rustc_metadata: Encode/decode DefPathHashes without an Option

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1319,7 +1319,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     ) -> DefPathHash {
         *def_path_hashes
             .entry(index)
-            .or_insert_with(|| self.root.tables.def_path_hashes.get(self, index).unwrap())
+            .or_insert_with(|| self.root.tables.def_path_hashes.get(self, index))
     }
 
     #[inline]

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -478,13 +478,13 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 let def_key = self.lazy(table.def_key(def_index));
                 let def_path_hash = table.def_path_hash(def_index);
                 self.tables.def_keys.set_some(def_index, def_key);
-                self.tables.def_path_hashes.set_some(def_index, def_path_hash);
+                self.tables.def_path_hashes.set(def_index, def_path_hash);
             }
         } else {
             for (def_index, def_key, def_path_hash) in table.enumerated_keys_and_path_hashes() {
                 let def_key = self.lazy(def_key);
                 self.tables.def_keys.set_some(def_index, def_key);
-                self.tables.def_path_hashes.set_some(def_index, *def_path_hash);
+                self.tables.def_path_hashes.set(def_index, *def_path_hash);
             }
         }
     }

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -350,6 +350,7 @@ define_tables! {
     is_macro_rules: Table<DefIndex, bool>,
     is_type_alias_impl_trait: Table<DefIndex, bool>,
     attr_flags: Table<DefIndex, AttrFlags>,
+    def_path_hashes: Table<DefIndex, DefPathHash>,
     explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,
     inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
     inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
@@ -403,7 +404,6 @@ define_tables! {
     // `DefPathTable` up front, since we may only ever use a few
     // definitions from any given crate.
     def_keys: Table<DefIndex, LazyValue<DefKey>>,
-    def_path_hashes: Table<DefIndex, DefPathHash>,
     proc_macro_quoted_spans: Table<usize, LazyValue<Span>>,
     generator_diagnostic_data: Table<DefIndex, LazyValue<GeneratorDiagnosticData<'static>>>,
     variant_data: Table<DefIndex, LazyValue<VariantData>>,

compiler/rustc_metadata/src/rmeta/table.rs

@@ -44,6 +44,12 @@ impl<T> IsDefault for LazyArray<T> {
     }
 }
 
+impl IsDefault for DefPathHash {
+    fn is_default(&self) -> bool {
+        self.0 == Fingerprint::ZERO
+    }
+}
+
 /// Helper trait, for encoding to, and decoding from, a fixed number of bytes.
 /// Used mainly for Lazy positions and lengths.
 /// Unchecked invariant: `Self::default()` should encode as `[0; BYTE_LEN]`,
@@ -191,21 +197,18 @@ fixed_size_enum! {
 }
 
 // We directly encode `DefPathHash` because a `LazyValue` would incur a 25% cost.
-impl FixedSizeEncoding for Option<DefPathHash> {
+impl FixedSizeEncoding for DefPathHash {
     type ByteArray = [u8; 16];
 
     #[inline]
     fn from_bytes(b: &[u8; 16]) -> Self {
-        // NOTE: There's a collision between `None` and `Some(0)`.
-        Some(DefPathHash(Fingerprint::from_le_bytes(*b)))
+        DefPathHash(Fingerprint::from_le_bytes(*b))
     }
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 16]) {
-        match self {
-            None => unreachable!(),
-            Some(DefPathHash(fingerprint)) => *b = fingerprint.to_le_bytes(),
-        }
+        debug_assert!(!self.is_default());
+        *b = self.0.to_le_bytes();
     }
 }
 

compiler/rustc_span/src/def_id.rs

@@ -119,6 +119,12 @@ impl DefPathHash {
     }
 }
 
+impl Default for DefPathHash {
+    fn default() -> Self {
+        DefPathHash(Fingerprint::ZERO)
+    }
+}
+
 impl Borrow<Fingerprint> for DefPathHash {
     #[inline]
     fn borrow(&self) -> &Fingerprint {