From 88788f775d05b148b2d67fc307fcad29872bfd10 Mon Sep 17 00:00:00 2001
From: Ryu <114303361+ryuapp@users.noreply.github.com>
Date: Sun, 15 Mar 2026 19:47:44 +0900
Subject: [PATCH] fix(fmt): ensure formatter always emits a trailing newline

---
 README.md               |  2 +-
 examples/hello-world.zy |  2 +-
 src/commands/check.rs   | 12 +++++++++---
 src/fmt.rs              |  3 +++
 src/tests/fmt.rs        | 21 +++++++++++++++++++++
 5 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 10d09d7..5f92c7e 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ A statically typed programming language.
 
 ```zy
 const std = import("std")
-std.debug.print("Hello World!")
+std.debug.print("Hello, 世界")
 ```
 
 ## License
diff --git a/examples/hello-world.zy b/examples/hello-world.zy
index c0d42eb..80cfc59 100644
--- a/examples/hello-world.zy
+++ b/examples/hello-world.zy
@@ -1,2 +1,2 @@
 const std = import("std")
-std.debug.print("Hello World!")
+std.debug.print("Hello, 世界")
diff --git a/src/commands/check.rs b/src/commands/check.rs
index 9417594..465d350 100644
--- a/src/commands/check.rs
+++ b/src/commands/check.rs
@@ -165,8 +165,14 @@ fn fmt_diff(source: &str, formatted: &str) -> String {
             }
         }
     }
-    let trimmed_len = out.trim_end().len();
-    out.truncate(trimmed_len);
+    if out.is_empty() && source != formatted {
+        // Only difference is trailing newline — show as an added empty line
+        let ln = source.lines().count() + 1;
+        out.push_str(&crate::colors::green(&format!("{} | +", ln)));
+    } else {
+        let trimmed_len = out.trim_end().len();
+        out.truncate(trimmed_len);
+    }
     out
 }
 
@@ -196,7 +202,7 @@ fn check_and_report(
     let mut changed = false;
     if parse_errors.is_empty() {
         let formatted = crate::fmt::format_program(&ast, &blanks);
-        if formatted.trim() != source.trim() {
+        if formatted != source {
             if fix {
                 std::fs::write(path, &formatted)
                     .unwrap_or_else(|e| panic!("Failed to write '{}': {}", path, e));
diff --git a/src/fmt.rs b/src/fmt.rs
index 73c9833..7bdb398 100644
--- a/src/fmt.rs
+++ b/src/fmt.rs
@@ -362,5 +362,8 @@ fn precedence(op: &BinOp) -> u8 {
 pub fn format_program(program: &crate::parser::Program, blanks: &[bool]) -> String {
     let mut f = Formatter::new();
     f.format_program(program, blanks);
+    if !f.buf.ends_with('\n') {
+        f.buf.push('\n');
+    }
     f.buf
 }
diff --git a/src/tests/fmt.rs b/src/tests/fmt.rs
index 4bf7d11..337bcad 100644
--- a/src/tests/fmt.rs
+++ b/src/tests/fmt.rs
@@ -296,6 +296,27 @@ fn test_fmt_modulo() {
     idempotent(&out);
 }
 
+#[test]
+fn test_fmt_trailing_newline() {
+    let out = fmt("const x = 1");
+    assert!(
+        out.ends_with('\n'),
+        "expected trailing newline, got: {:?}",
+        out
+    );
+}
+
+#[test]
+fn test_fmt_trailing_newline_not_doubled() {
+    let out = fmt("const x = 1\n");
+    assert!(out.ends_with('\n'), "expected trailing newline");
+    assert!(
+        !out.ends_with("\n\n"),
+        "trailing newline doubled: {:?}",
+        out
+    );
+}
+
 #[test]
 fn test_fmt_fixtures_idempotent() {
     let fixtures = std::fs::read_dir("src/tests/fixtures")