Allow destructure in loops.

When looping over an iterable of tuples, it should be possible
to destructure the tuple into individual variables directly in
the loop construct.

Suggested by @nubis in #14.

examples/simple/src/main.rs

@@ -45,6 +45,13 @@ fn test_list() {
                "<ul>\n  \n    <li>foo</li>\n  \n    <li>bar</li>\n  </ul>\n");
 }
 
+#[test]
+fn test_list_destructure() {
+    assert_eq!(r2s(|o| list_destructure(o, &["foo", "bar"])),
+               "<ul>\n  \n    <li>0: foo</li>\n  \n    \
+                <li>1: bar</li>\n  </ul>\n");
+}
+
 #[test]
 fn test_uselist() {
     assert_eq!(r2s(|o| uselist(o)),

examples/simple/templates/list_destructure.rs.html

@@ -0,0 +1,7 @@
+@(items: &[&str])
+
+<ul>
+  @for (n, item) in items.iter().enumerate() {
+    <li>@n: @item</li>
+  }
+</ul>

src/expression.rs

@@ -39,7 +39,7 @@ named!(pub expression<&[u8], String>,
                }) >>
            (format!("{}{}{}", from_utf8(pre).unwrap(), name, post))));
 
-named!(comma_expressions<&[u8], String>,
+named!(pub comma_expressions<&[u8], String>,
        map!(separated_list!(tag!(", "), expression),
             |list: Vec<_>| list.join(", ")));
 

src/templateexpression.rs

@@ -1,4 +1,4 @@
-use expression::{expression, rust_name};
+use expression::{comma_expressions, expression, rust_name};
 use spacelike::{comment, spacelike};
 use std::fmt::{self, Display};
 use std::str::from_utf8;
@@ -188,8 +188,14 @@ named!(pub template_expression<&[u8], TemplateExpression>,
                    }))) |
                Some(b"for") => do_parse!(
                    spacelike >>
-                   name: return_error!(err_str!("Expected loop variable name"),
-                                       rust_name) >>
+                   name: return_error!(err_str!("Expected loop variable name \
+                                                 or destructuring tuple"),
+                                       alt!(rust_name |
+                                            do_parse!(tag!("(") >>
+                                                      args: comma_expressions >>
+                                                      tag!(")") >>
+                                                      (format!("({})", args)))
+                                            )) >>
                    spacelike >>
                    return_error!(err_str!("Expected \"in\""), tag!("in")) >>
                    spacelike >>