Add cargo xtask clippy (#8722)

This PR sets up a `cargo xtask clippy` command for running `cargo
clippy` with our defined set of options.

The intent is to make this easier to manage as we start enabling more
Clippy rules.

Release Notes:

- N/A

tooling/xtask/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[dependencies]
+anyhow.workspace = true
+clap = { workspace = true, features = ["derive"] }

tooling/xtask/LICENSE-GPL

@@ -0,0 +1 @@
+../../LICENSE-GPL

tooling/xtask/src/main.rs

@@ -0,0 +1,80 @@
+use std::process::Command;
+
+use anyhow::{bail, Context, Result};
+use clap::{Parser, Subcommand};
+
+#[derive(Parser)]
+#[command(name = "cargo xtask")]
+struct Args {
+    #[command(subcommand)]
+    command: CliCommand,
+}
+
+#[derive(Subcommand)]
+enum CliCommand {
+    /// Runs `cargo clippy`.
+    Clippy(ClippyArgs),
+}
+
+fn main() -> Result<()> {
+    let args = Args::parse();
+
+    match args.command {
+        CliCommand::Clippy(args) => run_clippy(args),
+    }
+}
+
+#[derive(Parser)]
+struct ClippyArgs {
+    /// Whether to deny warnings (`clippy --deny warnings`).
+    #[arg(long)]
+    deny_warnings: bool,
+}
+
+fn run_clippy(args: ClippyArgs) -> Result<()> {
+    let cargo = std::env::var("CARGO").unwrap_or_else(|_| "cargo".to_string());
+
+    let mut clippy_command = Command::new(&cargo);
+    clippy_command
+        .arg("clippy")
+        .arg("--workspace")
+        .arg("--release")
+        .arg("--all-targets")
+        .arg("--all-features");
+
+    clippy_command.arg("--");
+
+    if args.deny_warnings {
+        clippy_command.args(["--deny", "warnings"]);
+    }
+
+    // Allow all Clippy lints by default, as we have a lot of violations at the moment.
+    // We can tighten things up once we have a better handle on them.
+    clippy_command.args(["--allow", "clippy::all"]);
+
+    // Deny `dbg!` and `todo!`s.
+    clippy_command
+        .args(["--deny", "clippy::dbg_macro"])
+        .args(["--deny", "clippy::todo"]);
+
+    eprintln!(
+        "running: {cargo} {}",
+        clippy_command
+            .get_args()
+            .map(|arg| format!("{}", arg.to_str().unwrap()))
+            .collect::<Vec<_>>()
+            .join(" ")
+    );
+
+    let exit_status = clippy_command
+        .spawn()
+        .context("failed to spawn child process")?
+        .wait()
+        .context("failed to wait for child process")?;
+
+    if !exit_status.success() {
+        bail!("clippy failed: {}", exit_status);
+    }
+
+    Ok(())
+}