message: step.rs: improve comment for clarity
diff: @@ -200,10 +200,10 @@ impl<'a, 'tcx, M: Machine<'tcx>> EvalContext<'a, 'tcx, M> {
  }
  }
  -// WARNING: make sure that any methods implemented on this type don't ever access ecx.stack
  -// this includes any method that might access the stack
  -// basically don't call anything other than `load_mir`, `alloc_ptr`, `push_stack_frame`
  -// The reason for this is, that `push_stack_frame` modifies the stack out of obvious reasons
  +// WARNING: This code pushes new stack frames. Make sure that any methods implemented on this
  +// type don't ever access ecx.stack[ecx.cur_frame()], as that will change. This includes, e.g.,
  +// using the current stack frame's substitution.
  +// Basically don't call anything other than `load_mir`, `alloc_ptr`, `push_stack_frame`.
  struct ConstantExtractor<'a, 'b: 'a, 'tcx: 'b, M: Machine<'tcx> + 'a> {
  span: Span,
  ecx: &'a mut EvalContext<'b, 'tcx, M>,
  