Preserve single \ in raw string literals

[MichaReiser]

# Input
pattern_esc = r"'\', '\\'"

# Ruff
pattern_esc = r"'', '\\'"

Expected: Same as input

Probably relevant code:

ruff/crates/ruff_python_formatter/src/expression/string.rs

Lines 413 to 481 in 0301889

	fn normalize_string(input: &str, quotes: StringQuotes) -> (Cow<str>, ContainsNewlines) {
	// The normalized string if `input` is not yet normalized.
	// `output` must remain empty if `input` is already normalized.
	let mut output = String::new();
	// Tracks the last index of `input` that has been written to `output`.
	// If `last_index` is `0` at the end, then the input is already normalized and can be returned as is.
	let mut last_index = 0;
	let mut newlines = ContainsNewlines::No;
	let style = quotes.style;
	let preferred_quote = style.as_char();
	let opposite_quote = style.invert().as_char();
	let mut chars = input.char_indices();
	while let Some((index, c)) = chars.next() {
	if c == '\r' {
	output.push_str(&input[last_index..index]);
	// Skip over the '\r' character, keep the `\n`
	if input.as_bytes().get(index + 1).copied() == Some(b'\n') {
	chars.next();
	}
	// Replace the `\r` with a `\n`
	else {
	output.push('\n');
	}
	last_index = index + '\r'.len_utf8();
	newlines = ContainsNewlines::Yes;
	} else if c == '\n' {
	newlines = ContainsNewlines::Yes;
	} else if !quotes.triple {
	if c == '\\' {
	if let Some(next) = input.as_bytes().get(index + 1).copied().map(char::from) {
	#[allow(clippy::if_same_then_else)]
	if next == opposite_quote {
	// Remove the escape by ending before the backslash and starting again with the quote
	chars.next();
	output.push_str(&input[last_index..index]);
	last_index = index + '\\'.len_utf8();
	} else if next == preferred_quote {
	// Quote is already escaped, skip over it.
	chars.next();
	} else if next == '\\' {
	// Skip over escaped backslashes
	chars.next();
	}
	}
	} else if c == preferred_quote {
	// Escape the quote
	output.push_str(&input[last_index..index]);
	output.push('\\');
	output.push(c);
	last_index = index + preferred_quote.len_utf8();
	}
	}
	}
	let normalized = if last_index == 0 {
	Cow::Borrowed(input)
	} else {
	output.push_str(&input[last_index..]);
	Cow::Owned(output)
	};
	(normalized, newlines)
	}

Playground

[harupy]

@MichaReiser Can I work on this?

[MichaReiser]

@MichaReiser Can I work on this?

Sure!