Rollup merge of #105829 - the8472:tidy-style, r=jyn514

Speed up tidy

This can be reviewed commit by commit since they contain separate optimizations.

```
# master
$ taskset -c 0-5 hyperfine './x test tidy'
Benchmark #1: ./x test tidy
  Time (mean ± σ):      4.857 s ±  0.064 s    [User: 12.967 s, System: 2.014 s]
  Range (min … max):    4.779 s …  4.997 s    10 runs

# PR
$ taskset -c 0-5 hyperfine './x test tidy'
Benchmark #1: ./x test tidy
  Time (mean ± σ):      3.672 s ±  0.035 s    [User: 10.524 s, System: 2.029 s]
  Range (min … max):    3.610 s …  3.725 s    10 runs
```

1 files changed, 19 insertions, 4 deletions

diff --git a/src/bootstrap/format.rs b/src/bootstrap/format.rs
index 5e7264fe765..b2f6afead79 100644
--- a/src/bootstrap/format.rs
+++ b/src/bootstrap/format.rs
@@ -8,7 +8,7 @@ use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::sync::mpsc::SyncSender;
 
-fn rustfmt(src: &Path, rustfmt: &Path, paths: &[PathBuf], check: bool) -> impl FnMut() {
+fn rustfmt(src: &Path, rustfmt: &Path, paths: &[PathBuf], check: bool) -> impl FnMut(bool) -> bool {
     let mut cmd = Command::new(&rustfmt);
     // avoid the submodule config paths from coming into play,
     // we only allow a single global config for the workspace for now
@@ -23,7 +23,13 @@ fn rustfmt(src: &Path, rustfmt: &Path, paths: &[PathBuf], check: bool) -> impl F
     let cmd_debug = format!("{:?}", cmd);
     let mut cmd = cmd.spawn().expect("running rustfmt");
     // poor man's async: return a closure that'll wait for rustfmt's completion
-    move || {
+    move |block: bool| -> bool {
+        if !block {
+            match cmd.try_wait() {
+                Ok(Some(_)) => {}
+                _ => return false,
+            }
+        }
         let status = cmd.wait().unwrap();
         if !status.success() {
             eprintln!(
@@ -34,6 +40,7 @@ fn rustfmt(src: &Path, rustfmt: &Path, paths: &[PathBuf], check: bool) -> impl F
             );
             crate::detail_exit(1);
         }
+        true
     }
 }
 
@@ -146,15 +153,23 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
             let child = rustfmt(&src, &rustfmt_path, paths.as_slice(), check);
             children.push_back(child);
 
+            // poll completion before waiting
+            for i in (0..children.len()).rev() {
+                if children[i](false) {
+                    children.swap_remove_back(i);
+                    break;
+                }
+            }
+
             if children.len() >= max_processes {
                 // await oldest child
-                children.pop_front().unwrap()();
+                children.pop_front().unwrap()(true);
             }
         }
 
         // await remaining children
         for mut child in children {
-            child();
+            child(true);
         }
     });