Let a portion of DefPathHash uniquely identify the DefPath's crate.

This allows to directly map from a DefPathHash to the crate it
originates from, without constructing side tables to do that mapping.

It also allows to reliably and cheaply check for DefPathHash collisions.

1 files changed, 13 insertions, 2 deletions

diff --git a/compiler/rustc_data_structures/src/fingerprint.rs b/compiler/rustc_data_structures/src/fingerprint.rs
index 08c3419a842..fecc51324cc 100644
--- a/compiler/rustc_data_structures/src/fingerprint.rs
+++ b/compiler/rustc_data_structures/src/fingerprint.rs
@@ -7,19 +7,30 @@ use std::hash::{Hash, Hasher};
 use std::mem::{self, MaybeUninit};
 
 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
+#[repr(C)]
 pub struct Fingerprint(u64, u64);
 
 impl Fingerprint {
     pub const ZERO: Fingerprint = Fingerprint(0, 0);
 
     #[inline]
+    pub fn new(_0: u64, _1: u64) -> Fingerprint {
+        Fingerprint(_0, _1)
+    }
+
+    #[inline]
     pub fn from_smaller_hash(hash: u64) -> Fingerprint {
         Fingerprint(hash, hash)
     }
 
     #[inline]
     pub fn to_smaller_hash(&self) -> u64 {
-        self.0
+        // Even though both halves of the fingerprint are expected to be good
+        // quality hash values, let's still combine the two values because the
+        // Fingerprints in DefPathHash have the StableCrateId portion which is
+        // the same for all DefPathHashes from the same crate. Combining the
+        // two halfs makes sure we get a good quality hash in such cases too.
+        self.0.wrapping_mul(3).wrapping_add(self.1)
     }
 
     #[inline]
@@ -93,7 +104,7 @@ impl FingerprintHasher for crate::unhash::Unhasher {
     #[inline]
     fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
         // `Unhasher` only wants a single `u64`
-        self.write_u64(fingerprint.0);
+        self.write_u64(fingerprint.0.wrapping_add(fingerprint.1));
     }
 }