feat: add Preserve option to match_arm_wrapping (rust-lang#4896)

src/config/options.rs

@@ -448,8 +448,11 @@ pub enum MatchArmLeadingPipe {
 pub enum MatchArmWrapping {
     /// Follow the Style Guide Prescription
     Default,
-    /// Don't block wrap when the first line can't fit
+    /// Same as Default, except don't block wrap match arms when the opening line of its body
+    /// can't fit on the same line as the `=>`.
     NoBlockFirstLine,
-    /// Always wrap match arms
+    /// Always block wrap match arms
     Always,
+    /// Preserve the block wrapping on match arms
+    Preserve,
 }

src/matches.rs

@@ -326,6 +326,10 @@ fn flatten_arm_body<'a>(
             if let ast::ExprKind::Block(..) = expr.kind {
                 flatten_arm_body(context, expr, None)
             } else {
+                if context.config.match_arm_wrapping() == MatchArmWrapping::Preserve {
+                    return (false, body);
+                }
+
                 let cond_becomes_muti_line = opt_shape
                     .and_then(|shape| rewrite_cond(context, expr, shape))
                     .map_or(false, |cond| cond.contains('\n'));
@@ -416,7 +420,8 @@ fn rewrite_match_body(
 
         let indent_str = shape.indent.to_string_with_newline(context.config);
         let (body_prefix, body_suffix) = match context.config.match_arm_wrapping() {
-            MatchArmWrapping::Default | MatchArmWrapping::Always if !context.inside_macro() => {
+            MatchArmWrapping::NoBlockFirstLine => ("", String::from(",")),
+            _ if !context.inside_macro() => {
                 let comma = if context.config.match_block_trailing_comma() {
                     ","
                 } else {

tests/source/configs/match_arm_wrapping/always.rs

@@ -3,16 +3,16 @@
 
 fn main() {
     match lorem {
-        1 => foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(x),
-        2 => {
+        1000 => foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(x),
+        2000 => {
             println!("{}", sit)
         }
-        3 => panic!(),
-        4 => (),
-        y => {
+        3000 => panic!(),
+        4000 => (),
+        ipsum => {
             // Some comment
-            let ipsum = y - 1;
-            println!("{}", ipsum);
+            let dolor = ipsum % 2;
+            println!("{}", dolor);
         }
     }
 }

tests/source/configs/match_arm_wrapping/preserve.rs

@@ -0,0 +1,20 @@
+// rustfmt-match_arm_wrapping: Preserve
+// Wrap match-arms
+
+fn main() {
+    match lorem {
+        1000 => foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(x),
+        2000 => {
+            println!("{}", sit)
+        }
+        3000 => panic!(),
+        4000 => {
+            ()
+        }
+        ipsum => {
+            // Some comment
+            let dolor = ipsum % 2;
+            println!("{}", dolor);
+        }
+    }
+}

tests/target/configs/match_arm_wrapping/always.rs

@@ -3,22 +3,22 @@
 
 fn main() {
     match lorem {
-        1 => {
+        1000 => {
             foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(x)
         }
-        2 => {
+        2000 => {
             println!("{}", sit)
         }
-        3 => {
+        3000 => {
             panic!()
         }
-        4 => {
+        4000 => {
             ()
         }
-        y => {
+        ipsum => {
             // Some comment
-            let ipsum = y - 1;
-            println!("{}", ipsum);
+            let dolor = ipsum % 2;
+            println!("{}", dolor);
         }
     }
 }

tests/target/configs/match_arm_wrapping/preserve.rs

@@ -0,0 +1,22 @@
+// rustfmt-match_arm_wrapping: Preserve
+// Wrap match-arms
+
+fn main() {
+    match lorem {
+        1000 => {
+            foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(x)
+        }
+        2000 => {
+            println!("{}", sit)
+        }
+        3000 => panic!(),
+        4000 => {
+            ()
+        }
+        ipsum => {
+            // Some comment
+            let dolor = ipsum % 2;
+            println!("{}", dolor);
+        }
+    }
+}