Trailing commas for match block arms

Attempt to implement an option for trailing commas for block based match arms (issue
173). Put in place test files to verify this behaviour.

src/config.rs

@@ -308,4 +308,6 @@ create_config! {
     hard_tabs: bool, false, "Use tab characters for indentation, spaces for alignment";
     wrap_comments: bool, false, "Break comments to fit on the line";
     wrap_match_arms: bool, true, "Wrap multiline match arms in blocks";
+    match_block_trailing_comma: bool, false,
+        "Put a trailing comma after a block based match arm (non-block arms are not affected)";
 }

src/expr.rs

@@ -823,7 +823,7 @@ fn rewrite_match(context: &RewriteContext,
             // We couldn't format the arm, just reproduce the source.
             let snippet = context.snippet(mk_sp(arm_start_pos(arm), arm_end_pos(arm)));
             result.push_str(&snippet);
-            result.push_str(arm_comma(&arm.body));
+            result.push_str(arm_comma(context, &arm.body));
         }
     }
     // BytePos(1) = closing match brace.
@@ -854,8 +854,10 @@ fn arm_end_pos(arm: &ast::Arm) -> BytePos {
     arm.body.span.hi
 }
 
-fn arm_comma(body: &ast::Expr) -> &'static str {
+fn arm_comma(context: &RewriteContext, body: &ast::Expr) -> &'static str {
-    if let ast::ExprBlock(ref block) = body.node {
+    if context.config.match_block_trailing_comma {
+        ","
+    } else if let ast::ExprBlock(ref block) = body.node {
         if let ast::DefaultBlock = block.rules {
             ""
         } else {
@@ -950,7 +952,7 @@ impl Rewrite for ast::Arm {
             ref x => x,
         };
 
-        let comma = arm_comma(body);
+        let comma = arm_comma(context, body);
 
         // Let's try and get the arm body on the same line as the condition.
         // 4 = ` => `.len()
@@ -961,7 +963,8 @@ impl Rewrite for ast::Arm {
 
             match rewrite {
                 Some(ref body_str) if !body_str.contains('\n') || !context.config.wrap_match_arms ||
-                                      comma.is_empty() => {
+                                      comma.is_empty() ||
+                                      context.config.match_block_trailing_comma => {
                     return Some(format!("{}{} => {}{}",
                                         attr_str.trim_left(),
                                         pats_str,

tests/source/match-block-trailing-comma.rs

@@ -0,0 +1,13 @@
+// rustfmt-match_block_trailing_comma: true
+// Match expressions, no unwrapping of block arms or wrapping of multiline
+// expressions.
+
+fn foo() {
+    match x {
+        a => {
+            "line1";
+            "line2"
+        }
+        b => "bar",
+    }
+}

tests/target/match-block-trailing-comma.rs

@@ -0,0 +1,13 @@
+// rustfmt-match_block_trailing_comma: true
+// Match expressions, no unwrapping of block arms or wrapping of multiline
+// expressions.
+
+fn foo() {
+    match x {
+        a => {
+            "line1";
+            "line2"
+        },
+        b => "bar",
+    }
+}