Fix handling of newlines in empty files (#7473)

crates/ruff_formatter/src/lib.rs

@@ -429,7 +429,6 @@ pub type FormatResult<F> = Result<F, FormatError>;
 /// impl Format<SimpleFormatContext> for Paragraph {
 ///     fn fmt(&self, f: &mut Formatter<SimpleFormatContext>) -> FormatResult<()> {
 ///         write!(f, [
-///             hard_line_break(),
 ///             text(&self.0, None),
 ///             hard_line_break(),
 ///         ])
@@ -440,7 +439,7 @@ pub type FormatResult<F> = Result<F, FormatError>;
 /// let paragraph = Paragraph(String::from("test"));
 /// let formatted = format!(SimpleFormatContext::default(), [paragraph])?;
 ///
-/// assert_eq!("\ntest\n", formatted.print()?.as_code());
+/// assert_eq!("test\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 || self.state.buffer.is_empty() {
+                    if self.state.line_width > 0 {
                         self.print_char('\n');
                     }
 

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

@@ -0,0 +1,3 @@
+
+
+

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

@@ -0,0 +1 @@
+

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

@@ -0,0 +1 @@
+    

crates/ruff_python_formatter/src/module/mod_module.rs

@@ -1,5 +1,6 @@
 use ruff_formatter::write;
 use ruff_python_ast::ModModule;
+use ruff_python_trivia::lines_after;
 
 use crate::comments::SourceComment;
 use crate::prelude::*;
@@ -11,8 +12,18 @@ pub struct FormatModModule;
 
 impl FormatNodeRule<ModModule> for FormatModModule {
     fn fmt_fields(&self, item: &ModModule, f: &mut PyFormatter) -> FormatResult<()> {
-        let ModModule { range: _, body } = item;
+        let ModModule { range, body } = item;
 
+        if body.is_empty() {
+            // Only preserve an empty line if the source contains an empty line too.
+            if !f.context().comments().has_leading(item)
+                && lines_after(range.end(), f.context().source()) != 0
+            {
+                empty_line().fmt(f)
+            } else {
+                Ok(())
+            }
+        } else {
             write!(
                 f,
                 [
@@ -22,6 +33,7 @@ impl FormatNodeRule<ModModule> for FormatModModule {
                 ]
             )
         }
+    }
 
     fn fmt_dangling_comments(
         &self,

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

@@ -0,0 +1,18 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/empty_multiple_trailing_newlines.py
+---
+## Input
+```py
+
+
+
+```
+
+## Output
+```py
+
+```
+
+
+

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

@@ -0,0 +1,14 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/empty_now_newline.py
+---
+## Input
+```py
+```
+
+## Output
+```py
+```
+
+
+

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

@@ -0,0 +1,16 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/empty_trailing_newline.py
+---
+## Input
+```py
+
+```
+
+## Output
+```py
+
+```
+
+
+

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

@@ -0,0 +1,14 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/empty_whitespace.py
+---
+## Input
+```py
+    ```
+
+## Output
+```py
+```
+
+
+

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

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