First draft for supporting window functions and other aggregate funct…

…ion expressions

See
[here](https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS)
and
[here](https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-AGGREGATES)
for the relevant postgresql documentation

This PR implements mainly the relevant DSL structure for this types. As
of today this should support most of the relevant API surface.

This PR is not finished yet, I mainly push this now to get some early
feedback on the exposed user API. See the two dummy tests for how to use
the new functionality.

What's still missing:

- [ ] Support for other backends
- [ ] Tests, a lot of tests
- [ ] Compile tests to verify that we do not generate invalid SQL
- [ ] Documentation

diesel/src/backend.rs

@@ -317,6 +317,20 @@ pub trait SqlDialect: self::private::TrustedBackend {
         doc = "See [`sql_dialect::alias_syntax`] for provided default implementations"
     )]
     type AliasSyntax;
+
+    /// Configures how this backend support the `GROUP` frame unit for window functions
+    #[cfg_attr(
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes",
+        doc = "See [`sql_dialect::window_frame_clause_group_support`] for provided default implementations"
+    )]
+    type WindowFrameClauseGroupSupport;
+
+    /// Configures how this backend supports aggregate function expressions
+    #[cfg_attr(
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes",
+        doc = "See [`sql_dialect::window_frame_clause_group_support`] for provided default implementations"
+    )]
+    type AggregateFunctionExpressions;
 }
 
 /// This module contains all options provided by diesel to configure the [`SqlDialect`] trait.
@@ -539,6 +553,34 @@ pub(crate) mod sql_dialect {
         #[derive(Debug, Copy, Clone)]
         pub struct AsAliasSyntax;
     }
+
+    /// This module contains all reusable options to configure [`SqlDialect::WindowFrameClauseGroupSupport`]
+    #[diesel_derives::__diesel_public_if(
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
+    )]
+    pub mod window_frame_clause_group_support {
+        /// Indicates that this backend does not support the `GROUPS` frame unit
+        #[derive(Debug, Copy, Clone)]
+        pub struct NoGroupWindowFrameUnit;
+
+        /// Indicates that this backend does support the `GROUPS` frame unit as specified by the standard
+        #[derive(Debug, Copy, Clone)]
+        pub struct IsoGroupWindowFrameUnit;
+    }
+
+    /// This module contains all reusable options to configure [`SqlDialect::AggregateFunctionExpressions`]
+    #[diesel_derives::__diesel_public_if(
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
+    )]
+    pub mod aggregate_function_expressions {
+        /// Indicates that this backend does not support aggregate function expressions
+        #[derive(Debug, Copy, Clone)]
+        pub struct NoAggregateFunctionExpressions;
+
+        /// Indicates that this backend supports aggregate function expressions similar to PostgreSQL
+        #[derive(Debug, Copy, Clone)]
+        pub struct PostgresLikeAggregateFunctionExpressions;
+    }
 }
 
 // These traits are not part of the public API

diesel/src/connection/instrumentation.rs

@@ -356,6 +356,12 @@ impl DynInstrumentation {
     #[diesel_derives::__diesel_public_if(
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     )]
+    #[cfg(any(
+        feature = "postgres",
+        feature = "sqlite",
+        feature = "mysql",
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
+    ))]
     pub(crate) fn default_instrumentation() -> Self {
         Self {
             inner: get_default_instrumentation(),
@@ -367,6 +373,12 @@ impl DynInstrumentation {
     #[diesel_derives::__diesel_public_if(
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     )]
+    #[cfg(any(
+        feature = "postgres",
+        feature = "sqlite",
+        feature = "mysql",
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
+    ))]
     pub(crate) fn none() -> Self {
         Self {
             inner: None,
@@ -378,6 +390,12 @@ impl DynInstrumentation {
     #[diesel_derives::__diesel_public_if(
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     )]
+    #[cfg(any(
+        feature = "postgres",
+        feature = "sqlite",
+        feature = "mysql",
+        feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
+    ))]
     pub(crate) fn on_connection_event(&mut self, event: InstrumentationEvent<'_>) {
         // This implementation is not necessary to be able to call this method on this object
         // because of the already existing Deref impl.

