Hypothetical code if RFC 1935 were implemented

diesel/src/expression/expression_methods/eq_all.rs

@@ -14,24 +14,28 @@ pub trait EqAll<Rhs> {
     fn eq_all(self, rhs: Rhs) -> Self::Output;
 }
 
-impl<L1, L2, LTail, R1, R2, RTail> EqAll<Cons<R1, Cons<R2, RTail>>>
-    for Cons<L1, Cons<L2, LTail>> where
+impl<L1, L2, LTail, R1, R2, RTail> EqAll<(R1, R2, ...RTail)>
+    for (L1, L2, ...LTail) where
         L1: EqAll<R1>,
-        Cons<L2, LTail>: EqAll<Cons<R2, RTail>>,
+        (L2, ...LTail): EqAll<(R2, ...RTail)>,
+        LTail: Tuple,
+        RTail: Tuple,
 {
-    type Output = And<<L1 as EqAll<R1>>::Output, <Cons<L2, LTail> as EqAll<Cons<R2, RTail>>>::Output>;
+    type Output = And<<L1 as EqAll<R1>>::Output, <(L1, ...LTail) as EqAll<(R1, ...RTail)>>::Output>;
 
-    fn eq_all(self, rhs: Cons<R1, Cons<R2, RTail>>) -> Self::Output {
-        self.0.eq_all(rhs.0).and(self.1.eq_all(rhs.1))
+    fn eq_all(self, rhs: (R1, R2, ...RTail)) -> Self::Output {
+        let (lhead, ...ltail) = self;
+        let (rhead, ...rtail) = rhs;
+        lhead.eq_all(rhead).and(ltail.eq_all(rtail))
     }
 }
 
-impl<Left, Right> EqAll<Cons<Right, Nil>> for Cons<Left, Nil> where
+impl<Left, Right> EqAll<(Right,)> for Cons<(Left,)> where
     Left: EqAll<Right>,
 {
     type Output = <Left as EqAll<Right>>::Output;
 
-    fn eq_all(self, rhs: Cons<Right, Nil>) -> Self::Output {
+    fn eq_all(self, rhs: (Right,)) -> Self::Output {
         self.0.eq_all(rhs.0)
     }
 }

diesel/src/expression/mod.rs

@@ -82,15 +82,17 @@ impl<'a, T: Expression + ?Sized> Expression for &'a T {
     type SqlType = T::SqlType;
 }
 
-impl<Head, Tail> Expression for Cons<Head, Tail> where
+impl<Head, Tail> Expression for (Head, ...Tail) where
     Head: Expression + NonAggregate,
     Tail: Expression + NonAggregate,
+    Tail: Tuple,
+    Tail::SqlType: Tuple,
 {
-    type SqlType = Cons<Head::SqlType, Tail::SqlType>;
+    type SqlType = (Head::SqlType, ...Tail::SqlType);
 }
 
-impl Expression for Nil {
-    type SqlType = Nil;
+impl Expression for () {
+    type SqlType = ();
 }
 
 /// Describes how a type can be represented as an expression for a given type.
@@ -142,16 +144,18 @@ impl<'a, T: ?Sized, QS> SelectableExpression<QS> for &'a T where
     type SqlTypeForSelect = T::SqlTypeForSelect;
 }
 
-impl<Head, Tail, QS> SelectableExpression<QS> for Cons<Head, Tail> where
+impl<Head, Tail, QS> SelectableExpression<QS> for (Head, ...Tail) where
     Head: SelectableExpression<QS>,
     Tail: SelectableExpression<QS>,
-    Cons<Head, Tail>: Expression,
+    (Head, ...Tail): Expression,
+    Tail: Tuple,
+    Tail::SqlTypeForSelect: Tuple,
 {
-    type SqlTypeForSelect = Cons<Head::SqlTypeForSelect, Tail::SqlTypeForSelect>;
+    type SqlTypeForSelect = (Head::SqlTypeForSelect, ...Tail::SqlTypeForSelect);
 }
 
-impl<QS> SelectableExpression<QS> for Nil {
-    type SqlTypeForSelect = Nil;
+impl<QS> SelectableExpression<QS> for () {
+    type SqlTypeForSelect = ();
 }
 
