Add back str sentinels

Mark-Simulacrum · Mark-Simulacrum · commit 6ca3c44af89a · 2024-01-19T19:59:04.000-05:00
diff --git a/rustc-hash/src/lib.rs b/rustc-hash/src/lib.rs
@@ -165,10 +165,12 @@ impl Hasher for FxHasher {
         self.add_to_hash(i);
     }
 
-    // Avoid hashing 0xFF sentinel; we don't need prefix-free hashes.
     #[inline]
     fn write_str(&mut self, s: &str) {
         self.write(s.as_bytes());
+        // FIXME: Not having this seems to add significant cost to Symbol::intern(?), but it's not
+        // very clear why. That's interning &str, so it's not obvious why this matters.
+        self.write_u8(0xFF);
     }
 
     // Avoid hashing length prefixes. For our purposes the actual data gives us plenty of entropy

Original file line number	Diff line number	Diff line change
`@@ -165,10 +165,12 @@ impl Hasher for FxHasher {`
`165`	`165`	`self.add_to_hash(i);`
`166`	`166`	`}`
`167`	`167`
`168`		`- // Avoid hashing 0xFF sentinel; we don't need prefix-free hashes.`
`169`	`168`	`#[inline]`
`170`	`169`	`fn write_str(&mut self, s: &str) {`
`171`	`170`	`self.write(s.as_bytes());`
	`171`	`+ // FIXME: Not having this seems to add significant cost to Symbol::intern(?), but it's not`
	`172`	`+ // very clear why. That's interning &str, so it's not obvious why this matters.`
	`173`	`+ self.write_u8(0xFF);`
`172`	`174`	`}`
`173`	`175`
`174`	`176`	`// Avoid hashing length prefixes. For our purposes the actual data gives us plenty of entropy`