Implement #[sql_name] for sql_function!

The docs for this have already been written, it just hadn't been
implemented. The primary use case is to support variadic functions.

Example:

```rust
sql_function! {
    #[sql_name = "COALESCE"]
    fn coalesce2<T>(x: T, y: T) -> T;
}

sql_function! {
    #[sql_name = "COALESCE"]
    fn coalesce3<T>(x: T, y: T, Z: T) -> T;
}

sql_function! {
    #[sql_name = "COALESCE"]
    fn or_default<T: NotNull>(x: Nullable<T>, y: T) -> T;
}
```

Author: sgrif

CHANGELOG.md

@@ -20,6 +20,11 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
   `NonAggregate` for your function. See [the documentation for
   `sql_function!`][sql-function-1-3-0] for more details.
 
+* `sql_function!` now supports renaming the function by annotating it with
+  `#[sql_name = "SOME_FUNCTION"]`. This can be used to support functions with
+  multiple signatures such as coalesce, by defining multiple rust functions
+  (with different names) that have the same `#[sql_name]`.
+
 * Added `sqlite-bundled` feature to `diesel_cli` to make installing on
   some platforms easier.
 

diesel/src/expression/functions/mod.rs

@@ -125,15 +125,16 @@ macro_rules! __diesel_sql_function_body {
         }
     };
 
-    // Searching for `#[aggregate]`, found it.
+    // Searching for `#[aggregate]`, found it. Search for `sql_name`.
     (
         unchecked_meta = (#[aggregate] $($unchecked:tt)*),
         meta = ($($meta:tt)*),
         $($rest:tt)*
     ) => {
         __diesel_sql_function_body! {
             aggregate = yes,
-            meta = ($($meta)* $($unchecked)*),
+            unchecked_meta = ($($meta)* $($unchecked)*),
+            meta = (),
             $($rest)*
         }
     };
@@ -151,21 +152,70 @@ macro_rules! __diesel_sql_function_body {
         }
     };
 
-    // Done searching for `#[aggregate]`.
+    // Done searching for `#[aggregate]`. Search for `sql_name`
     (
         unchecked_meta = (),
         meta = $meta:tt,
         $($rest:tt)*
     ) => {
         __diesel_sql_function_body! {
             aggregate = no,
+            unchecked_meta = $meta,
+            meta = (),
+            $($rest)*
+        }
+    };
+
+    // Searching for `#[sql_name]`, found it.
+    (
+        aggregate = $aggregate:tt,
+        unchecked_meta = (#[sql_name = $sql_name:expr] $($unchecked:tt)*),
+        meta = ($($meta:tt)*),
+        $($rest:tt)*
+    ) => {
+        __diesel_sql_function_body! {
+            aggregate = $aggregate,
+            sql_name = $sql_name,
+            meta = ($($meta)* $($unchecked)*),
+            $($rest)*
+        }
+    };
+
+    // Searching for `#[sql_name]`. Didn't find it.
+    (
+        aggregate = $aggregate:tt,
+        unchecked_meta = (#$checked:tt $($unchecked:tt)*),
+        meta = ($($meta:tt)*),
+        $($rest:tt)*
+    ) => {
+        __diesel_sql_function_body! {
+            aggregate = $aggregate,
+            unchecked_meta = ($($unchecked)*),
+            meta = ($($meta)* #$checked),
+            $($rest)*
+        }
+    };
+
+    // Done searching for `#[sql_name]`.
+    (
+        aggregate = $aggregate:tt,
+        unchecked_meta = (),
+        meta = $meta:tt,
+        fn_name = $fn_name:ident,
+        $($rest:tt)*
+    ) => {
+        __diesel_sql_function_body! {
+            aggregate = $aggregate,
+            sql_name = stringify!($fn_name),
             meta = $meta,
+            fn_name = $fn_name,
             $($rest)*
         }
     };
 
     (
         aggregate = $aggregate:tt,
+        sql_name = $sql_name:expr,
         meta = ($($meta:tt)*),
         fn_name = $fn_name:ident,
         type_args = ($($type_args:ident,)*),
@@ -222,7 +272,7 @@ macro_rules! __diesel_sql_function_body {
                 for<'a> ($(&'a $arg_name),*): $crate::query_builder::QueryFragment<DB>,
             {
                 fn walk_ast(&self, mut out: $crate::query_builder::AstPass<DB>) -> $crate::result::QueryResult<()> {
-                    out.push_sql(concat!(stringify!($fn_name), "("));
+                    out.push_sql(concat!($sql_name, "("));
                     $crate::query_builder::QueryFragment::walk_ast(
                         &($(&self.$arg_name),*), out.reborrow())?;
                     out.push_sql(")");
@@ -460,6 +510,7 @@ macro_rules! __diesel_sqlite_register_fn {
 ///
 /// sql_function! {
 ///     #[aggregate]
+///     #[sql_name = "SUM"]
 ///     fn sum<ST: Foldable>(expr: ST) -> ST::Sum;
 /// }
 ///

diesel/src/lib.rs

@@ -125,6 +125,8 @@
                  used_underscore_binding))]
 #![cfg_attr(all(test, feature = "clippy"), allow(option_unwrap_used, result_unwrap_used))]
 
+#![recursion_limit = "256"]
+
 #[cfg(feature = "postgres")]
 #[macro_use]
 extern crate bitflags;