gh-143201: Remove unused truncated variable from warnings

[AZero13]

It is not used and hasn't been, so we can remove it.

• Issue: warnings writes the full source line instead of the truncated one #143201

[cmaloney]

👍 on just removing.

cc: @vstinner who last worked on this code as part of bpo-19424 (a4c704b). Looks like that was a translation of older code.

[cmaloney]

@AZero13 please do not force push to CPython PR branches: https://devguide.python.org/getting-started/pull-request-lifecycle/#don-t-force-push

[vstinner]

The change is not correct: the function must skip the indentation.

I introduced a bug in a4c704b (in 2013, 13 years ago!): PyFile_WriteObject(sourceline, f_stderr, Py_PRINT_RAW) should be PyFile_WriteObject(truncated, f_stderr, Py_PRINT_RAW) instead.

Example:

import sys
import warnings

# Force the usage of the C implementation
del warnings._showwarnmsg

def current_lineno():
    return sys._getframe(1).f_lineno

def func():
              # looooong indentation
              lineno = current_lineno() + 1
              warnings.warn_explicit('text', Warning,
                                     __file__, lineno,
                                     module_globals=globals())

func()

The output is:

$ ./python x.py 
/home/vstinner/python/main/x.py:13: Warning: text
              warnings.warn_explicit('text', Warning,

The long indentation is not replaced with 2 spaces.

If I comment del warnings._showwarnmsg line, the indentation is replaced with 2 spaces as expected:

$ ./python x.py 
/home/vstinner/python/main/x.py:13: Warning: text
  warnings.warn_explicit('text', Warning,

Well, IMO the first problem here is that the C implementation (C show_warning() function) is not covered by tests.

[cmaloney]

@AZero13 could you update this PR for vstinner's requested changes? He's a lot more authoratative on this code than I am. Sorry for leading you back and forth.