Add if-while-let_multiple_patterns RFC

text/0000-if-while-let_multiple_patterns.md

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+- Feature Name: if_while_let_multiple_patterns
+- Start Date: 2015-03-04
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Allow `if let` and `while let` to match on multiple patterns with `|`.
+
+# Motivation
+
+The goal of this RFC is to enable `if let` and `while let` statements to match on multiple patterns, just
+like `match` is able to.
+
+Currently, the `if let` and `while let` notation is inconsistent with the `match` notation,
+even though they are is just sugaring for `match`. Specifically, `if let` and `while let` do not
+allow matching on multiple patterns.
+
+For instance, the following piece of code is allowed:
+
+```rust
+enum Foo {
+  One(u8),
+  Two(u8),
+  Three
+}
+use Foo::*;
+
+fn main () {
+    let x = One(42);
+    match x {
+        One(n) | Two(n) => {
+            println!("Got one or two with val: {}", n);
+        }
+        _ => {}
+    }
+}
+```
+
+but this isn't:
+
+
+```rust
+enum Foo {
+  One(u8),
+  Two(u8),
+  Three
+}
+use Foo::*;
+
+fn main () {
+    let x = One(42);
+    if let One(n) | Two(n) = x {
+        println!("Got one or two with val: {}", n);
+    }
+}
+```
+
+This RFC proposes to extend the notation for `if let` and `while let` to also cover multiple
+patterns separated by pipes (`|`).
+
+# Detailed design
+
+Basically take the design proposed in the original RFC for [if-let](0160-if-let.md) and extend the notation to allow multiple patterns.
+
+Specifically, modify the grammar for `if-cond` to:
+
+```
+if-cond = 'let' pattern [ '|' pattern ] * '=' expression
+```
+
+and ditto for `while let`.
+
+# Drawbacks
+
+It's an additional feature in the language.
+
+# Alternatives
+
+Not doing anything and require multiple `if let` statements or an actual `match`.
+
+# Unresolved questions
+
+We could consider extending `if let` and `while let` to also include guards, but that has opens up the question about syntax (`if let Some(n) = x if n > 0` is not extremely pretty), so we should probably save that for an RFC of its own.