Add xtask crate (#4293)

Replaces only the cargo_deny.sh script for now. Can be expanded over
time to replace the other shell and python scripts, so only Rust is
needed to work with the repository.

Closes <https://github.com/emilk/egui/issues/2887>
Closes <https://github.com/emilk/egui/issues/4373>

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

xtask/src/deny.rs

@@ -0,0 +1,60 @@
+//! Run `cargo deny`
+//!
+//! Also installs the subcommand if it is not already installed.
+
+use std::process::Command;
+
+use super::DynError;
+
+pub fn deny(args: &[&str]) -> Result<(), DynError> {
+    if !args.is_empty() {
+        return Err(format!("Invalid arguments: {args:?}").into());
+    }
+    install_cargo_deny()?;
+    let targets = [
+        "aarch64-apple-darwin",
+        "aarch64-linux-android",
+        "i686-pc-windows-gnu",
+        "i686-pc-windows-msvc",
+        "i686-unknown-linux-gnu",
+        "wasm32-unknown-unknown",
+        "x86_64-apple-darwin",
+        "x86_64-pc-windows-gnu",
+        "x86_64-pc-windows-msvc",
+        "x86_64-unknown-linux-gnu",
+        "x86_64-unknown-linux-musl",
+        "x86_64-unknown-redox",
+    ];
+    for target in targets {
+        let mut cmd = Command::new("cargo");
+        cmd.args([
+            "deny",
+            "--all-features",
+            "--log-level",
+            "error",
+            "--target",
+            target,
+            "check",
+        ]);
+        super::utils::print_cmd(&cmd);
+        let status = cmd.status()?;
+        if !status.success() {
+            return Err(status.to_string().into());
+        }
+    }
+    Ok(())
+}
+
+fn install_cargo_deny() -> Result<(), DynError> {
+    let already_installed = Command::new("cargo")
+        .args(["deny", "--version"])
+        .output()
+        .is_ok_and(|out| out.status.success());
+    if already_installed {
+        return Ok(());
+    }
+    let mut cmd = Command::new("cargo");
+    cmd.args(["+stable", "install", "--quiet", "--locked", "cargo-deny"]);
+    let reason = "install cargo-deny";
+    super::utils::ask_to_run(cmd, true, reason)
+}