Remove Ordering from the prelude.

Author: steveklabnik

text/0000-remove-ordering-from-prelude.md

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+- Start Date: 2014-12-05
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Currently, `Ordering` and its variants are exported in the prelude. We should not
+do that any more.
+
+# Motivation
+
+In the past, enums were not namespaced. To use each variant, they must also be
+`use`d.
+
+`Ordering` and its variants were in the prelude before my time, and so I'm not 100% sure
+why they're in the prelude. I would imagine, like most things, it comes down to
+convenience: importing four things to `match` on `==` is kind of a hassle.
+
+The prelude adds names to every Rust program, and so the standard for being in
+the prelude is currently 'clearly useful in every Rust program.' However,
+`match`ing on an `Ordering` is not particularly common:
+
+```bash
+$ git grep --name-only Greater src 
+src/doc/guide.md
+src/doc/reference.md
+src/etc/unicode.py
+src/etc/vim/syntax/rust.vim
+src/liballoc/rc.rs
+src/libcollections/bit.rs
+src/libcollections/btree/node.rs
+src/libcollections/btree/set.rs
+src/libcollections/slice.rs
+src/libcollections/str.rs
+src/libcollections/tree/map.rs
+src/libcollections/tree/set.rs
+src/libcollections/trie/set.rs
+src/libcore/cmp.rs
+src/libcore/iter.rs
+src/libcore/prelude.rs
+src/libcore/ptr.rs
+src/libcore/slice.rs
+src/libcore/str.rs
+src/libcoretest/cmp.rs
+src/libcoretest/tuple.rs
+src/libregex/vm.rs
+src/librustc/middle/typeck/infer/region_inference/mod.rs
+src/librustc_trans/trans/_match.rs
+src/librustdoc/html/render.rs
+src/libstd/prelude.rs
+src/libtest/stats.rs
+src/libunicode/normalize.rs
+src/libunicode/tables.rs
+src/test/bench/shootout-k-nucleotide-pipes.rs
+src/test/run-pass/bool.rs
+src/test/run-pass/deriving-self-lifetime-totalord-totaleq.rs
+```
+
+Note the first result there, the Guide. A full quarter of the instances of `Greater` are
+used in the Guide, because `Ordering` is used to teach about enums. Due to shadowing,
+this leads to https://github.com/rust-lang/rust/issues/17967, which trips up many newbies.
+See [this comment](https://github.com/rust-lang/rust/issues/17967#issuecomment-61572399)
+from @bstrie, especially. Removing `Ordering` from the prelude fixes this issue
+nicely.
+
+# Detailed design
+
+Remove these four lines:
+
+* https://github.com/rust-lang/rust/blob/master/src/libstd/prelude.rs#L66-L67
+* https://github.com/rust-lang/rust/blob/master/src/libcore/prelude.rs#L51-L52
+
+And the fix the fallout by adding some `use` statements to the affected files. I would
+be willing to do said implementation.
+
+# Drawbacks
+
+This is another `[breaking-change]`. It is easy to fix, though, so I do not consider
+this drawback to be significant.
+
+# Alternatives
+
+We could remove the export of just the variants. This would not fix my Guide issue, but
+would still be more conformant to our style guidelines.
+
+# Unresolved questions
+
+No technical ones, this is entirely a social change.