[swift-evolution] [Proposal] Make `didSet` also available for `let` property, or something new such as `didInit`

[shengjia wang]

Since swift v1.2, we can initialize `let` property in `init()` instead of
being forced to give a value when declare it. This is great !

But every time I run into the case such as the example below:

let view: UIView {
  didSet {
    /**
     * This time, `view` did set ( a.k.a initialized in case of `let`
property), so I want to bind some other actions just after, such as
`setBackgroundColor`. But actually I can't, compiler will complain that
`let` declaration can not be observing properties. So I have to either move
all these "actions" to `init()` or change `view` to a `var` property which
is not necessary at all.
     */
  }
}

Actually in swift, I think it's quite commun issue that people run into a
large `init()` method. This approach could make it way better in most cases.

So, I'm wondering why not make `didSet` also available for `let` property,
or maybe even better to add new keyword such as "didInit" which only get
called for first set.

- Victor Wang
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