Infer indentation with imports when logical indent is absent (#11608)

## Summary

In an `__init__.py` file, it's not uncommon to lack a logical indent
(since it may just contain imports). In such cases, we were always
falling back to four-space indent. This PR adds detection for indents
within import groups.

Closes https://github.com/astral-sh/ruff/issues/11606.

crates/ruff_linter/resources/test/fixtures/isort/two_space.py

@@ -0,0 +1,10 @@
+# If the file doesn't contain a logical indent token, we should still detect two-space indentation on imports.
+from math import (
+  sin,
+  tan,
+  cos,
+  nan,
+  pi,
+)
+
+del sin, cos, tan, pi, nan

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py

@@ -29,6 +29,11 @@ print("%#o" % (123,))
 
 print("brace {} %s" % (1,))
 
+print((
+    "foo %s "
+    "bar %s" % (x, y)
+))
+
 print(
   "%s" % (
     "trailing comma",
@@ -52,10 +57,6 @@ print("%(ab)s" % {"a" "b": 1})
 
 print("%(a)s" % {"a"  :  1})
 
-print((
-    "foo %s "
-    "bar %s" % (x, y)
-))
 
 print(
     "foo %(foo)s "

crates/ruff_linter/src/rules/isort/mod.rs

@@ -341,6 +341,7 @@ mod tests {
     #[test_case(Path::new("split.py"))]
     #[test_case(Path::new("star_before_others.py"))]
     #[test_case(Path::new("trailing_suffix.py"))]
+    #[test_case(Path::new("two_space.py"))]
     #[test_case(Path::new("type_comments.py"))]
     #[test_case(Path::new("unicode.py"))]
     fn default(path: &Path) -> Result<()> {

crates/ruff_linter/src/rules/isort/snapshots/ruff_linter__rules__isort__tests__two_space.py.snap

@@ -0,0 +1,32 @@
+---
+source: crates/ruff_linter/src/rules/isort/mod.rs
+---
+two_space.py:2:1: I001 [*] Import block is un-sorted or un-formatted
+   |
+ 1 |   # If the file doesn't contain a logical indent token, we should still detect two-space indentation on imports.
+ 2 | / from math import (
+ 3 | |   sin,
+ 4 | |   tan,
+ 5 | |   cos,
+ 6 | |   nan,
+ 7 | |   pi,
+ 8 | | )
+ 9 | | 
+10 | | del sin, cos, tan, pi, nan
+   | |_^ I001
+   |
+   = help: Organize imports
+
+ℹ Safe fix
+1 1 | # If the file doesn't contain a logical indent token, we should still detect two-space indentation on imports.
+2 2 | from math import (
+3   |-  sin,
+4   |-  tan,
+5 3 |   cos,
+6 4 |   nan,
+7 5 |   pi,
+  6 |+  sin,
+  7 |+  tan,
+8 8 | )
+9 9 | 
+10 10 | del sin, cos, tan, pi, nan

crates/ruff_python_codegen/src/stylist.rs

@@ -6,7 +6,7 @@ use once_cell::unsync::OnceCell;
 
 use ruff_python_ast::{str::Quote, StringFlags};
 use ruff_python_parser::lexer::LexResult;
-use ruff_python_parser::Tok;
+use ruff_python_parser::{Tok, TokenKind};
 use ruff_source_file::{find_newline, LineEnding, Locator};
 
 #[derive(Debug, Clone)]
@@ -86,6 +86,38 @@ fn detect_indention(tokens: &[LexResult], locator: &Locator) -> Indentation {
 
         Indentation(whitespace.to_string())
     } else {
+        // If we can't find a logical indent token, search for a non-logical indent, to cover cases
+        // like:
+        //```python
+        // from math import (
+        //   sin,
+        //   tan,
+        //   cos,
+        // )
+        // ```
+        let mut depth = 0usize;
+        for (token, range) in tokens.iter().flatten() {
+            match token.kind() {
+                TokenKind::Lpar | TokenKind::Lbrace | TokenKind::Lsqb => {
+                    depth = depth.saturating_add(1);
+                }
+                TokenKind::Rpar | TokenKind::Rbrace | TokenKind::Rsqb => {
+                    depth = depth.saturating_sub(1);
+                }
+                TokenKind::NonLogicalNewline => {
+                    let line = locator.line(range.end());
+                    let indent_index = line.chars().position(|c| !c.is_whitespace());
+                    if let Some(indent_index) = indent_index {
+                        if indent_index > 0 {
+                            let whitespace = &line[..indent_index];
+                            return Indentation(whitespace.to_string());
+                        }
+                    }
+                }
+                _ => {}
+            }
+        }
+
         Indentation::default()
     }
 }
@@ -177,7 +209,6 @@ if True:
             &Indentation("\t".to_string())
         );
 
-        // TODO(charlie): Should non-significant whitespace be detected?
         let contents = r"
 x = (
   1,
@@ -189,7 +220,7 @@ x = (
         let tokens: Vec<_> = lex(contents, Mode::Module).collect();
         assert_eq!(
             Stylist::from_tokens(&tokens, &locator).indentation(),
-            &Indentation::default()
+            &Indentation("  ".to_string())
         );
 
         // formfeed indent, see `detect_indention` comment.