config: Stabilize binop_separator again.

See reasoning in rust-lang#4144, which still applies. Other projects like bindgen
etc also use it by default, and this would allow them to move to stable
rust.

Fixes rust-lang#3368 again.

Author: emilio

Diff for: CHANGELOG.md

@@ -14,6 +14,7 @@
 ### Changed
 
 - Properly sort imports containing raw identifiers [#3791](https://github.com/rust-lang/rustfmt/issues/3791) (note this is change version gated, and not applied by default)
+- `binop_separator` is now stable ([#3368](https://github.com/rust-lang/rustfmt/issues/3368)).
 
 ### Added
 

Diff for: Configurations.md

@@ -47,7 +47,7 @@ Where to put a binary operator when a binary expression goes multiline.
 
 - **Default value**: `"Front"`
 - **Possible values**: `"Front"`, `"Back"`
-- **Stable**: No (tracking issue: [#3368](https://github.com/rust-lang/rustfmt/issues/3368))
+- **Stable**: Yes
 
 #### `"Front"` (default):
 

Diff for: src/config/mod.rs

@@ -105,7 +105,7 @@ create_config! {
     space_before_colon: bool, false, false, "Leave a space before the colon";
     space_after_colon: bool, true, false, "Leave a space after the colon";
     spaces_around_ranges: bool, false, false, "Put spaces around the  .. and ..= range operators";
-    binop_separator: SeparatorPlace, SeparatorPlace::Front, false,
+    binop_separator: SeparatorPlace, SeparatorPlace::Front, true,
         "Where to put a binary operator when a binary expression goes multiline";
 
     // Misc.