Create dummy format CLI (#4453)

* Create dummy format CLI

* Hide format from clap, too

Missed that this is a separate option from `#[doc(hidden)]`

* Remove cargo feature and replace with warning

* No-alloc files parameter matching

* beta warning: warn -> warn_user_once

* Rephrase warning

crates/ruff_cli/src/lib.rs

@@ -1,10 +1,11 @@
-use std::io::{self, BufWriter};
-use std::path::PathBuf;
+use std::io::{self, stdout, BufWriter, Write};
+use std::path::{Path, PathBuf};
 use std::process::ExitCode;
 use std::sync::mpsc::channel;
 
-use anyhow::Result;
+use anyhow::{Context, Result};
 use clap::CommandFactory;
+use log::warn;
 use notify::{recommended_watcher, RecursiveMode, Watcher};
 
 use ruff::logging::{set_up_logging, LogLevel};
@@ -13,6 +14,7 @@ use ruff::settings::{flags, CliSettings};
 use ruff::{fs, warn_user_once};
 
 use crate::args::{Args, CheckArgs, Command};
+use crate::commands::run_stdin::read_from_stdin;
 use crate::printer::{Flags as PrinterFlags, Printer};
 
 pub mod args;
@@ -117,11 +119,41 @@ quoting the executed command, along with the relevant file contents and `pyproje
             shell.generate(&mut Args::command(), &mut io::stdout());
         }
         Command::Check(args) => return check(args, log_level),
+        Command::Format { files } => return format(&files),
     }
 
     Ok(ExitStatus::Success)
 }
 
+fn format(files: &[PathBuf]) -> Result<ExitStatus> {
+    warn_user_once!(
+        "`ruff format` is a work-in-progress, subject to change at any time, and intended for \
+        internal use only."
+    );
+
+    // dummy
+    let format_code = |code: &str| code.replace("# DEL", "");
+
+    match &files {
+        // Check if we should read from stdin
+        [path] if path == Path::new("-") => {
+            let unformatted = read_from_stdin()?;
+            let formatted = format_code(&unformatted);
+            stdout().lock().write_all(formatted.as_bytes())?;
+        }
+        _ => {
+            for file in files {
+                let unformatted = std::fs::read_to_string(file)
+                    .with_context(|| format!("Could not read {}: ", file.display()))?;
+                let formatted = format_code(&unformatted);
+                std::fs::write(file, formatted)
+                    .with_context(|| format!("Could not write to {}, exiting", file.display()))?;
+            }
+        }
+    }
+    Ok(ExitStatus::Success)
+}
+
 fn check(args: CheckArgs, log_level: LogLevel) -> Result<ExitStatus> {
     #[cfg(feature = "ecosystem_ci")]
     let ecosystem_ci = args.ecosystem_ci;