Anywhere where we require commas to separate items in some kind of list, we should accept (but not require) a trailing comma after the last element in the list.
[ ] List literals
[ ] Tuple literals
[ ] Destructuring left-hand-side patterns
[ ] Enum members
[ ] List of variables in a let statement
[ ] Function and method definition formal argument lists
[ ] Function and method call actual argument lists
Anywhere where we require commas to separate items in some kind of list, we should accept (but not require) a trailing comma after the last element in the list.