mir: use FiniteBitSet<u32> in polymorphization

This commit changes polymorphization to return a `FiniteBitSet<u32>`
rather than a `FiniteBitSet<u64>` because most functions do not use
anywhere near sixty-four generic parameters so keeping a `u64` around is
unnecessary in most cases.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_index/bit_set.rs

@@ -1034,6 +1034,30 @@ pub trait FiniteBitSetTy:
     fn checked_shr(self, rhs: u32) -> Option<Self>;
 }
 
+impl FiniteBitSetTy for u32 {
+    const DOMAIN_SIZE: u32 = 32;
+
+    const FILLED: Self = Self::MAX;
+    const EMPTY: Self = Self::MIN;
+
+    const ONE: Self = 1u32;
+    const ZERO: Self = 0u32;
+
+    fn checked_shl(self, rhs: u32) -> Option<Self> {
+        self.checked_shl(rhs)
+    }
+
+    fn checked_shr(self, rhs: u32) -> Option<Self> {
+        self.checked_shr(rhs)
+    }
+}
+
+impl std::fmt::Debug for FiniteBitSet<u32> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:032b}", self.0)
+    }
+}
+
 impl FiniteBitSetTy for u64 {
     const DOMAIN_SIZE: u32 = 64;
 

src/librustc_metadata/rmeta/decoder.rs

@@ -1150,7 +1150,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .decode((self, tcx))
     }
 
-    fn get_unused_generic_params(&self, id: DefIndex) -> FiniteBitSet<u64> {
+    fn get_unused_generic_params(&self, id: DefIndex) -> FiniteBitSet<u32> {
         self.root
             .tables
             .unused_generic_params

src/librustc_metadata/rmeta/mod.rs

@@ -285,7 +285,7 @@ define_tables! {
     super_predicates: Table<DefIndex, Lazy!(ty::GenericPredicates<'tcx>)>,
     mir: Table<DefIndex, Lazy!(mir::Body<'tcx>)>,
     promoted_mir: Table<DefIndex, Lazy!(IndexVec<mir::Promoted, mir::Body<'tcx>>)>,
-    unused_generic_params: Table<DefIndex, Lazy<FiniteBitSet<u64>>>,
+    unused_generic_params: Table<DefIndex, Lazy<FiniteBitSet<u32>>>,
 }
 
 #[derive(Copy, Clone, RustcEncodable, RustcDecodable)]

src/librustc_middle/query/mod.rs

@@ -1319,7 +1319,7 @@ rustc_queries! {
         query codegen_unit(_: Symbol) -> &'tcx CodegenUnit<'tcx> {
             desc { "codegen_unit" }
         }
-        query unused_generic_params(key: DefId) -> FiniteBitSet<u64> {
+        query unused_generic_params(key: DefId) -> FiniteBitSet<u32> {
             cache_on_disk_if { key.is_local() }
             desc {
                 |tcx| "determining which generic parameters are unused by `{}`",

src/librustc_mir/monomorphize/polymorphize.rs

@@ -28,7 +28,7 @@ pub fn provide(providers: &mut Providers) {
 /// Determine which generic parameters are used by the function/method/closure represented by
 /// `def_id`. Returns a bitset where bits representing unused parameters are set (`is_empty`
 /// indicates all parameters are used).
-fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u64> {
+fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u32> {
     debug!("unused_generic_params({:?})", def_id);
 
     if !tcx.sess.opts.debugging_opts.polymorphize {
@@ -53,7 +53,7 @@ fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u64> {
     // Create a bitset with N rightmost ones for each parameter.
     let generics_count: u32 =
         generics.count().try_into().expect("more generic parameters than can fit into a `u32`");
-    let mut unused_parameters = FiniteBitSet::<u64>::new_empty();
+    let mut unused_parameters = FiniteBitSet::<u32>::new_empty();
     unused_parameters.set_range(0..generics_count);
     debug!("unused_generic_params: (start) unused_parameters={:?}", unused_parameters);
     mark_used_by_default_parameters(tcx, def_id, generics, &mut unused_parameters);
@@ -84,7 +84,7 @@ fn mark_used_by_default_parameters<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
     generics: &'tcx ty::Generics,
-    unused_parameters: &mut FiniteBitSet<u64>,
+    unused_parameters: &mut FiniteBitSet<u32>,
 ) {
     if !tcx.is_trait(def_id) && (tcx.is_closure(def_id) || tcx.type_of(def_id).is_generator()) {
         for param in &generics.params {
@@ -110,11 +110,11 @@ fn mark_used_by_default_parameters<'tcx>(
 fn mark_used_by_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
-    unused_parameters: &mut FiniteBitSet<u64>,
+    unused_parameters: &mut FiniteBitSet<u32>,
 ) {
     let def_id = tcx.closure_base_def_id(def_id);
 
-    let is_self_ty_used = |unused_parameters: &mut FiniteBitSet<u64>, self_ty: Ty<'tcx>| {
+    let is_self_ty_used = |unused_parameters: &mut FiniteBitSet<u32>, self_ty: Ty<'tcx>| {
         debug!("unused_generic_params: self_ty={:?}", self_ty);
         if let ty::Param(param) = self_ty.kind {
             !unused_parameters.contains(param.index).unwrap_or(false)
@@ -123,7 +123,7 @@ fn mark_used_by_predicates<'tcx>(
         }
     };
 
-    let mark_ty = |unused_parameters: &mut FiniteBitSet<u64>, ty: Ty<'tcx>| {
+    let mark_ty = |unused_parameters: &mut FiniteBitSet<u32>, ty: Ty<'tcx>| {
         let mut vis = UsedGenericParametersVisitor { tcx, def_id, unused_parameters };
         ty.visit_with(&mut vis);
     };
@@ -159,7 +159,7 @@ fn emit_unused_generic_params_error<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
     generics: &'tcx ty::Generics,
-    unused_parameters: &FiniteBitSet<u64>,
+    unused_parameters: &FiniteBitSet<u32>,
 ) {
     debug!("emit_unused_generic_params_error: def_id={:?}", def_id);
     let base_def_id = tcx.closure_base_def_id(def_id);
@@ -195,7 +195,7 @@ fn emit_unused_generic_params_error<'tcx>(
 struct UsedGenericParametersVisitor<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
-    unused_parameters: &'a mut FiniteBitSet<u64>,
+    unused_parameters: &'a mut FiniteBitSet<u32>,
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for UsedGenericParametersVisitor<'a, 'tcx> {