Use unused_generic_params from crate metadata

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

@@ -226,7 +226,15 @@ fn into_args(self) -> (DefId, SimplifiedType) {
     lookup_default_body_stability => { table }
     lookup_deprecation_entry => { table }
     params_in_repr => { table }
-    unused_generic_params => { 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)))
+    }
     opt_def_kind => { table_direct }
     impl_parent => { table }
     impl_polarity => { table_direct }

compiler/rustc_middle/src/query/keys.rs

@@ -63,7 +63,7 @@ impl<'tcx> Key for ty::InstanceDef<'tcx> {
 
     #[inline(always)]
     fn query_crate_is_local(&self) -> bool {
-        true
+        self.def_id().is_local()
     }
 
     fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
@@ -76,7 +76,7 @@ impl<'tcx> Key for ty::Instance<'tcx> {
 
     #[inline(always)]
     fn query_crate_is_local(&self) -> bool {
-        true
+        self.def_id().is_local()
     }
 
     fn default_span(&self, tcx: TyCtxt<'_>) -> Span {

compiler/rustc_monomorphize/src/polymorphize.rs

@@ -36,6 +36,8 @@ fn unused_generic_params<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: ty::InstanceDef<'tcx>,
 ) -> UnusedGenericParams {
+    assert!(instance.def_id().is_local());
+
     if !tcx.sess.opts.unstable_opts.polymorphize {
         // If polymorphization disabled, then all parameters are used.
         return UnusedGenericParams::new_all_used();
@@ -100,13 +102,6 @@ fn should_polymorphize<'tcx>(
         return false;
     }
 
-    // Polymorphization results are stored in cross-crate metadata only when there are unused
-    // parameters, so assume that non-local items must have only used parameters (else this query
-    // would not be invoked, and the cross-crate metadata used instead).
-    if !def_id.is_local() {
-        return false;
-    }
-
     // Foreign items have no bodies to analyze.
     if tcx.is_foreign_item(def_id) {
         return false;

tests/codegen-units/polymorphization/auxiliary/poly-dep.rs

@@ -0,0 +1,4 @@
+// compile-flags: -Zpolymorphize=on
+
+#[inline(never)]
+pub fn foo<T>() {}

tests/codegen-units/polymorphization/poly-foreign.rs

@@ -0,0 +1,11 @@
+// aux-build:poly-dep.rs
+// compile-flags: --crate-type=lib -Zprint-mono-items=eager -Zpolymorphize=on
+
+extern crate poly_dep;
+
+pub static FN1: fn() = poly_dep::foo::<i32>;
+pub static FN2: fn() = poly_dep::foo::<u32>;
+
+//~ MONO_ITEM static FN1
+//~ MONO_ITEM static FN2
+//~ MONO_ITEM fn poly_dep::foo::<T>