[ruff] Preserve relative whitespace in multi-line expressions (RUF033) (#19647)

## Summary

Fixes #19581

I decided to add in a `indent_first_line` function into
[`textwrap.rs`](https://github.com/astral-sh/ruff/blob/main/crates/ruff_python_trivia/src/textwrap.rs),
as it solely focuses on text manipulation utilities. It follows the same
design as `indent()`, and there may be situations in the future where it
can be reused as well.

---------

Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>
Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/resources/test/fixtures/ruff/RUF033.py

@@ -124,3 +124,19 @@ def fun_with_python_syntax():
             ...
 
     return Foo
+
+
+@dataclass
+class C:
+    def __post_init__(self, x: tuple[int, ...] = (
+        1,
+        2,
+    )) -> None:
+        self.x = x
+
+
+@dataclass
+class D:
+    def __post_init__(self, x: int = """
+    """) -> None:
+        self.x = x

crates/ruff_linter/src/rules/ruff/rules/post_init_default.rs

@@ -186,7 +186,7 @@ fn use_initvar(
 
     let indentation = indentation_at_offset(post_init_def.start(), checker.source())
         .context("Failed to calculate leading indentation of `__post_init__` method")?;
-    let content = textwrap::indent(&content, indentation);
+    let content = textwrap::indent_first_line(&content, indentation);
 
     let initvar_edit = Edit::insertion(
         content.into_owned(),

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF033_RUF033.py.snap

@@ -455,3 +455,57 @@ help: Use `dataclasses.InitVar` instead
 122 123 |             ,
 123 124 |         ) -> None:
 124 125 |             ...
+
+RUF033 [*] `__post_init__` method with argument defaults
+   --> RUF033.py:131:50
+    |
+129 |   @dataclass
+130 |   class C:
+131 |       def __post_init__(self, x: tuple[int, ...] = (
+    |  __________________________________________________^
+132 | |         1,
+133 | |         2,
+134 | |     )) -> None:
+    | |_____^
+135 |           self.x = x
+    |
+help: Use `dataclasses.InitVar` instead
+
+ℹ Unsafe fix
+128 128 | 
+129 129 | @dataclass
+130 130 | class C:
+131     |-    def __post_init__(self, x: tuple[int, ...] = (
+    131 |+    x: InitVar[tuple[int, ...]] = (
+132 132 |         1,
+133 133 |         2,
+134     |-    )) -> None:
+    134 |+    )
+    135 |+    def __post_init__(self, x: tuple[int, ...]) -> None:
+135 136 |         self.x = x
+136 137 | 
+137 138 | 
+
+RUF033 [*] `__post_init__` method with argument defaults
+   --> RUF033.py:140:38
+    |
+138 |   @dataclass
+139 |   class D:
+140 |       def __post_init__(self, x: int = """
+    |  ______________________________________^
+141 | |     """) -> None:
+    | |_______^
+142 |           self.x = x
+    |
+help: Use `dataclasses.InitVar` instead
+
+ℹ Unsafe fix
+137 137 | 
+138 138 | @dataclass
+139 139 | class D:
+140     |-    def __post_init__(self, x: int = """
+141     |-    """) -> None:
+    140 |+    x: InitVar[int] = """
+    141 |+    """
+    142 |+    def __post_init__(self, x: int) -> None:
+142 143 |         self.x = x

crates/ruff_python_trivia/src/textwrap.rs

@@ -71,6 +71,66 @@ pub fn indent<'a>(text: &'a str, prefix: &str) -> Cow<'a, str> {
     Cow::Owned(result)
 }
 
+/// Indent only the first line by the given prefix.
+///
+/// This function is useful when you want to indent the first line of a multi-line
+/// expression while preserving the relative indentation of subsequent lines.
+///
+/// # Examples
+///
+/// ```
+/// # use ruff_python_trivia::textwrap::indent_first_line;
+///
+/// assert_eq!(indent_first_line("First line.\nSecond line.\n", "  "),
+///            "  First line.\nSecond line.\n");
+/// ```
+///
+/// When indenting, trailing whitespace is stripped from the prefix.
+/// This means that empty lines remain empty afterwards:
+///
+/// ```
+/// # use ruff_python_trivia::textwrap::indent_first_line;
+///
+/// assert_eq!(indent_first_line("\n\n\nSecond line.\n", "  "),
+///            "\n\n\nSecond line.\n");
+/// ```
+///
+/// Leading and trailing whitespace coming from the text itself is
+/// kept unchanged:
+///
+/// ```
+/// # use ruff_python_trivia::textwrap::indent_first_line;
+///
+/// assert_eq!(indent_first_line(" \t  Foo   ", "->"), "-> \t  Foo   ");
+/// ```
+pub fn indent_first_line<'a>(text: &'a str, prefix: &str) -> Cow<'a, str> {
+    if prefix.is_empty() {
+        return Cow::Borrowed(text);
+    }
+
+    let mut lines = text.universal_newlines();
+    let Some(first_line) = lines.next() else {
+        return Cow::Borrowed(text);
+    };
+
+    let mut result = String::with_capacity(text.len() + prefix.len());
+
+    // Indent only the first line
+    if first_line.trim_whitespace().is_empty() {
+        result.push_str(prefix.trim_whitespace_end());
+    } else {
+        result.push_str(prefix);
+    }
+    result.push_str(first_line.as_full_str());
+
+    // Add remaining lines without indentation
+    for line in lines {
+        result.push_str(line.as_full_str());
+    }
+
+    Cow::Owned(result)
+}
+
 /// Removes common leading whitespace from each line.
 ///
 /// This function will look at each non-empty line and determine the
@@ -409,6 +469,61 @@ mod tests {
         assert_eq!(dedent(text), text);
     }
 
+    #[test]
+    fn indent_first_line_empty() {
+        assert_eq!(indent_first_line("\n", "  "), "\n");
+    }
+
+    #[test]
+    #[rustfmt::skip]
+    fn indent_first_line_nonempty() {
+        let text = [
+            "  foo\n",
+            "bar\n",
+            "  baz\n",
+        ].join("");
+        let expected = [
+            "//   foo\n",
+            "bar\n",
+            "  baz\n",
+        ].join("");
+        assert_eq!(indent_first_line(&text, "// "), expected);
+    }
+
+    #[test]
+    #[rustfmt::skip]
+    fn indent_first_line_empty_line() {
+        let text = [
+            "  foo",
+            "bar",
+            "",
+            "  baz",
+        ].join("\n");
+        let expected = [
+            "//   foo",
+            "bar",
+            "",
+            "  baz",
+        ].join("\n");
+        assert_eq!(indent_first_line(&text, "// "), expected);
+    }
+
+    #[test]
+    #[rustfmt::skip]
+    fn indent_first_line_mixed_newlines() {
+        let text = [
+            "  foo\r\n",
+            "bar\n",
+            "  baz\r",
+        ].join("");
+        let expected = [
+            "//   foo\r\n",
+            "bar\n",
+            "  baz\r",
+        ].join("");
+        assert_eq!(indent_first_line(&text, "// "), expected);
+    }
+
     #[test]
     #[rustfmt::skip]
     fn adjust_indent() {