Add prototype of ruff format for projects

**Summary** Add recursive formatting based on `ruff check` file discovery for `ruff format`, as a prototype for the formatter alpha. This allows e.g. `format ../projects/django/`. It's still lacking support for any settings except line length.

Error handling works in my manual tests:

```
$  target/debug/ruff format scripts/
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended for internal use only.
```
(the above changes `add_rule.py` where we have the wrong bin op breaking)

```
$ ruff format ../projects/django/
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended for internal use only.
Failed to format /home/konsti/projects/django/tests/test_runner_apps/tagged/tests_syntax_error.py: source contains syntax errors: ParseError { error: UnrecognizedToken(Name { name: "syntax_error" }, None), offset: 131, source_path: "<filename>" }
```

```
$ target/debug/ruff format a
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended for internal use only.
Failed to read /home/konsti/ruff/a/d.py: Permission denied (os error 13)
```

**Test Plan** Missing! I'm not sure if it's worth building tests at this stage or how they should look like.

crates/ruff_cli/Cargo.toml

@@ -24,6 +24,7 @@ doc = false
 ruff = { path = "../ruff", features = ["clap"] }
 ruff_cache = { path = "../ruff_cache" }
 ruff_diagnostics = { path = "../ruff_diagnostics" }
+ruff_formatter = { path = "../ruff_formatter" }
 ruff_macros = { path = "../ruff_macros" }
 ruff_python_ast = { path = "../ruff_python_ast" }
 ruff_python_formatter = { path = "../ruff_python_formatter" }
@@ -59,6 +60,8 @@ serde_json = { workspace = true }
 shellexpand = { workspace = true }
 similar = { workspace = true }
 strum = { workspace = true, features = [] }
+thiserror = { workspace = true }
+tracing = { workspace = true }
 walkdir = { version = "2.3.2" }
 wild = { version = "2" }
 

crates/ruff_cli/src/commands/format.rs

@@ -0,0 +1,100 @@
+#![allow(clippy::print_stderr)]
+
+use crate::args::{Arguments, Overrides};
+use crate::resolve::resolve;
+use crate::ExitStatus;
+use anyhow::bail;
+use colored::Colorize;
+use rayon::iter::{IntoParallelIterator, ParallelIterator};
+use ruff::resolver::python_files_in_path;
+use ruff_formatter::LineWidth;
+use ruff_python_ast::PySourceType;
+use ruff_python_formatter::{format_module, FormatModuleError, PyFormatOptions};
+use std::io;
+use std::path::{Path, PathBuf};
+use thiserror::Error;
+use tracing::span;
+use tracing::Level;
+
+/// The inner errors are all flat, i.e. none of them has a source
+#[derive(Error, Debug)]
+enum FormatterIterationError {
+    #[error("Failed to traverse the inputs paths: {0}")]
+    Ignore(#[from] ignore::Error),
+    #[error("Failed to read {0}: {1}")]
+    Read(PathBuf, io::Error),
+    #[error("Failed to write {0}: {1}")]
+    Write(PathBuf, io::Error),
+    #[error("Failed to format {0}: {1}")]
+    FormatModule(PathBuf, FormatModuleError),
+}
+
+pub(crate) fn format(cli: &Arguments, overrides: &Overrides) -> anyhow::Result<ExitStatus> {
+    let pyproject_config = resolve(
+        cli.isolated,
+        cli.config.as_deref(),
+        overrides,
+        cli.stdin_filename.as_deref(),
+    )?;
+    let (paths, resolver) = python_files_in_path(&cli.files, &pyproject_config, overrides)?;
+    if paths.is_empty() {
+        bail!("no python files in TODO(@konstin) pass them in")
+    }
+
+    let all_success = paths
+        .into_par_iter()
+        .map(|dir_entry| {
+            let dir_entry = dir_entry?;
+            let path = dir_entry.path();
+            let source_type = PySourceType::from(path);
+            if !(source_type.is_python() || source_type.is_stub())
+                || path
+                    .extension()
+                    .is_some_and(|extension| extension == "toml")
+            {
+                return Ok(());
+            }
+
+            let line_length = resolver.resolve(path, &pyproject_config).line_length;
+            // TODO(@konstin): Unify `LineWidth` and `LineLength`
+            let line_width = LineWidth::try_from(
+                u16::try_from(line_length.get()).expect("Line shouldn't be larger than 2**16"),
+            )
+            .expect("Configured line length is too large for the formatter");
+            let options = PyFormatOptions::from_extension(path).with_line_width(line_width);
+
+            format_path(path, options)
+        })
+        .map(|result| {
+            match result {
+                Ok(()) => true,
+                Err(err) => {
+                    // The inner errors are all flat, i.e. none of them has a source
+                    eprintln!("{}", err.to_string().red().bold());
+                    false
+                }
+            }
+        })
+        .all(|success| success);
+
+    if all_success {
+        Ok(ExitStatus::Success)
+    } else {
+        Ok(ExitStatus::Error)
+    }
+}
+
+#[tracing::instrument(skip_all, fields(path = %path.display()))]
+fn format_path(path: &Path, options: PyFormatOptions) -> Result<(), FormatterIterationError> {
+    let unformatted = std::fs::read_to_string(path)
+        .map_err(|err| FormatterIterationError::Read(path.to_path_buf(), err))?;
+    let formatted = {
+        let span = span!(Level::TRACE, "format_path_without_io", path = %path.display());
+        let _enter = span.enter();
+        format_module(&unformatted, options)
+            .map_err(|err| FormatterIterationError::FormatModule(path.to_path_buf(), err))?
+    };
+    std::fs::write(path, formatted.as_code().as_bytes())
+        .map_err(|err| FormatterIterationError::Write(path.to_path_buf(), err))?;
+    Ok(())
+}

