Update format.rs to display correct message for already formatted fil…

…es (#9153)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

New messages for "format" mode. 
Fixes #9132 

## Test Plan

<!-- How was it tested? -->

I ran the tests specified in `CONTRIBUTING.md`
```bash
cargo run -p ruff_cli -- check /path/to/some_files.py --no-cache
cargo run -p ruff_cli -- format --check /path/to/some_files.py --no-cache

cargo clippy --workspace --all-targets --all-features -- -D warnings
RUFF_UPDATE_SCHEMA=1 cargo test
pre-commit run --all-files --show-diff-on-failure
```

**Note:** In case no files are detected, either correctly formatted,
changed, or unchanged, it does not display a message. Wouldn't it be
better to show some message in this case?

crates/ruff_cli/src/commands/format.rs

@@ -515,7 +515,7 @@ impl<'a> FormatResults<'a> {
         if changed > 0 && unchanged > 0 {
             writeln!(
                 f,
-                "{} file{} {}, {} file{} left unchanged",
+                "{} file{} {}, {} file{} {}",
                 changed,
                 if changed == 1 { "" } else { "s" },
                 match self.mode {
@@ -524,6 +524,10 @@ impl<'a> FormatResults<'a> {
                 },
                 unchanged,
                 if unchanged == 1 { "" } else { "s" },
+                match self.mode {
+                    FormatMode::Write => "left unchanged",
+                    FormatMode::Check | FormatMode::Diff => "already formatted",
+                },
             )
         } else if changed > 0 {
             writeln!(
@@ -539,9 +543,13 @@ impl<'a> FormatResults<'a> {
         } else if unchanged > 0 {
             writeln!(
                 f,
-                "{} file{} left unchanged",
+                "{} file{} {}",
                 unchanged,
                 if unchanged == 1 { "" } else { "s" },
+                match self.mode {
+                    FormatMode::Write => "left unchanged",
+                    FormatMode::Check | FormatMode::Diff => "already formatted",
+                },
             )
         } else {
             Ok(())

crates/ruff_cli/tests/format.rs

@@ -255,7 +255,7 @@ fn mixed_line_endings() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    2 files left unchanged
+    2 files already formatted
     "###);
     Ok(())
 }
@@ -328,6 +328,60 @@ OTHER = "OTHER"
     Ok(())
 }
 
+#[test]
+fn messages() -> Result<()> {
+    let tempdir = TempDir::new()?;
+
+    fs::write(
+        tempdir.path().join("main.py"),
+        r#"
+from test import say_hy
+
+if __name__ == "__main__":
+    say_hy("dear Ruff contributor")
+"#,
+    )?;
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .current_dir(tempdir.path())
+        .args(["format", "--no-cache", "--isolated", "--check"])
+        .arg("main.py"), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    Would reformat: main.py
+    1 file would be reformatted
+
+    ----- stderr -----
+    "###);
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .current_dir(tempdir.path())
+        .args(["format", "--no-cache", "--isolated"])
+        .arg("main.py"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    1 file reformatted
+
+    ----- stderr -----
+    "###);
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .current_dir(tempdir.path())
+        .args(["format", "--no-cache", "--isolated"])
+        .arg("main.py"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    1 file left unchanged
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
 #[test]
 fn force_exclude() -> Result<()> {
     let tempdir = TempDir::new()?;
@@ -876,7 +930,7 @@ fn test_diff() {
 
 
         ----- stderr -----
-        2 files would be reformatted, 1 file left unchanged
+        2 files would be reformatted, 1 file already formatted
         "###);
     });
 }