Add the ability to work with boxed expressions

There's a few limitations that I'm not happy with at the moment:

- The type of the expression has to be repeated twice
  (https://github.com/rust-lang/rust/isues/30056)
- It won't work with joins yet. I might be able to work around this in
  the short term, but it may require
  rust-lang/rust#29864 or specialization to
  express properly

src/expression/mod.rs

@@ -119,6 +119,18 @@ pub trait Expression {
     }
 }
 
+impl<T: Expression + ?Sized> Expression for Box<T> {
+    type SqlType = T::SqlType;
+
+    fn to_sql(&self, out: &mut QueryBuilder) -> BuildQueryResult {
+        Expression::to_sql(&**self, out)
+    }
+
+    fn to_insert_sql(&self, out: &mut QueryBuilder) -> BuildQueryResult {
+        Expression::to_insert_sql(&**self, out)
+    }
+}
+
 pub trait AsExpression<T: NativeSqlType> {
     type Expression: Expression<SqlType=T>;
 
@@ -139,5 +151,24 @@ pub trait SelectableExpression<
 >: Expression {
 }
 
+impl<T: ?Sized, ST, QS> SelectableExpression<QS, ST> for Box<T> where
+    T: SelectableExpression<QS, ST>,
+    ST: NativeSqlType,
+    Box<T>: Expression,
+{
+}
+
 pub trait NonAggregate: Expression {
 }
+
+impl<T: NonAggregate + ?Sized> NonAggregate for Box<T> {
+}
+
+pub trait BoxableExpression<QS, ST: NativeSqlType>: Expression + SelectableExpression<QS, ST> + NonAggregate {
+}
+
+impl<QS, T, ST> BoxableExpression<QS, ST> for T where
+    ST: NativeSqlType,
+    T: Expression + SelectableExpression<QS, ST> + NonAggregate,
+{
+}

src/lib.rs

@@ -25,7 +25,7 @@ pub mod helper_types {
 mod macros;
 
 pub use connection::{Connection, Cursor};
-pub use expression::{Expression, SelectableExpression};
+pub use expression::{Expression, SelectableExpression, BoxableExpression};
 pub use query_dsl::*;
 pub use query_source::{QuerySource, Queriable, Table, Column, JoinTo};
 pub use result::{TransactionError, TransactionResult, ConnectionError, ConnectionResult};

tests/filter.rs

@@ -230,3 +230,23 @@ fn or_doesnt_mess_with_precidence_of_previous_statements() {
 
     assert_eq!(Some(0), count);
 }
+
+sql_function!(lower, lower_t, (x: VarChar) -> VarChar);
+
+#[test]
+fn filter_by_boxed_predicate() {
+    fn by_name(name: &str) -> Box<BoxableExpression<users::table, types::Bool, SqlType=types::Bool>> {
+        Box::new(lower(users::name).eq(name.to_string()))
+    }
+
+    let connection = connection_with_sean_and_tess_in_users_table();
+    let sean = User::new(1, "Sean");
+    let tess = User::new(2, "Tess");
+    let queried_sean = users::table.filter(by_name("sean")).first(&connection)
+        .unwrap();
+    let queried_tess = users::table.filter(by_name("tess")).first(&connection)
+        .unwrap();
+
+    assert_eq!(Some(sean), queried_sean);
+    assert_eq!(Some(tess), queried_tess);
+}