Fix spelling errors.

src/libsyntax/ext/base.rs

@@ -472,7 +472,7 @@ impl<'a> ExtCtxt<'a> {
     }
     /// Emit `msg` attached to `sp`, and stop compilation immediately.
     ///
-    /// `span_err` should be strongly prefered where-ever possible:
+    /// `span_err` should be strongly preferred where-ever possible:
     /// this should *only* be used when
     /// - continuing has a high risk of flow-on errors (e.g. errors in
     ///   declaring a macro would cause all uses of that macro to

src/libsyntax/ext/deriving/generic/mod.rs

@@ -834,7 +834,7 @@ impl<'a> MethodDef<'a> {
                                 generic `deriving`");
             }
 
-            // `ref` inside let matches is buggy. Causes havoc wih rusc.
+            // `ref` inside let matches is buggy. Causes havoc with rusc.
             // let (variant_index, ref self_vec) = matches_so_far[0];
             let (variant, self_vec) = match matches_so_far.get(0) {
                 &(_, v, ref s) => (v, s)

src/libsyntax/ext/expand.rs

@@ -1295,7 +1295,7 @@ mod test {
     }
 
     // create a really evil test case where a $x appears inside a binding of $x
-    // but *shouldnt* bind because it was inserted by a different macro....
+    // but *shouldn't* bind because it was inserted by a different macro....
     // can't write this test case until we have macro-generating macros.
 
     // FIXME #9383 : lambda var hygiene