Refactor fix_trailing_whitespace

[tmdhhl]

Closes #294
Closes #223

This PR refactors fix_trailing_whitespace.rs, improves the handling of trailing whitespace, especially for Markdown files where exactly two trailing spaces at the end of a line indicate a line break. Related to #294.

Main changes

• Introduced a Chars type to encapsulate a custom list of characters to trim, resolving an issue where clap cannot correctly parse --chars= \t into Vec<char>.
• Optimized the fix_file function logic:
  • Only preserve and normalize to exactly two trailing spaces on non-empty Markdown lines that already have two trailing spaces; it does not add missing trailing spaces.
  • Normalize lines with more than two trailing spaces to exactly two spaces.
  • Remove all trailing whitespace on blank lines or non-Markdown files, with no extra spaces preserved.
• Added extensive unit tests covering:
  • Markdown lines with excessive or insufficient trailing spaces.
  • Forced Markdown mode.
  • Custom trim character sets.
  • Different line ending formats (including CRLF).
  • Files without trailing newline.
  • Wildcard Markdown extensions.
• Only write back files if actual content changes occur, avoiding unnecessary writes.

About Chars type encapsulation and clap parsing issue

When using clap to parse command-line arguments, directly parsing a parameter like --chars=" \t" into a Vec<char> runs into issues because:

• clap splits arguments by whitespace by default, causing strings containing spaces and tabs to be misparsed or treated as a single element.
• For example, passing --chars=" \t" may not correctly convert into a character vector containing space and tab characters.

To solve this, we implemented a custom Chars type with the FromStr trait:

• Chars wraps a Vec<char> internally and manually converts the input string into a vector of characters.
• This approach allows correct parsing regardless of spaces, tabs, or other special characters included in the input.

This workaround bypasses clap's default parsing limitations and enables flexible handling of user-defined trim character sets.

Trailing two-space preservation rules and reference

• Only lines that already have exactly two trailing spaces will preserve these spaces as Markdown line breaks.
• Markdown lines without two trailing spaces will have their trailing whitespace removed normally, without automatically adding spaces.
• This rule prevents unnecessary modifications and aligns with Markdown syntax standards.

This behavior is consistent with the original implementation in the open-source project pre-commit. For example:

@pytest.mark.parametrize(
    ('input_s', 'expected'),
    (
        ('foo \nbar \n', 'foo\nbar\n'),
        ('bar\t\nbaz\t\n', 'bar\nbaz\n'),
    ),
)
def test_fixes_trailing_whitespace(input_s, expected, tmpdir):
    path = tmpdir.join('file.md')
    path.write(input_s)
    assert main((str(path),)) == 1
    assert path.read() == expected

[Copilot]

Pull Request Overview

This PR refactors the fix_trailing_whitespace.rs module to improve handling of trailing whitespace, particularly for Markdown files where two trailing spaces indicate a line break. The refactor resolves a clap parsing issue with custom character sets and optimizes the file processing logic.

Key changes:

• Introduced a custom Chars type to work around clap's inability to parse --chars=" \t" correctly
• Rewrote the file processing logic to use async I/O with temporary files and only write back when changes occur
• Added comprehensive test coverage for various scenarios including CRLF handling, Unicode characters, and edge cases