Generate the README's --help output automatically via cargo +nightly dev generate-all (#1483)

ruff_dev/src/generate_cli_help.rs

@@ -0,0 +1,34 @@
+//! Generate CLI help.
+
+use anyhow::Result;
+use clap::{Args, CommandFactory};
+use ruff::cli::Cli as MainCli;
+
+use crate::utils::replace_readme_section;
+
+const HELP_BEGIN_PRAGMA: &str = "<!-- Begin auto-generated cli help. -->";
+const HELP_END_PRAGMA: &str = "<!-- End auto-generated cli help. -->";
+
+#[derive(Args)]
+pub struct Cli {
+    /// Write the generated help to stdout (rather than to `README.md`).
+    #[arg(long)]
+    pub(crate) dry_run: bool,
+}
+
+pub fn main(cli: &Cli) -> Result<()> {
+    let mut cmd = MainCli::command();
+    let output = cmd.render_help().to_string();
+
+    if cli.dry_run {
+        print!("{output}");
+    } else {
+        replace_readme_section(
+            &format!("```shell\n{output}\n```\n"),
+            HELP_BEGIN_PRAGMA,
+            HELP_END_PRAGMA,
+        )?;
+    }
+
+    Ok(())
+}