diesel/src/expression/count.rs

@@ -29,6 +29,7 @@ define_sql_function! {
     /// # }
     /// ```
     #[aggregate]
+    #[window]
     fn count<T: SqlType + SingleValue>(expr: T) -> BigInt;
 }
 

diesel/src/expression/functions/aggregate_expressions.rs

@@ -0,0 +1,264 @@
+use crate::backend::Backend;
+use crate::expression::{AsExpression, ValidGrouping};
+use crate::query_builder::{AstPass, NotSpecialized, QueryFragment, QueryId};
+use crate::sql_types::Bool;
+use crate::{AppearsOnTable, Expression, QueryResult, SelectableExpression};
+
+macro_rules! empty_clause {
+    ($name: ident) => {
+        #[derive(Debug, Clone, Copy, QueryId)]
+        pub struct $name;
+
+        impl<DB> crate::query_builder::QueryFragment<DB> for $name
+        where
+            DB: crate::backend::Backend + crate::backend::DieselReserveSpecialization,
+        {
+            fn walk_ast<'b>(
+                &'b self,
+                _pass: crate::query_builder::AstPass<'_, 'b, DB>,
+            ) -> crate::QueryResult<()> {
+                Ok(())
+            }
+        }
+    };
+}
+
+mod aggregate_filter;
+mod aggregate_order;
+pub(crate) mod frame_clause;
+mod over_clause;
+mod partition_by;
+mod prefix;
+mod within_group;
+
+use self::aggregate_filter::{FilterDsl, NoFilter};
+use self::aggregate_order::{NoOrder, OrderAggregateDsl, OrderWindowDsl};
+use self::frame_clause::{FrameDsl, NoFrame};
+pub use self::over_clause::OverClause;
+use self::over_clause::{NoWindow, OverDsl};
+use self::partition_by::PartitionByDsl;
+use self::prefix::{All, AllDsl, DistinctDsl, NoPrefix};
+use self::within_group::{NoWithin, WithinGroupDsl};
+
+#[derive(QueryId, Debug)]
+pub struct AggregateExpression<
+    Fn,
+    Prefix = NoPrefix,
+    Order = NoOrder,
+    Filter = NoFilter,
+    Within = NoWithin,
+    Window = NoWindow,
+> {
+    prefix: Prefix,
+    function: Fn,
+    order: Order,
+    filter: Filter,
+    within_group: Within,
+    window: Window,
+}
+
+impl<Fn, Prefix, Order, Filter, Within, Window, DB> QueryFragment<DB>
+    for AggregateExpression<Fn, Prefix, Order, Filter, Within, Window>
+where
+    DB: crate::backend::Backend + crate::backend::DieselReserveSpecialization,
+    Fn: FunctionFragment<DB>,
+    Prefix: QueryFragment<DB>,
+    Order: QueryFragment<DB>,
+    Filter: QueryFragment<DB>,
+    Within: QueryFragment<DB>,
+    Window: QueryFragment<DB> + WindowFunctionFragment<Fn, DB>,
+{
+    fn walk_ast<'b>(&'b self, mut pass: AstPass<'_, 'b, DB>) -> QueryResult<()> {
+        pass.push_sql(Fn::FUNCTION_NAME);
+        pass.push_sql("(");
+        self.prefix.walk_ast(pass.reborrow())?;
+        self.function.walk_arguments(pass.reborrow())?;
+        self.order.walk_ast(pass.reborrow())?;
+        pass.push_sql(")");
+        self.within_group.walk_ast(pass.reborrow())?;
+        self.filter.walk_ast(pass.reborrow())?;
+        self.window.walk_ast(pass.reborrow())?;
+        Ok(())
+    }
+}
+
+impl<Fn, Prefix, Order, Filter, Within, GB> ValidGrouping<GB>
+    for AggregateExpression<Fn, Prefix, Order, Filter, Within>
+where
+    Fn: ValidGrouping<GB>,
+{
+    type IsAggregate = <Fn as ValidGrouping<GB>>::IsAggregate;
+}
+
+impl<Fn, Prefix, Order, Filter, Within, GB, Partition, WindowOrder, Frame> ValidGrouping<GB>
+    for AggregateExpression<
+        Fn,
+        Prefix,
+        Order,
+        Filter,
+        Within,
+        OverClause<Partition, WindowOrder, Frame>,
+    >
+where
+    Fn: IsWindowFunction,
+    Fn::ArgTypes: ValidGrouping<GB>,
+{
+    // not sure about that, check this
+    type IsAggregate = <Fn::ArgTypes as ValidGrouping<GB>>::IsAggregate;
+}
+
+impl<Fn, Prefix, Order, Filter, Within, Window> Expression
+    for AggregateExpression<Fn, Prefix, Order, Filter, Within, Window>
+where
+    Fn: Expression,
+{
+    type SqlType = <Fn as Expression>::SqlType;
+}
+
+impl<Fn, Prefix, Order, Filter, Within, Window, QS> AppearsOnTable<QS>
+    for AggregateExpression<Fn, Prefix, Order, Filter, Within, Window>
+where
+    Self: Expression,
+    Fn: AppearsOnTable<QS>,
+{
+}
+
+impl<Fn, Prefix, Order, Filter, Within, Window, QS> SelectableExpression<QS>
+    for AggregateExpression<Fn, Prefix, Order, Filter, Within, Window>
+where
+    Self: Expression,
+    Fn: SelectableExpression<QS>,
