Fix #2480

This commit replaces the recursive trait bounds on our SqlType impls
for tuples with explicit bounds for all tuple sizes. This reduces the
compile time for the 32-column feature for me from 17mins to ~40s.

diesel/src/type_impls/tuples.rs

@@ -395,13 +395,45 @@ macro_rules! impl_from_sql_row {
 }
 
 macro_rules! impl_sql_type {
-    ($T1: ident, $($T: ident,)+) => {
-        impl<$T1, $($T,)+> SqlType for ($T1, $($T,)*)
-        where $T1: SqlType,
-              ($($T,)*): SqlType,
-              $T1::IsNull: OneIsNullable<<($($T,)*) as SqlType>::IsNull>,
+    (
+        @build
+        start_ts = [$($ST: ident,)*],
+        ts = [$T1: ident,],
+        bounds = [$($bounds: tt)*],
+        is_null = [$($is_null: tt)*],
+    )=> {
+        impl<$($ST,)*> SqlType for ($($ST,)*)
+        where
+            $($ST: SqlType,)*
+            $($bounds)*
+            $T1::IsNull: OneIsNullable<$($is_null)*>,
         {
-            type IsNull = <$T1::IsNull as OneIsNullable<<($($T,)*) as SqlType>::IsNull>>::Out;
+            type IsNull = <$T1::IsNull as OneIsNullable<$($is_null)*>>::Out;
+        }
+
+    };
+    (
+        @build
+        start_ts = [$($ST: ident,)*],
+        ts = [$T1: ident, $($T: ident,)+],
+        bounds = [$($bounds: tt)*],
+        is_null = [$($is_null: tt)*],
+    )=> {
+        impl_sql_type!{
+            @build
+            start_ts = [$($ST,)*],
+            ts = [$($T,)*],
+            bounds = [$($bounds)* $T1::IsNull: OneIsNullable<$($is_null)*>,],
+            is_null = [<$T1::IsNull as OneIsNullable<$($is_null)*>>::Out],
+        }
+    };
+    ($T1: ident, $($T: ident,)+) => {
+        impl_sql_type!{
+            @build
+            start_ts = [$T1, $($T,)*],
+            ts = [$($T,)*],
+            bounds = [],
+            is_null = [$T1::IsNull],
         }
     };
     ($T1: ident,) => {