Comment by TeMPOraL

Yes, I want to overlay the code directly into the parent method, preferably with appropriate syntax highlighting and whatever other goodies the IDE/editor provides normally. It would be read-only to indicate that it's just a transient overlay, and not an actual code change.

So, if I have a code like:

  auto Foo(Bar b) {
    return b.knob();
  }

  auto Frob(Frobbable f) {
    auto q = Foo(f.thing());
    return q.quux(f.otherthing());
  }

  auto DoSth(Frobbable frobbie) {
    auto a = Frob(frobbie);
    return a.magic();
  }

Then I want to mark the last function, and automatically turn it into:

  auto DoSth(Frobbable frobbie) {
    auto foo_1 = frobbie.thing();
    auto q_1 = foo_1.knob();
    auto frob_1 = frobbie.otherthing();
    auto a = q_1.quux(frob_1);
    return a.magic(); 
  }

Or something equivalent, possibly with highlights/synthetic comments telling me which bits of code came from where. I want to be able to keep inlining function calls like this, until I hit a boundary layer like the standard library, or a third-party library. I might want to expand past that, but I don't think I'd do that much. I'd also like to be able to re-fold code I'm not interested in, to reduce noise.

What such tool would do is automating the process I'm currently doing manually - jumping around the tiny functions calling other tiny functions, in order to reassemble the actual sequence of lower-level operations.

I don't want this to be a tooltip, because I want to keep expanding past the first level, and have the overlay stay in place until I'm done with it.

EDIT: languages in the Lisp family - like Common Lisp or Emacs Lisp - feature a tool called "macroexpander". Emacs/SLIME wraps it into an interactive "macrostepper" that behaves pretty much exactly like the tool I described in this discussion thread.

EDIT2: See the excellent demo upthread by ' emilprogviz - https://news.ycombinator.com/item?id=27306118. That's the kind of tool I had in mind.