Only compute the crate hash when necessary.

The crate hash is needed:
- if debug assertions are enabled, or
- if incr. comp. is enabled, or
- if metadata is being generated, or
- if `-C instrumentation-coverage` is enabled.

This commit avoids computing the crate hash when these conditions are
all false, such as when doing a release build of a binary crate.

It uses `Option` to store the hashes when needed, rather than
computing them on demand, because some of them are needed in multiple
places and computing them on demand would make compilation slower.

The commit also removes `Owner::hash_without_bodies`. There is no
benefit to pre-computing that one, it can just be done in the normal
fashion.

1 files changed, 24 insertions, 31 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index b20157f2c7c..234d88a1ac6 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -463,8 +463,10 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
         rustc_span::hygiene::clear_syntax_context_map();
     }
 
-    let hir_hash = compute_hir_hash(tcx, &owners);
-    hir::Crate { owners, hir_hash }
+    // Don't hash unless necessary, because it's expensive.
+    let opt_hir_hash =
+        if tcx.sess.needs_crate_hash() { Some(compute_hir_hash(tcx, &owners)) } else { None };
+    hir::Crate { owners, opt_hir_hash }
 }
 
 #[derive(Copy, Clone, PartialEq, Debug)]
@@ -657,42 +659,33 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         bodies.sort_by_key(|(k, _)| *k);
         let bodies = SortedMap::from_presorted_elements(bodies);
-        let (hash_including_bodies, hash_without_bodies) = self.hash_owner(node, &bodies);
-        let (nodes, parenting) =
-            index::index_hir(self.tcx.sess, &*self.tcx.definitions_untracked(), node, &bodies);
-        let nodes = hir::OwnerNodes { hash_including_bodies, hash_without_bodies, nodes, bodies };
-        let attrs = {
-            let hash = self.tcx.with_stable_hashing_context(|mut hcx| {
+
+        // Don't hash unless necessary, because it's expensive.
+        let (opt_hash_including_bodies, attrs_hash) = if self.tcx.sess.needs_crate_hash() {
+            self.tcx.with_stable_hashing_context(|mut hcx| {
+                let mut stable_hasher = StableHasher::new();
+                hcx.with_hir_bodies(node.def_id(), &bodies, |hcx| {
+                    node.hash_stable(hcx, &mut stable_hasher)
+                });
+                let h1 = stable_hasher.finish();
+
                 let mut stable_hasher = StableHasher::new();
                 attrs.hash_stable(&mut hcx, &mut stable_hasher);
-                stable_hasher.finish()
-            });
-            hir::AttributeMap { map: attrs, hash }
+                let h2 = stable_hasher.finish();
+
+                (Some(h1), Some(h2))
+            })
+        } else {
+            (None, None)
         };
+        let (nodes, parenting) =
+            index::index_hir(self.tcx.sess, &*self.tcx.definitions_untracked(), node, &bodies);
+        let nodes = hir::OwnerNodes { opt_hash_including_bodies, nodes, bodies };
+        let attrs = hir::AttributeMap { map: attrs, opt_hash: attrs_hash };
 
         self.arena.alloc(hir::OwnerInfo { nodes, parenting, attrs, trait_map })
     }
 
-    /// Hash the HIR node twice, one deep and one shallow hash. This allows to differentiate
-    /// queries which depend on the full HIR tree and those which only depend on the item signature.
-    fn hash_owner(
-        &mut self,
-        node: hir::OwnerNode<'hir>,
-        bodies: &SortedMap<hir::ItemLocalId, &'hir hir::Body<'hir>>,
-    ) -> (Fingerprint, Fingerprint) {
-        self.tcx.with_stable_hashing_context(|mut hcx| {
-            let mut stable_hasher = StableHasher::new();
-            hcx.with_hir_bodies(node.def_id(), bodies, |hcx| {
-                node.hash_stable(hcx, &mut stable_hasher)
-            });
-            let hash_including_bodies = stable_hasher.finish();
-            let mut stable_hasher = StableHasher::new();
-            hcx.without_hir_bodies(|hcx| node.hash_stable(hcx, &mut stable_hasher));
-            let hash_without_bodies = stable_hasher.finish();
-            (hash_including_bodies, hash_without_bodies)
-        })
-    }
-
     /// This method allocates a new `HirId` for the given `NodeId` and stores it in
     /// the `LoweringContext`'s `NodeId => HirId` map.
     /// Take care not to call this method if the resulting `HirId` is then not