Remove the [] notation for taking a whole slice.

0000-remove-whole-slice.md

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+- Start Date: 2014-11-29
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Remove the `[]` notation for taking a whole slice.
+
+# Motivation
+
+Since we accepted [RFC 235](text/0235-collections-conventions.md) we can take a
+slice of a vector or string (e.g., `x`) by writing `&*x`. In the future (#241)
+this may become `&x`. It therefore seems unnecessary to have the additional
+`x[]` syntax for getting a slice. In [RFC 439](text/0439-cmp-ops-reform.md), we
+change the implementation of slice notation to use a first class range. There is
+then no obvious way to represent whole slices. Personally, I have found `x[]`
+hard to read in code and easily confused with array notation.
+
+To avoid a hackey range fix, remove duplication, and avoid backwards
+compatibility risk, we should remove the `[]` notation. (We can always add it
+back in later without breaking backwards compatibility, if we decide this was a
+mistake).
+
+# Detailed design
+
+`expr[]` would no longer parse as an expression. The corresponding methods in the
+`Slice` traits would be removed.
+
+# Drawbacks
+
+Using `&*` might be off-putting/unintuitive for new users (although they'll have
+to get used to it for cross-borrowing pretty quickly, and hopefully this will
+only be short term until we get deref coercions).
+
+If a collection implements `Slice` but not `Deref`, then there is no nice way to
+get a slice representation (presumably the collection would allow `as_slice()`).
+I imagine most collections that can be represented as slices should implement
+`Deref` though, in order to be consistent with `Vec` and `String`.
+
+# Alternatives
+
+Leave as is.
+
+Replace `[]` with `[..]`, which is at least easier to read, although doesn't
+solve the problem with duplication.
+
+# Unresolved questions
+
+What is the impact of this change? We didn't mass convert code in the compiler
+or libraries to use `[]`, so the impact should be quite small, although it has
+been used to some extent. I don't know how much it is used in other code. It is
+quite hard to find and replace, since `[]` is also used for an empty array. It
+would be easy to instrument the compiler to gather this info though (and to give
+deprecation warnings and/or advice for fixing bustage).