[Pitch] `With` functions in the standard library

Evolution Pitches
140

Maybe just make do an expression?

let widget = do {
  var w = Widget()
  w.identifier = newId
  w.color = .red
  w.width = 100
  w.angle = 2
  w
}

Potentially it could be enriched by an optional init keyword:

let widget = do init Widget {
  self.init()
  identifier = newId
  color = .red
  width = 100
  angle = 2
}

which is in fact an inline version of init extension:

extension Widget {
  init(identifier: Int, color: Color, width: CGFloat, angle: Double) {
    self.init()
    self.identifier = identifier
    self.color = color
    self.width = width
    self.angle = angle
  }
}

But why to stop on inits? We could support inline extensions in general:

do init Type { ... } // equivalent of extension init
do with existingValue { ... } // equivalent of extension func, the return type is inferred by the body
do mutating existingValue { ... } // equivalent of extension mutating func, the return type is inferred by the body
do consuming existingValue { ... } // equivalent of extension consuming func, the return type is inferred by the body
do modifying existingValue { ... } // equivalent of extension func that is forced to return self