Adjust some comments on individual use declarations.

When we do the big `use` reformatting there are a tiny number of cases
where rustfmt moves a comment from one `use` item to another in an
undesirable way. This commit pre-emptively rearranges things to prevent
this from happening.

compiler/rustc_data_structures/src/sip128.rs

@@ -1,12 +1,13 @@
 //! This is a copy of `core::hash::sip` adapted to providing 128 bit hashes.
 
-// This code is very hot and uses lots of arithmetic, avoid overflow checks for performance.
-// See https://github.com/rust-lang/rust/pull/119440#issuecomment-1874255727
-use rustc_serialize::int_overflow::{DebugStrictAdd, DebugStrictSub};
 use std::hash::Hasher;
 use std::mem::{self, MaybeUninit};
 use std::ptr;
 
+// This code is very hot and uses lots of arithmetic, avoid overflow checks for performance.
+// See https://github.com/rust-lang/rust/pull/119440#issuecomment-1874255727
+use rustc_serialize::int_overflow::{DebugStrictAdd, DebugStrictSub};
+
 #[cfg(test)]
 mod tests;
 

library/alloc/src/testing/crash_test.rs

@@ -1,5 +1,4 @@
-// We avoid relying on anything else in the crate, apart from the `Debug` trait.
-use crate::fmt::Debug;
+use crate::fmt::Debug; // the `Debug` trait is the only thing we use from `crate::fmt`
 use std::cmp::Ordering;
 use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};