Avoid extra comma in vertical single-field struct patterns (#1403)

* Add (failing) test for #1397

* Fix for #1397

Specifically, we end up double-adding a trailing comma for single-member
struct patterns that are arranged vertically. One is added by write_list
(since such structs return true for needs_trailing_separator), and
another is added by the if in the old code.

src/patterns.rs

@@ -13,8 +13,8 @@ use codemap::SpanUtils;
 use config::{IndentStyle, MultilineStyle};
 use rewrite::{Rewrite, RewriteContext};
 use utils::{wrap_str, format_mutability};
-use lists::{format_item_list, itemize_list, ListItem, struct_lit_shape, struct_lit_tactic,
-            shape_for_tactic, struct_lit_formatting, write_list};
+use lists::{DefinitiveListTactic, format_item_list, itemize_list, ListItem, struct_lit_shape,
+            struct_lit_tactic, shape_for_tactic, struct_lit_formatting, write_list};
 use expr::{rewrite_unary_prefix, rewrite_pair};
 use types::{rewrite_path, PathContext};
 use super::Spanned;
@@ -167,7 +167,13 @@ fn rewrite_struct_pat(path: &ast::Path,
             fields_str.push_str("..");
         } else {
             if !fields_str.is_empty() {
-                fields_str.push_str(", ");
+                // there are preceeding struct fields being matched on
+                if fmt.tactic == DefinitiveListTactic::Vertical {
+                    // if the tactic is Vertical, write_list already added a trailing ,
+                    fields_str.push_str(" ");
+                } else {
+                    fields_str.push_str(", ");
+                }
             }
             fields_str.push_str("..");
         }

tests/target/issue-1397.rs

@@ -0,0 +1,23 @@
+pub enum TransactionState {
+    Committed(i64),
+}
+
+pub enum Packet {
+    Transaction { state: TransactionState },
+}
+
+fn baz(p: Packet) {
+    loop {
+        loop {
+            loop {
+                loop {
+                    if let Packet::Transaction {
+                               state: TransactionState::Committed(ts, ..), ..
+                           } = p {
+                        unreachable!()
+                    }
+                }
+            }
+        }
+    }
+}