Speaking of “#” …what about this?
var x: [3 # Int] = [1, 2, 3]
It even works with the count, element type, or both being inferred:
var x: [# Int] = [1, 2, 3]
var y: [3 #] = [1, 2, 3]
var z: [#] = [1, 2, 3]
The proposal mentions# as an option among the “arbitrary punctuation”, but… it’s the number sign, so it’s not really arbitrary.