Need better diagnostics for multi-value assignment with missing parens

Given var a = 1, b = 2, in some other languages you can swap them with

a, b = b, a

In Swift, parens are required:

(a, b) = (b, a)

If you forget the parens, though, the errors aren't very helpful:

Consecutive statements on a line must be separated by ';'
Expected expression

a, b = b, a
 ~
 ;

It'd be nice to recognize this and suggest adding parentheses.