crates/ruff_cli/src/commands/mod.rs

@@ -1,6 +1,7 @@
 pub(crate) mod add_noqa;
 pub(crate) mod clean;
 pub(crate) mod config;
+pub(crate) mod format;
 pub(crate) mod linter;
 pub(crate) mod rule;
 pub(crate) mod run;

crates/ruff_cli/src/lib.rs

@@ -4,8 +4,9 @@ use std::path::{Path, PathBuf};
 use std::process::ExitCode;
 use std::sync::mpsc::channel;
 
-use anyhow::{Context, Result};
+use anyhow::Result;
 use clap::CommandFactory;
+use clap::FromArgMatches;
 use log::warn;
 use notify::{recommended_watcher, RecursiveMode, Watcher};
 
@@ -154,32 +155,34 @@ quoting the executed command, along with the relevant file contents and `pyproje
     Ok(ExitStatus::Success)
 }
 
-fn format(files: &[PathBuf]) -> Result<ExitStatus> {
+fn format(paths: &[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."
+        "`ruff format` is a work-in-progress, subject to change at any time, and intended only for \
+        experimentation."
     );
 
-    match &files {
+    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)
         }
         _ => {
-            for file in files {
-                let unformatted = std::fs::read_to_string(file)
-                    .with_context(|| format!("Could not read {}: ", file.display()))?;
-                let options = PyFormatOptions::from_extension(file);
-                let formatted = format_module(&unformatted, options)?;
-                std::fs::write(file, formatted.as_code().as_bytes())
-                    .with_context(|| format!("Could not write to {}, exiting", file.display()))?;
-            }
+            // 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)
         }
     }
-    Ok(ExitStatus::Success)
 }
 
 pub fn check(args: CheckArgs, log_level: LogLevel) -> Result<ExitStatus> {

crates/ruff_python_formatter/README.md

@@ -3,6 +3,15 @@
 The goal of our formatter is to be compatible with Black except for rare edge cases (mostly
 involving comment placement).
 
+You can try an experimental version of the formatter on your project with:
+
+```shell
+cargo run --bin ruff -- format path/to/your/project
+```
+
+Note that currently the only supported option is `line-length` and that both the CLI and the
+formatting are a work-in-progress and will change before the stable release.
+
 ## Dev tools
 
 **Testing your changes** You can use the `ruff_python_formatter` binary to format individual files

crates/ruff_python_formatter/src/lib.rs

@@ -103,9 +103,9 @@ where
 
 #[derive(Error, Debug)]
 pub enum FormatModuleError {
-    #[error("source contains syntax errors (lexer error): {0:?}")]
+    #[error("source contains syntax errors: {0:?}")]
     LexError(LexicalError),
-    #[error("source contains syntax errors (parser error): {0:?}")]
+    #[error("source contains syntax errors: {0:?}")]
     ParseError(ParseError),
     #[error(transparent)]
     FormatError(#[from] FormatError),