Let's fix `if let` syntax

Evolution Pitches
49

I don't love the current if let syntax, but this feels like it is fixing a very specific special case, where I'd rather see a more general addition to the language which would let us fix this ourselves

for example; If a function was allowed to access it's caller, then we could write

func ifPresent(_ block: (Any)->Void) {
    if let nonNil = #caller {
        block(nonNil)
    }
}

which could be used as

foo?.ifPresent() {
  $0.rect = .zero
}

for bonus points, we could have a way of defining default variable names when calling functions. Throw in access to the caller's string name, and you have

func ifPresent(_ block: (Any)->Void) {
    if let nonNil = #caller {
        block(nonNil @VariableDefaultName(#caller_string_name))
    }
}

which could be used as

foo?.ifPresent() {
  foo.rect = .zero
}

or

foo?.ifPresent() {
  preferredName in
  preferredName.rect = .zero
}

there are a bunch of other places where @VariableDefaultName would be great.

for example

networkRequest.run {
   /// @VariableDefaultName has already implicitly written
   /// request, data, error in 
  if error.isNil {
   data.process()
 }
}

note: we wouldn't need access to #caller if there was a way to write

extension Any {
  func someFunc() {
   //I can now access self, rather than needing #caller
  }
}

but I don't think that is currently possible.

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