Normalize '\r' in string literals to '\n'

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## Summary

This PR normalizes line endings inside of strings to `\n` as required by the printer.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

I added a new test using `\r\n` and ran the ecosystem check. There are no remaining end of line panics. 


https://gist.github.com/MichaReiser/8f36b1391ca7b48475b3a4f592d74ff4

<!-- How was it tested? -->

crates/ruff_python_formatter/resources/test/fixtures/ruff/carriage_return/.editorconfig

@@ -0,0 +1,2 @@
+[*.py]
+end_of_line = crlf

crates/ruff_python_formatter/resources/test/fixtures/ruff/carriage_return/.gitattributes

@@ -0,0 +1 @@
+* text=auto eol=crlf

crates/ruff_python_formatter/resources/test/fixtures/ruff/carriage_return/string.py

@@ -0,0 +1,6 @@
+'This string will not include \
+backslashes or newline characters.'
+
+"""Multiline
+String \"
+"""

crates/ruff_python_formatter/src/expression/string.rs

@@ -199,12 +199,11 @@ impl Format<PyFormatContext<'_>> for FormatStringPart {
         let raw_content_range = relative_raw_content_range + self.part_range.start();
 
         let raw_content = &string_content[relative_raw_content_range];
-        let (preferred_quotes, contains_newlines) =
-            preferred_quotes(raw_content, quotes, f.options().quote_style());
+        let preferred_quotes = preferred_quotes(raw_content, quotes, f.options().quote_style());
 
         write!(f, [prefix, preferred_quotes])?;
 
-        let normalized = normalize_quotes(raw_content, preferred_quotes);
+        let (normalized, contains_newlines) = normalize_string(raw_content, preferred_quotes);
 
         match normalized {
             Cow::Borrowed(_) => {
@@ -294,9 +293,7 @@ fn preferred_quotes(
     input: &str,
     quotes: StringQuotes,
     configured_style: QuoteStyle,
-) -> (StringQuotes, ContainsNewlines) {
-    let mut contains_newlines = ContainsNewlines::No;
-
+) -> StringQuotes {
     let preferred_style = if quotes.triple {
         // True if the string contains a triple quote sequence of the configured quote style.
         let mut uses_triple_quotes = false;
@@ -305,7 +302,6 @@ fn preferred_quotes(
         while let Some(c) = chars.next() {
             let configured_quote_char = configured_style.as_char();
             match c {
-                '\n' | '\r' => contains_newlines = ContainsNewlines::Yes,
                 '\\' => {
                     if matches!(chars.peek(), Some('"' | '\\')) {
                         chars.next();
@@ -358,10 +354,6 @@ fn preferred_quotes(
                     double_quotes += 1;
                 }
 
-                '\n' | '\r' => {
-                    contains_newlines = ContainsNewlines::Yes;
-                }
-
                 _ => continue,
             }
         }
@@ -384,13 +376,10 @@ fn preferred_quotes(
         }
     };
 
-    (
-        StringQuotes {
-            triple: quotes.triple,
-            style: preferred_style,
-        },
-        contains_newlines,
-    )
+    StringQuotes {
+        triple: quotes.triple,
+        style: preferred_style,
+    }
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -435,30 +424,56 @@ impl Format<PyFormatContext<'_>> for StringQuotes {
 
 /// Adds the necessary quote escapes and removes unnecessary escape sequences when quoting `input`
 /// with the provided `style`.
-fn normalize_quotes(input: &str, quotes: StringQuotes) -> Cow<str> {
-    if quotes.triple {
-        Cow::Borrowed(input)
-    } else {
-        // 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;
+///
+/// Returns the normalized string and whether it contains new lines.
+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 style = quotes.style;
-        let preferred_quote = style.as_char();
-        let opposite_quote = style.invert().as_char();
+    let mut newlines = ContainsNewlines::No;
 
-        let mut chars = input.char_indices();
+    let style = quotes.style;
+    let preferred_quote = style.as_char();
+    let opposite_quote = style.invert().as_char();
 
-        while let Some((index, c)) = chars.next() {
+    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)) = chars.next() {
+                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 {
@@ -469,12 +484,14 @@ fn normalize_quotes(input: &str, quotes: StringQuotes) -> Cow<str> {
                 last_index = index + preferred_quote.len_utf8();
             }
         }
-
-        if last_index == 0 {
-            Cow::Borrowed(input)
-        } else {
-            output.push_str(&input[last_index..]);
-            Cow::Owned(output)
-        }
     }
+
+    let normalized = if last_index == 0 {
+        Cow::Borrowed(input)
+    } else {
+        output.push_str(&input[last_index..]);
+        Cow::Owned(output)
+    };
+
+    (normalized, newlines)
 }

crates/ruff_python_formatter/tests/snapshots/format@carriage_return__string.py.snap

@@ -0,0 +1,26 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/carriage_return/string.py
+---
+## Input
+```py
+'This string will not include \
+backslashes or newline characters.'
+
+"""Multiline
+String \"
+"""
+```
+
+## Output
+```py
+"This string will not include \
+backslashes or newline characters."
+
+"""Multiline
+String \"
+"""
+```
+
+
+