SVH: Don't hash the HIR twice when once is enough.

The SVH (Strict Version Hash) of a crate is currently computed
by hashing the ICHes (Incremental Computation Hashes) of the
crate's HIR. This is fine, expect that for incr. comp. we compute
two ICH values for each HIR item, one for the complete item and
one that just includes the item's interface. The two hashes are
are needed for dependency tracking but if we are compiling
non-incrementally and just need the ICH values for the SVH,
one of them is enough, giving us the opportunity to save some
work in this case.

src/librustc_incremental/calculate_svh/mod.rs

@@ -99,6 +99,13 @@ impl<'a, 'tcx: 'a> ComputeItemHashesVisitor<'a, 'tcx> {
                                               item_like: T)
         where T: HashStable<StableHashingContext<'a, 'tcx>>
     {
+        if !hash_bodies && !self.hcx.tcx().sess.opts.build_dep_graph() {
+            // If we just need the hashes in order to compute the SVH, we don't
+            // need have two hashes per item. Just the one containing also the
+            // item's body is sufficient.
+            return
+        }
+
         let mut hasher = IchHasher::new();
         self.hcx.while_hashing_hir_bodies(hash_bodies, |hcx| {
             item_like.hash_stable(hcx, &mut hasher);
@@ -143,7 +150,7 @@ impl<'a, 'tcx: 'a> ComputeItemHashesVisitor<'a, 'tcx> {
                                (item_dep_node, item_hash)
                            })
                            .collect();
-            item_hashes.sort(); // avoid artificial dependencies on item ordering
+            item_hashes.sort_unstable(); // avoid artificial dependencies on item ordering
             item_hashes.hash(&mut crate_state);
         }
 

src/librustc_incremental/lib.rs

@@ -23,6 +23,7 @@
 #![feature(staged_api)]
 #![feature(rand)]
 #![feature(conservative_impl_trait)]
+#![feature(sort_unstable)]
 #![cfg_attr(stage0, feature(pub_restricted))]
 
 extern crate graphviz;