Don't require any imports for #[derive(AsChangeset)]

I had accidentally relied on `use diesel::prelude::*` occurring, as I've
pretty much never had a place that didn't do that. Still, I make every
effort to fully qualify as much as humanly possible in these macros, so
we shouldn't rely on it being in scope. This shouldn't cause issues even
if the trait is already imported (our integration tests would fail if it
were a problem).

Fixes diesel-rs#536.

Author: sgrif

CHANGELOG.md

@@ -11,6 +11,9 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
 * Added missing impls for loading `chrono::NaiveDateTime` from a column of type
   `Timestamptz`
 
+* `#[derive(AsChangeset)]` no longer assumes that `use diesel::prelude::*` has
+  been done.
+
 ## [0.9.0] - 2016-12-08
 
 ### Added

diesel/src/macros/as_changeset.rs

@@ -228,6 +228,7 @@ macro_rules! impl_AsChangeset {
 
                 #[allow(non_shorthand_field_patterns)]
                 fn as_changeset(self) -> Self::Changeset {
+                    use $crate::prelude::ExpressionMethods;
                     let $self_to_columns = *self;
                     ($(
                         AsChangeset_column_expr!(
@@ -305,3 +306,25 @@ macro_rules! AsChangeset_column_expr {
         $column.eq($field_access)
     };
 }
+
+#[cfg(test)]
+mod using_as_changeset_with_any_imports {
+    table!(users {
+        id -> Integer,
+        name -> VarChar,
+    });
+
+    #[allow(missing_debug_implementations)]
+    struct Changes {
+        id: i32,
+        name: String,
+    }
+
+    impl_AsChangeset! {
+        (users)
+        struct Changes {
+            id: i32,
+            name: String,
+        }
+    }
+}