No config to disable formatting of macro_rules! matchers

[sgrif]

Attempting to use the version of rustfmt from 1.26.1 results in a lot of suboptimal formatting in Diesel's macro definitions. It's removing newlines where we want them, adding them where we don't, and doing some really weird stuff like removing spaces after commas (which I'm assuming is a bug?).

We generally format our matching rules with the same formatting they'd be invoked with. Since rustfmt doesn't touch macro invocations unless they look like a Rust function, I'd expect the same to be true for matchers. If nothing else, I'd like to have a config to disable this new behavior, as it's blocking our ability to update.

Some examples of poor formatting:

diff --git a/diesel/src/expression/operators.rs b/diesel/src/expression/operators.rs
index 5fd306a..310d9d6 100644
--- a/diesel/src/expression/operators.rs
+++ b/diesel/src/expression/operators.rs
@@ -106,30 +106,18 @@ macro_rules! __diesel_operator_body {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! __diesel_operator_to_sql {
-    (
-        notation = infix,
-        operator_expr = $op:expr,
-        field_exprs = ($left:expr, $right:expr),
-    ) => {
+    (notation = infix,operator_expr = $op:expr,field_exprs = ($left:expr, $right:expr),) => {
         $left;
         $op;
         $right;
     };

-    (
-        notation = postfix,
-        operator_expr = $op:expr,
-        field_exprs = ($expr:expr),
-    ) => {
+    (notation = postfix,operator_expr = $op:expr,field_exprs = ($expr:expr),) => {
         $expr;
         $op;
     };

-    (
-        notation = prefix,
-        operator_expr = $op:expr,
-        field_exprs = ($expr:expr),
-    ) => {
+    (notation = prefix,operator_expr = $op:expr,field_exprs = ($expr:expr),) => {
         $op;
         $expr;
     };

--- a/diesel/src/macros/mod.rs
+++ b/diesel/src/macros/mod.rs
@@ -839,7 +839,7 @@ macro_rules! __diesel_table_impl {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! __diesel_table_query_source_impl {
-    ($table_struct:ident, public, $table_name:expr) => {
+    ($table_struct:ident,public, $table_name:expr) => {
         impl QuerySource for $table_struct {
             type FromClause = Identifier<'static>;
             type DefaultSelection = <Self as Table>::AllColumns;
@@ -966,12 +966,13 @@ macro_rules! joinable_inner {
     };

     (
-        left_table_ty = $left_table_ty:ty,
-        right_table_ty = $right_table_ty:ty,
-        right_table_expr = $right_table_expr:expr,
-        foreign_key = $foreign_key:path,
-        primary_key_ty = $primary_key_ty:ty,
-        primary_key_expr = $primary_key_expr:expr,
+        left_table_ty =
+        $left_table_ty:ty,right_table_ty =
+        $right_table_ty:ty,right_table_expr =
+        $right_table_expr:expr,foreign_key =
+        $foreign_key:path,primary_key_ty =
+        $primary_key_ty:ty,primary_key_expr =
+        $primary_key_expr:expr,
     ) => {
         impl $crate::JoinTo<$right_table_ty> for $left_table_ty {
             type FromClause = $right_table_ty;