Add a macro for defaulted enum encoding, use it for hir::Defaultness and deduplicate the logic for other similar enums

compiler/rustc_hir/src/hir.rs

@@ -3841,8 +3841,12 @@ impl IsAsync {
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable, Decodable, HashStable_Generic)]
+#[derive(Default)]
 pub enum Defaultness {
-    Default { has_value: bool },
+    Default {
+        has_value: bool,
+    },
+    #[default]
     Final,
 }
 
@@ -4231,8 +4235,8 @@ impl fmt::Display for Safety {
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable, Decodable, HashStable_Generic)]
 #[derive(Default)]
 pub enum Constness {
-    Const,
     #[default]
+    Const,
     NotConst,
 }
 

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1753,7 +1753,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let item = tcx.associated_item(def_id);
 
         if matches!(item.container, AssocContainer::Trait | AssocContainer::TraitImpl(_)) {
-            self.tables.defaultness.set_some(def_id.index, item.defaultness(tcx));
+            self.tables.defaultness.set(def_id.index, item.defaultness(tcx));
         }
 
         record!(self.tables.assoc_container[def_id] <- item.container);
@@ -2155,7 +2155,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 let header = tcx.impl_trait_header(def_id);
                 record!(self.tables.impl_trait_header[def_id] <- header);
 
-                self.tables.defaultness.set_some(def_id.index, tcx.defaultness(def_id));
+                self.tables.defaultness.set(def_id.index, tcx.defaultness(def_id));
 
                 let trait_ref = header.trait_ref.instantiate_identity();
                 let simplified_self_ty = fast_reject::simplify_type(

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -403,6 +403,7 @@ define_tables! {
     asyncness: Table<DefIndex, ty::Asyncness>,
     constness: Table<DefIndex, hir::Constness>,
     safety: Table<DefIndex, hir::Safety>,
+    defaultness: Table<DefIndex, hir::Defaultness>,
 
 - optional:
     attributes: Table<DefIndex, LazyArray<hir::Attribute>>,
@@ -436,7 +437,6 @@ define_tables! {
     thir_abstract_const: Table<DefIndex, LazyValue<ty::EarlyBinder<'static, ty::Const<'static>>>>,
     impl_parent: Table<DefIndex, RawDefId>,
     const_conditions: Table<DefIndex, LazyValue<ty::ConstConditions<'static>>>,
-    defaultness: Table<DefIndex, hir::Defaultness>,
     // FIXME(eddyb) perhaps compute this on the fly if cheap enough?
     coerce_unsized_info: Table<DefIndex, LazyValue<ty::adjustment::CoerceUnsizedInfo>>,
     mir_const_qualif: Table<DefIndex, LazyValue<mir::ConstQualifs>>,

compiler/rustc_metadata/src/rmeta/table.rs

@@ -25,33 +25,6 @@ impl IsDefault for bool {
     }
 }
 
-impl IsDefault for ty::Asyncness {
-    fn is_default(&self) -> bool {
-        match self {
-            ty::Asyncness::Yes => false,
-            ty::Asyncness::No => true,
-        }
-    }
-}
-
-impl IsDefault for hir::Constness {
-    fn is_default(&self) -> bool {
-        match self {
-            hir::Constness::Const => false,
-            hir::Constness::NotConst => true,
-        }
-    }
-}
-
-impl IsDefault for hir::Safety {
-    fn is_default(&self) -> bool {
-        match self {
-            hir::Safety::Safe => false,
-            hir::Safety::Unsafe => true,
-        }
-    }
-}
-
 impl IsDefault for u32 {
     fn is_default(&self) -> bool {
         *self == 0
@@ -139,6 +112,43 @@ macro_rules! fixed_size_enum {
     }
 }
 