 /// Marker trait to indicate that an expression does not include any aggregate
@@ -167,13 +171,13 @@ impl<T: NonAggregate + ?Sized> NonAggregate for Box<T> {
 impl<'a, T: NonAggregate + ?Sized> NonAggregate for &'a T {
 }
 
-impl<Head, Tail> NonAggregate for Cons<Head, Tail> where
+impl<Head, Tail> NonAggregate for (Head, ...Tail) where
     Head: NonAggregate,
-    Tail: NonAggregate,
+    Tail: NonAggregate + Tuple,
 {
 }
 
-impl NonAggregate for Nil {
+impl NonAggregate for () {
 }
 
 use query_builder::{QueryFragment, QueryId};

diesel/src/insertable/hlist_impls.rs

@@ -2,9 +2,10 @@ use hlist::*;
 use super::*;
 use query_builder::QueryFragment;
 
-impl<Head, Tail, DB> InsertValues<DB> for Cons<Head, Tail> where
+impl<Head, Tail, DB> InsertValues<DB> for (Head, ...Tail) where
     DB: Backend,
-    Cons<Head, Tail>: InsertValuesRecursive<DB>,
+    (Head, ...Tail): InsertValuesRecursive<DB>,
+    Tail: Tuple,
 {
     fn column_names(&self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
         InsertValuesRecursive::<DB>::column_names(self, false, out)
@@ -23,14 +24,14 @@ impl<Head, Tail, DB> InsertValues<DB> for Cons<Head, Tail> where
 }
 
 #[doc(hidden)]
-pub trait InsertValuesRecursive<DB: Backend> {
+pub trait InsertValuesRecursive<DB: Backend>: Tuple {
     fn column_names(&self, comma_needed: bool, out: &mut DB::QueryBuilder) -> BuildQueryResult;
     fn values_clause(&self, comma_needed: bool, out: &mut DB::QueryBuilder) -> BuildQueryResult;
     fn values_bind_params(&self, out: &mut DB::BindCollector) -> QueryResult<()>;
 }
 
 impl<Col, Expr, Tail, DB> InsertValuesRecursive<DB>
-    for Cons<ColumnInsertValue<Col, Expr>, Tail> where
+    for (ColumnInsertValue<Col, Expr>, ...Tail) where
         DB: Backend + SupportsDefaultKeyword,
         Col: Column,
         Col::SqlType: IntoNullable,
@@ -71,7 +72,7 @@ use sqlite::Sqlite;
 
 #[cfg(feature="sqlite")]
 impl<Col, Expr, Tail> InsertValuesRecursive<Sqlite>
-    for Cons<ColumnInsertValue<Col, Expr>, Tail> where
+    for (ColumnInsertValue<Col, Expr>, ...Tail) where
         Col: Column,
         Col::SqlType: IntoNullable,
         Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable> + QueryFragment<Sqlite>,
@@ -118,7 +119,7 @@ impl<Col, Expr, Tail> InsertValuesRecursive<Sqlite>
     }
 }
 
-impl<DB: Backend> InsertValuesRecursive<DB> for Nil {
+impl<DB: Backend> InsertValuesRecursive<DB> for () {
     fn column_names(&self, _: bool, _: &mut DB::QueryBuilder) -> BuildQueryResult {
         Ok(())
     }

diesel/src/query_builder/mod.rs

@@ -132,30 +132,40 @@ impl<DB: Backend> QueryFragment<DB> for () {
     }
 }
 
-impl<Head, Head2, Tail, DB> QueryFragment<DB> for Cons<Head, Cons<Head2, Tail>> where
+trait QueryFragmentTuple<DB>: Tuple {
+    fn to_sql(self, out: &mut DB::QueryBuilder) -> BuildQueryResult;
+    fn collect_binds(self, out: &mut DB::BindCollector) -> QueryResult<()>;
+    fn is_safe_to_cache_prepared(self) -> bool;
+}
+
+impl<Head, Head2, Tail, DB> QueryFragmentTuple<DB> for (Head, Head2, ...Tail) where
     DB: Backend,
     Head: QueryFragment<DB>,
-    Cons<Head2, Tail>: QueryFragment<DB>,
+    Tail: Tuple,
+    (Head2, ...Tail): QueryFragmentTuple<DB>,
 {
-    fn to_sql(&self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
-        try!(self.0.to_sql(out));
+    fn to_sql(self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
+        let (head, ...tail) = self;
+        try!(head.to_sql(out));
         out.push_sql(", ");
-        try!(self.1.to_sql(out));
+        try!(tail.to_sql(out));
         Ok(())
     }
 
