Let empty files be empty files (reprise)

src/formatting.rs

@@ -229,15 +229,15 @@ impl<'a, T: FormatHandler + 'a> FormatContext<'a, T> {
             &visitor.buffer
         );
 
-        // For some reason, the source_map does not include terminating
-        // newlines so we must add one on for each file. This is sad.
+        // The source map does not include terminating newlines, so normalize the file ending.
+        // Style Edition 2027 and later leave an empty formatted buffer empty.
         let num_newlines = count_newlines(&visitor.buffer);
         if self
             .config
             .file_lines()
             .contains_line(&path, num_newlines + 1)
         {
-            source_file::append_newline(&mut visitor.buffer);
+            source_file::append_newline(&mut visitor.buffer, self.config.style_edition());
         }
 
         format_lines(

src/source_file.rs

@@ -3,10 +3,10 @@ use std::io::{self, Write};
 use std::path::Path;
 use std::sync::Arc;
 
-use crate::NewlineStyle;
 use crate::config::FileName;
 use crate::emitter::{self, Emitter};
 use crate::parse::session::ParseSess;
+use crate::{NewlineStyle, StyleEdition};
 
 #[cfg(test)]
 use crate::config::Config;
@@ -15,8 +15,11 @@ use crate::create_emitter;
 #[cfg(test)]
 use crate::formatting::FileRecord;
 
-// Append a newline to the end of each file.
-pub(crate) fn append_newline(s: &mut String) {
+// Append a final newline, except when Style Edition 2027 or later produces an empty buffer.
+pub(crate) fn append_newline(s: &mut String, style_edition: StyleEdition) {
+    if style_edition >= StyleEdition::Edition2027 && s.is_empty() {
+        return;
+    }
     s.push('\n');
 }
 

src/test/mod.rs

@@ -49,6 +49,13 @@ const FILE_SKIP_LIST: &[&str] = &[
     "cfg_mod/bar.rs",
     "cfg_mod/foo.rs",
     "cfg_mod/wasm32.rs",
+    // Empty and newline-only files cannot declare nightly-only 2027 style edition inline.
+    "empty_file_style_edition_2027.rs",
+    "newline_only_file_style_edition_2027.rs",
+    // These roots are tested with child traversal disabled by
+    // reorder_modules_style_edition_2027_tests.
+    "reorder_modules/disabled_style_edition_2027.rs",
+    "reorder_modules/enabled_style_edition_2027.rs",
     "skip/foo.rs",
 ];
 
@@ -528,6 +535,28 @@ fn self_tests() {
     assert_eq!(warnings, 0, "Rustfmt's code generated {warnings} warnings");
 }
 
+#[nightly_only_test]
+#[test]
+fn empty_files_style_edition_2027_tests() {
+    init_log();
+    run_test_with(&TestSetting::default(), || {
+        let files = vec![
+            PathBuf::from("tests/source/empty_file_style_edition_2027.rs"),
+            PathBuf::from("tests/source/newline_only_file_style_edition_2027.rs"),
+        ];
+        let config = Some(PathBuf::from(
+            "tests/config/empty_file_style_edition_2027.toml",
+        ));
+        let (_reports, count, fails) = check_files(files, &config);
+
+        println!("Ran {count} empty_files_style_edition_2027 tests.");
+        assert_eq!(
+            fails, 0,
+            "{fails} empty_files_style_edition_2027 tests failed"
+        );
+    });
+}
+
 #[test]
 fn format_files_find_new_files_via_cfg_if() {
     init_log();
@@ -605,6 +634,42 @@ fn format_files_find_new_files_via_cfg_select() {
     });
 }
 
+#[nightly_only_test]
+#[test]
+fn reorder_modules_style_edition_2027_tests() {
+    init_log();
+    run_test_with(&TestSetting::default(), || {
+        let test_cases = [
+            (
+                "disabled_style_edition_2027.rs",
+                "tests/config/reorder_modules_disabled_style_edition_2027.toml",
+            ),
+            (
+                "enabled_style_edition_2027.rs",
+                "tests/config/reorder_modules_enabled_style_edition_2027.toml",
+            ),
+        ];
+        let mut count = 0;
+        let mut fails = 0;
+
+        for (test_file, config_file) in test_cases {
+            let files = vec![
+                PathBuf::from("tests/source/reorder_modules").join(test_file),
+                PathBuf::from("tests/target/reorder_modules").join(test_file),
+            ];
+            let (_, case_count, case_fails) = check_files(files, &Some(PathBuf::from(config_file)));
+            count += case_count;
+            fails += case_fails;
+        }
+
+        println!("Ran {count} reorder_modules_style_edition_2027 tests.");
+        assert_eq!(
+            fails, 0,
+            "{fails} reorder_modules_style_edition_2027 tests failed"
+        );
+    });
+}
+
 #[test]
 fn stdin_formatting_smoke_test() {
     init_log();

tests/config/empty_file_style_edition_2027.toml

@@ -0,0 +1 @@
+style_edition = "2027"

tests/config/newline_file.toml

@@ -0,0 +1 @@
+style_edition = "2024"

tests/config/reorder_modules_disabled_style_edition_2027.toml

@@ -0,0 +1,3 @@
+reorder_modules = false
+skip_children = true
+style_edition = "2027"

tests/config/reorder_modules_enabled_style_edition_2027.toml

@@ -0,0 +1,3 @@
+reorder_modules = true
+skip_children = true
+style_edition = "2027"

tests/source/newline_file.rs

@@ -0,0 +1 @@
+

tests/source/newline_only_file_style_edition_2027.rs

@@ -0,0 +1 @@
+

tests/target/newline_file.rs

@@ -0,0 +1 @@
+