Allow use of pipe operator in patterns.

text/0000-pipe-in-sub-expressions.md

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+- Feature Name: pipe_in_sub_expressions.
+- Start Date: 2017-02-03
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+## Summary
+this RFC proposes allowing the `|` operator to be used within patterns in match
+statements, to allow for pattern matching with less boilerplate.
+
+## Detailed Design
+The current pattern matching in `match` statements is very powerful, however
+there is a lot of boilerplate code needed to fully take advantage of it.
+Consider a situation where you are building a state machine that is iterating
+through `chars_indices`, and when you see `' '`, `'\n'`, `'\r'` or `'\f'`, you
+want to change the state. Currently your match statement would look something
+like the following example. There is no great way of reducing that boilerplate,
+if anything that boilerplate only grows worse as you have more cases, and bigger
+tuples.  Conservatively this feature would be simple syntactic sugar that just
+expands to the old syntax.
+
+```rust
+match iter.next() {
+    Some(_, ' ') | Some(_, '\n') | Some(_, '\r') | Some(_, '\u{21A1}') => {
+        // Change state
+    }
+    Some(index, ch) => {
+        // Look at char
+    }
+    None => return Err(Eof),
+}
+```
+
+The solution to this would be to allow for `|` to be used within tuples. This
+will significantly reduce how much boilerplate is required to have conditional
+matches with tuples.
+
+```rust
+match iter.next() {
+    Some(_, ' ' | '\n' | '\r' | '\u{21A1}') => {
+        // Change state
+    }
+    Some(index, ch) => {
+        // Look at char
+    }
+    None => return Err(Eof),
+}
+```
+In terms of Rust's grammar `pats_or` would probably be changed to the following.
+Which should allow for `Some(1 | 2 | 3, ' ' | '\n' | '\r')`.
+
+```yacc
+pats_or
+: pat              { $$ = mk_node("Pats", 1, $1); }
+| pat '|' pat  { $$ = ext_node($1, 1, $3); }
+;
+```
+
+## How We Teach This
+I think all that would be required is additional examples in sections on
+pattern matching. The syntax is very intuitive.
+
+## Drawbacks
+None that I can think of.
+
+## Alternatives
+Keep syntax as is.
+
+## Unresolved Questions
+None at the moment.