Use on-disk-hash-table format for DefPathHashMap in hir::definitions.

compiler/rustc_hir/src/def_path_hash_map.rs

@@ -0,0 +1,37 @@
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_span::def_id::{DefIndex, DefPathHash};
+
+#[derive(Clone, Default)]
+pub struct Config;
+
+impl odht::Config for Config {
+    type Key = DefPathHash;
+    type Value = DefIndex;
+
+    type EncodedKey = [u8; 16];
+    type EncodedValue = [u8; 4];
+
+    type H = odht::UnHashFn;
+
+    #[inline]
+    fn encode_key(k: &DefPathHash) -> [u8; 16] {
+        k.0.to_le_bytes()
+    }
+
+    #[inline]
+    fn encode_value(v: &DefIndex) -> [u8; 4] {
+        v.as_u32().to_le_bytes()
+    }
+
+    #[inline]
+    fn decode_key(k: &[u8; 16]) -> DefPathHash {
+        DefPathHash(Fingerprint::from_le_bytes(*k))
+    }
+
+    #[inline]
+    fn decode_value(v: &[u8; 4]) -> DefIndex {
+        DefIndex::from_u32(u32::from_le_bytes(*v))
+    }
+}
+
+pub type DefPathHashMap = odht::HashTableOwned<Config>;