+{
+}
+
+/// A helper marker trait that this function is a window function
+/// This is only used to provide the gate the `WindowExpressionMethods`
+/// trait onto, not to check if the construct is valid for a given backend
+/// This check is postponed to building the query via `QueryFragment`
+/// (We have access to the DB type there)
+pub trait IsWindowFunction {
+    /// A tuple of all arg types
+    type ArgTypes;
+}
+
+/// A helper marker trait that this function is a valid window function
+/// for the given backend
+/// this trait is used to transport information that
+/// a certain function can be used as window function for a specific
+/// backend
+/// We allow to specialize this function for different SQL dialects
+pub trait WindowFunctionFragment<Fn, DB: Backend, SP = NotSpecialized> {}
+
+/// A helper marker trait that this function as a aggregate function
+/// This is only used to provide the gate the `AggregateExpressionMethods`
+/// trait onto, not to check if the construct is valid for a given backend
+/// This check is postponed to building the query via `QueryFragment`
+/// (We have access to the DB type there)
+pub trait IsAggregateFunction {}
+
+/// A specialized QueryFragment helper trait that allows us to walk the function name
+/// and the function arguments in seperate steps
+pub trait FunctionFragment<DB: Backend> {
+    /// The name of the sql function
+    const FUNCTION_NAME: &'static str;
+
+    /// Walk the function argument part (everything between ())
+    fn walk_arguments<'b>(&'b self, pass: AstPass<'_, 'b, DB>) -> QueryResult<()>;
+}
+
+// TODO: write helper types for all functions
+// TODO: write doc tests for all functions
+/// Expression methods to build aggregate function expressions
+pub trait AggregateExpressionMethods: Sized {
+    /// `DISTINCT` modifier
+    fn distinct(self) -> Self::Output
+    where
+        Self: DistinctDsl,
+    {
+        <Self as DistinctDsl>::distinct(self)
+    }
+
+    /// `ALL` modifier
+    fn all(self) -> Self::Output
+    where
+        Self: AllDsl,
+    {
+        <Self as AllDsl>::all(self)
+    }
+
+    /// Add an aggregate filter
+    fn filter_aggregate<P>(self, f: P) -> Self::Output
+    where
+        P: AsExpression<Bool>,
+        Self: FilterDsl<P::Expression>,
+    {
+        <Self as FilterDsl<P::Expression>>::filter(self, f.as_expression())
+    }
+
+    /// Add an aggregate order
+    fn order_aggregate<O>(self, o: O) -> Self::Output
+    where
+        Self: OrderAggregateDsl<O>,
+    {
+        <Self as OrderAggregateDsl<O>>::order(self, o)
+    }
+
+    // todo: restrict this to order set aggregates
+    // (we don't have any in diesel yet)
+    #[doc(hidden)] // for now
+    fn within_group<O>(self, o: O) -> Self::Output
+    where
+        Self: WithinGroupDsl<O>,
+    {
+        <Self as WithinGroupDsl<O>>::within_group(self, o)
+    }
+}
+
+impl<T> AggregateExpressionMethods for T {}
+
+/// Methods to construct a window function call
+pub trait WindowExpressionMethods: Sized {
+    /// Turn a function call into a window function call
+    fn over(self) -> Self::Output
+    where
+        Self: OverDsl,
+    {
+        <Self as OverDsl>::over(self)
+    }
+
+    /// Add a filter to the current window function
+    // todo: do we want `or_filter` as well?
+    fn filter_window<P>(self, f: P) -> Self::Output
+    where
+        P: AsExpression<Bool>,
+        Self: FilterDsl<P::Expression>,
+    {
+        <Self as FilterDsl<P::Expression>>::filter(self, f.as_expression())
+    }
+
+    /// Add a partition clause to the current window function
+    fn partition_by<E>(self, expr: E) -> Self::Output
+    where
+        Self: PartitionByDsl<E>,
+    {
+        <Self as PartitionByDsl<E>>::partition_by(self, expr)
+    }
+
+    /// Add a order clause to the current window function
+    fn window_order<E>(self, expr: E) -> Self::Output
+    where
+        Self: OrderWindowDsl<E>,
+    {
+        <Self as OrderWindowDsl<E>>::order(self, expr)
+    }
+
+    /// Add a frame clause to the current window function
+    fn frame_by<E>(self, expr: E) -> Self::Output
+    where
+        Self: FrameDsl<E>,
+    {
+        <Self as FrameDsl<E>>::frame(self, expr)
+    }
+}
+
+impl<T> WindowExpressionMethods for T {}