auto merge of #6972 : artagnon/rust/bad-for-loop, r=bstrie

I noticed this while reading the tutorial.

[1/2] adds tests to guard against regressions.
[2/2] corrects the tutorial.

Please let me know if you have a standard place to put tests: they seem to be all over the place currently.

doc/tutorial.md

@@ -1613,18 +1613,19 @@ loop. Like `do`, `for` is a nice syntax for describing control flow
 with closures.  Additionally, within a `for` loop, `break`, `loop`,
 and `return` work just as they do with `while` and `loop`.
 
-Consider again our `each` function, this time improved to
-break early when the iteratee returns `false`:
+Consider again our `each` function, this time improved to return
+immediately when the iteratee returns `false`:
 
 ~~~~
-fn each(v: &[int], op: &fn(v: &int) -> bool) {
+fn each(v: &[int], op: &fn(v: &int) -> bool) -> bool {
    let mut n = 0;
    while n < v.len() {
        if !op(&v[n]) {
-           break;
+           return false;
        }
        n += 1;
    }
+   return true;
 }
 ~~~~
 

src/test/compile-fail/bad-for-loop-2.rs

@@ -0,0 +1,15 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    fn quux(_: &fn(&int) -> bool) -> () { }
+    for quux |_| { } //~ ERROR expected `for` closure to return
+                     // `bool`, but found `()`
+}

src/test/compile-fail/bad-for-loop.rs

@@ -9,7 +9,8 @@
 // except according to those terms.
 
 fn main() {
-    fn baz(_x: &fn(y: int) -> int) {}
-    for baz |_e| { } //~ ERROR A `for` loop iterator should expect a closure that returns `bool`
-                     //~^ ERROR expected `for` closure to return `bool`
+    fn quux(_: &fn(&int) -> int) -> bool { true }
+    for quux |_| { } //~ ERROR A `for` loop iterator should expect a
+                     // closure that returns `bool`.  This iterator
+                     // expects a closure that returns `int`.
 }