Improve the comments in Symbol::interner.

Author: nnethercote

compiler/rustc_span/src/symbol.rs

@@ -1716,8 +1716,9 @@ pub(crate) struct Interner(Lock<InternerInner>);
 // found that to regress performance up to 2% in some cases. This might be
 // revisited after further improvements to `indexmap`.
 //
-// This type is private to prevent accidentally constructing more than one `Interner` on the same
-// thread, which makes it easy to mixup `Symbol`s between `Interner`s.
+// This type is private to prevent accidentally constructing more than one
+// `Interner` on the same thread, which makes it easy to mixup `Symbol`s
+// between `Interner`s.
 #[derive(Default)]
 struct InternerInner {
     arena: DroplessArena,
@@ -1743,14 +1744,20 @@ impl Interner {
 
         let name = Symbol::new(inner.strings.len() as u32);
 
-        // `from_utf8_unchecked` is safe since we just allocated a `&str` which is known to be
-        // UTF-8.
+        // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
+        // and immediately convert the clone back to `&[u8], all because there
+        // is no `inner.arena.alloc_str()` method. This is clearly safe.
         let string: &str =
             unsafe { str::from_utf8_unchecked(inner.arena.alloc_slice(string.as_bytes())) };
-        // It is safe to extend the arena allocation to `'static` because we only access
-        // these while the arena is still alive.
+
+        // SAFETY: we can extend the arena allocation to `'static` because we
+        // only access these while the arena is still alive.
         let string: &'static str = unsafe { &*(string as *const str) };
         inner.strings.push(string);
+
+        // This second hash table lookup can be avoided by using `RawEntryMut`,
+        // but this code path isn't hot enough for it to be worth it. See
+        // #91445 for details.
         inner.names.insert(string, name);
         name
     }