Rollup merge of #108772 - jyn514:faster-tidy, r=the8472

Speed up tidy quite a lot

I highly recommend reviewing this commit-by-commit. Based on #106440 for convenience.

## Timings

These were collected by running `x test tidy -v` to copy paste the command, then using [`samply record`](https://github.com/mstange/samply).

before (8 threads)
![image](https://user-images.githubusercontent.com/23638587/222965319-352ad2c8-367c-4d74-960a-e4bb161a6aff.png)

after (8 threads) ![image](https://user-images.githubusercontent.com/23638587/222965323-fa846f4e-727a-4bf8-8e3b-1b7b40505cc3.png)

before (64 threads) ![image](https://user-images.githubusercontent.com/23638587/222965302-dc88020c-19e9-49d9-a87d-cad054d717f3.png)
after (64 threads) ![image](https://user-images.githubusercontent.com/23638587/222965335-e73d7622-59de-41d2-9cc4-1bd67042a349.png)

The last commit makes tidy use more threads, so comparing "before (8 threads)" to "after (64 threads)" is IMO the most realistic comparison. Locally, that brings the time for me to run tidy down from 4 to .9 seconds, i.e. the majority of the time for `x test tidy` is now spend running `fmt --check`.

r? `@the8472`

1 files changed, 5 insertions, 1 deletions

diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index f5d680df113..baddc9da48d 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -1118,7 +1118,11 @@ impl Step for Tidy {
         cmd.arg(&builder.src);
         cmd.arg(&builder.initial_cargo);
         cmd.arg(&builder.out);
-        cmd.arg(builder.jobs().to_string());
+        // Tidy is heavily IO constrained. Still respect `-j`, but use a higher limit if `jobs` hasn't been configured.
+        let jobs = builder.config.jobs.unwrap_or_else(|| {
+            8 * std::thread::available_parallelism().map_or(1, std::num::NonZeroUsize::get) as u32
+        });
+        cmd.arg(jobs.to_string());
         if builder.is_verbose() {
             cmd.arg("--verbose");
         }