-    fn collect_binds(&self, out: &mut DB::BindCollector) -> QueryResult<()> {
-        try!(self.0.collect_binds(out));
-        try!(self.1.collect_binds(out));
+    fn collect_binds(self, out: &mut DB::BindCollector) -> QueryResult<()> {
+        let (head, ...tail) = self;
+        try!(head.collect_binds(out));
+        try!(tail.collect_binds(out));
         Ok(())
     }
 
-    fn is_safe_to_cache_prepared(&self) -> bool {
-        self.0.is_safe_to_cache_prepared() && self.1.is_safe_to_cache_prepared()
+    fn is_safe_to_cache_prepared(self) -> bool {
+        let (head, ...tail) = self;
+        head.is_safe_to_cache_prepared() && tail.is_safe_to_cache_prepared()
     }
 }
 
-impl<Head, DB> QueryFragment<DB> for Cons<Head, Nil> where
+impl<Head, DB> QueryFragmentTuple<DB> for (Head,) where
     DB: Backend,
     Head: QueryFragment<DB>,
 {
@@ -172,6 +182,24 @@ impl<Head, DB> QueryFragment<DB> for Cons<Head, Nil> where
     }
 }
 
+impl<T, DB> QueryFragment<DB> for (...T) where
+    DB: Backend,
+    T: Tuple,
+    for<'a> T::AsRefs<'a>: QueryFragmentTuple<DB>,
+{
+    fn to_sql(&self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
+        self.elements_as_ref().to_sql(out)
+    }
+
+    fn collect_binds(&self, out: &mut DB::BindCollector) -> QueryResult<()> {
+        self.elements_as_ref().collect_binds(out)
+    }
+
+    fn is_safe_to_cache_prepared(&self) -> bool {
+        self.elements_as_ref().is_safe_to_cache_prepared(out)
+    }
+}
+
 /// Types that can be converted into a complete, typed SQL query. This is used
 /// internally to automatically add the right select clause when none is
 /// specified, or to automatically add `RETURNING *` in certain contexts

diesel/src/query_builder/query_id.rs

@@ -48,8 +48,25 @@ impl<DB> QueryId for QueryFragment<DB> {
     }
 }
 
-impl_query_id!(Cons<T, U>);
-impl_query_id!(Nil);
+impl<Head, ...Tail> QueryId for (Head, ...Tail) where
+    Head: QueryId,
+    Tail: Tuple + QueryId,
+    Tail::QueryId: Tuple,
+{
+    type QueryId = (Head::QueryId, ...Tail::QueryId);
+
+    fn has_static_query_id() -> bool {
+        Head::has_static_query_id() && Tail::has_static_query_id()
+    }
+}
+
+impl QueryId for () {
+    type QueryId = ();
+
+    fn has_static_query_id() -> bool {
+        true
+    }
+}
 
 #[cfg(test)]
 mod tests {

diesel/src/query_builder/update_statement/changeset.rs

@@ -62,75 +62,99 @@ impl<T: Changeset<DB>, DB: Backend> Changeset<DB> for Option<T> {
     }
 }
 
-impl<Head, Head2, Tail> AsChangeset for Cons<Head, Cons<Head2, Tail>> where
+impl<Head, Head2, Tail> AsChangeset for (Head, Head2, ...Tail) where
     Head: AsChangeset,
-    Cons<Head2, Tail>: AsChangeset<Target=Head::Target>,
+    Tail: Tuple,
+    (Head2, ...Tail): AsChangeset<Target=Head::Target>,
+    <(Head2, ...Tail) as AsChangeset>::Changeset: Tuple,
 {
     type Target = Head::Target;
-    type Changeset = Cons<
+    type Changeset = (
         Head::Changeset,
-        <Cons<Head2, Tail> as AsChangeset>::Changeset,
-    >;
+        ...<(Head2, ...Tail) as AsChangeset>::Changeset,
+    );
 
     fn as_changeset(self) -> Self::Changeset {
-        Cons(
-            self.0.as_changeset(),
-            self.1.as_changeset(),
-        )
+        let (head, ...tail) = self;
+        (head.as_changeset(), ...tail.as_changeset())
     }
 }
 
