refactor: remove unnecessary unsafe (#5802)

unsafe impls for `Send` and `Sync` should not be added by default. in
the case of `SlotGuard` removing them does not cause any issues, as the
compiler automatically derives those.

This PR adds requirement to document the unsafety (see
[clippy::undocumented_unsafe_blocks]) and opportunistically adds
`#![deny(unsafe_code)]` to most places where we don't have unsafe code
right now.

TRPL on Send and Sync:
https://doc.rust-lang.org/book/ch16-04-extensible-concurrency-sync-and-send.html

[clippy::undocumented_unsafe_blocks]:
https://rust-lang.github.io/rust-clippy/master/#/undocumented_unsafe_blocks

libs/utils/src/id.rs

@@ -120,6 +120,8 @@ impl Id {
             chunk[0] = HEX[((b >> 4) & 0xf) as usize];
             chunk[1] = HEX[(b & 0xf) as usize];
         }
+
+        // SAFETY: vec constructed out of `HEX`, it can only be ascii
         unsafe { String::from_utf8_unchecked(buf) }
     }
 }

libs/utils/src/lib.rs

@@ -1,5 +1,6 @@
 //! `utils` is intended to be a place to put code that is shared
 //! between other crates in this repository.
+#![deny(clippy::undocumented_unsafe_blocks)]
 
 pub mod backoff;
 

libs/utils/src/shutdown.rs

@@ -1,6 +1,7 @@
 /// Immediately terminate the calling process without calling
 /// atexit callbacks, C runtime destructors etc. We mainly use
 /// this to protect coverage data from concurrent writes.
-pub fn exit_now(code: u8) {
+pub fn exit_now(code: u8) -> ! {
+    // SAFETY: exiting is safe, the ffi is not safe
     unsafe { nix::libc::_exit(code as _) };
 }