Do not break empty blocks in match arm patterns

Closes rust-lang#4125

Author: ayazhafiz

rustfmt-core/rustfmt-lib/src/matches.rs

@@ -248,8 +248,17 @@ fn rewrite_match_arm(
     };
 
     // Patterns
-    // 5 = ` => {`
-    let pat_shape = shape.sub_width(5)?.offset_left(pipe_offset)?;
+    let is_empty_block = if let ast::ExprKind::Block(ref block, _) = arm.body.kind {
+        is_empty_block(context, block, Some(&arm.body.attrs))
+    } else {
+        false
+    };
+    let sub_width = if is_empty_block {
+        " => {}".len()
+    } else {
+        " => {".len()
+    };
+    let pat_shape = shape.sub_width(sub_width)?.offset_left(pipe_offset)?;
 
     let pats_str = arm.pat.rewrite(context, pat_shape)?;
 
@@ -479,7 +488,7 @@ fn rewrite_match_body(
 
         match rewrite {
             Some(ref body_str)
-                if is_block
+                if (is_block && (!is_empty_block || !body_str.contains('\n')))
                     || (!body_str.contains('\n')
                         && unicode_str_width(body_str) <= body_shape.width) =>
             {

rustfmt-core/rustfmt-lib/tests/source/issue-4125.rs

@@ -0,0 +1,19 @@
+// rustfmt-max_width: 60
+
+fn main() {
+    match (pat, num) {
+        (PatternOne, 1) | (RoomForBlockOnSameLine, 10) => {}
+
+        (PatternOne, 1) | (PatternStretchedToBounds, 2) => {}
+
+        UnsplitableVeryLongArmPatternWithRoomForBlock0 => {}
+
+        UnsplitableVeryLongArmPatternWithBraceAtEndOfLn => {}
+
+        UnsplitableVeryLongArmPatternWithArrowAtColnWidth => {}
+
+        UnsplitableVeryLongArmPatternWithArrowPastColnWidth => {}
+
+        UnsplitableVeryLongArmPatternGoingBeyondMaxColumnWidth => {}
+    }
+}

rustfmt-core/rustfmt-lib/tests/target/issue-4125.rs

@@ -0,0 +1,24 @@
+// rustfmt-max_width: 60
+
+fn main() {
+    match (pat, num) {
+        (PatternOne, 1) | (RoomForBlockOnSameLine, 10) => {}
+
+        (PatternOne, 1)
+        | (PatternStretchedToBounds, 2) => {}
+
+        UnsplitableVeryLongArmPatternWithRoomForBlock0 => {}
+
+        UnsplitableVeryLongArmPatternWithBraceAtEndOfLn =>
+            {}
+
+        UnsplitableVeryLongArmPatternWithArrowAtColnWidth =>
+            {}
+
+        UnsplitableVeryLongArmPatternWithArrowPastColnWidth =>
+            {}
+
+        UnsplitableVeryLongArmPatternGoingBeyondMaxColumnWidth =>
+            {}
+    }
+}

rustfmt-core/rustfmt-lib/tests/target/match.rs

@@ -124,8 +124,8 @@ fn issue339() {
         // collapsing here is safe
         ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff => {}
         // collapsing here exceeds line length
-        ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffg => {
-        }
+        ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffg =>
+            {}
         h => { // comment above block
         }
         i => {} // comment below block