Added where_trailing_comma option.

The default is 'false', since a lot of the time there's only a single
predicate.

src/config.rs

@@ -316,6 +316,7 @@ fn default() -> Config {
     where_layout: ListTactic, ListTactic::Vertical, "Element layout inside a where clause";
     where_pred_indent: BlockIndentStyle, BlockIndentStyle::Visual,
         "Indentation style of a where predicate";
+    where_trailing_comma: bool, false, "Put a trailing comma on where clauses";
     generics_indent: BlockIndentStyle, BlockIndentStyle::Visual, "Indentation of generics";
     struct_trailing_comma: SeparatorTactic, SeparatorTactic::Vertical,
         "If there is a trailing comma on structs";

src/items.rs

@@ -1530,7 +1530,7 @@ fn rewrite_where_clause(context: &RewriteContext,
     let fmt = ListFormatting {
         tactic: tactic,
         separator: ",",
-        trailing_separator: SeparatorTactic::Never,
+        trailing_separator: SeparatorTactic::from_bool(context.config.where_trailing_comma),
         indent: offset,
         width: budget,
         ends_with_newline: true,

tests/config/small_tabs.toml

@@ -12,6 +12,7 @@ where_density = "Tall"
 where_indent = "Tabbed"
 where_layout = "Vertical"
 where_pred_indent = "Visual"
+where_trailing_comma = false
 generics_indent = "Visual"
 struct_trailing_comma = "Vertical"
 struct_lit_trailing_comma = "Vertical"