Support for pure setters

[za_creature (Radu Dan)]

I ended up here after following Set-only subscripts and not being convinced by Dave's argument that x[b] = y implies that x[b].mutatingMethod() may be called. I've had similar difficulties in grasping that a mutating method can reassign self which I, ignoring the underlying practicalities, still consider a weird and very narrow use case.

@Chris_Lattner

Swift fundamentally assumes that something mutable can also be read.

and

@Joe_Groff

var tmp = foo // invoke getter
tmp.x = value
foo = tmp // invoke setter

But how is this still true with non-copyable types? Yes, foo can be read but it can't be copied without a consume operator, in which case it's no longer possible to reassign it since it's out of scope. Doing otherwise is essentially the same as a set-only property, no?