printf: fix formatting for zero value when precision is zero

[nickorlow]

Fixes issue #7509

According to the C/C++ reference, "if both the value and precision are 0, the conversion results in no characters. "

This PR also includes code to handle the following exceptions:

• If width isn't zero, the result should still be padded, but empty
• If the alternate octal output is selected, a 0 should be outputted
• The positive sign should be printed if specified

[hz2]

For clippy related issues, it may be helpful to run locally:

cargo clippy --all-targets --all-features -- -D warnings

[nickorlow]

Ah, forgot to check lint. Thank you.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/printf/printf. tests/printf/printf is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[nickorlow]

Will look into failing tests + other lints I didn't pick up earlier

[nickorlow]

Reran lint and addressed issues there. The failing GNU test case was because I didn't switch the precision parser to return a None for a negative value instead of Some(0usize).

Should be good to run workflow again.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[nickorlow]

Updated the one test case. Not sure there's much I can do about the other one that's complaining about linting issues in the zerocopy crate.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/du/files0-from is no longer failing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[RenjiSann]

Can you squash your commits in a single one for better readability in the main branch ?
Thank you !

[nickorlow]

@RenjiSann Done! Sorry it took so long to get back on that.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[nickorlow]

Just rebased on top of main.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[hz2]

nit: noticed #7510 was closed, do we think we can remove this ignore now?

[sylvestre]

Using width = 0 here seems unnecessary since you're not actually using width
padding when precision is None. Could be simplified to just s = format!("{prefix}{s}");
no ?

[sylvestre]

what about something like ?

  s = match (self.precision, x) {
      (Some(0), 0) => match self.variant {
          UnsignedIntVariant::Octal(Prefix::Yes) => String::from("0"),
          _ => String::new(),
      },
      (Some(p), _) => format!("{prefix}{s:0>p$}"),
      (None, _) => format!("{prefix}{s}"),
  };

[sylvestre]

Same

  let s = match (self.precision, abs) {
      (Some(0), 0) => String::new(),
      (Some(p), _) if p > 0 => format!("{abs:0>p$}"),
      _ => abs.to_string(),
  };