Introduce a #[declare_sql_function] attribute macro

This commit introduces a new `#[declare_sql_function]` attribute macro
that can be applied to `extern "SQL"` blocks. This is essentially the
same as the existing `define_sql_function!` function like macro in terms
of functionality.

I see the following advantages of using an attribute macro + an `extern
"SQL"` block instead:

* This is closer to rust syntax, so rustfmt will understand that and
work correctly inside these blocks
* This allows to put several functions into the same block
* Maybe in the future this also allows to apply attributes to the whole
block instead of to each item

The downside of this change is that we then have three variants to
declare sql functions:

* `sql_function!()` (deprectated)
* `define_sql_function!()` (introduced in 2.2)
* The new attribute macro

Author: weiznich

CHANGELOG.md

@@ -21,7 +21,14 @@ Increasing the minimal supported Rust version will always be coupled at least wi
 * Support for postgres multirange type
 * Added `diesel::r2d2::TestCustomizer`, which allows users to customize their `diesel::r2d2::Pool`s
 in a way that makes the pools suitable for use in parallel tests.
+* Added support for built-in PostgreSQL range operators and functions
+* Added support for various built-in PostgreSQL array functions
 * Added `Json` and `Jsonb` support for the SQLite backend.
+* Added a `#[diesel::declare_sql_function]` attribute macro to easily define support for 
+  multiple sql functions at once via an `extern "SQL"` block
+
+### Fixed 
+
 * Fixed diesel thinking `a.eq_any(b)` was non-nullable even if `a` and `b` were nullable.
 * Generate `InstrumentationEvent::BeginTransaction` for immediate and exclusive transactions in SQLite
 * Added `wasm32-unknown-unknown` target support for sqlite backend.

diesel/src/connection/statement_cache/strategy.rs

@@ -149,7 +149,6 @@ mod testing_utils {
 mod tests_pg {
     use crate::connection::CacheSize;
     use crate::dsl::sql;
-    use crate::expression::functions::define_sql_function;
     use crate::insertable::Insertable;
     use crate::pg::Pg;
     use crate::sql_types::{Integer, VarChar};
@@ -159,6 +158,11 @@ mod tests_pg {
 
     use super::testing_utils::{count_cache_calls, RecordCacheEvents};
 
+    #[crate::declare_sql_function]
+    extern "SQL" {
+        fn lower(x: VarChar) -> VarChar;
+    }
+
     table! {
         users {
             id -> Integer,
@@ -210,8 +214,6 @@ mod tests_pg {
         assert_eq!(2, count_cache_calls(connection));
     }
 
-    define_sql_function!(fn lower(x: VarChar) -> VarChar);
-
     #[diesel_test_helper::test]
     fn queries_with_identical_types_and_binds_but_different_sql_are_cached_separately() {
         let connection = &mut connection();

diesel/src/expression/count.rs

@@ -1,6 +1,6 @@
 use std::marker::PhantomData;
 
-use super::functions::define_sql_function;
+use super::functions::declare_sql_function;
 use super::{is_aggregate, AsExpression};
 use super::{Expression, ValidGrouping};
 use crate::backend::Backend;
@@ -9,7 +9,8 @@ use crate::result::QueryResult;
 use crate::sql_types::{BigInt, DieselNumericOps, SingleValue, SqlType};
 use crate::{AppearsOnTable, SelectableExpression};
 
-define_sql_function! {
+#[declare_sql_function]
+extern "SQL" {
     /// Creates a SQL `COUNT` expression
     ///
     /// As with most bare functions, this is not exported by default. You can import

diesel/src/expression/functions/aggregate_folding.rs

@@ -1,7 +1,8 @@
-use crate::expression::functions::define_sql_function;
+use crate::expression::functions::declare_sql_function;
 use crate::sql_types::Foldable;
 
-define_sql_function! {
+#[declare_sql_function]
+extern "SQL" {
     /// Represents a SQL `SUM` function. This function can only take types which are
     /// Foldable.
     ///
@@ -19,9 +20,7 @@ define_sql_function! {
     /// ```
     #[aggregate]
     fn sum<ST: Foldable>(expr: ST) -> ST::Sum;
-}
 
-define_sql_function! {
     /// Represents a SQL `AVG` function. This function can only take types which are
     /// Foldable.
     ///

diesel/src/expression/functions/aggregate_ordering.rs

@@ -1,7 +1,8 @@
 use self::private::SqlOrdAggregate;
-use crate::expression::functions::define_sql_function;
+use crate::expression::functions::declare_sql_function;
 
-define_sql_function! {
+#[declare_sql_function]
+extern "SQL" {
     /// Represents a SQL `MAX` function. This function can only take types which are
     /// ordered.
     ///
@@ -18,9 +19,7 @@ define_sql_function! {
     /// # }
     #[aggregate]
     fn max<ST: SqlOrdAggregate>(expr: ST) -> ST::Ret;
-}
 
-define_sql_function! {
     /// Represents a SQL `MIN` function. This function can only take types which are
     /// ordered.
     ///

diesel/src/expression/functions/date_and_time.rs

@@ -1,6 +1,6 @@
 use crate::backend::Backend;
 use crate::expression::coerce::Coerce;
-use crate::expression::functions::define_sql_function;
+use crate::expression::functions::declare_sql_function;
 use crate::expression::{AsExpression, Expression, ValidGrouping};
 use crate::query_builder::*;
 use crate::result::QueryResult;
@@ -27,7 +27,9 @@ impl_selectable_expression!(now);
 
 operator_allowed!(now, Add, add);
 operator_allowed!(now, Sub, sub);
-define_sql_function! {
+
+#[declare_sql_function]
+extern "SQL" {
     /// Represents the SQL `DATE` function. The argument should be a Timestamp
     /// expression, and the return value will be an expression of type Date.
     ///

diesel/src/expression/functions/mod.rs

@@ -1,5 +1,8 @@
 //! Helper macros to define custom sql functions
 
+#[doc(inline)]
+pub use diesel_derives::declare_sql_function;
+
 #[doc(inline)]
 pub use diesel_derives::define_sql_function;
 

diesel/src/lib.rs

@@ -734,6 +734,8 @@ pub mod prelude {
     // That issue can be avoided by also importing it anonymously:
     pub use crate::expression::IntoSql as _;
 
+    #[doc(inline)]
+    pub use crate::expression::functions::declare_sql_function;
     #[doc(inline)]
     pub use crate::expression::functions::define_sql_function;
     #[cfg(all(feature = "with-deprecated", not(feature = "without-deprecated")))]