Can you guess why importing the attached x.py does nothing, without
printing "hello" at all?

The real issue shown in that example is that 'return' and 'yield'
outside a function are ignored instead of giving a SyntaxError if they
are optimized away by 'if 0:'.  

(Hint about x.py: the yield sets the CO_GENERATOR flag before it gets
optimized away.  So clearly, that's a way to comment out a whole module
-- just put "if 0: yield" anywhere at top-level...)

See also issue #1920

This was corrected by Benjamin with r69158.
Now the yield statement is still optimized away, but at least the
CO_GENERATOR flag is set only when compiling a function.

...which does not really solve anything, as "if 0: yield" at
module-level is now just ignored instead of raising a SyntaxError (e.g.
like "if False: yield").

Here is a patch that properly raises SyntaxError when 'return' or  
'yield' statements appear outside a function.

I did not bother to update the (deprecated) compiler package: it seems 
to be completely foreign to this kind of checks...

>>> dis.dis(compiler.compile("return 1", "module.py", "exec"))
  1           0 LOAD_CONST               1 (1)
              3 RETURN_VALUE
              4 LOAD_CONST               0 (None)
              7 RETURN_VALUE

You should remove the error logic compile.c for "return" and "yield" in
function calls.

The problem with this approach is that every SyntaxError generated
during bytecode compilation must be moved to earlier. For example, I can
still use "break" and "continue" outside loops with your patch. The only
way I can think of around this is to compile the block anyway and remove
the extra code later. Maybe this optimization could be moved to the peep
holer?

msg81023 indicates that more work is needed on this.