex, and not well-loved. Remove features is seen as a valid way to reduce complexity. Second, slice matching in particular, is difficult to implement, while also being of only moderate utility (there are many types of collections - slices just happen to be built into the language). Finally, the exhaustiveness check is not correct for slice patterns - because of their complexity; it's not known that it
+can be done correctly, nor whether it is worth the effort even if.
+
+# Detailed design
+
+The `advanced_slice_patterns` feature gate will be added. When the compiler encounters slice pattern matches in head or middle position it will emit a warning or error accourding to the current settings.
+
+# Drawbacks
+
+It removes two features that some people like.
+
+# Alternatives
+
+Fixing the exhaustiveness check would allow the feature to remain.
+
+# Unresolved questions
+
+N/A