From: Geoffry Song <goffrie@dropbox.com>
Date: Fri, 22 Sep 2017 06:04:17 +0000 (-0700)
Subject: Use memoized hashes when hashing Fingerprint.
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Use memoized hashes when hashing Fingerprint.

The recursive hashing of dependencies can cause exponential blowup. We
already have a memoized hash available, so use that, Merkle-tree-style.
---

diff --git a/src/cargo/ops/cargo_rustc/fingerprint.rs b/src/cargo/ops/cargo_rustc/fingerprint.rs
index 38cef8f1c..e4d2dd124 100644
--- a/src/cargo/ops/cargo_rustc/fingerprint.rs
+++ b/src/cargo/ops/cargo_rustc/fingerprint.rs
@@ -297,7 +297,14 @@ impl hash::Hash for Fingerprint {
             memoized_hash: _,
             ref rustflags,
         } = *self;
-        (rustc, features, target, profile, deps, local, rustflags).hash(h)
+        (rustc, features, target, profile, local, rustflags).hash(h);
+
+        h.write_usize(deps.len());
+        for &(ref name, ref fingerprint) in deps {
+            name.hash(h);
+            // use memoized dep hashes to avoid exponential blowup
+            h.write_u64(Fingerprint::hash(fingerprint));
+        }
     }
 }