Preparations to allow boxed dynamic select clauses

diesel/src/query_builder/mod.rs

@@ -41,7 +41,9 @@ pub use self::insert_statement::{
 };
 pub use self::query_id::QueryId;
 #[doc(inline)]
-pub use self::select_clause::{SelectClauseExpression, SelectClauseQueryFragment};
+pub use self::select_clause::{
+    IntoBoxedSelectClause, SelectClauseExpression, SelectClauseQueryFragment,
+};
 #[doc(hidden)]
 pub use self::select_statement::{BoxedSelectStatement, SelectStatement};
 pub use self::sql_query::{BoxedSqlQuery, SqlQuery};

diesel/src/query_builder/select_clause.rs

@@ -66,3 +66,41 @@ where
         source.default_selection().walk_ast(pass)
     }
 }
+
+/// An internal helper trait to convert different select clauses
+/// into their boxed counter part.
+///
+/// You normally don't need this trait, at least as long as you
+/// don't implement your own select clause representation
+pub trait IntoBoxedSelectClause<'a, DB, QS> {
+    /// The sql type of the select clause
+    type SqlType;
+
+    /// Convert the select clause into a the boxed representation
+    fn into_boxed(self, source: &QS) -> Box<dyn QueryFragment<DB> + 'a>;
+}
+
+impl<'a, DB, T, QS> IntoBoxedSelectClause<'a, DB, QS> for SelectClause<T>
+where
+    T: QueryFragment<DB> + SelectableExpression<QS> + 'a,
+    DB: Backend,
+{
+    type SqlType = T::SqlType;
+
+    fn into_boxed(self, _source: &QS) -> Box<dyn QueryFragment<DB> + 'a> {
+        Box::new(self.0)
+    }
+}
+
+impl<'a, DB, QS> IntoBoxedSelectClause<'a, DB, QS> for DefaultSelectClause
+where
+    QS: QuerySource,
+    QS::DefaultSelection: QueryFragment<DB> + 'a,
+    DB: Backend,
+{
+    type SqlType = <QS::DefaultSelection as Expression>::SqlType;
+
+    fn into_boxed(self, source: &QS) -> Box<dyn QueryFragment<DB> + 'a> {
+        Box::new(source.default_selection())
+    }
+}

diesel/src/query_builder/select_statement/dsl_impls.rs

@@ -339,12 +339,11 @@ impl<F, S, D, W, O, L, Of, G, LC, LM, Modifier> ModifyLockDsl<Modifier>
     }
 }
 
-impl<'a, F, S, D, W, O, L, Of, G, DB> BoxedDsl<'a, DB>
-    for SelectStatement<F, SelectClause<S>, D, W, O, L, Of, G>
+impl<'a, F, S, D, W, O, L, Of, G, DB> BoxedDsl<'a, DB> for SelectStatement<F, S, D, W, O, L, Of, G>
 where
     Self: AsQuery,
     DB: Backend,
-    S: QueryFragment<DB> + SelectableExpression<F> + 'a,
+    S: IntoBoxedSelectClause<'a, DB, F>,
     D: QueryFragment<DB> + 'a,
     W: Into<BoxedWhereClause<'a, DB>>,
     O: Into<Option<Box<dyn QueryFragment<DB> + 'a>>>,
@@ -356,36 +355,7 @@ where
 
     fn internal_into_boxed(self) -> Self::Output {
         BoxedSelectStatement::new(
-            Box::new(self.select.0),
-            self.from,
-            Box::new(self.distinct),
-            self.where_clause.into(),
-            self.order.into(),
-            Box::new(self.limit),
-            Box::new(self.offset),
-            Box::new(self.group_by),
-        )
-    }
-}
-
-impl<'a, F, D, W, O, L, Of, G, DB> BoxedDsl<'a, DB>
-    for SelectStatement<F, DefaultSelectClause, D, W, O, L, Of, G>
-where
-    Self: AsQuery,
-    DB: Backend,
-    F: QuerySource,
-    F::DefaultSelection: QueryFragment<DB> + 'a,
-    D: QueryFragment<DB> + 'a,
-    W: Into<BoxedWhereClause<'a, DB>>,
-    O: Into<Option<Box<dyn QueryFragment<DB> + 'a>>>,
-    L: QueryFragment<DB> + 'a,
-    Of: QueryFragment<DB> + 'a,
-    G: QueryFragment<DB> + 'a,
-{
-    type Output = BoxedSelectStatement<'a, <F::DefaultSelection as Expression>::SqlType, F, DB>;
-    fn internal_into_boxed(self) -> Self::Output {
-        BoxedSelectStatement::new(
-            Box::new(self.from.default_selection()),
+            self.select.into_boxed(&self.from),
             self.from,
             Box::new(self.distinct),
             self.where_clause.into(),