Fix dangling module comments (#7456)

crates/ruff_formatter/src/lib.rs

@@ -440,7 +440,7 @@ pub type FormatResult<F> = Result<F, FormatError>;
 /// let paragraph = Paragraph(String::from("test"));
 /// let formatted = format!(SimpleFormatContext::default(), [paragraph])?;
 ///
-/// assert_eq!("test\n", formatted.print()?.as_code());
+/// assert_eq!("\ntest\n", formatted.print()?.as_code());
 /// # Ok(())
 /// # }
 /// ```

crates/ruff_formatter/src/printer/mod.rs

@@ -124,7 +124,7 @@ impl<'a> Printer<'a> {
                     self.flush_line_suffixes(queue, stack, Some(element));
                 } else {
                     // Only print a newline if the current line isn't already empty
-                    if self.state.line_width > 0 {
+                    if self.state.line_width > 0 || self.state.buffer.is_empty() {
                         self.print_char('\n');
                     }
 

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/module_comment.py

@@ -0,0 +1,4 @@
+
+
+
+# hehehe >:)a

crates/ruff_python_formatter/src/comments/placement.rs

@@ -2,7 +2,7 @@ use std::cmp::Ordering;
 
 use ruff_python_ast::node::AnyNodeRef;
 use ruff_python_ast::whitespace::indentation;
-use ruff_python_ast::{self as ast, Comprehension, Expr, MatchCase, Parameters};
+use ruff_python_ast::{self as ast, Comprehension, Expr, MatchCase, ModModule, Parameters};
 use ruff_python_trivia::{
     find_only_token_in_range, indentation_at_offset, BackwardsTokenizer, CommentRanges,
     SimpleToken, SimpleTokenKind, SimpleTokenizer,
@@ -229,8 +229,12 @@ fn handle_enclosed_comment<'a>(
                 CommentPlacement::Default(comment)
             }
         }
-        AnyNodeRef::ModModule(_) => {
-            handle_module_level_own_line_comment_before_class_or_function_comment(comment, locator)
+        AnyNodeRef::ModModule(module) => {
+            handle_trailing_module_comment(module, comment).or_else(|comment| {
+                handle_module_level_own_line_comment_before_class_or_function_comment(
+                    comment, locator,
+                )
+            })
         }
         AnyNodeRef::WithItem(_) => handle_with_item_comment(comment, locator),
         AnyNodeRef::PatternMatchSequence(pattern_match_sequence) => {
@@ -882,6 +886,35 @@ fn handle_trailing_binary_like_comment<'a>(
     }
 }
 
+/// Handles trailing comments after the last statement in a module.
+/// Ruff's parser sets the module range to exclude trailing comments and the result is that
+/// [`CommentPlacement::Default`] makes these comments dangling comments.
+///
+/// This method overrides the handling to make these comments trailing comments of the last
+/// statement instead.
+///
+/// ```python
+/// a
+///
+/// # trailing comment
+/// ```
+///
+/// Comments of an all empty module are leading module comments
+fn handle_trailing_module_comment<'a>(
+    module: &'a ModModule,
+    comment: DecoratedComment<'a>,
+) -> CommentPlacement<'a> {
+    if comment.preceding_node().is_none() && comment.following_node().is_none() {
+        if let Some(last_statement) = module.body.last() {
+            CommentPlacement::trailing(last_statement, comment)
+        } else {
+            CommentPlacement::leading(comment.enclosing_node(), comment)
+        }
+    } else {
+        CommentPlacement::Default(comment)
+    }
+}
+
 /// Handles own line comments on the module level before a class or function statement.
 /// A comment only becomes the leading comment of a class or function if it isn't separated by an empty
 /// line from the class. Comments that are separated by at least one empty line from the header of the

crates/ruff_python_formatter/src/comments/snapshots/ruff_python_formatter__comments__tests__only_comments.snap

@@ -8,8 +8,7 @@ expression: comments.debug(test_case.source_code)
         range: 0..0,
         source: ``,
     }: {
-        "leading": [],
-        "dangling": [
+        "leading": [
             SourceComment {
                 text: "# Some comment",
                 position: OwnLine,
@@ -21,6 +20,7 @@ expression: comments.debug(test_case.source_code)
                 formatted: false,
             },
         ],
+        "dangling": [],
         "trailing": [],
     },
 }

crates/ruff_python_formatter/src/module/mod_module.rs

@@ -1,24 +1,22 @@
-use ruff_formatter::prelude::hard_line_break;
-use ruff_formatter::{Buffer, FormatResult};
+use ruff_formatter::write;
 use ruff_python_ast::ModModule;
 
-use crate::comments::{trailing_comments, SourceComment};
+use crate::comments::SourceComment;
+use crate::prelude::*;
 use crate::statement::suite::SuiteKind;
-use crate::{write, AsFormat, FormatNodeRule, PyFormatter};
+use crate::FormatNodeRule;
 
 #[derive(Default)]
 pub struct FormatModModule;
 
 impl FormatNodeRule<ModModule> for FormatModModule {
     fn fmt_fields(&self, item: &ModModule, f: &mut PyFormatter) -> FormatResult<()> {
         let ModModule { range: _, body } = item;
-        let comments = f.context().comments().clone();
 
         write!(
             f,
             [
                 body.format().with_options(SuiteKind::TopLevel),
-                trailing_comments(comments.dangling(item)),
                 // Trailing newline at the end of the file
                 hard_line_break()
             ]

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

@@ -17,6 +17,7 @@ magic-trailing-comma    = Respect
 ```
 
 ```py
+
 ```
 
 
@@ -30,6 +31,7 @@ magic-trailing-comma    = Respect
 ```
 
 ```py
+
 ```
 
 
@@ -43,6 +45,7 @@ magic-trailing-comma    = Respect
 ```
 
 ```py
+
 ```
 
 

crates/ruff_python_formatter/tests/snapshots/black_compatibility@simple_cases__whitespace.py.snap → crates/ruff_python_formatter/tests/snapshots/format@statement__module_comment.py.snap

@@ -1,31 +1,19 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
-input_file: crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/whitespace.py
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/module_comment.py
 ---
 ## Input
-
 ```py
 
-```
 
-## Black Differences
 
-```diff
---- Black
-+++ Ruff
-@@ -1 +0,0 @@
--
+# hehehe >:)a
 ```
 
-## Ruff Output
-
+## Output
 ```py
+# hehehe >:)a
 ```
 
-## Black Output
-
-```py
-
-```