cli: add a fake code formatter tool for testing

cli/Cargo.toml

@@ -39,6 +39,11 @@ name = "fake-diff-editor"
 path = "testing/fake-diff-editor.rs"
 required-features = ["test-fakes"]
 
+[[bin]]
+name = "fake-formatter"
+path = "testing/fake-formatter.rs"
+required-features = ["test-fakes"]
+
 [[test]]
 name = "runner"
 

cli/testing/fake-formatter.rs

@@ -0,0 +1,93 @@
+// Copyright 2024 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::fs::OpenOptions;
+use std::io::Write;
+use std::path::PathBuf;
+use std::process::ExitCode;
+
+use clap::{arg, Parser};
+use itertools::Itertools;
+
+/// A fake code formatter, useful for testing
+///
+/// `fake-formatter` is similar to `cat`.
+/// `fake-formatter --reverse` is similar to `rev` (not `tac`).
+/// `fake-formatter --stdout foo` is similar to `echo foo`.
+/// `fake-formatter --stdout foo --stderr bar --fail` is similar to
+///   `echo foo; echo bar >&2; false`.
+/// `fake-formatter --tee foo` is similar to `tee foo`).
+///
+/// This program acts as a portable alternative to that class of shell commands.
+#[derive(Parser, Debug)]
+struct Args {
+    /// Exit with non-successful status.
+    #[arg(long, default_value_t = false)]
+    fail: bool,
+
+    /// Reverse the characters in each line when reading stdin
+    #[arg(long, default_value_t = false)]
+    reverse: bool,
+
+    /// Write this string to stdout, and ignore stdin.
+    #[arg(long)]
+    stdout: Option<String>,
+
+    /// Write this string to stderr.
+    #[arg(long)]
+    stderr: Option<String>,
+
+    /// Duplicate stdout into this file.
+    #[arg(long)]
+    tee: Option<PathBuf>,
+}
+
+fn main() -> ExitCode {
+    let args: Args = Args::parse();
+    // Code formatters tend to print errors before printing the result.
+    if let Some(data) = args.stderr {
+        eprint!("{}", data);
+    }
+    let stdout = if let Some(data) = args.stdout {
+        data // --reverse doesn't apply to --stdout.
+    } else {
+        std::io::stdin()
+            .lines()
+            .map(|line| {
+                format!(
+                    "{}\n",
+                    if args.reverse {
+                        line.unwrap().chars().rev().collect()
+                    } else {
+                        line.unwrap()
+                    }
+                )
+            })
+            .join("")
+    };
+    print!("{}", stdout);
+    if let Some(path) = args.tee {
+        let mut file = OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(path)
+            .unwrap();
+        write!(file, "{}", stdout).unwrap();
+    }
+    if args.fail {
+        ExitCode::FAILURE
+    } else {
+        ExitCode::SUCCESS
+    }
+}