It would be useful to have a way to specify that a Signal is driven by a specific Elaboratable.

Consider this example, which presupposes a driven_by attribute on Signal().

class MyCounter(Elaboratable):
  def __init__(self):
    self.advance = Signal()
    self.count = Signal(16, driven_by=self)

  def elaborate(self, platform):
    m = Module()
    with m.If(self.advance):
      m.d.sync += count.eq(count + 1)

Here, specifying driven_by=self on the count signal performs two functions:

1. It acts as documentation of the MyCounter interface, making it clear that count is an output. Since advance is not specified to be driven by this Elaboratable, it can be assumed to be an input.
2. It provides a mechanism to find some class of bugs, such as forgetting to drive the signal from this module or also driving it from a second module.

It is probably only worth adding a driven_by type property if it is programatically checked.

Not every Signal would need to have a specified driver. Probably the vast majority of uses would be as above: documenting the outputs of an Elaboratable by specifying driven_by=self on those Signals.