Add support for tidy linting via external tools for non-rust files

This change adds the flag `--check-extras` to `tidy`. It accepts a comma
separated list of any of the options:

- py (test everything applicable for python files)
- py:lint (lint python files using `ruff`)
- py:fmt (check formatting for python files using `black`)
- shell or shell:lint (lint shell files using `shellcheck`)

Specific files to check can also be specified via positional args.
Examples:

- `./x test tidy --check-extras=shell,py`
- `./x test tidy --check-extras=py:fmt -- src/bootstrap/bootstrap.py`
- `./x test tidy --check-extras=shell -- src/ci/*.sh`
- Python formatting can be applied with bless:
  `./x test tidy --ckeck-extras=py:fmt --bless`

`ruff` and `black` need to be installed via pip; this tool manages these
within a virtual environment at `build/venv`. `shellcheck` needs to be
installed on the system already.

src/bootstrap/builder/tests.rs

@@ -587,6 +587,7 @@ mod dist {
             run: None,
             only_modified: false,
             skip: vec![],
+            extra_checks: None,
         };
 
         let build = Build::new(config);
@@ -658,6 +659,7 @@ mod dist {
             pass: None,
             run: None,
             only_modified: false,
+            extra_checks: None,
         };
         // Make sure rustfmt binary not being found isn't an error.
         config.channel = "beta".to_string();

src/bootstrap/flags.rs

@@ -336,6 +336,10 @@ pub enum Subcommand {
         /// whether to automatically update stderr/stdout files
         bless: bool,
         #[arg(long)]
+        /// comma-separated list of other files types to check (accepts py, py:lint,
+        /// py:fmt, shell)
+        extra_checks: Option<String>,
+        #[arg(long)]
         /// rerun tests even if the inputs are unchanged
         force_rerun: bool,
         #[arg(long)]
@@ -473,6 +477,13 @@ impl Subcommand {
         }
     }
 
+    pub fn extra_checks(&self) -> Option<&str> {
+        match *self {
+            Subcommand::Test { ref extra_checks, .. } => extra_checks.as_ref().map(String::as_str),
+            _ => None,
+        }
+    }
+
     pub fn only_modified(&self) -> bool {
         match *self {
             Subcommand::Test { only_modified, .. } => only_modified,

src/bootstrap/test.rs

@@ -4,6 +4,7 @@
 //! our CI.
 
 use std::env;
+use std::ffi::OsStr;
 use std::ffi::OsString;
 use std::fs;
 use std::iter;
@@ -1094,6 +1095,14 @@ impl Step for Tidy {
         if builder.config.cmd.bless() {
             cmd.arg("--bless");
         }
+        if let Some(s) = builder.config.cmd.extra_checks() {
+            cmd.arg(format!("--extra-checks={s}"));
+        }
+        let mut args = std::env::args_os();
+        if let Some(_) = args.find(|arg| arg == OsStr::new("--")) {
+            cmd.arg("--");
+            cmd.args(args);
+        }
 
         if builder.config.channel == "dev" || builder.config.channel == "nightly" {
             builder.info("fmt check");