Don't trim last empty line in docstrings (#9813)

crates/ruff_python_formatter/resources/test/fixtures/ruff/docstring_newlines.py

@@ -0,0 +1,55 @@
+# Tests that Ruff correctly preserves newlines before the closing quote
+
+def test():
+    """a, b
+c"""
+
+
+def test2():
+    """a, b
+    c
+"""
+
+def test3():
+    """a, b
+
+"""
+
+
+def test4():
+    """
+    a, b
+
+"""
+
+
+def test5():
+    """
+    a, b
+a"""
+
+def test6():
+    """
+    a, b
+
+    c
+"""
+
+
+
+def test7():
+    """
+    a, b
+
+
+
+
+"""
+
+def test7():
+    """
+    a, b
+
+
+
+    """

crates/ruff_python_formatter/src/string/docstring.rs

@@ -6,6 +6,7 @@ use std::{borrow::Cow, collections::VecDeque};
 
 use ruff_formatter::printer::SourceMapGeneration;
 use ruff_python_parser::ParseError;
+
 use {once_cell::sync::Lazy, regex::Regex};
 use {
     ruff_formatter::{write, FormatOptions, IndentStyle, LineWidth, Printed},
@@ -114,7 +115,10 @@ pub(crate) fn format(normalized: &NormalizedString, f: &mut PyFormatter) -> Form
     // is_borrowed is unstable :/
     let already_normalized = matches!(docstring, Cow::Borrowed(_));
 
-    let mut lines = docstring.lines().peekable();
+    // Use `split` instead of `lines` to preserve the closing quotes on their own line
+    // if they have no indentation (in which case the last line is `\n` which
+    // `lines` omit for the last element).
+    let mut lines = docstring.split('\n').peekable();
 
     // Start the string
     write!(f, [normalized.prefix, normalized.quotes])?;
@@ -259,7 +263,7 @@ impl<'ast, 'buf, 'fmt, 'src> DocstringLinePrinter<'ast, 'buf, 'fmt, 'src> {
     /// iterator given contains all lines except for the first.
     fn add_iter(
         &mut self,
-        mut lines: std::iter::Peekable<std::str::Lines<'src>>,
+        mut lines: std::iter::Peekable<std::str::Split<'src, char>>,
     ) -> FormatResult<()> {
         while let Some(line) = lines.next() {
             let line = InputDocstringLine {

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

@@ -0,0 +1,125 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/docstring_newlines.py
+---
+## Input
+```python
+# Tests that Ruff correctly preserves newlines before the closing quote
+
+def test():
+    """a, b
+c"""
+
+
+def test2():
+    """a, b
+    c
+"""
+
+def test3():
+    """a, b
+
+"""
+
+
+def test4():
+    """
+    a, b
+
+"""
+
+
+def test5():
+    """
+    a, b
+a"""
+
+def test6():
+    """
+    a, b
+
+    c
+"""
+
+
+
+def test7():
+    """
+    a, b
+
+
+
+
+"""
+
+def test7():
+    """
+    a, b
+
+
+
+    """
+```
+
+## Output
+```python
+# Tests that Ruff correctly preserves newlines before the closing quote
+
+
+def test():
+    """a, b
+    c"""
+
+
+def test2():
+    """a, b
+    c
+    """
+
+
+def test3():
+    """a, b"""
+
+
+def test4():
+    """
+    a, b
+
+    """
+
+
+def test5():
+    """
+        a, b
+    a"""
+
+
+def test6():
+    """
+    a, b
+
+    c
+    """
+
+
+def test7():
+    """
+    a, b
+
+
+
+
+    """
+
+
+def test7():
+    """
+    a, b
+
+
+
+    """
+```
+
+
+