Is pattern matching to variables only allowed during variable definitions, and not during var assignments?

E.g., this works:
val t = (1, 2)
var (a, b) = t
a = 1, b = 2

But,
var x = 10
var y = 20
(x, y) = t
Illegal start of definitio

Seems to be that pattern based assignment would also be quite useful???

Further comments: (from Burak on the mailing list)
This particular change would probably not require any changes to the matcher, rather to the frontend (parsing - desugaring - typechecking).

It would be translated as something like

var x = 10
var y = 20
match {
(x1, y1) => x = x1; y = y1;
}

The pattern matcher would never need to be aware of something being a mutable variable.

Burak