Use stdin for formatter when --stdin-filename is provided (#6926)

## Summary

Just making the formatter CLI more consistent with the linter -- e.g.,
we now use stdin on invocations like `cat foo.py | cargo run -p ruff_cli
-- format -- --stdin-filename=foo.py`, instead of _only_ relying on the
`-` file (and use the same helper as the linter to facilitate this).

crates/ruff_cli/src/lib.rs

@@ -146,7 +146,7 @@ quoting the executed command, along with the relevant file contents and `pyproje
         Command::Linter { format } => commands::linter::linter(format)?,
         Command::Clean => commands::clean::clean(log_level)?,
         Command::GenerateShellCompletion { shell } => {
-            shell.generate(&mut Args::command(), &mut io::stdout());
+            shell.generate(&mut Args::command(), &mut stdout());
         }
         Command::Check(args) => return check(args, log_level),
         Command::Format { files } => return format(&files),
@@ -161,27 +161,28 @@ fn format(paths: &[PathBuf]) -> Result<ExitStatus> {
         experimentation."
     );
 
-    match &paths {
-        // Check if we should read from stdin
-        [path] if path == Path::new("-") => {
-            let unformatted = read_from_stdin()?;
-            let options = PyFormatOptions::from_extension(Path::new("stdin.py"));
-            let formatted = format_module(&unformatted, options)?;
-            stdout().lock().write_all(formatted.as_code().as_bytes())?;
-            Ok(ExitStatus::Success)
-        }
-        _ => {
-            // We want to use the same as `ruff check <files>`, but we don't actually want to allow
-            // any of the linter settings.
-            // TODO(@konstin): Refactor this to allow getting config and resolver without going
-            // though clap.
-            let args_matches = CheckArgs::command()
-                .no_binary_name(true)
-                .get_matches_from(paths);
-            let check_args: CheckArgs = CheckArgs::from_arg_matches(&args_matches)?;
-            let (cli, overrides) = check_args.partition();
-            commands::format::format(&cli, &overrides)
-        }
+    // We want to use the same as `ruff check <files>`, but we don't actually want to allow
+    // any of the linter settings.
+    // TODO(konstin): Refactor this to allow getting config and resolver without going
+    // though clap.
+    let args_matches = CheckArgs::command()
+        .no_binary_name(true)
+        .get_matches_from(paths);
+    let check_args: CheckArgs = CheckArgs::from_arg_matches(&args_matches)?;
+    let (cli, overrides) = check_args.partition();
+
+    if is_stdin(&cli.files, cli.stdin_filename.as_deref()) {
+        let unformatted = read_from_stdin()?;
+        let options = cli
+            .stdin_filename
+            .as_deref()
+            .map(PyFormatOptions::from_extension)
+            .unwrap_or_default();
+        let formatted = format_module(&unformatted, options)?;
+        stdout().lock().write_all(formatted.as_code().as_bytes())?;
+        Ok(ExitStatus::Success)
+    } else {
+        commands::format::format(&cli, &overrides)
     }
 }