Mark built-in providers as hashable

Built-in providers always have an immutable `hashCode`. Making them hashable makes it easier to deduplicate a list of providers, which is sometimes required as the list of provider instances returned by a rule must not contain two instances for a given provider.

src/main/java/com/google/devtools/build/lib/packages/BuiltinProvider.java

@@ -65,6 +65,11 @@ public final int hashCode() {
     return getClass().hashCode();
   }
 
+  @Override
+  public void checkHashable() {
+    // The hash code is based on the class, so it is hashable.
+  }
+
   @Override
   public boolean isExported() {
     return true;

src/test/java/com/google/devtools/build/lib/starlark/StarlarkRuleImplementationFunctionsTest.java

@@ -78,6 +78,7 @@
 import java.util.regex.Pattern;
 import net.starlark.java.annot.Param;
 import net.starlark.java.annot.StarlarkMethod;
+import net.starlark.java.eval.Dict;
 import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Mutability;
 import net.starlark.java.eval.Printer;
@@ -4025,4 +4026,11 @@ def _impl(ctx):
     assertThat(a2.getRoot().getExecPathString())
         .matches(getRelativeOutputPath() + "/[\\w\\-]+\\-exec/bin");
   }
+
+  @Test
+  public void testHashableProviders() throws Exception {
+    ev.execAndExport("p = provider()");
+    Dict<?, ?> dict = (Dict<?, ?>) ev.eval("{k: None for k in [DefaultInfo, p, DefaultInfo, p]}");
+    assertThat(dict.size()).isEqualTo(2);
+  }
 }