Auto merge of #142770 - tgross35:rollup-w74w39t, r=tgross35

Rollup of 8 pull requests

Successful merges:

 - rust-lang/rust#138291 (rewrite `optimize` attribute to use new attribute parsing infrastructure)
 - rust-lang/rust#140920 (Extract some shared code from codegen backend target feature handling)
 - rust-lang/rust#141990 (Implement send_signal for unix child processes)
 - rust-lang/rust#142668 (vec_deque/fmt/vec tests: remove static mut)
 - rust-lang/rust#142687 (Reduce uses of `hir_crate`.)
 - rust-lang/rust#142699 (Update books)
 - rust-lang/rust#142714 (add comment to `src/bootstrap/build.rs`)
 - rust-lang/rust#142753 (Update library dependencies)

r? `@ghost`
`@rustbot` modify labels: rollup

src/bootstrap/build.rs

@@ -1,6 +1,7 @@
 use std::env;
 
 fn main() {
+    // this is needed because `HOST` is only available to build scripts.
     let host = env::var("HOST").unwrap();
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-env=BUILD_TRIPLE={host}");