Format empty lines in stub files closer to black's preview style

crates/ruff_python_formatter/resources/test/fixtures/ruff/stub_files/suite.pyi

@@ -1,5 +1,5 @@
 """Tests for empty line rules in stub files, mostly inspired by typeshed.
-The rules follow no discernible pattern. See also
+The rules are a list of nested exceptions. See also
 https://github.com/psf/black/blob/c160e4b7ce30c661ac4f2dfa5038becf1b8c5c33/src/black/lines.py#L576-L744
 """
 

crates/ruff_python_formatter/src/statement/suite.rs

@@ -331,8 +331,6 @@ impl FormatRule<Suite, PyFormatContext<'_>> for FormatSuite {
 ///
 /// These rules are ported from black (preview mode at time of writing) using the stubs test case:
 /// <https://github.com/psf/black/blob/c160e4b7ce30c661ac4f2dfa5038becf1b8c5c33/src/black/lines.py#L576-L744>
-/// The reasons for these rules here and in [`stub_suite_empty_line`] are not commented as i did
-/// not follow them
 fn stub_file_empty_lines(
     kind: SuiteKind,
     preceding: &Stmt,
@@ -342,25 +340,20 @@ fn stub_file_empty_lines(
     f: &mut PyFormatter,
 ) -> FormatResult<()> {
     let source = f.context().source();
-
+    // Preserve the empty line if the definitions are separated by a comment
+    let empty_line_condition = preceding_comments.has_trailing()
+        || following_comments.has_leading()
+        || !stub_suite_can_omit_empty_line(preceding, following, f);
     match kind {
         SuiteKind::TopLevel => {
-            // Preserve the empty line if the definitions are separated by a comment
-            if preceding_comments.has_trailing()
-                || following_comments.has_leading()
-                || stub_suite_empty_line(preceding, following, f)
-            {
+            if empty_line_condition {
                 empty_line().fmt(f)
             } else {
                 hard_line_break().fmt(f)
             }
         }
         SuiteKind::Class | SuiteKind::Other | SuiteKind::Function => {
-            if (preceding_comments.has_trailing()
-                || following_comments.has_leading()
-                || stub_suite_empty_line(preceding, following, f))
-                && lines_after_ignoring_trivia(preceding.end(), source) > 1
-            {
+            if empty_line_condition && lines_after_ignoring_trivia(preceding.end(), source) > 1 {
                 empty_line().fmt(f)
             } else {
                 hard_line_break().fmt(f)
@@ -369,9 +362,9 @@ fn stub_file_empty_lines(
     }
 }
 
-/// Add an empty line except when one of the three reason to skip them is given
-fn stub_suite_empty_line(preceding: &Stmt, following: &Stmt, f: &mut PyFormatter) -> bool {
-    // Two subsequent classes that both have an ellipsis only body
+/// Only a function to compute it lazily
+fn stub_suite_can_omit_empty_line(preceding: &Stmt, following: &Stmt, f: &mut PyFormatter) -> bool {
+    // Two subsequent class definitions that both have an ellipsis only body
     // ```python
     // class A: ...
     // class B: ...
@@ -402,7 +395,7 @@ fn stub_suite_empty_line(preceding: &Stmt, following: &Stmt, f: &mut PyFormatter
             .as_class_def_stmt()
             .is_some_and(|class| !class.decorator_list.is_empty());
 
-    // Two subsequent functions where at least the preceding has an ellipsis only body
+    // A function definition following a stub function definition
     // ```python
     // def test(): ...
     // def b(): a
@@ -412,10 +405,9 @@ fn stub_suite_empty_line(preceding: &Stmt, following: &Stmt, f: &mut PyFormatter
         .is_some_and(|function| contains_only_an_ellipsis(&function.body, f.context().comments()))
         && following.is_function_def_stmt();
 
-    // Add an empty line except when one of the three reason to skip them is given
-    !(class_sequences_with_ellipsis_only
+    class_sequences_with_ellipsis_only
         || class_decorator_instead_of_empty_line
-        || function_with_ellipsis)
+        || function_with_ellipsis
 }
 
 /// Returns `true` if a function or class body contains only an ellipsis with no comments.

crates/ruff_python_formatter/tests/snapshots/format@stub_files__suite.pyi.snap

@@ -5,7 +5,7 @@ input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/stub_files
 ## Input
 ```py
 """Tests for empty line rules in stub files, mostly inspired by typeshed.
-The rules follow no discernible pattern. See also
+The rules are a list of nested exceptions. See also
 https://github.com/psf/black/blob/c160e4b7ce30c661ac4f2dfa5038becf1b8c5c33/src/black/lines.py#L576-L744
 """
 
@@ -158,7 +158,7 @@ class ComplexStatements:
 ## Output
 ```py
 """Tests for empty line rules in stub files, mostly inspired by typeshed.
-The rules follow no discernible pattern. See also
+The rules are a list of nested exceptions. See also
 https://github.com/psf/black/blob/c160e4b7ce30c661ac4f2dfa5038becf1b8c5c33/src/black/lines.py#L576-L744
 """