Implement FixedSizeEncoding for UnusedGenericParams.

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -226,15 +226,7 @@ provide! { tcx, def_id, other, cdata,
     lookup_default_body_stability => { table }
     lookup_deprecation_entry => { table }
     params_in_repr => { table }
-    // FIXME: Could be defaulted, but `LazyValue<UnusedGenericParams>` is not `FixedSizeEncoding`..
-    unused_generic_params => {
-        cdata
-            .root
-            .tables
-            .unused_generic_params
-            .get(cdata, def_id.index)
-            .map_or_else(|| ty::UnusedGenericParams::new_all_used(), |lazy| lazy.decode((cdata, tcx)))
-    }
+    unused_generic_params => { cdata.root.tables.unused_generic_params.get(cdata, def_id.index) }
     opt_def_kind => { table_direct }
     impl_parent => { table }
     impl_polarity => { table_direct }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1440,9 +1440,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             let instance =
                 ty::InstanceDef::Item(ty::WithOptConstParam::unknown(def_id.to_def_id()));
             let unused = tcx.unused_generic_params(instance);
-            if !unused.all_used() {
-                record!(self.tables.unused_generic_params[def_id.to_def_id()] <- unused);
-            }
+            self.tables.unused_generic_params.set(def_id.local_def_index, unused);
         }
 
         // Encode all the deduced parameter attributes for everything that has MIR, even for items

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -356,6 +356,7 @@ define_tables! {
     inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
     associated_items_for_impl_trait_in_trait: Table<DefIndex, LazyArray<DefId>>,
     opt_rpitit_info: Table<DefIndex, Option<LazyValue<ty::ImplTraitInTraitData>>>,
+    unused_generic_params: Table<DefIndex, UnusedGenericParams>,
 
 - optional:
     attributes: Table<DefIndex, LazyArray<ast::Attribute>>,
@@ -398,7 +399,6 @@ define_tables! {
     trait_def: Table<DefIndex, LazyValue<ty::TraitDef>>,
     trait_item_def_id: Table<DefIndex, RawDefId>,
     expn_that_defined: Table<DefIndex, LazyValue<ExpnId>>,
-    unused_generic_params: Table<DefIndex, LazyValue<UnusedGenericParams>>,
     params_in_repr: Table<DefIndex, LazyValue<BitSet<u32>>>,
     repr_options: Table<DefIndex, LazyValue<ReprOptions>>,
     // `def_keys` and `def_path_hashes` represent a lazy version of a

compiler/rustc_metadata/src/rmeta/table.rs

@@ -3,7 +3,7 @@ use crate::rmeta::*;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_hir::def::{CtorKind, CtorOf};
 use rustc_index::vec::Idx;
-use rustc_middle::ty::ParameterizedOverTcx;
+use rustc_middle::ty::{ParameterizedOverTcx, UnusedGenericParams};
 use rustc_serialize::opaque::FileEncoder;
 use rustc_serialize::Encoder as _;
 use rustc_span::hygiene::MacroKind;
@@ -50,6 +50,16 @@ impl IsDefault for DefPathHash {
     }
 }
 
+impl IsDefault for UnusedGenericParams {
+    fn is_default(&self) -> bool {
+        // UnusedGenericParams encodes the *un*usedness as a bitset.
+        // This means that 0 corresponds to all bits used, which is indeed the default.
+        let is_default = self.bits() == 0;
+        debug_assert_eq!(is_default, self.all_used());
+        is_default
+    }
+}
+
 /// 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]`,
@@ -271,6 +281,21 @@ impl FixedSizeEncoding for bool {
     }
 }
 
+impl FixedSizeEncoding for UnusedGenericParams {
+    type ByteArray = [u8; 4];
+
+    #[inline]
+    fn from_bytes(b: &[u8; 4]) -> Self {
+        let x: u32 = u32::from_bytes(b);
+        UnusedGenericParams::from_bits(x)
+    }
+
+    #[inline]
+    fn write_to_bytes(self, b: &mut [u8; 4]) {
+        self.bits().write_to_bytes(b);
+    }
+}
+
 // 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`.

compiler/rustc_middle/src/ty/instance.rs

@@ -781,6 +781,12 @@ fn needs_fn_once_adapter_shim(
 #[derive(Debug, Copy, Clone, Eq, PartialEq, Decodable, Encodable, HashStable)]
 pub struct UnusedGenericParams(FiniteBitSet<u32>);
 
+impl Default for UnusedGenericParams {
+    fn default() -> Self {
+        UnusedGenericParams::new_all_used()
+    }
+}
+
 impl UnusedGenericParams {
     pub fn new_all_unused(amount: u32) -> Self {
         let mut bitset = FiniteBitSet::new_empty();
@@ -807,4 +813,12 @@ impl UnusedGenericParams {
     pub fn all_used(&self) -> bool {
         self.0.is_empty()
     }
+
+    pub fn bits(&self) -> u32 {
+        self.0.0
+    }
+
+    pub fn from_bits(bits: u32) -> UnusedGenericParams {
+        UnusedGenericParams(FiniteBitSet(bits))
+    }
 }