+macro_rules! defaulted_enum {
+    ($ty:ty { $(($($pat:tt)*))* } $( unreachable { $(($($upat:tt)*))+ } )?) => {
+        impl FixedSizeEncoding for $ty {
+            type ByteArray = [u8; 1];
+
+            #[inline]
+            fn from_bytes(b: &[u8; 1]) -> Self {
+                use $ty::*;
+                let val = match b[0] {
+                    $(${index()} => $($pat)*,)*
+                    _ => panic!("Unexpected {} code: {:?}", stringify!($ty), b[0]),
+                };
+                // Make sure the first entry is always the default value,
+                // and none of the other values are the default value
+                debug_assert_ne!((b[0] != 0), IsDefault::is_default(&val));
+                val
+            }
+
+            #[inline]
+            fn write_to_bytes(self, b: &mut [u8; 1]) {
+                debug_assert!(!IsDefault::is_default(&self));
+                use $ty::*;
+                b[0] = match self {
+                    $($($pat)* => ${index()},)*
+                    $($($($upat)*)|+ => unreachable!(),)?
+                };
+                debug_assert_ne!(b[0], 0);
+            }
+        }
+        impl IsDefault for $ty {
+            fn is_default(&self) -> bool {
+                <$ty as Default>::default() == *self
+            }
+        }
+    }
+}
+
 // Workaround; need const traits to construct bitflags in a const
 macro_rules! const_macro_kinds {
     ($($name:ident),+$(,)?) => (MacroKinds::from_bits_truncate($(MacroKinds::$name.bits())|+))
@@ -205,7 +215,7 @@ fixed_size_enum! {
     }
 }
 
-fixed_size_enum! {
+defaulted_enum! {
     hir::Defaultness {
         ( Final                        )
         ( Default { has_value: false } )
@@ -213,6 +223,27 @@ fixed_size_enum! {
     }
 }
 
+defaulted_enum! {
+    ty::Asyncness {
+        ( No  )
+        ( Yes )
+    }
+}
+
+defaulted_enum! {
+    hir::Constness {
+        ( Const    )
+        ( NotConst )
+    }
+}
+
+defaulted_enum! {
+    hir::Safety {
+        ( Unsafe )
+        ( Safe   )
+    }
+}
+
 fixed_size_enum! {
     hir::CoroutineKind {
         ( Coroutine(hir::Movability::Movable)                                          )
@@ -301,72 +332,6 @@ impl FixedSizeEncoding for bool {
     }
 }
 
-impl FixedSizeEncoding for ty::Asyncness {
-    type ByteArray = [u8; 1];
-
-    #[inline]
-    fn from_bytes(b: &[u8; 1]) -> Self {
-        match b[0] {
-            0 => ty::Asyncness::No,
-            1 => ty::Asyncness::Yes,
-            _ => unreachable!(),
-        }
-    }
-
-    #[inline]
-    fn write_to_bytes(self, b: &mut [u8; 1]) {
-        debug_assert!(!self.is_default());
-        b[0] = match self {
-            ty::Asyncness::No => 0,
-            ty::Asyncness::Yes => 1,
-        }
-    }
-}
-
-impl FixedSizeEncoding for hir::Constness {
-    type ByteArray = [u8; 1];
-
-    #[inline]
-    fn from_bytes(b: &[u8; 1]) -> Self {
-        match b[0] {
-            0 => hir::Constness::NotConst,
-            1 => hir::Constness::Const,
-            _ => unreachable!(),
-        }
-    }
-
-    #[inline]
-    fn write_to_bytes(self, b: &mut [u8; 1]) {
-        debug_assert!(!self.is_default());
-        b[0] = match self {
-            hir::Constness::NotConst => 0,
-            hir::Constness::Const => 1,
-        }
-    }
-}
-
-impl FixedSizeEncoding for hir::Safety {
-    type ByteArray = [u8; 1];
-
-    #[inline]
-    fn from_bytes(b: &[u8; 1]) -> Self {
-        match b[0] {
-            0 => hir::Safety::Unsafe,
-            1 => hir::Safety::Safe,
-            _ => unreachable!(),
-        }
-    }
-
-    #[inline]
-    fn write_to_bytes(self, b: &mut [u8; 1]) {
-        debug_assert!(!self.is_default());
-        b[0] = match self {
-            hir::Safety::Unsafe => 0,
-            hir::Safety::Safe => 1,
-        }
-    }
-}
-
 // NOTE(eddyb) there could be an impl for `usize`, which would enable a more
 // generic `LazyValue<T>` impl, but in the general case we might not need / want
 // to fit every `usize` in `u32`.