-impl<Head> AsChangeset for Cons<Head, Nil> where
+impl<Head> AsChangeset for (Head,) where
     Head: AsChangeset,
 {
     type Target = Head::Target;
-    type Changeset = Cons<Head::Changeset, Nil>;
+    type Changeset = (Head::Changeset,);
 
     fn as_changeset(self) -> Self::Changeset {
-        Cons(
-            self.0.as_changeset(),
-            Nil,
-        )
+        (self.0.as_changeset(),)
     }
 }
 
-impl<Head, Tail, DB> Changeset<DB> for Cons<Head, Tail> where
+trait ChangesetTuple<DB: Backend>: Tuple {
+    fn is_noop(self) -> bool;
+    fn to_sql(self, out: &mut DB::QueryBuilder) -> BuildQueryResult;
+    fn collect_binds(self, out: &mut DB::BindCollector) -> QueryResult<()>;
+}
+
+impl<Head, Tail, DB> ChangesetTuple<DB> for (Head, ...Tail) where
     DB: Backend,
     Head: Changeset<DB>,
-    Tail: Changeset<DB>,
+    Tail: ChangesetTuple<DB>,
 {
-    fn is_noop(&self) -> bool {
-        self.0.is_noop() && self.1.is_noop()
+    fn is_noop(self) -> bool {
+        let (head, ...tail) = self;
+        head.is_noop() && tail.is_noop()
     }
 
-    fn to_sql(&self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
+    fn to_sql(self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
         use query_builder::QueryBuilder;
 
-        if !self.0.is_noop() {
-            try!(self.0.to_sql(out));
-            if !self.1.is_noop() {
+        let (head, ...tail) = self;
+        if !head.is_noop() {
+            try!(head.to_sql(out));
+            if !tail.is_noop() {
                 out.push_sql(", ");
             }
         }
-        self.1.to_sql(out)
+        tail.to_sql(out)
     }
 
-    fn collect_binds(&self, out: &mut DB::BindCollector) -> QueryResult<()> {
-        try!(self.0.collect_binds(out));
-        self.1.collect_binds(out)
+    fn collect_binds(self, out: &mut DB::BindCollector) -> QueryResult<()> {
+        let (head, ...tail) = self;
+        try!(head.collect_binds(out));
+        tail.collect_binds(out)
     }
 }
 
-impl<DB: Backend> Changeset<DB> for Nil {
-    fn is_noop(&self) -> bool {
+impl<DB: Backend> ChangesetTuple<DB> for () {
+    fn is_noop(self) -> bool {
         true
     }
 
-    fn to_sql(&self, _: &mut DB::QueryBuilder) -> BuildQueryResult {
+    fn to_sql(self, _: &mut DB::QueryBuilder) -> BuildQueryResult {
         Ok(())
     }
 
-    fn collect_binds(&self, _: &mut DB::BindCollector) -> QueryResult<()> {
+    fn collect_binds(self, _: &mut DB::BindCollector) -> QueryResult<()> {
         Ok(())
     }
 }
+
+impl<DB, T> Changeset<DB> for (...T) where
+    DB: Backend,
+    T: Tuple,
+    for<'a> T::AsRefs<'a>: ChangesetTuple<DB>,
+{
+    fn is_noop(&self) -> bool {
+        self.elements_as_ref().is_noop()
+    }
+
+    fn to_sql(&self, out: &mut DB::QueryBuilder) -> BuildQueryResult {
+        self.elements_as_ref().to_sql(out)
+    }
+
+    fn collect_binds(&self, out: &mut DB::BindCollector) -> QueryResult<()> {
+        self.elements_as_ref().collect_binds(out)
+    }
+}