Type specification in `for`

[Erik Demaine]

I think we should allow specifying the type of declarations within for loops. Examples:

for x: T of y
for x: T in y
for each x: T of y
for let [x, y]: [A, B] of y

Some of these currently have a different meaning (e.g. for {x: T} of y). An alternative that would preserve this implicit object destructuring meaning would be able to allow type specifiers only when there's an explicit const/let/var:

for const x: T of y