Memory-map the dep-graph instead of reading it up front

Cargo.lock

@@ -4371,6 +4371,7 @@ dependencies = [
 name = "rustc_query_system"
 version = "0.0.0"
 dependencies = [
+ "hashbrown",
  "parking_lot 0.11.2",
  "rustc-rayon-core",
  "rustc_arena",

compiler/rustc_incremental/src/persist/load.rs

@@ -204,7 +204,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
                     return LoadResult::DataOutOfDate;
                 }
 
-                let dep_graph = SerializedDepGraph::decode(&mut decoder);
+                let dep_graph = SerializedDepGraph::decode(bytes);
 
                 LoadResult::Ok { data: (dep_graph, prev_work_products) }
             }

compiler/rustc_query_system/Cargo.toml

@@ -8,6 +8,7 @@ doctest = false
 
 [dependencies]
 rustc_arena = { path = "../rustc_arena" }
+hashbrown = { version = "0.12.0", features = ["raw"] }
 tracing = "0.1"
 rustc-rayon-core = { version = "0.4.0", optional = true }
 rustc_ast = { path = "../rustc_ast" }

compiler/rustc_query_system/src/dep_graph/dep_node.rs

@@ -50,7 +50,7 @@ use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use std::fmt;
 use std::hash::Hash;
 
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Encodable, Decodable)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct DepNode<K> {
     pub kind: K,
     pub hash: PackedFingerprint,

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -731,8 +731,7 @@ impl<K: DepKind> DepGraph<K> {
         debug_assert_eq!(data.previous.index_to_node(prev_dep_node_index), *dep_node);
 
         let prev_deps = data.previous.edge_targets_from(prev_dep_node_index);
-
-        for &dep_dep_node_index in prev_deps {
+        for dep_dep_node_index in prev_deps {
             self.try_mark_parent_green(tcx, data, dep_dep_node_index, dep_node)?
         }
 
@@ -1181,8 +1180,7 @@ impl<K: DepKind> CurrentDepGraph<K> {
                     prev_graph.fingerprint_by_index(prev_index),
                     prev_graph
                         .edge_targets_from(prev_index)
-                        .iter()
-                        .map(|i| prev_index_to_index[*i].unwrap())
+                        .map(|i| prev_index_to_index[i].unwrap())
                         .collect(),
                 );
                 prev_index_to_index[prev_index] = Some(dep_node_index);

compiler/rustc_query_system/src/dep_graph/mod.rs

@@ -13,7 +13,7 @@ pub use serialized::{SerializedDepGraph, SerializedDepNodeIndex};
 
 use crate::ich::StableHashingContext;
 use rustc_data_structures::profiling::SelfProfilerRef;
-use rustc_serialize::{opaque::FileEncoder, Encodable};
+use rustc_serialize::MmapSafe;
 use rustc_session::Session;
 
 use std::fmt;
@@ -84,7 +84,7 @@ impl FingerprintStyle {
 }
 
 /// Describe the different families of dependency nodes.
-pub trait DepKind: Copy + fmt::Debug + Eq + Hash + Send + Encodable<FileEncoder> + 'static {
+pub trait DepKind: Copy + fmt::Debug + Eq + Hash + Send + MmapSafe + 'static {
     /// DepKind to use when incr. comp. is turned off.
     const NULL: Self;